Guide for Use of 

Wood Preservatives 

in Historic Structures

Stan Lebow

Ronald W. Anthony

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General 

Technical 

Report

FPL–GTR–217

Independence Mine Historical Park, Alaska
October 2012

Lebow, Stan; Anthony, Ronald W. 2012. Guide for use of wood preservatives in historic structures. General Technical Report FPL-GTR-217. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products 

Laboratory. 59 p.

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Abstract

This document provides guidance on wood preservation 

options in the context of historic preservation. Preserving 

wooden building materials is critical to historic preservation 

practitioners. Biodeterioration can be minimized through 

design, construction practices, maintenance, and, if necessary, by use of wood preservatives. Moisture is the primary 

cause of biodeterioration, and if exposure to moisture cannot 

be prevented, the application of preservatives or use of 

pressure-treated wood may be warranted. The Secretary of 

Interior’s Standards for the Treatment of Historic Properties emphasize retaining the historic character of a property, 

including distinctive materials, features, and spatial relationships. Existing conditions should be carefully evaluated to 

determine the appropriate level of intervention. 

Wood preservatives are generally grouped into two categories: preservatives used for in-place field (remedial) 

treatment and preservatives used for pressure treatments. 

A limitation of in-place treatments is that they cannot be 

forced deeply into the wood under pressure. However, they 

can be applied into the center of large wooden members 

via treatment holes. These preservatives may be available 

as liquids, rods, or pastes. Pressure-treated wood has much 

deeper and more uniform preservative penetration than 

wood treated with other methods. The type of pressuretreated wood is often dependent on the requirements of the 

specific application. To guide selection of pressure-treated 

wood, the American Wood Protection Association developed 

Use Category System standards. Other preservative characteristics, such as color, odor, and surface oiliness may also 

be relevant. Guidelines for selection and application of field 

treatments and for selection and specification of pressuretreated wood are provided in this document.

Keywords: Guide, historic structures, preservative treatments, remedial treatments, pressure treatments

Acknowledgments

This document was developed under a grant from the 

National Park Service and the National Center for Preservation Technology and Training. Its contents are solely the 

responsibility of the authors and do not necessarily represent 

the official position or policies of the National Park Service 

or the National Center for Preservation Technology and 

Training. The authors express their appreciation to Kimberly 

D. Dugan and Deborah J. Anthony for their contributions to 

the technical content, providing photographs and assistance 

with editing of this document. The authors also thank Ms. 

Mary Striegel from the National Center for Preservation 

Technology and Training for her constructive comments. We 

also gratefully acknowledge the assistance of Madelon Wise 

and Tivoli Gough (USDA, Forest Products Laboratory) for 

their assistance in producing and refining the publication. 

Finally, we express our appreciation to the reviewers whose 

comments greatly improved the content and clarity of the 

guide.

Conversion table

English unit Conversion factor SI unit

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)
Contents

Executive Summary............................................................1

Introduction.........................................................................1

 Intended Audience ..........................................................2

 Wood Preservatives and Historic Preservation 

 Philosophy.......................................................................2

 Wood Preservatives and Pressure-Treated Wood in 

 Historic Preservation.......................................................4

Assessing the Need for Preservative Treatment..................4

 Causes of Wood Degradation...........................................4

 Role of Wood Cellular Structure................................... 11

 Problem Areas for Deterioration in Historic 

 Structures ...................................................................... 11

 Nonpreservative Approaches to Preventing 

 Deterioration ................................................................. 15

Wood Preservative Overview............................................ 17

 What Is a Wood Preservative? ...................................... 17

 When Is Application of Preservatives Appropriate?..... 17

 Historical Use of Wood Preservatives........................... 20

Options for In-Place Preservative Treatment.................... 21

 Characteristics of In-Place Treatments ......................... 21

 Application Guidelines.................................................. 24

 Summary of In-Place Treatment Application 

 Concepts........................................................................ 27

 Example In-Place Treatment Applications.................... 28

 Who Can Apply In-Place Preservative Treatments?..... 30

Using Pressure-Treated Wood........................................... 32

 When to Consider Using Pressure-Treated Wood......... 32

 Preservative Penetration................................................ 33

 Treated Wood Use Category System ............................ 34

 Standardization ............................................................. 34

 Treatment Specifications............................................... 35

 Environmental Considerations for Pressure-Treated 

 Wood ............................................................................. 37

 Alternatives to Pressure-Treated Wood......................... 38

Summary........................................................................... 39

Sources of Information ..................................................... 41

Appendix A—Pressure-Treated Wood Dichotomous 

Key.................................................................................... 44

 A. Sawn Lumber or Timbers......................................... 44

 B. Sawn Lumber or Timbers for Highway 

 Construction............................................................. 46

 C. Round Piles .............................................................. 46

 D. Round Posts.............................................................. 46

 E. Poles ......................................................................... 47

 F. Glued-Laminated Members...................................... 48

 G. Plywood ................................................................... 49

 H. Permanent Wood Foundations ................................. 50

 I. Wood in Seawater...................................................... 51

Appendix B—Description of Pressure Treatment 

Preservatives (Grouped by Exposure Hazard).................. 52

 Applications Protected from Moisture .......................... 52

 Applications Above Ground with Partial Protection..... 52

 Applications Above Ground but Fully Exposed to 

 the Weather.................................................................... 53

 Applications in Direct Contact with the Ground or 

 Fresh Water ................................................................... 54

 Marine (Seawater) Applications ................................... 59

Guide for Use of Wood Preservatives 

in Historic Structures

Stan Lebow, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin

Ronald W. Anthony, Wood Scientist

Anthony & Associates, Inc., Fort Collins, Colorado

Executive Summary

Extending the life of (preserving) wooden building materials 

is critical to historic preservation practitioners. The susceptibility of wood to biodeterioration can be minimized through 

design, construction practices, maintenance, and in some 

cases through treatment of structural members with wood 

preservatives. The goals of this document are to provide a 

foundation for understanding wood preservatives in the context of historic preservation and offer realistic preservation 

options for historic preservation practitioners.

The Secretary of Interior’s Standards for the Treatment of 

Historic Properties place emphasis on retaining the historic 

character of a property, including distinctive materials, 

features, and spatial relationships. Accordingly, a careful 

evaluation of existing conditions should be conducted to 

determine the appropriate level of intervention. Moisture is 

the source of most forms of biodeterioration, and mitigation 

of the moisture conditions is the most effective treatment. If 

continued exposure to moisture is expected, the application 

of preservatives or use of preservative-treated wood may be 

warranted. For distinctive features with severe deterioration, 

repair or limited replacement is preferred over full replacement. Overall, the preservation approach should use the 

gentlest means possible.

Wood preservative treatments are generally grouped into 

two categories. Remedial or in-place field treatments use 

nonpressure preservatives in applications other than pressure treatments. The objective of all these treatments is 

to distribute preservative into areas of a structure that are 

vulnerable to moisture accumulation or not protected by the 

original pressure treatment (if any). A major limitation of 

in-place treatments is that they cannot be forced deep into 

the wood under pressure as is done in pressure treatment 

processes. However, they can be applied into the center of 

large, wooden members via treatment holes. These preservatives may be available as liquids, rods, or pastes. Guidelines 

for selection and application of these treatments are provided in this document.

Preservatives used for pressure treatment represent the second category of wood preservatives. Pressure-treated wood 

has much deeper and more uniform preservative penetration 

than wood treated with other methods. The type of 

preservative applied is often dependent on the requirements 

of the specific application. To guide selection of the types 

of preservatives and loadings appropriate to a specific enduse, the American Wood Protection Association (AWPA) 

developed Use Category System (UCS) standards. Other 

preservative characteristics, such as color, odor, and surface 

oiliness may also be relevant. Best Management Practices 

(BMPs) have been developed to minimize potential environmental impacts from pressure-treated wood. Guidelines for 

selection and specification of preservative-treated wood are 

provided in this document.

Introduction

Wood, as an abundant resource throughout most of the 

world, has been used for thousands of years as a building 

material. The vast majority of the historic buildings in the 

United States have been built primarily of wood, and even 

masonry and stone buildings generally have wooden elements. The preservation of wood as a common historic 

building material is therefore critical to historic preservation 

practitioners. As a biological material, wood is both incredibly complex and yet generally durable if properly used 

and maintained. Susceptibility to biodeterioration can be 

minimized through design, construction, and maintenance 

practices and in some cases through treatment of wooden 

members with wood preservatives.

Wood preservatives and pressure-treated wood repairs appeal to historic preservation practitioners as methods to 

extend the service life of wood elements and historic wood 

buildings. Additionally, these treatments and products are 

sometimes aggressively marketed to foster the erroneous assumption that such treatments or materials represent a cureall for the maintenance needs of wooden buildings. Navigating the vast number of products and marketing claims to 

determine if a treatment or product is suitable for historic 

preservation applications can be a daunting task.

This document seeks to address some of the complex issues 

that can arise when considering the need for, application of, 

and maintenance of field-applied wood preservatives and 

pressure-treated wood in historic preservation applications. 

This manual discusses the suitability of wood preservatives and pressure-treated material within the context of 

the Secretary of the Interior’s Standards for the Treatment 
General Technical Report FPL–GTR–217

2

of Historic Properties, the need for wood preservatives or 

pressure-treated replacement material, the long-term costs 

and maintenance requirements of wood preservatives and 

pressure-treated wood, and the various types of field-applied 

preservatives and pressure-treated wood options available 

for use today. A decision tree has been included in Appendix A to facilitate decisions regarding application or use of 

preservatives and pressure-treated wood in historic building 

projects.

Intended Audience

This document is intended to serve as a reference manual 

for historic preservation practitioners seeking to conserve 

and extend the service life of wood products and structures 

in their care. Many of these approaches can be used by laypersons with minimal technical training. Other preservation 

options that require higher levels of maintenance or a skilled 

technician are also discussed. The goals of this document 

are to provide a foundation for understanding wood preservatives in the context of historic preservation and offer realistic preservation options for historic preservation 

practitioners.

Wood Preservatives and Historic 

Preservation Philosophy

The Secretary of Interior’s Standards for the Treatment of 

Historic Properties (the Standards) provide a philosophical framework for responsible preservation practices for 

all historic resource types. The Secretary of the Interior’s 

Guidelines for Preserving, Rehabilitating, Restoring, and 

Reconstructing Historic Buildings (the Guidelines) apply 

specifically to structural resources (buildings), and were 

developed to facilitate the application of the Standards. The 

Guidelines provide recommended work treatments and techniques that are consistent with the Standards. The Standards 

are based on four treatment options for historic buildings. 

The four treatment options are Preservation, Rehabilitation, Restoration, and Reconstruction. The Standards differ 

for each treatment option, and the subsequent Guidelines 

vary as well. Use of wood preservatives or pressure-treated 

wood as repair or replacement material may or may not be 

an acceptable work treatment depending upon the treatment 

option.

The Standards for each treatment option are reprinted below. 

The Guidelines have been summarized to reflect the suitability of wood preservatives and/or pressure-treated wood 

within each treatment option. Full Guidelines can 

be found on the National Park Service website 

(www.nps.gov/index.htm).

Standards for Preservation

•	 A property will be used as it was historically, or be given a new use that maximizes the retention of distinctive 

materials, features, spaces, and spatial relationships. 

Where treatment and use have not been identified, a 

property will be protected and, if necessary, stabilized 

until additional work may be undertaken.

•	 The historic character of a property will be retained 

and preserved. The replacement of intact or repairable 

historic materials or alteration of features, spaces, and 

spatial relationships that characterize a property will be 

avoided.

•	 Each property will be recognized as a physical record 

of its time, place, and use. Work needed to stabilize, 

consolidate, and conserve existing historic materials 

and features will be physically and visually compatible, 

identifiable upon close inspection, and properly documented for future research.

•	 Changes to a property that have acquired historic significance in their own right will be retained and preserved.

•	 Distinctive materials, features, finishes, and construction techniques or examples of craftsmanship that characterize a property will be preserved.

•	 The existing condition of historic features will be evaluated to determine the appropriate level of intervention 

needed. Where the severity of deterioration requires 

repair or limited replacement of a distinctive feature, 

the new material will match the old in composition, 

design, color, and texture.

•	 Chemical or physical treatments, if appropriate, will be 

undertaken using the gentlest means possible. Treatments that cause damage to historic materials will not 

be used.

•	 Archeological resources will be protected and preserved 

in place. If such resources must be disturbed, mitigation 

measures will be undertaken.

The preservation treatment option is based on an assumption that the historic features of a building remain essentially 

intact. The primary goal of the preservation approach is to 

retain historic fabric through maintenance and repair work; 

replacement of historic fabric should be minimized.

Standards for Rehabilitation

•	 A property will be used as it was historically or be 

given a new use that requires minimal change to its 

distinctive materials, features, spaces, and spatial 

relationships.

•	 The historic character of a property will be retained 

and preserved. The removal of distinctive materials or 

alteration of features, spaces, and spatial relationships 

that characterize a property will be avoided.

•	 Each property will be recognized as a physical record 

of its time, place, and use. Changes that create a false 

sense of historical development, such as adding conjectural features or elements from other historic properties, 

will not be undertaken.
Guide for Use of Wood Preservatives in Historic Structures

3

•	 Changes to a property that have acquired historic significance in their own right will be retained and preserved.

•	 Distinctive materials, features, finishes, and construction techniques or examples of craftsmanship that characterize a property will be preserved.

•	 Deteriorated historic features will be repaired rather 

than replaced. Where the severity of deterioration requires replacement of a distinctive feature, the new 

feature will match the old in design, color, texture, and, 

where possible, materials. Replacement of missing features will be substantiated by documentary and physical 

evidence.

•	 Chemical or physical treatments, if appropriate, will be 

undertaken using the gentlest means possible. Treatments that cause damage to historic materials will not 

be used.

•	 Archeological resources will be protected and preserved 

in place. If such resources must be disturbed, mitigation 

measures will be undertaken.

•	 New additions, exterior alterations, or related new construction will not destroy historic materials, features, 

and spatial relationships that characterize the property. 

The new work shall be differentiated from the old and 

will be compatible with the historic materials, features, 

size, scale and proportion, and massing to protect the 

integrity of the property and its environment.

•	 New additions and adjacent or related new construction 

will be undertaken in such a manner that, if removed in 

the future, the essential form and integrity of the historic property and its environment would be unimpaired.

Rehabilitation treatment is similar in many respects to the 

preservation treatment option, except that it is assumed that 

the historic fabric does not survive intact and that more 

repair and some replacement of material will be necessary. 

Rehabilitation also allows for alterations and additions for 

modernization and alternate uses.

Standards for Restoration

•	 A property will be used as it was historically or be 

given a new use that reflects the property’s restoration 

period.

•	 Materials and features from the restoration period will 

be retained and preserved. The removal of materials or 

alteration of features, spaces, and spatial relationships 

that characterize the period will not be undertaken.

•	 Each property will be recognized as a physical record of 

its time, place, and use. Work needed to stabilize, consolidate, and conserve materials and features from the 

restoration period will be physically and visually compatible, identifiable upon close inspection, and properly 

documented for future research.

•	 Materials, features, spaces, and finishes that characterize other historical periods will be documented prior to 

their alteration or removal.

•	 Distinctive materials, features, finishes, and construction techniques or examples of craftsmanship that characterize the restoration period will be preserved.

•	 Deteriorated features from the restoration period will 

be repaired rather than replaced. Where the severity of 

deterioration requires replacement of a distinctive feature, the new feature will match the old in design, color, 

texture, and, where possible, materials.

•	 Replacement of missing features from the restoration 

period will be substantiated by documentary and physical evidence. A false sense of history will not be created by adding conjectural features, features from other 

properties, or by combining features that never existed 

together historically.

•	 Chemical or physical treatments, if appropriate, will be 

undertaken using the gentlest means possible. Treatments that cause damage to historic materials will not 

be used.

•	 Archeological resources affected by a project will be 

protected and preserved in place. If such resources must 

be disturbed, mitigation measures will be undertaken.

•	 Designs that were never executed historically will not 

be constructed.

In contrast to the preservation and rehabilitation treatment 

options, the intent in restoration is to return a building to its 

original appearance at its most historically significant time 

period. Restoration allows for the removal of historic fabric 

that does not date to the period of significance and allows 

for the replacement of missing features from the restoration 

period.

Standards for Reconstruction

•	 Reconstruction will be used to depict vanished or nonsurviving portions of a property when documentary 

and physical evidence is available to permit accurate 

reconstruction with minimal conjecture, and such reconstruction is essential to the public understanding of 

the property.

•	 Reconstruction of a landscape, building, structure, or 

object in its historic location will be preceded by a 

thorough archeological investigation to identify and 

evaluate those features and artifacts that are essential 

to an accurate reconstruction. If such resources must be 

disturbed, mitigation measures will be undertaken.

•	 Reconstruction will include measures to preserve any 

remaining historic materials, features, and spatial 

relationships.
General Technical Report FPL–GTR–217

4

•	 Reconstruction will be based on the accurate duplication of historic features and elements substantiated by 

documentary or physical evidence rather than on conjectural designs or the availability of different features 

from other historic properties. A reconstructed property 

will re-create the appearance of the nonsurviving historic property in materials, design, color, and texture.

•	 A reconstruction will be clearly identified as a 

contemporary re-creation.

•	 Designs that were never executed historically will not 

be constructed.

The reconstruction treatment option is applied when it is 

necessary to re-create a building that no longer exists. Similar to restoration, the intent is to build a structure that accurately depicts the original building in its most historically 

significant time period. This treatment option is undertaken 

only rarely and has extensive documentation requirements.

The suitability of using wood preservatives or pressuretreated wood as repair or replacement material is determined 

by the treatment philosophy being applied to a specific 

building and by the Guidelines. It is important to note that 

the Guidelines are intended to provide general parameters 

of acceptable and unacceptable work techniques and treatments. Each historic building is unique, and decisions concerning the use of wood preservatives or pressure-treated 

wood must be reached by considering the historical significance of the material to be treated, repaired, or replaced, as 

well as the parameters outlined by the Standards and 

Guidelines.

General Principles for All Treatment Options

Although the four treatment options vary in intent and expressed goals, some common themes exist. Retaining the 

historic character and maximizing the retention of distinctive materials, architectural features, spaces, and spatial 

relationships is integral to all of the treatment options. Another common theme is the evaluation of existing conditions 

to determine the appropriate level of intervention. For distinctive features with severe deterioration, repair or limited 

replacement should be undertaken rather than full replacement. For all treatment options, new material should match 

the old in design, composition, color, and texture as much as 

possible, but compatible substitute materials may be acceptable. No matter the treatment option being followed, chemical or physical treatments, if determined to be appropriate, 

must use the gentlest means possible. Additionally, for all 

treatment options, archaeological resources must be protected and preserved in place or, if they must be disturbed, 

appropriate mitigation measures must be followed.

Rehabilitation-Specific Criteria

The rehabilitation treatment option is the most commonly 

applied and is the only approach that allows for alterations 

and additions to be made. Because of this, rehabilitation has 

special criteria relating to additions and alterations. New 

additions, exterior alterations, or related new construction 

must not destroy historic materials, features, or spatial relationships that characterize the property, and new work must 

be distinguishable from the historic. Additionally, new work 

must be compatible with the historic materials, features, 

size, scale and proportion, and massing to protect and retain 

the integrity of the property and the environment.

Wood Preservatives and Pressure-Treated 

Wood in Historic Preservation

For most historic structures, use of wood preservatives or 

pressure-treated wood becomes a consideration when deterioration has been identified and when there are concerns 

about the long-term serviceability of the wooden elements. 

If moisture problems and subsequent deterioration were 

caused by a lack of maintenance, there is generally no need 

to apply wood preservatives or repair materials with pressure-treated wood, unless the maintenance issues cannot be 

addressed or the project is to be mothballed for a significant 

period of time. If the building has poor drainage conditions 

that cannot be mitigated, or if construction or design flaws 

have led to deterioration, the application of preservatives 

and the use of pressure-treated wood for repairs may be 

warranted.

It is important to note that there will be costs associated with 

wood preservatives beyond the initial product purchase and 

application. Treated historic material and pressure-treated 

replacement materials will require regular inspection and 

maintenance. When undertaking a treatment program or 

when deciding to use pressure-treated materials, it is important to budget for the long-term maintenance costs of the 

treatment or product. In today’s volatile market, new products become available frequently, whereas older products 

are often discontinued. Be aware of the potential for product discontinuity and insure the compatibility of any new 

preservative treatments with old treatments if they are no 

longer available. Additionally, preservative treatments and 

pressure-treated materials contain pesticides that are subject 

to environmental regulations. As perceptions regarding pesticides change, some of the products currently available may 

be restricted. Planning for future changes in environmental 

regulations is an essential step when making the decision to 

apply wood preservatives or use pressure-treated wood as a 

repair or replacement material.

Assessing the Need for Preservative 

Treatment

Causes of Wood Degradation

There are many causes of wood degradation, and often multiple types of degradation can interact to affect a wooden 

member in a structure. Appropriately selected and applied 
Guide for Use of Wood Preservatives in Historic Structures

5

preservative treatments can be highly effective in preventing or stopping some types of degradation, but may be less 

effective or unnecessary for protection against other degradation mechanisms. As with many products, wood preservatives have both risks and benefits and should only be applied 

when the derived benefit outweighs the possible negative 

consequences. Accordingly, some understanding of the 

causes of wood degradation is necessary when considering 

the need for preservative treatments.

Importance of Moisture in Deterioration

Moisture serves as a catalyst for many forms of deterioration and is an integral component of weathering (including 

freeze-thaw action), mold, decay, and insect attack. Moisture stains are not necessarily an indication of damage to 

the wood, but are a record of the wood being exposed to 

water either repeatedly throughout its life or for an extended 

period of time. Moisture can cause nails, screws, and other 

metal fasteners to rust, which can cause additional staining of the wood. Moisture aids in the weathering process 

by causing wood to swell or shrink, thus generating checks 

and splits as the wood fibers expand or contract. Wood that 

is not exposed to environmental weathering or in contact 

with a source of moisture can remain stable for decades or 

centuries.

The role of moisture in biodeterioration, and especially fungal decay, cannot be over-emphasized. Decay fungi require a 

moisture content of at least 20% to sustain any growth, and 

higher moisture contents (over 29%) are required for initial 

spore germination. Most brown- and white-rot decay fungi 

prefer wood in the moisture content range of 40% to 80%. 

Previously established fungi are not necessarily eliminated 

at low moisture contents. Decay fungi have been reported to 

survive (without further growth) for up to 9 years on wood 

at moisture contents around 12%. As the moisture content 

exceeds 80%, void spaces in the wood are increasingly filled 

with water. The subsequent lack of oxygen and build-up of 

carbon dioxide in free water limits fungal growth. Soft-rot 

fungi tolerate higher moisture contents but still cannot colonize wood that is completely saturated. Thus, wood that is 

continually immersed does not suffer damage from decay 

fungi, although it can very slowly degrade because of bacterial growth. This accounts for the longevity of wood in some 

types of structures and the subsequent onset of decay when 

moisture is removed. An example of this phenomenon has 

been occurring along the shore of Lake Michigan in recent 

years, where lowering water levels have allowed decay in 

untreated piles that had previously been immersed.

Moisture also plays a role in damage by insects, although 

some insects can attack wood at lower moisture contents 

than required by fungus. The role of moisture in termite attack varies with termite species. Dampwood termites require 

wood with high moisture content and typically only attack 

wood that is in direct contact with the ground. As a result, 

their impact on wooden structures is relatively minor. Native 

subterranean termites require moisture to prevent desiccation, but can attack wood with moisture content well below 

the fiber saturation point by building shelter tubes from the 

soil and periodically returning to the soil to replenish water 

lost from their bodies. Formosan subterranean termites also 

require a source of moisture to attack wood above ground, 

but are less reliant on proximity to soil for survival. They 

may establish colonies on upper floors of buildings if a consistent source of moisture is present. Drywood termites are 

so named because they are able to survive in wood above 

ground, and can often derive sufficient moisture solely from 

the wood.

Wood-boring beetles can often colonize drier wood than 

either termites or fungi. The most destructive groups, the 

powderpost beetles, can colonize wood at moisture contents 

of 13% or above. Wood indoors in a climate-controlled environment is typically too dry for attack, but wood in poorly 

ventilated areas or in exterior walls may be vulnerable. Less 

is known about the moisture requirements of carpenter ants, 

although they generally only become established if some 

portion of the structure has a high moisture content. Once 

established in a moist area, however, they can expand into 

adjacent areas that do not have excessive moisture.

Weathering

Weathering is often the primary mode of deterioration of 

exterior wood in historic buildings, as siding, shingles, and 

external additions are typically exposed to precipitation and 

direct ultraviolet light. Weathering is readily apparent from 

the grey and brown surfaces of the wood and the small splits 

that develop during the weathering process. Weathering of 

wood is the result of the action of cyclic wetting and drying, exposure to ultraviolet (UV) light, and erosion of the 

wood through wind-blown debris (a process similar to sand 

blasting). Initially, the wood grays or darkens and small 

seasoning checks and splits begin to develop on the wood 

surface that allow for moisture penetration. These turn into 

longer splits due to cyclic wetting and drying of the wood 

or freeze–thaw action (Fig. 1). The weathering process 

changes the appearance of wood and gradually erodes the 

wood fibers, but the process is slow enough that collapse 

of a wood member because decay or insect attack generally 

occurs long before weathering becomes a major factor in the 

wood failure. Weathered wood may be considered aesthetically pleasing because it adds an air of authenticity to historic buildings, and, unlike decay or insect attack, it seldom 

damages the wood enough to require replacement, with the 

exception of thinner wood elements such as shingles and 

clapboard siding.

Prevention of weathering is not the primary purpose of 

wood preservatives, but those dissolved in oil and those 

containing water repellents may lessen moisture-related 

problems for a number of years. Preservatives that have 
General Technical Report FPL–GTR–217

6

some degree of opacity may offer partial protection against 

UV degradation.

Mold, Mildew, and Stain Fungi

Molds (also called mildew) and stain fungi are types of 

fungi that do not deteriorate wood but can cause surface 

discoloration. Most molds and mildews are green, orange, or 

black and are powdery in appearance (Fig. 2). If spores are 

present, they can grow very quickly on moist wood or wood 

in very humid conditions. Because the conditions that are 

favorable for growth of molds and mildews are the same as 

for more destructive decay fungi, the wood-discoloring 

organisms should be considered as warning signs of potential problems. Sapstain fungi grow deep within the wood 

structure, causing blue or black discolorations (Fig. 3). They 

are often seen in the sapwood of pine species and can be 

quite apparent after application of a clear finish. These fungi 

typically colonize the wood before it is initially dried after 

harvesting and perish once the wood is dried and placed 

in service. Although the color remains in the wood indefinitely, the fungi are much less likely than mold to reappear 

with subsequent wetting. Thus, their presence in a historic 

structure does not necessarily indicate a moisture problem. 

Sapstain fungi can increase the wood’s permeability, making 

it more likely to absorb liquids. This can increase susceptibility to decay during subsequent exposure to moisture and 

affect the finishing properties. Some types of wood preservatives are highly 

effective against mold and stain fungi, while others are 

ineffective or only moderately effective.

Lichens, Moss, and Algae

Lichens, mosses, and algae are distinctly different types of 

organisms that are often grouped when discussing their relationship to wood durability. Lichens are unique organisms 

that can grow on wood but typically do not harm the wood 

fibers (Fig. 4). Lichens are typically only found on exterior 

wood elements. Lichens grow from spores and tend to grow 

very slowly. They need an undisturbed surface, indirect 

sunlight, and moisture to develop. The fungal components 

of the lichen do not parasitize living plant cells, break down 

wood cells, or provide gateways for other pathogens to enter 

Figure 1—Weathering has caused cracking and loosened 

the fibers on the upper surface of this deck board.

Figure 2—Typical black mold on a softwood (left) and 

hardwood (right).

Figure 3—Sapstain penetrates deeply into the wood and 

cannot be removed by sanding.
Guide for Use of Wood Preservatives in Historic Structures

7

wood fibers. Because most lichens are extremely firmly embedded in their substrates, forcible removal of lichens can 

cause significant surface damage to wood materials.

Mosses are nonvascular plants that can thrive on a variety of 

porous, moisture-retentive surfaces such as brick and wood 

(Fig. 5). Mosses grow from spores that are distributed by air 

currents and are generally found in damp, low-light conditions. Most mosses require near-constant moisture levels 

to survive. Mosses do not damage wood fibers; however, 

the presence of moss is an indication of a continuous highmoisture environment, and the sponge-like composition of 

the moss plant traps moisture at the wood surface. If mosses 

are present on wood elements of a historic building, moisture levels are likely to be very high and decay fungi are 

probable. Moss can be easily removed with natural bristle 

brushes and careful cleaning, but mechanical removal will 

spread rhizomes and spores, so unless underlying conditions are altered, the moss will likely return. While biocides 

are effective for killing mosses, chemical applications can 

cause staining of the wood surface, and have the potential to 

harm adjacent plants. Additionally, chemical treatments do 

not alter the conditions that make it favorable for moss (and 

wood-decay fungi) growth. Mosses can be more effectively 

controlled by improving the underlying conditions that lead 

to moss growth (high moisture content and low-light conditions). Alterations made to improve ground drainage and 

irrigation system modifications can reduce the amount of 

moisture contributing to moss growth and trimming trees 

and vegetation that create shady conditions can increase the 

amount of direct sunlight to help deter moss growth.

Green algae are also commonly observed on wood surfaces 

that are moist and shaded (Fig. 6). The algae is confined 

to the surface and does not does damage the wood but like 

moss, algae is an indicator of moisture conditions conducive 

to decay. Like moss, algae can be removed with bleach or 

other chemical treatments but will reappear unless conditions are altered.

Decay Fungi

All wood is subject to a variety of deterioration mechanisms, the most prominent of which is wood-decay fungi. 

Figure 4—Lichen (shown here) are sometimes confused 

with the fruiting bodies of decay fungi. Photograph 

courtesy of Anthony & Associates, Inc.

Figure 5—Moss on a cedar shake roof. Photograph 

courtesy of Anthony & Associates, Inc.

Figure 6—Green algae are commonly found on wood 

surfaces in moist, shady locations.
General Technical Report FPL–GTR–217

8

In part, the high degree of damage by wood-decay fungi is 

caused by their ubiquitous presence in all locations. Given 

suitable conditions, attack by some type of wood decay 

fungus is assured. Wood-decay fungi excrete enzymes 

that break down wood fibers, which can ultimately lead to 

strength loss and the inability of wood to perform its intended function. Most wood-decay fungi are only able to grow 

on wood with a moisture content greater than 20% and are 

unable to damage adjacent dry wood. However, two types of 

fungi are able to destroy dry wood by pulling water through 

several feet of root-like strands (called rhizomorphs) to 

moisten the wood enough to allow for decay processes to 

occur. Fortunately, these destructive dry-rot fungi are rare 

and found in limited geographic areas of the northeastern 

United States.

Common white-rot, soft-rot, and brown-rot fungi are the 

typical causes of wood deterioration. Both white-rot and 

brown-rot fungi can produce a cottony white growth on the 

surface of the wood that should not be confused with nondestructive white mold or mildew. Wood that has white- or 

brown-rot decay fungi will tend to be soft, friable, and easily penetrated. Brown-rot fungi will cause wood to darken 

and appear brittle and cracked with cubical checking 

(Fig. 7). Wood affected by brown-rot fungi will ultimately 

shrink, twist, and become dry and powdery. White-rot 

fungi leads to fibrous, spongy wood that appears bleached 

or drained of color. Wood affected by white-rot begins to 

shrink only after advanced decay has occurred. Soft-rot 

fungi generally occur in wood with high water and nitrogen contents and are commonly found in fence posts and 

foundation posts that are in contact with the ground and can 

“recruit” nitrogen from the soil (Fig. 8). Soft-rot acts as its 

name implies and destroys the structural integrity of wood 

by degrading the cellulose and hemicelluloses, the materials 

in wood that form the wood “skeleton.”

Larger wood members will frequently rot on the interior 

with no externally visible sign of deterioration. Moisture 

absorption though the end grain of the post or beam, seasoning checks, or drilled holes provide a highly favorable environment for decay fungi to attack the interior of the wood 

member. Deterioration through decay is a particular concern 

where the wood is in contact with the ground or other materials, such as masonry, that may facilitate moisture absorption into the wood.

Decay fungi break down wood components over time. The 

early stage of decay (incipient decay) is characterized by 

discoloration and an initial loss of integrity of the wood. No 

voids are present. At this stage of decay, probing with an 

awl or blunt implement may reveal the wood to be soft or 

punky. Punky wood is spongy wood that has experienced a 

loss of strength and structural integrity because of decomposition of connective fibers. As decay progresses, the cellular 

integrity of the wood deteriorates until small voids develop. 

These small voids continue to extend primarily along the 

wood grain (where it is easier for moisture to move through 

the wood) but can also progress across the grain.

Larger voids can develop where the decay started and the 

boundaries of the incipient decay will continue to extend, 

reducing the integrity of the wood and, potentially, compromising the ability of the wood to provide the structural support required. Advanced decay, the ultimate result of moisture intrusion, is a severe threat to the long-term viability of 

wood components of historic structures.

Appropriately selected and applied wood preservatives can 

be highly effective in protecting wood from attack by decay 

fungi. Halting or preventing growth of decay fungi is one of 

the primary purposes of most wood preservatives.

Figure 7—Typical brown-rot decay.

Figure 8—Soft-rot typically occurs under very moist 

conditions. Photograph courtesy of Anthony & Associates, 

Inc.
Guide for Use of Wood Preservatives in Historic Structures

9

Insects

Insect attack is generally a minor contributing factor to the 

deterioration of dry wood, as most insects seek out wood 

that has already been compromised by high moisture levels. 

However, a number of wood-boring insect species can cause 

significant damage to historic buildings and are likely to be 

of concern to preservationists in areas where wood-damaging insects are present. In the southeastern United States and 

other humid coastal regions, in particular, insects are more 

likely to be an issue than in other parts of the country. The 

diversity of insect species that can damage wood is quite 

broad, so only the most common and most damaging of 

these insect pests are discussed here.

Insect attack by termites or other wood borers will reduce 

the cross section of a wood member by either digesting or 

tunneling through the wood. With decay, there is usually 

a gradual transition from sound wood to punky wood to a 

total loss of wood fiber (a void). Unlike decay, insect damage tends to have an abrupt transition between affected and 

unaffected areas of the wood.

Termites are the primary wood-attacking insect, and structures should be monitored to identify potential infestations 

by closely examining for bore holes, frass (wood substance 

removed by the boring action of the insect), mud tubes, live 

insects, or other evidence of activity. A number of termite 

species can damage wood in historic structures (Fig. 9). 

These species include native subterranean termites, Formosan subterranean termites, drywood termites, and dampwood 

termites. While termite species found in the United States 

can be difficult to distinguish from one another, especially 

when swarming, each species does have specific identifying 

characteristics. Any suspected termite infestation should be 

handled by a professional exterminator, preferably one with 

experience in historic preservation.

The Eastern native subterranean termite is the most common 

wood-attacking termite in the United States and is found 

in every state except Alaska. These termites require moist 

wood to survive and typically damage the interior core of 

wood members first, so an infestation often goes unnoticed 

until the damage has become severe. Subterranean termites 

tend to consume softer earlywood first, leaving latewood 

in ridges around their galleries. These termites often enter 

wood members through wood in contact with the soil, but 

they can survive in wood with no soil contact provided the 

wood remains moist. A common visual indicator of subterranean termites is the presence of mud shelter tubes on the 

surface of the wood or heavily channeled wood compacted 

with mud. Termite shelter tubes can cross mortar and brick. 

Prolonged infestation can lead to a significant loss of wood 

cross section and structural integrity.

The Formosan subterranean termite is an invasive termite 

species larger and more aggressive than native North 

American subterranean termites. Native to southern China, 

Taiwan, and Japan, Formosan termite populations were 

established in South Africa, Hawaii, and in the continental 

United States by the mid-1900s. A highly destructive insect 

species, Formosan termites live in extremely large colonies 

that can contain up to several million termites with a foraging range up to 300 ft in soil. Because of its population size 

and foraging range, the presence of Formosan termites poses 

serious threats to historic wood elements and buildings, 

particularly along the Gulf Coast, southern California, and 

Hawaii. There may be little to no external evidence of infestation, so historic buildings with wood elements in states 

known to have active Formosan termite populations should 

periodically be closely inspected to identify potential termite activity. An exterminator skilled in Formosan termite 

extermination and with familiarity with historic preservation 

requirements should be called in cases where Formosan termites are suspected. Formosan termites have been reported 

in Alabama, Arizona, California, Florida, Georgia, Hawaii, 

Louisiana, Mississippi, New Mexico, North Carolina, South 

Carolina, Tennessee, Texas, and Virginia.

Drywood termite infestations have been recorded in Alabama, Arizona, California, Georgia, Florida, Louisiana, Mississippi, New Mexico, North Carolina, South Carolina, Texas, and Utah. Drywood termites do not require contact with 

soil or other sources of moisture within the wood. Colonies 

can reside in nondecayed wood with low moisture contents. 

Drywood termites live in small social colonies with as few 

Figure 9—Damage caused by termites after only 6 months 

of soil contact in Louisiana.
General Technical Report FPL–GTR–217

10

as 50 insects to over 3,000 insects for a mature colony. They 

remain entirely above ground and do not connect their nests 

to the ground with mud tubes or galleries. Typically, the 

first sign of a drywood termite infestation is dry fecal pellets 

collecting at or near the base of wood members. The fecal 

pellets are hard, angular, less than 1 mm in length, and vary 

in color from light gray or tan to very dark brown. Interior 

galleries tend to be broad pockets or chambers connected by 

smaller tunnels that cut across latewood. Irreparable damage 

to wooden elements can be caused by drywood termites in 

2 to 4 years, depending on the size of the element and the 

size of the infestation.

Dampwood termites, most commonly found along the Pacific Coast, have been identified in Washington, California, 

Nevada, Oregon, and Montana. Some less destructive dampwood termite species also live in Florida. Although typically not as destructive as subterranean termites, with ideal 

conditions they can cause significant damage. Dampwood 

termites are larger than subterranean termites, and unlike 

subterranean termites, they usually build their colonies in 

wood that is already in the early stages of decay. As long as 

the wood has a high moisture content, the colony will not 

require contact with the ground. In relatively sound wood, 

the galleries will tend to follow the softer earlywood, however, if decay is more advanced, the galleries tend to become larger and cut through harder latewood. Fecal pellets 

tend to be the same color as the wood being eaten and, in 

very damp wood, stick to the sides of the galleries in amorphous clumps.

Another wood-boring insect species, the carpenter ant, can 

cause damage to wood in historic buildings. Unlike termites, 

however, carpenter ants do not feed on wood but rather 

burrow into wood to make nests. Carpenter ant infestation 

is most typically identified by the presence of large (6- to 

13-mm- (0.25- to 0.5-in.-)) long ants that can range in color 

(depending on species) from dull black with reddish legs 

and golden hairs covering the abdomen to a combination 

of red and black or completely red, black, or brown. Damage to the wood is typically in the interior, but there may be 

piles of fibrous, sawdust-like frass in or around checks and 

splits. Galleries within the wood generally follow the grain.

Carpenter bees can also damage wood in historic buildings. Carpenter bees have a world-wide geographic range 

and vary in size and shape from small, 6-mm- (0.25-in.-) 

long bees to large, hairy bees that resemble nonwoodboring bumblebees. There are approximately 500 species 

of carpenter bees, many of which build their nests in dead 

wood, bamboo, or structural timbers. In the United States, 

21 species of small carpenter bees can be found across the 

country, as well as seven species of large carpenter bees that 

range across the southern states from Arizona to Florida and 

along the east coast as far north as Virginia. Typically, only 

the larger species of carpenter bees create nesting galleries 

in solid wood and pose a risk to exterior wood surfaces of 

historic buildings. These large carpenter bee species are, on 

average, 13 mm (0.5 in.) or longer in length and can range 

in color from yellow to black and resemble nonwood-boring 

bumblebees. In several species, the females live in tunnels 

alongside their offspring in loose social groups. Carpenter 

bees typically create shallow tunnels that do not cause 

significant structural damage for wooden buildings or 

structures.

Several types of wood-boring beetles can cause damage 

in historic structures, with the most destructive following 

into the species referred to as “powderpost beetles.” Unlike 

decay fungi, powderpost beetles are capable of attacking 

wood that is well below the fiber saturation point, allowing them to attack members in historic structures that are 

protected from direct wetting. Powderpost beetles lay their 

eggs on the surface of sapwood of the desired species. The 

eggs hatch into larvae that tunnel into the wood, leaving 

little evidence of their presence inside the wood. The larvae 

tunnel extensively through the wood over periods extending for 1 to 7 or more years. Once the larvae have obtained 

a sufficient amount of energy, the larvae pupate to become 

adults. These adults then exit the wood, leaving small round 

exit holes on the wood surface. This is often the first visible 

sign of an infestation (Fig. 10). The inside of powderpost 

damaged wood tends to be crumbly and powdery. Another 

type of beetle that can be problematic, especially along the 

Atlantic Coast, is the old house borer. Like powderpost 

beetles, the old house borer can attack relatively dry wood, 

but unlike the powderpost beetle, which primarily attacks 

hardwoods, the old house borer attacks only softwoods. As 

with the powderpost beetles, there is little evidence of attack until the adult emerges 2 to 15 years after eggs are laid 

on the wood surface. Most wood preservatives can prevent 

attack by most types of beetles, but surface treatments may 

not be effective against existing infestations. Finishes may 

be as effective as wood preservatives in preventing attack by 

the most troublesome types of beetles.

Figure 10—Piles of very fine sawdust are indicative of an 

active powderpost beetle infestation.
Guide for Use of Wood Preservatives in Historic Structures

11

Role of Wood Cellular Structure

Wood structure affects both susceptibility to decay and 

the movement of preservative through the wood. On the 

most basic level, wood can be thought of as a collection 

of elongated, hollow straws arranged in a series of parallel 

circles along the length of the tree (Fig. 11). Because of this 

structure, fluids move much more readily along than across 

the wood grain. Exposed end-grain serves as conduit for 

rapid movement of moisture deep inside large members. 

This structure also allows preservatives to move more readily along than across the wood grain. Although the majority 

of wood cells are aligned to maximize flow parallel to the 

grain, the wood structure does allow some flow across the 

grain. This transverse flow is accomplished through ray cells 

and through openings between longitudinal cells. As a tree 

develops, new cells grow around the outer circumference 

of the stem forming the conductive tissues that comprise 

the sapwood. Tree growth is fastest in the spring, producing 

relatively thin-walled cells (earlywood), while thick-walled 

cells are formed late in the season (latewood). These alternating bands of thick- and thin-walled cells form growth 

rings, or annual rings. The older, inner sapwood cells 

eventually stop functioning and form a darker core of nonconductive tissues called heartwood. The thickness of this 

sapwood band varies greatly by species. Heartwood differs 

from sapwood most notably in its much higher extractive 

content and much lower permeability. 

Problem Areas for Deterioration in Historic 

Structures

Historic structures vary greatly in design, condition, and 

exposure, but some generalizations can be applied to problem areas in most types of structures. Significant decay can 

occur in any untreated portion of a structure where wood 

moisture content is above 20% to 25% and oxygen is 

present for sustained periods. Sufficient oxygen and moisture for decay are almost always present in wooden members placed in contact with the ground or the waterline area 

of members placed in water (Figs. 12, 13). But, in most climates, there is also sufficient moisture for decay in members 

that are not directly in contact with soil or water or protected 

from precipitation. In general, larger wooden members are 

most prone to developing decay because water becomes 

trapped inside the wood during precipitation events and is 

slow to dry during subsequent dry weather. Liquid water is 

rapidly absorbed in end-grain during rain events, and subsequent drying can be slowed if air movement is limited in 

that area. Unfortunately these conditions commonly exist 

at connections where members are joined by fasteners or 

Figure 11—Typical cellular structure of softwood species. Figure 12—Ground contact greatly increases the risk of 

biodeterioration.

Figure 13—Even in very dry climates, such as Keane 

Wonder Mine in Death Valley, wood below ground has 

sufficient moisture for decay. Photographs courtesy of 

Anthony & Associates, Inc.
General Technical Report FPL–GTR–217

12

at interfaces with other materials (such as beam pockets in 

masonry walls).

In general the structural members of most historic structures 

were not treated with wood preservatives before installation; they can therefore be more vulnerable than treated 

wood to biodeterioration in areas with sustained exposure to 

moisture. However, the open construction typical of historic 

structures, coupled with the likely use of old growth timber 

containing substantial heartwood, generally makes the structural framing in historic structures fairly resistant to deterioration. The open method of construction makes it possible 

for the wood to dry quickly if it gets wet and thus reduces 

the likelihood of biodeterioration; however, sometimes 

moisture does get trapped and can lead to deterioration. 

Modern alterations to open construction, such as insulation 

to provide energy efficiency, can increase the likelihood of 

decay by reducing air circulation (and therefore moisture 

evaporation) around wood members.

Historic structures are likely to have several problem areas, 

including the following:

•	 Wood in contact with the ground

•	 Wood that exhibits moisture stains

•	 Wood with visible decay

•	 Roof penetrations, such as around chimneys and vents

•	 Attic sheathing, framing lumber, and timbers

•	 Sill beams and wall plates, particularly those in contact 

with masonry

•	 Floor joists and girders, particularly where they rest on 

exterior walls

•	 Openings (doors and windows)

•	 Material interfaces, such as wood and masonry, particularly beam pockets

•	 Exterior woodwork, including cladding, shingles, and 

soffits

•	 Porches

•	 Crawl spaces and basements

•	 Areas of the structure that have been altered

One of the most common areas of deterioration is the roof 

sheathing, framing, or timbers. Deterioration of these members is common in buildings and structures that have been 

neglected or abandoned or have lacked sufficient maintenance. Roofs serve important roles in protecting structural 

elements from moisture intrusion and deterioration and are 

therefore critical to the long-term survival of a structure. 

Any penetration in the roof envelope, such as a chimney or a 

vent, can create an avenue for water intrusion. Missing roof 

shingles can also allow water into a structure (Fig. 14). The 

typical pattern of deterioration results from missing shingles 

allowing water to penetrate and damage the roof sheathing; 

once the roof sheathing has been compromised, the tops of 

roof purlins or roof rafters are exposed to moisture intrusion 

and wood-decay fungi, and active leaks can occur that can 

damage other structural members within the roof and exterior walls (Fig. 15). 

Woodwork around windows and doors is also a common location for decay because precipitation and condensation can 

become trapped in the joints. Flat, horizontal surfaces such 

as door and window sills can also allow moisture to collect 

(Figs. 16–18). Often, these surfaces are painted and moisture intrusion problems are generally indicated by flaking or 

peeling paint. Occasionally, the painted surface may show 

Figure 14—The lack of shingles allows moisture 

penetration into the roof and wall framing members of 

this mine building. Photograph courtesy of Anthony & 

Associates, Inc.

Figure 15—These rafter tails exhibit discoloration 

and green mold growth indicative of a high-moisture 

environment. Photograph courtesy of Anthony & 

Associates, Inc.
Guide for Use of Wood Preservatives in Historic Structures

13

no signs of moisture intrusion but the wood underneath may 

be completely saturated.

Exterior wall cladding and sheathing can be subjected to 

biodeterioration as well. Often, the wall siding or cladding 

can remain intact despite significant or repeated wetting 

episodes because moisture can evaporate relatively quickly 

from the exposed surfaces. Moisture that penetrates to 

the wall sheathing can become trapped and with little opportunity to evaporate, can lead to deterioration (Fig. 19). 

The extent of roof overhang in relation to the height of the 

structure is often a key factor influencing the extent of decay 

that develops in siding and other exposed members (such as 

rafter tails), as overhangs serve to protect the surfaces from 

significant wetting episodes.

Another common area of deterioration is at the interface between wood and another material, such as beam pockets for 

joists or girders (Fig. 20). Biodeterioration most commonly 

occurs within the beam pockets along exterior walls where 

moisture can wick from porous masonry or mortar joints 

into the end grain of structural timbers. Additionally, because beam pockets are often enclosed, it can be difficult for 

moisture to evaporate, leading to accelerated rates of decay.

Crawl spaces and basements often have moisture intrusion 

issues that can lead to deterioration of the structural wood 

members. In general, any wood that is below grade will 

have higher moisture content than wood above grade and 

a greater propensity for deterioration. Wood that rests on 

foundation walls is susceptible to moisture wicking from 

porous stone or concrete (Figs. 21– 22). Wood in close 

proximity to the soil of an unlined crawl space is exposed 

to higher humidity as water evaporates from the earth 

(Fig. 23). Areas of poor drainage around a building’s foundation can lead to localized areas of damage as well.

If it is kept dry, wood in historic structures can typically perform as intended for hundreds of years. Poor construction 

Figure 16—Windows and other millwork that can trap 

moisture can be vulnerable to decay.

Figure 17—A severely deteriorated door threshold. 

Photograph courtesy of Anthony & Associates, Inc.

Figure 18—A moisture meter on this window sill shows 

that the sill is saturated. Photograph courtesy of Anthony 

& Associates, Inc.

Figure 19—Deteriorated wall sheathing behind clapboard 

siding. Photograph courtesy of Anthony & Associates, Inc.
General Technical Report FPL–GTR–217

14

detailing, lack of maintenance, plumbing failures, flooding 

episodes, changes in grade or surface drainage patterns, and 

severe weather events can trigger moisture intrusion that 

leads to subsequent biodeterioration. The most common 

areas of deterioration tend to be where support members 

contact the ground or foundation. Wood in these areas tends 

to be more susceptible to decay and other forms of biodeterioration because of the proximity of the wood to a ready 

source of moisture found in the soil, combined with an 

ability of most foundation materials to wick moisture and a 

general lack of air circulation. In these areas, effective moisture mitigation is critical to ensure the survival of the wood 

members. In cases where the moisture cannot be mitigated, 

however, using preservative-treated wood as replacement 

material may be an option.

As discussed in the section “Using Pressure-Treated Wood,” 

structures that were built with pressure-treated wood are 

also not immune to biodeterioration. The preservative treatments used on older structures were generally very effective 

in protecting the treated wood. However, in many cases, and 

especially with larger members, the preservative does not 

penetrate all the way to the center of each piece. This barrier 

can be compromised, either during the original construction 

or as a result of checks and cracks from normal weathering 

and moisture changes. One of the most common sources of 

exposure of untreated wood is drilling holes or cutting members to length during construction. In larger members, this 

practice may expose untreated wood and if this exposed surface is left unprotected, there is an increased probability that 

internal decay will develop. Attempts to protect this cut 

surface may only be partially successful. Cut-off posts, 

poles, or piles that do not appear to have been adequately 

protected are among the most likely candidates for 

Figure 20—A structural beam with decay within the beam 

pocket. Photograph courtesy of Anthony & Associates, Inc.

Figure 21—High moisture content and decay led to the 

crushing failure of this large timber column under design 

loads. Photograph courtesy of Anthony & Associates, Inc.

Figure 22—A sill plate resting on a concrete foundation 

below grade with evidence of deterioration. Photograph 

courtesy of Anthony & Associates, Inc.

Figure 23—Wood girder and joists in a flooded crawl 

space; the evaporation of the water into the air and 

lack of air circulation greatly increases the likelihood of 

deterioration of these members. Photograph courtesy of 

Anthony & Associates, Inc.
Guide for Use of Wood Preservatives in Historic Structures

15

application of field treatments. Check (crack) formation in 

both round and large sawn timbers is another route for exposure of untreated wood in the center of members. These 

checks also allow water to collect and be trapped within 

the wood. Small drying checks also may not be a concern 

if they do not penetrate past the treated zone. However, the 

appearance of large drying checks in timbers or logs can be 

an indication of conditions favorable for internal decay, and 

these are areas that warrant closer inspection and possible 

field treatment.

Nonpreservative Approaches to Preventing 

Deterioration

Nonpreservative approaches involve changing the exposure 

environment so that conditions are less favorable for wood 

degradation. These approaches are often the most effective 

and long-lasting means of preservation and should be considered before the application of wood preservatives.

Keep It Dry

As previously discussed, moisture is the primary means 

through which weathering, decay fungi, and insect 

infestation cause wood deterioration. Where compatible 

with historic preservation philosophy, taking measures to 

protect wood from wetting is generally the most effective 

approach to wood protection. Although the importance of 

moisture in wood deterioration is widely recognized, 

conditions that can lead to moisture-related problems are 

common.

In many structures, the roof is the primary (and often only) 

defense against moisture intrusion, and thus the integrity 

of the roof system is critical. Unfortunately, maintenance 

of roofs on historic structures can be costly and technically 

challenging (Fig. 24). Although roof problems may be obvious, smaller leaks can go unnoticed for years. Sources 

of moisture from openings in the roof or siding can occur 

almost anywhere in a structure and are not always easy to 

detect. Water stains or general discoloration may be visible, 

but may not be immediately adjacent to the place through 

which water enters the structure. In some cases, the roof 

may be intact but the overhang may not provide adequate 

protection for either original or replacement structural members. Management of water running off the roof can also 

be a source of moisture exposure for lower portions of the 

structure. Lack of flashing or inadequate flashing is another 

source of moisture intrusion, especially in structures with 

minimal roof overhang (Fig. 25).

In addition to properly maintaining or repairing roofing and 

flashing leaks on the roof and building envelope, it is important to assess other sources of moisture. Check rain gutters, 

downspouts, interior plumbing, and spigots for leaks and 

note the location of these elements relative to the structure 

(Fig. 26). Spigots that are located near wooden elements 

should be monitored when in use to identify any potential 

Figure 24—Repairing missing or damaged roofing material 

is a priority in preventing deterioration.

Figure 25—Improperly installed or damaged flashing can 

direct water into the structure and promote decay.

Figure 26—A leaking downspout led to decay in the corner 

of this historic cabin.
General Technical Report FPL–GTR–217

16

leaks (such as from a loose hose connection) that could 

lead to deterioration of structural elements. The direction 

of spray from water sprinklers should also be assessed and 

alterations to the direction and intensity of flow should be 

made if necessary to prevent water saturation of the ground 

near structural wood members and to prevent wooden elements from getting wet. Plumbing fixtures and pipe connections within structures should be assessed to identify and 

repair any potential leaks that could damage structural wood 

members as well.

Poor drainage around a structure may be mitigated through 

the re-grading of the surrounding soil or by installing a 

French drain around the perimeter or a portion of the perimeter of a structure. As this requires disruption and modification of the ground around a historic structure, such a step 

may require State Historic Preservation Officer (SHPO) 

approval and archaeological monitoring to identify and 

document any archaeological material uncovered during 

excavations. This type of moisture mitigation is quite common for historic structures and can be effective at improving 

drainage conditions if done correctly. This step should be 

considered for log buildings and vernacular structures with 

loose stone or no foundations.

Other areas of structures may become vulnerable to moisture as a result of vandalism. Vandalism is a frequent cause 

of water intrusion in covered bridges, where cladding may 

be repeatedly removed to allow access for fishing or swimming (Fig. 27). Any portion of a bridge where the cladding 

has been lost for an extended period, or even for several 

shorter periods, may be vulnerable to decay.

Vegetation can also be a contributing factor in moisture 

problems. Shade prevents wood from drying after rain and 

can lead to growth of moss and lichens that further trap 

water. Vines and brush growing close to structures increase 

humidity and slow drying, and in some cases can physically 

damage roofing or siding (Fig. 28). Dense clusters of vegetation drop leaves that release nitrogen as they decompose 

and attract decay fungi. Increased vegetative cover also 

often attracts insects and rodents that can damage wooden 

elements. Preventing or removing vegetation can increase 

the durability of the structure. 

Minimize Contact with Soil and Organic Material

Soil and organic matter can provide ideal conditions for 

colonization of wood by fungi and termites. Soil provides 

both moisture and the micronutrients that these organisms 

need for optimal growth. Many vernacular structures were 

built with wood in direct contact with soil. Although other 

structures may not have been not originally designed or constructed to place nondurable wood in direct contact with soil 

or organic material, these conditions can develop over time 

(Fig. 29). Human activities, animal activities, erosion, and 

other forces can all lead to changing soil lines at the base of 

the structure, and organic debris can accumulate in aboveground areas of some structures. The latter is particularly 

true in structures adjacent to trees or similar vegetation. 

Accumulated organic debris traps water, and like soil, can 

provide micro-nutrients that aid the growth of decay fungi. 

Unfortunately, this debris tends to accumulate in joints and 

connections, where the risk of decay is already relatively 

high. Although it is often not practical to remove all of this 

material, it is beneficial to remove obvious accumulations. 

Areas to inspect for organic debris accumulation include the 

tops of exposed fence posts and pergolas, roof transitions 

and angles, and any exposed timber joints. Foundation walls 

and column and post bases should also be inspected for 

organic debris and soil build-up. In some cases, it may be 

necessary to contact the state SHPO office prior to removing 

soil build-up.

Figure 27—Vandals removed cladding from this covered 

bridge, exposing the large support beams to moisture. Figure 28—In addition to possible mechanical damage, 

vines create shading and trap moisture.
Guide for Use of Wood Preservatives in Historic Structures

17

Wood Preservative Overview

What Is a Wood Preservative?

When considered in its broadest context, a wood preservative is any substance or material that, when applied to 

wood, extends the useful service life of the wood product. 

In more practical terms, wood preservatives are generally 

chemicals, applied as solids, liquids, or gases, that are either 

toxic to wood-degrading organisms or cause some change 

in wood properties that renders the wood less vulnerable 

to degradation. Most wood preservatives contain pesticide 

ingredients, and as such must have registration with the U.S. 

Environmental Protection Agency (EPA). However, some 

preservatives such as those based on water repellents work 

on the basis of moisture exclusion and do not contain pesticides. Preservatives that do contain pesticides are required 

to provide information on the type and concentration of pesticide on the label. Because the term “wood preservative” is 

applied to a broad range of products there is often confusion 

or misunderstanding about the types of products being described, and some degree of specificity is needed.

Remedial, In-Place, Field-Applied, Supplemental, 

or Nonpressure Preservatives

This catch-all category of preservatives includes all types 

of preservative applications other than pressure treatments. 

Examples range from finishes, to boron rods, to fumigants 

(Table 1). The objective of all these treatments is to distribute preservative into areas of a structure that are vulnerable 

to moisture accumulation or not protected by the original 

pressure treatment. A major limitation of in-place treatments 

is that they cannot be forced deep into the wood under pressure as is done in pressure-treatment processes. However, 

they can be applied into the center of large members via 

treatment holes. In-place treatments are often available in 

several forms. For example, borate treatments can be applied as liquids, pastes, gels, and rods.

Pressure-Treatment Preservatives and PressureTreated Wood

The greatest volume of wood preservatives is used in the 

pressure treatment of wood at specialized treatment facilities. In these treatment plants, bundles of wood products are 

placed into large pressure cylinders and combinations of 

vacuum, pressure, and sometimes heat are used to force the 

preservative deeply into the wood. Pressure-treated wood 

and the pressure-treatment preservatives differ from 

nonpressure preservatives in three important ways. 

(1) Pressure-treated wood has much deeper and more uniform preservative penetration than wood treated in other 

manners. (2) Most preservatives used in pressure treatment 

are not available for application by the public. In some 

cases, such as with the older preservatives, this is because 

the U.S. EPA considers them too toxic to be handled by 

the general public. In other cases, the preservatives are not 

highly toxic, but the supplier has not taken the additional 

steps needed to introduce the preservative into the retail 

market. (3) Pressure-treatment preservatives and pressuretreated wood undergo review by standard-setting organizations to ensure that the resulting product will be sufficiently 

durable in the intended end-use. Standards also apply to 

treatment processes and require specific quality control and 

quality assurance procedures for the treated wood product. 

This level of oversight is needed because pressure-treated 

wood is used in applications where it is expected to provide 

service for decades, and where premature failure could result in injury or death. In contrast, nonpressure preservatives 

may undergo relatively little review, other than the U.S. EPA 

evaluation of pesticide toxicity.

When Is Application of Preservatives 

Appropriate?

There is no simple answer to this question, but some general 

guidelines do apply. Wood moisture is a key consideration. 

Although there are exceptions for termite and beetle attack, 

in general preservatives are not needed for wood that can 

be consistently protected from moisture. In contrast, wood 

that is moist (over 20% moisture content) for sustained periods is vulnerable to colonization by decay fungi and possibly other organisms. Researchers still do not completely 

understand the minimum periods of elevated moisture, or 

the frequency of elevated moisture, needed for decay to 

progress. The potential for wetting varies with climate, site 

conditions, and member dimensions. Large members can 

trap and hold moisture for much longer periods than thinner 

members. Connections and fasteners that trap moisture also 

play an important role. In historic structures, the condition 

of existing members provides insight into the need for preservative treatment. If a member is badly decayed and no 

action is taken to lessen exposure to moisture, then preservative treatment of the replacement member may be worthwhile. In contrast, if a member has survived largely intact 

for decades, then preservative treatment may not be justified 

Figure 29—Over time, soil has accumulated against the 

wall of this house, virtually guaranteeing severe decay.
General Technical Report FPL–GTR–217

18Table 1—Summary of supplemental preservative treatment properties and applications 

Applied as Actives Supplied as Dilution 

EPA hazard

category Uses 

Mobility

in wood 

Examples of 

trade name(s) 

Liquid 98% DOTa Powder Dilute to 10–15%

in water (by weight) 

Caution Surface spray, brush or foam,

internal injection, poured in holes 

High Board Defense, Borasol, 

Timbor, TimberSaver, Armour-guard

Liquid 25–40% DOTa Water/ 

glycol-based 

Dilute 1:1 with water 

Caution Surface spray or brush, 

poured into holes 

High Bora care, Bor-Ram, 

BoraThor, Shell-guard 

Liquid Copper naphthenate,1- 

2% as Cu 

Oil or 

waterbased 

RTU Warning Surface spray or brush, 

poured into holes, pads for bandages 

Low Tenino, Cuprinol No. 10 

Green Wood Preservative,Jasco, Termin-8, CU-89 RTU II 

Liquid 9.1% DOTa,

0.51% boric acid, 0.96% copper hydroxide(0.6% copper) 

Waterbased RTU Caution Surface spray, brush or foam,

internal injection 

B high, 

Cu low 

Genics CuB 

Liquid Copper naphthenate, 

5% as Cu 

Waterbased Dilute 1:4 or 

1:1.5 with water 

Danger Surface spray or brush, 

poured into holes 

Low Aqua-Nap 5 

Liquid Copper naphthenate, 

8% as Cu 

Oilbased Dilute 1:3.0–3.8 or

1:7.5–8 with oil 

Warning Surface spray or brush, 

poured into holes 

Low Cu-Nap Concentrate, 

COP-R-NAP

Liquid Copper-8-quinolinolate 

(0.675%) 

Oilbased RTU Caution Surface spray or brush, 

poured into holes 

Low Outlast Q8 Log Oil 

Liquid Zinc naphthenate, 

1–2% as Zn 

Oil or 

waterbased 

RTU Warning Surface spray or brush, 

poured into holes 

Low Jasco ZPW 

Liquid 33% sodium Nmethyldithiocarbamate Liquid fumigant RTU Danger Internal fumigant treatment, poured into holes Gas, very high WoodFume, SMDC- Fume, 

Pol Fume 

Rod 100% anhydrous 

disodium octaborate 

Rod RTU Caution Placed into holes High Impel Rod 

Rod 93% sodium fluoride Rod RTU Warning Placed into holes High FluRod Rod 90.6% DOTa,

4.7% boric acid, 2.6% Cu 

Rod RTU Caution Placed into holes B high, 

Cu low 

Cobra Rod 

Granules 98% Dazomet Granule RTU Danger Internal fumigant treatment,

placed into holes 

Gas, 

very high 

Dura-fume

Granules 98% Dazomet Granule RTU Danger Internal fumigant treatment,

placed into holes 

Gas, 

very high 

Super-Fume 
Guide for Use of Wood Preservatives in Historic Structures

19

Table 1—Summary of supplemental preservative treatment properties and applications (con.) 

Applied as Actives Supplied as Dilution 

EPA hazard

category Uses 

Mobility

in wood 

Examples of 

trade name(s) 

Capsule

(paper tube) 

98% Dazomet Capsule RTU Danger Internal fumigant treatment,

placed into holes 

Gas, 

very high 

Super-Fume 

Capsule 97% 

methylisothiocyanate 

Capsule RTU Danger, 

poison,

restricted 

Internal fumigant treatment,placed into holes 

Gas, 

very high 

MITC–FUME 

Paste 43.5% borax, 

3.1% copper hydroxide (2% Cu) 

Paste RTU Warning With exterior wrap 

for groundline area, spread under pile caps, injected into holes (caulking gun) 

Cu low, 

B high 

Cu-Bor

Paste 40% borax, 

18% copper naphthenate(2% Cu) 

Paste RTU Warning With exterior wrap 

for groundline area, spread under pile caps, injected into holes (caulking gun) 

Cu low, 

B high 

CuRap 20 

Paste 43.7% borax, 

0.2% tebuconazole, 0.04% bifenthrin, 0.3 % copper quinolinolate(0.05% Cu) 

Paste RTU Caution With exterior wrap 

for groundline area, spread under pile caps, injected into holes (caulking gun) 

B high, 

others low

MP400-EXT

Gel 40% DOTa Gel RTU Caution Internal, injected into holes High Jecta 

aDisodium octaborate tetradydrate. 
General Technical Report FPL–GTR–217

20

unless other factors are expected to contribute to additional 

risk of deterioration in the future. Some knowledge of local 

conditions and risks is also helpful. For example, if a structure is in a location where Formosan subterranean termites 

are present or nearby, there may be more justification for 

preservative treatment than in the past.

Even when conditions are favorable to deterioration, one 

must consider whether the treatment options available will 

be effective. Surface-applied treatments may not be effective in reaching decay-prone areas within large timbers, 

and if the circumstances do not allow replacement of that 

member with a pressure-treated member or drilling of holes 

to apply internal treatments, then there may not be sufficient 

benefit to using preservatives. In this type of situation other 

options, such as protecting the wood member from moisture 

or replacing the member with a naturally durable wood, may 

be preferable. One must also consider whether the choice of 

preservatives allowed for a project will be effective. For example, if an in-place treatment for decay must be colorless, 

odorless, and have very low toxicity, the current options are 

limited to borate formulations. But because borate formulations are leachable, they only provide long-term protection 

in applications with limited exposure to liquid water. In 

some cases, it may be more practical to take no action 

and plan for periodic replacement of members as they 

deteriorate.

Historical Use of Wood Preservatives

Some historical structures may contain wooden components 

that were originally treated with wood preservatives either 

through pressure treatment or other means. The purpose of 

this section is to summarize historical use of wood preservatives and to discuss options for replacement of these 

members.

Prior to the 1920s, preservative treatments were largely 

confined to treatment of railroad ties, bridge timbers, and 

fence posts. The primary preservative used during this time 

was creosote or a creosote–oil mixture, although other waterborne preservatives were used to some extent. Zinc chloride was used from the early 1900s to the early 1930s, with 

maximum use around 1920. Its primary use was in treatment 

of railroad ties. Sodium fluoride was also used for a limited 

time in the early 1900s. In the late 1920s and early 1930s, 

other water-based preservatives such as zinc-meta-arsenite 

and chromated zinc chloride began to be used, although 

creosote remained the primary preservative. The other important oil-type preservative, pentachlorophenol, began to 

be produced in the early 1930s, with initial uses in exterior 

millwork. Pentachlorophenol in low viscosity oils (such as 

mineral spirits) was sometimes also used for treatment of 

interior millwork. Pentachlorophenol in heavy oil also became widely used for pressure treatment of wooden utility 

poles. Another oil-borne preservative, copper naphthenate, 

was sometimes used for brush or dip applications, and came 

to the forefront during the creosote shortage of 1945–1947 

when it was mixed with creosote pressure-treatment 

solutions.

An important shift in preservative use began in the 1940s 

and early 1950s when ammoniacal copper arsenate (ACA) 

and an early version of chromated copper arsenate (CCA) 

were introduced. An arsenic-free formulation, acid copper 

chromate (ACC), was also introduced and was primarily 

used for above-ground applications. These water-based preservatives became increasingly used and displaced earlier 

water-based preservative formulations. Eventually, formulations of CCA surpassed even creosote as the dominant pressure-treatment preservative. This trend was amplified with 

the increased use of CCA-treated wood in residential applications (i.e., decking and fencing) beginning in the 1960s 

and 70s. Relative volumes of creosote used also declined as 

pentachlorophenol became the dominant oil-borne treatment 

for utility poles. However, it is notable that both pentachlorophenol and creosote were available and widely used for 

consumer brush-on treatments until the early 1980s.

From the late 1800s to 1960s, a variety of other preservative 

formulations or active ingredients have been used to a lesser 

extent or for specific applications. Examples include copper8-quinolinolate (also called oxine copper), fluorine–chromium–arsenate–phenol (FCAP) pastes, copper sulfate, nickel 

salts, mercuric chloride, and boron compounds. Although 

relatively minor, use of nontypical preservative formulations 

cannot be completely discounted. Prior to environmental 

regulations and widespread acceptance of preservative 

standards, there were few limitations on chemicals used for 

wood protection.

Depending on the age and type of structure, it is possible to 

encounter wood treated with one or more of these historical 

wood preservatives, and the presence of treated wood can 

present the historic preservationist with unique challenges. 

In some cases, the preservative used may no longer be commercially available or may not have an EPA registration. In 

other situations, the preservative may still be available but 

is no longer considered appropriate (or registered) for the 

original end use. For example, wood treated with creosote or 

pentachlorophenol is still available, but its use in areas with 

limited air exchange or frequent human skin contact is no 

longer considered acceptable. 

A primary consideration is the need for preservative treatment to maintain durability. In some cases, such as treatment of interior millwork, the original preservative treatment may not have been necessary. In this situation, replacement material can either be left untreated or treated with 

nonpreservative finish that imparts a similar appearance. If 

preservative treatment is needed for durability purposes, an 

appropriate commercially available alternative preservative 

should be considered (see section Using Pressure-Treated 

Wood). It may be possible to apply a finish or other modi-
Guide for Use of Wood Preservatives in Historic Structures

21

fication to an available type of pressure-treated wood to 

create a desired effect. A few pressure-treatment preservatives are also available with pigments incorporated into the 

treatment process. Surface applied preservatives can also 

sometimes be pigmented. For example, oil-borne copper 

naphthenate solutions can be darkened to create an appearance similar to creosote-treated wood.

Options for In-Place Preservative 

Treatment

Characteristics of In-Place Treatments

Diffusible Preservatives

Diffusible preservatives, or diffusible components of preservatives, move slowly through water within the wood 

structure. Diffusible preservatives do not react with or “fix” 

in the wood, and thus are able to diffuse through wood as 

long as sufficient moisture is present. The distance or extent of diffusion is a function of preservative concentration, 

wood moisture content, and grain direction. A concentration 

gradient is needed to drive diffusion, and concentration can 

become a limiting factor with surface- (spray-) applied treatments because the volume of actives ingredients applied to 

the surface is limited. The most commonly available diffusible preservatives are based on the use of some form of 

boron (Table 1).

Sodium fluoride is less widely used as a diffusible treatment. 

This chemical is effective against decay fungi, but less active against insects. It is currently available in the form 

of a solid rod and as a component of liquid or paste 

formulations.

Boron-based supplemental treatments have several advantages. Boron has efficacy against both decay fungi and 

insects and has relatively low toxicity to humans. The sodium borate formulations used as field treatments are also 

relatively simple to dilute with water prior to application. 

Borates are also odorless and colorless and when diluted 

typically do not interfere with subsequent application of 

finishes.

Borate field treatment preservatives are available in a range 

of forms including powders, gels, thickened glycol solutions, solid rods, and as one component of preservative 

pastes. The concentration of actives is usually expressed as 

a percentage of disodium octaborate tetrahydrate (DOT), 

although concentration is sometimes reported as a percentage of boric acid equivalents (BAE) or boric oxide (B2O3) 

equivalents. Typically, wood moisture contents of at least 

20% are thought to be necessary for boron diffusion to occur, and while this moisture level is often surpassed for 

wood exposed outdoors, wood members more protected 

from moisture may be below this moisture content. Diffusion appears to be substantially more rapid at wood moisture 

contents in excess of 40%. At higher moisture contents, 

diffusion is much greater along than across the wood grain, 

but this effect may be less apparent at lower moisture 

contents.

Powdered borates are typically 98% DOT and are often the 

least expensive on the basis of active ingredient purchased. 

The powder is mixed (by weight) with water for use in spray 

or brush applications. Solution concentrations in the range 

of 15% DOT (by weight) can be achieved with the combination of warm water and vigorous agitation. Powdered 

borates can also be poured or packed into holes for internal 

treatments, but this method of application can be labor intensive and increases the risk of spillage.

Thickened glycol–borate solutions are typically provided 

with a 40% DOT content, although one product contains 

50% DOT. The syrupy liquid is then diluted 1:1 or 1:2 with 

water, yielding a solution containing approximately 22% 

or 15% DOT. Lower concentrations can also be prepared if 

desired. The glycol formulations allow a greater borate solution concentration than powders, and the resulting dilutions 

tend to resist precipitation longer than those prepared from 

powders. Dilution by volume rather than by weight can also 

be advantageous in some situations. The more viscous and 

concentrated glycol-borate solutions are also thought to 

allow deposition of higher concentrations of boron on the 

wood surface during spray applications.

Glycol–borate solutions can be applied by spray or brush 

or used to flood cut-ends or holes. Because the solution 

contains water, some diffusion can occur even in dry wood. 

This effect is greatest for applications that provide a reservoir of solution, such as in filling treatment holes. With the 

addition of foaming agents and specialized equipment, these 

formulations can also be applied as foams. This approach 

has been used by the National Park Service in treatment of 

difficult-to-access areas of historic wooden vessels.

Borate gels are currently less widely available than other 

forms of borates but are provided by at least one manufacturer. The gel contains 40% DOT and is provided in tubes 

for application with standard caulking guns. An advantage 

of the gel formulation is that it can be applied to voids, 

cracks, and treatment holes, which are oriented horizontally 

or downward and would not contain liquid borates. They are 

also convenient to apply but are typically the most costly 

form of borates on the basis of active ingredient purchased.

Rods contain active diffusible preservatives compressed or 

fused into a solid for ready application into treatment holes 

(Fig. 30). The most common active ingredient is boron, 

although one product is composed of sodium fluoride 

(Fig. 31) and another contains small percentage of copper 

(Fig. 32). The advantage of rod formulations is their ease 

of application and low risk of spillage. They can also be 

applied to holes drilled upward from under a member. One 

disadvantage of the rods is that their application does not 

include water to assist the initial diffusion process. Because 
General Technical Report FPL–GTR–217

22

of this lack of moisture, some applicators will drill slightly 

oversized treatment holes and fill the void space around the 

rod with a borate solution.

Paste formulations typically contain at least one component 

that diffuses into the wood and at least one other component 

that is expected to provide long-term protection near the application. The most common diffusible component is some 

form of borate, although one formulation uses fluoride. The 

less mobile component is commonly some form of copper. 

Pastes tend to be a more complex mixture of actives than 

other types of supplemental treatments. The paste treatments 

are most commonly applied to the ground line area of 

poles or terrestrial piles. In some products, the paste is 

incorporated directly into a wrap for ease of application. 

Labeling also allows most of the paste products to be used 

for internal treatment of holes by application with a caulking gun. The paste would need to be loaded into refillable 

caulking tubes for application in this manner. The pastes can 

also be spread on the tops of cut piles before application of 

pile caps. Because of their copper components, pastes have 

a blue or green color and thus may not be appropriate for 

areas where maintenance of a natural or historic appearance 

is important. Pastes also leave a residue on the wood surface 

in their area of application.

In some instances, water-based external treatments that 

contain both nondiffusible and diffusible components may 

be injected under low pressure; these products are most effective when inspection determines that a void has formed 

in the wood. Water-based external treatments typically are 

viscous in nature and will not run out of the wood as quickly 

or easily as nondiffusible liquids.

Non-Diffusible Liquids

The oldest and simplest method for field treatment involves 

brushing or spraying a preservative onto the surface of the 

suspected problem area. These solutions do not penetrate 

more than 1 or 2 mm across the grain of the wood, although 

greater penetration is possible parallel to the grain of the 

wood. In general, however, these treatments should not be 

expected to move great distances from their point of application. The preservatives in this category are applied 

as liquids but have some mechanism that allows them to 

resist leaching once applied to the wood. The most typical 

examples are the oil-borne preservatives that resist leaching because of their low water-solubility. For decades, 

pentachlorophenol and creosote solutions were used for this 

purpose, but their use is now restricted to pressure-treatment 

facilities. Most current liquid treatments use some form of 

copper (e.g., copper-8-quinolinolate or copper naphthenate), 

although zinc naphthenate is also available in some areas.

Oil-based copper naphthenate is available in copper concentrations ranging from 1% to 8% (as elemental copper). The 

solution is typically applied at 1% to 2% copper concentration, and more concentrated solutions are diluted with mineral spirits, diesel, or a similar solvent. These solutions impart an obvious green color to the wood (Fig. 33), although 

some of the 1% copper solutions are available tinted to dark 

brown or black. They also have noticeable odor.

Water-based copper naphthenate is currently less widely 

used than the oilbased formulations. It is available as a 

concentrate containing 5% copper and can be diluted with 

water. The water-based formulation has a somewhat less 

noticeable odor, and the color is more blue than green. The 

water-based formulation is slightly more expensive than the 

oil-based form, and may not penetrate as deeply into the 

wood as the oil-based form.

Figure 30—Borate rods are available in a range of 

sizes including the 19-mm (0.75-in.) and 13-mm (0.5-in.) 

diameters shown here.

Figure 31—Sodium fluoride rod.

Figure 32—Example of a rod that contains both boron and 

copper. 
Guide for Use of Wood Preservatives in Historic Structures

23

Oil-based copper-8-quinolinolate was recently standardized by the AWPA for field treatment of cuts, holes, or other 

areas of untreated wood exposed during construction. It 

is available as a ready-to-use solution containing 0.675% 

copper-8-quinolinolate (0.12% as copper metal) as well as 

incorporated water repellents. It has a light greenish color, 

although it can be tinted to some extent. It can be applied by 

immersion, brushing, or spraying.

Zinc naphthenate is similar to copper naphthenate, but zinc 

is less effective than copper in preventing decay from wooddestroying fungi and mildew. However, an advantage of zinc 

naphthenate is that it is clear and does not impart the characteristic greenish color of copper naphthenate. It is available 

in both water-based and solvent-based formulations.

Fumigants

Fumigants are used to internally treat large logs or timbers. 

Like some diffusible formulations, fumigants are applied 

in liquid or solid form in predrilled holes. However, they 

then volatilize into a gas that moves much greater distances 

through the wood than do the diffusible treatments. One 

type of fumigant has been shown to move over 2.4 m 

(8 ft) along the grain from point of application in poles. To 

be most effective, a fumigant should be applied at locations 

where it will not readily volatilize out of the wood to the 

atmosphere. All but one of the commercial fumigants (chloropicrin) eventually decompose to produce the active ingredient methylisothiocyanate (MITC). One of the products is 

the solid melt form of 97% MITC that is encapsulated in 

aluminum tubes. Other MITC products use Vapam (sodium 

N-methyldithiocarbiamate), or the granular Dazomet (tetrahydro-3,5-dimethyl-2-H-1,3,5, thiodazine-6-thione). One 

of the Dazomet products is available in pre-packaged tubes 

that can be placed into treatment holes with minimal handling or risk of spillage. It and the solid-melt form of MITC 

have the advantage of placement in holes that are drilled 

upward. Chloropicrin is a very effective fumigant but also 

difficult to handle safely because of its volatility. Fumigant 

treatments are generally more toxic and more difficult to 

handle than the diffusible treatments. Some are considered 

to be Restricted Use Pesticides (RUP) by the U.S. EPA and 

require extra precautions. Fumigants are usually applied by 

specially trained personnel.

Liquid fumigants are poured into pre-drilled treatment holes, 

necessitating that they be applied from above. A fumigant 

commonly applied in liquid form is metham sodium (33% 

Sodium N-methyldithiocarbamate). Like several fumigants, 

this liquid formulation decomposes to produce the active 

ingredient methylisothiocyanate (MITC). It tends to be less 

expensive than other sources of MITC, but also contains 

a lower proportion of active ingredient. One of the oldest 

fumigants, chloropicrin, is only available in liquid form. It 

is a RUP, the use of which is generally confined to critical 

structures in rural areas.

Granular fumigants are poured into pre-drilled treatment 

holes in a manner similar to liquids. The current formulations use granular Dazomet (98% tetrahydro-3,5-dimethyl2-H-1,3,5, thiodazine-6-thione), which decomposes to 

produce MITC. The granular fumigant formulations offer 

relatively easy handling compared with the liquid metham 

sodium and also contain a higher percentage of active 

ingredient. However, they decompose to produce MITC 

more slowly than the liquids, and in some cases liquid additives are also poured into the treatment hole to promote 

decomposition.

Encapsulated fumigants are pre-packaged for convenient 

application and have the added advantage of allowing holes 

to be drilled from below. In addition to convenience, these 

encapsulated fumigants minimize the risk of spillage when 

applications are made over water or any other sensitive 

environments. One encapsulated product contains the same 

granular Dazomet that is poured into holes. It is encased 

in a tube-shaped, air-permeable membrane that contains 

the granules while allowing MITC gas to escape (Fig. 34). 

Another encapsulated product is comprised of an aluminum 

tube filled with solid 97% MITC (Fig. 35). At the time of 

Figure 33—The green color of copper naphthenate tends 

to weather to brown over time. The photograph on the 

top is soon after construction and that on the bottom was 

taken one year later. This wood was pressure treated with 

oil-borne copper naphthenate.
General Technical Report FPL–GTR–217

24

application, a special tool is used to remove the air-tight cap 

from the tube, and MITC vapors are released through this 

opening. Disadvantages of the encapsulated fumigants are 

their higher costs and that they require a minimum treatment 

hole diameter and depth for application.

Fumigants should not be applied into voids or when application holes intersect voids or checks to prevent accidental 

release of the product into the environment. Structures 

where fumigants have been applied should be marked to 

indicate its presence. Care and caution should be taken in 

the removal of wood structures that have been treated with 

fumigants to prevent exposure.

Application Guidelines

Internal Treatments

Decay may become established in large timbers because 

once moisture penetrates deeply into the wood, it is slow to 

dry. Large timbers are typically too thick to effectively treat 

the interior with surface application of preservatives. Internal treatments are typically applied by drilling holes into 

the wood, but there are many variations on this approach 

(Table 2).

Diffusible internal treatments—Diffusible internal treatments generally do not move as great distances through the 

wood as do fumigants, so their location and spacing is critical. Although they could be used to treat the length of logs 

or beams, they may be better suited to protection of specific 

vulnerable areas such as near connections and areas around 

fasteners. The extent of movement of these diffusible 

treatments has been shown to vary with wood moisture content and wood species, although wood moisture content is 

probably the most important factor. Wood moisture content 

is typically lower for wood above ground than wood used in 

ground contact, and studies of boron movement from internal treatments have indicated somewhat limited mobility in 

above-ground timbers with low moisture content.

Research indicates that solid boron rods applied to aboveground timbers generally need to be placed no more than 

51 mm (2 in.) apart across the grain and 305 mm (12 in.) 

apart along the grain. Tighter spacing may be needed for 

some less permeable species, as there is substantial variability in boron mobility in timbers treated with combinations 

of liquid and solid internal treatments. In more permeable 

Southern Pine timbers, spacing of approximately 76 mm 

(3 in.) across the grain and between 76 and 125 mm (3 to 

5 in.) along the grain may be sufficient to achieve overlapping boron penetration. The manufacturer of one of the boron rod products recommends parallel to the grain spacing 

of between 152 and 381 mm (6 to 15 in.) depending on the 

size of the timber and the size of the rod installed. They also 

recommend that across the grain distance between treatment 

holes not exceed 152 mm (6 in.).

Liquid borates may be applied in a similar manner to rods, 

except that their use is generally limited to holes oriented 

downward. The concentration of boron in the liquid treatments is not as great as that in the rods, but the potential for 

diffusion is greater at lower wood moisture contents. The 

liquid borates also provide protection more rapidly than the 

rods, but the duration of protection is more limited. Liquid 

borates also allow more flexibility in the size of the treatment hole, and in some cases it may be desirable to drill 

many small holes instead of a few large holes. The liquids 

can be readily applied to smaller treatment holes with 

squeeze or squirt bottles. In situations where the treatment 

holes are protected from precipitation and public access, the 

holes can be temporarily left unplugged to allow re-filling 

as the liquid moves out of the treatment hole and into the 

wood. Alternatively, a rod can be placed into the treatment 

hole after the liquid has drained into the wood. It is worth 

noting, however, that movement of liquid is slow through 

the heartwood of many wood species, and that the time required for the hole to empty may be longer than anticipated. 

Rods and liquid borates can also be simultaneously added to 

treatment holes by drilling holes slightly larger than needed 

to accommodate the rod. This approach can provide both 

an immediate boost of liquid boron as well as the longer 

term slow release from the rod, but it does require drilling a 

larger treatment hole than would otherwise be necessary.

Liquid borates have also been injected into small treatment 

holes in horizontal timbers using a low-pressure sprayer, 

with the nozzle pressed tightly against the treatment hole to 

prevent leakage. Under these conditions, a diamond pattern 

has been recommended with 305 mm (12 in.) between holes 

Figure 34—A granular fumigant pre-packaged in a vaporpermeable membrane.

Figure 35—A solid fumigant encapsulated in a metal tube. 

The cap is removed at installation.
Guide for Use of Wood Preservatives in Historic Structures

25

Table 2—Application characteristics for internal preservative treatments 

Type of treatment 

Target retention

in wood 

(oz/ft3

 and 

kg/m3

)

Hole dimensions 

(in. (mm)) 

Spacing of 

treatment holes 

Diameter Length Posts/piles Timbers 

Boron rod 1.7–5, as DOTa 5/16–13/16

(8–21)

2.5–13 

(64–330)

7–15 in. 

(178–381 mm) 

vertical, 

90°–120° intervals

6–14 in. 

(152–356 mm)

along the grain,

3–6 in. 

(76–152 mm) 

across the grain 

Boron/copper rod 1.7–5, as DOTa 1/4–3/4 

(6–19)

1.5–5.5 

(38–140)

Vertical spacing

not described, 

120° intervals 

6–14 in. 

(152–356 mm)

along the grain 

Sodium fluoride rod 1.4, as NaF 7/16–5/8 

(11–16)

3–5

(76–127)

6 in. 

(152 mm) 

vertical, 

90°–120° intervals

Not described 

Borate, liquid glycol 1.1, as DOTa

 Variable Variable 7–15 in. 

(178–381 mm) 

vertical, 

90°–120° intervals

12–16 in. 

(305–406 mm)

along the grain,

4–6 in. 

(102–152 mm)

across the grain 

CuNaph liquid 0.96–2.4, 

as Cu 

Variable Variable Not described Not described 

CuNaph/NaF liquid NA Variable To cavity Flood internal 

cavity 

Not labeled 

for this use 

Borate/copper 

hydroxide liquid 

NA 0.5 

(13)

To decay

pocket

Flood decay 

pockets 

Flood decay 

pockets 

Borax/copper 

hydroxide paste 

3.7–14.7, 

as borax 

+ Cu(OH)2

Up to 1 

(25)

Variable Not described Not described 

Borax/CuNaph paste Not provided 3/4 

(19)

Variable 24 in. 

(610 mm) 

vertical, 

90° intervals 

Not labeled 

for this use 

Borax, tebuconazole, 

bifenthrin, oxine, 

copper 

Not provided Variable Variable Not described Not described 

DOT gel 1.1, as DOTa

 Variable To center 12–24 in. 

(305–610 mm) 

vertical 

12–24 in. 

(305–610 mm)

along grain 

Fumigants Approximately

0.01 for 

MITC-based,

unknown

for chloropicrin 

3/4–7/8 

(19–22)

Through

center, 

12 (305)

minimum 

for

MITC-fume

6–12 in. 

(152–305 mm), 

90°–120° intervals

Maximum of 

4 ft (1.23 m) 

along grain 

a

Disodium octaborate tetrahydrate.

along the grain and 102 to 152 mm (4 to 6 in.) across the 

grain. It is likely that penetration achieved using this 

approach would depend greatly on wood permeability. 

Risk of spillage into the area below the structure is likely 

to be higher with this approach than with nonpressure 

applications.

Gels and paste products may also be applied as diffusible 

internal treatments in a manner similar to liquids and rods. 

Depending on the properties of the individual product, they 

may be applied to holes that are horizontal or even oriented 

upward. Application to treatment holes is typically accomplished with use of a caulking tube and caulking gun. In 
General Technical Report FPL–GTR–217

26

theory, these formulations provide somewhat of a compromise between the liquid formulations and the solid rods, 

with slower distribution than the liquids but more rapid 

distribution than rods. However, there is little published 

research comparing the penetration or longevity of these 

formulations to that of the other formulations.

There is also limited information on the mobility of internal 

diffusible preservatives other than boron. Both fluoride and 

copper have been incorporated into internal treatments, and 

fluoride has been used as a stand-alone preservative in a 

fused rod form. The mobility of copper when applied in this 

manner appears very limited, probably as a result of lower 

water solubility and its tendency to react with and “fix” to 

the wood structure. Fluoride is thought to have diffusion 

properties similar to boron, although this assumption is not 

well supported by research.

Fumigants—To be most effective, a fumigant should be 

applied at locations where it will not leak away or be lost by 

diffusion to the atmosphere. When fumigants are applied, 

the member should be inspected thoroughly to determine an 

optimal drilling pattern that avoids metal fasteners, seasoning checks, and severely rotted wood. Manufacturers have 

developed specific guidance for application of their products 

to round vertical members such as posts, poles, and piles. 

Although these application instructions vary somewhat between products, they generally specify drilling holes of 

19- to 22-mm (0.75- to 0.825-in.) diameter downward at angles of 45° to 60° through the center of the round member. 

The length of the hole is approximately 2.5 times the radius 

of the member. A minimum hole length of 305 mm (12 in.) 

is required for the use of the MITC-FUME tube, necessitating the use of a steeper drilling angle in smaller diameter 

members. In terrestrial applications, the first hole is drilled 

at or slightly below the ground line. Subsequent holes are 

drilled higher on the member, moving up and around in a 

spiral pattern. Depending on the product and diameter of the 

member, the holes should be spaced at either 90° or 120° 

around the circumference. The recommended vertical distance between treatment holes varies from 

152 to 305 mm (6 to 12 in.) near the groundline, with 

305-mm (12-in.) spacing used higher on the member. Allowable uses of fumigants for aquatic structures are not 

always specified on the product labels, but at a minimum 

the lowest part of a treatment hole should be above the 

waterline.

Much less information is available on application of fumigants to large timbers. Holes are typically drilled into a narrow face of the member (usually either the top or bottom). 

Holes can be drilled straight down or slanted; slanting may 

be preferable because it provides a larger surface area in the 

holes for escape of fumigant. As a rule, the holes should be 

extended to within about 51 mm (2 in.) of the top or bottom 

of the timber and should be no more than 1.22 m (4 ft) apart. 

With the encapsulated solid fumigants, the treatment holes 

can be drilled upward in a similar manner. Solid fumigants 

provide a substantial advantage in treatment of timbers and 

beams because access is often limited to the bottom face. A 

disadvantage of the pre-encapsulated fumigants is that they 

require a minimum size of treatment hole, and thus cannot 

be used on smaller members.

When treating with fumigants, the treatment hole should be 

plugged with a tight-fitting treated wood dowel or removable plastic plug immediately after application. Sufficient 

room must remain in the treating hole so the plug can be 

driven without squirting the chemical out of the hole or affecting the solid fumigant. The amount of fumigant needed 

and the size and number of treating holes required depend 

on timber size. Fumigants will eventually diffuse out of the 

wood, allowing decay fungi to recolonize. Additional fumigant can be applied to the same treatment hole, a process 

that is made easier with the use of removable plugs.

Non-diffusible liquids—Non-diffusible liquid treatments, 

typically containing copper, are sometimes used for internal treatments. Although these treatments do not diffuse 

in water within the wood, they can wick for several inches 

parallel to the wood grain. Movement across the grain is 

minimal. The advantage of these liquids relative to the 

diffusible treatments is their resistance to leaching. Thus, 

they may have applications where duration of efficacy is 

of greater importance than volume of wood protected. An 

example is treatment of connector holes when substantial 

untreated wood is exposed during fabrication. Treatment 

holes can also be drilled above existing connectors, filled 

with preservative, and plugged. Again, this type of treatment 

may be desirable if subsequent fabrication or construction 

activities will make that area difficult to access in the future. 

In large members, these preservative liquids may be used to 

flood internal voids such as decay pockets, but the risk of 

spillage makes this type of application less suitable for some 

applications.

External Treatments

External treatments generally have the greatest applicability 

for members that have not been pressure treated, but also 

have value in protecting pressure-treated wood when untreated wood is exposed by fabrication during construction. 

Many of the same formulations used for internal treatments 

can also be used for external treatment. Protection is generally limited to within a few millimeters of the wood surface, 

although greater movement does occur when solutions are 

applied to the end-grain of wood. Surface-applied diffusible treatments can also achieve deeper penetration under 

some conditions. However, broad-scale surface sprays can 

be highly problematic from the viewpoint of environmental 

contamination, and potential benefit from this approach 

must be weighed against this risk. In many cases, it may 

be more practical to limit surface applications to localized 

areas.
Guide for Use of Wood Preservatives in Historic Structures

27

Diffusible liquid preservatives (borates) are typically applied with low-pressure sprayers or by brushing in smaller 

areas. The greatest benefit is achieved by flooding checks, 

cracks, and other openings, potentially allowing diffusion 

into decay-prone areas where water precipitation has 

become trapped within the wood. Because of this, it is often 

desirable to apply the solution after a prolonged dry interval, when checking in the wood is at a maximum. Borates 

applied to the wood surface can be rapidly depleted if the 

wood is exposed to precipitation or other forms of liquid 

water. Borate depletion from exposed members can be 

slowed (but not completely prevented) with application of 

a water-repellent formulation after the borate treatment has 

dried. This may necessitate tarping or otherwise protecting 

the treated members until they have dried sufficiently to allow application of the water repellent. Use of preservativebased water repellent (for example, containing copper or 

zinc naphthenate) can provide further protection to the wood 

surface. This process can be repeated after the wood surface 

loses its water repellency. Surface application of non-diffusible liquid treatments is typically limited to exposed situations where their resistance to leaching is a key attribute. As 

mentioned above, oil-type non-diffusible liquids can also be 

applied after a diffusible treatment to slow leaching of the 

diffusible preservative and to provide long-term protection.

The most common external use of gels and pastes is in the 

protection of the ground-line area of support poles, posts, 

or timbers as part of a wrap system. Soil is excavated from 

around the support to a depth of approximately 0.46 m 

(18 in.), and the formulation is brushed or troweled onto 

the exposed wood to form a thick layer that extends 51 to 

76 mm (2 to 3 in.) above the ground line. The layer of preservative is then covered with a water-impervious wrap to 

hold the chemical against the wood, and the excavated area 

is refilled. The diffusible components of the formulation 

(for example, boron) gradually diffuse into the wood while 

the less mobile components remain near the wood surface. 

When these pastes are applied to pine sapwood, boron 

or fluoride may penetrate as much as 76 mm (3 in.) into 

the wood, and copper may penetrate up to 13 mm (0.5 in.). 

These treatments have been shown to offer substantial protection to the groundline area of untreated wood. This type 

of application must not be used in areas where standing 

water is expected. The same principal can also be used to 

protect wood above ground that is covered with metal or a 

simple barrier. For example, these products can be spread 

on to the timbers that are subsequently wrapped with metal 

flashing. Metal flashing can cause moisture to condense 

between the metal and the wood, so treatment in this area 

is desirable. However, many of these formulations are not 

colorless, and preservative that wicks along the grain and 

extends beyond the cover could slightly discolor untreated 

wood.

Summary of In-Place Treatment Application 

Concepts

Liquid Surface Treatments

Surface-applied liquid treatments should not be expected to 

penetrate more than a few millimeters across the grain of 

the wood, although those containing boron can diffuse more 

deeply under certain moisture conditions. They will not effectively protect the interior of large piles or timbers.

Liquid surface treatments are most efficiently used for 

flooding checks, exposed end-grain, or bolt holes. They may 

move several centimeters parallel to the grain of the wood if 

the member is allowed to soak in the solution.

Surface treatments with diffusible components will be 

washed away by precipitation if used in exposed members. 

However, their loss can be slowed if a water-repellent finish 

is applied after the diffusible treatment has dried.

Paste Surface Treatments

Paste surface treatments can provide a greater reservoir of 

active ingredients than liquids. When used in conjunction 

with a wrap or similar surface barrier, these treatments can 

result in several centimeters of diffusion across the grain 

into moist wood over time. They are typically used for the 

groundline area of posts or timber that are not usually exposed to standing water, but can also be applied to end-grain 

of connections or under flashing. Some formulations can be 

applied under low pressure as a void treatment.

Internal Treatments

Internal treatments are typically applied to the interior of 

larger members where trapped moisture is thought to be a 

current or future concern. They can be applied to smaller 

members in some situations.

Diffusible treatments move through moisture in the wood. 

They are generally easy to handle, but do not move as great 

a distance as fumigants and do not move in dry wood. The 

diffusion distance in moist wood is approximately 51 to 

102 mm (2 to 4 in.) across the grain and 152 to 305 mm 

(6 to 12 in.) along the grain. Diffusible treatments may be 

best suited for focusing on specific problem areas such as 

near exposed end-grain, connections, or fasteners.

Rod diffusible treatments provide a longer, slower release of 

chemicals while liquid diffusible treatments provide a more 

rapid, but less long-lasting dose of preservative. Paste and 

gel internal treatments fall somewhere between rods and 

liquids in regard to speed of release.

Fumigant treatments move as a gas through the wood. They 

have the potential to move several feet along the grain of the 

wood, but have increased handling safety and application 

concerns compared with other internal treatments 

(Tables 1, 2). 
General Technical Report FPL–GTR–217

28

Example In-Place Treatment Applications

Log Cabins and Similar Structures

Log structures have several characteristics that can contribute to the potential for deterioration. Because of their large 

size, logs almost invariably form deep drying checks that 

allow moisture to penetrate to the center of the log. This 

moisture is slow to dry, increasing the likelihood that conditions will be conducive for decay development. In many 

structures, the logs at corners also protrude to such an extent 

that they have minimal protection from the roof overhang, 

and the large area of exposed end-grain aids moisture 

absorption. The bottom course of logs is also likely to be exposed to wetting either from wind-blown rain or from splash 

from water draining off the roof.

Possible approaches to protecting log structures include 

placement of boron or copper-boron rods into the ends of 

the logs nearer the ground (Figs. 36, 37). To minimize visibility, these holes can be drilled upward at an angle from 

below the logs. Borate solutions can be applied to the 

end-grain and other log surfaces, with emphasis on joints, 

checks, and other moisture-trapping surfaces. If chinking 

is to be replaced as part of the project, more visible treatments such as preservative gels could be used in the area 

to be covered by the chinking. Holes could also be drilled 

in this area for application of diffusible preservative rods 

or liquid borate or both. In conventional log homes, borate 

solutions are sometimes sprayed onto the entire outer wall 

after checking and before application of a water-repellent 

finish. Although the borates are leachable, the application of 

a water-repellent finish after the borate spray can slow boron 

loss.

Wooden Windows and Similar Millwork Applications

Millwork and windows in particular are one of the most 

common problem areas in both historic and contemporary 

wooden structures. Window woodwork may be subject to 

wetting from both precipitation and condensation, and the 

joint areas and their associated connections are well suited 

for absorbing and trapping moisture. Because of its high 

visibility, millwork can be difficult to protect with preservatives without affecting aesthetics. In some windows, holes 

can be drilled upward from below the sash to install smalldiameter, short-diffusible preservatives rods (Fig. 38). In 

other cases, if the woodwork will subsequently be painted, 

thin rods are installed from the upper surface followed by 

wooden plugs of a matching material. Filler and sanding 

may be needed to create a uniform appearance. Alternatively, small-diameter holes can be drilled into the problem area 

and repeatedly flooded with preservative. Again, this approach requires the use of some type of filler and surfacing 

before painting. The simplest and least damaging option is 

to apply concentrated liquid borate solution into the window 

corners. The extent of penetration achieved will be limited 

for coated wood, but substantial end-grain penetration is 

possible when the solution is applied to bare wood.

Figure 36—Example approach to field 

treatment of a log cabin corner. (a) Application of concentrated borate solution to 

area where logs meet. (b) Boron-copper 

or boron rods applied upward into logs. 

(c) Brush application of concentrated 

borate solution to log ends and checks 

in top of logs. (d) Application of borate 

solution to outer surfaces with garden 

sprayer. (e) Application of concentrated 

borate gel prior to chinking. (f) Downward 

application of boron or boron-copper rod 

prior to chinking.
Guide for Use of Wood Preservatives in Historic Structures

29

diffusible internal treatments can be applied into the narrow 

face of each member on each side of the connection 

(Fig. 39). Rods can be purchased in various diameters 

allowing use of relatively small-diameter treatment holes. 

Liquids, pastes, and gels can also be applied to smalldiameter holes, and drilling holes downward from the upper face allows use of liquid treatments either alone or in 

combination with rods. However, drilling from the top of 

the member may also create a more visible treatment hole 

for members below eye level. Visibility of the holes can be 

minimized by drilling downward for connections above eye 

level and upward for connections below eye level, but drilling upward limits treatments to solid rods. Drilling holes 

with diameter sufficient for fumigant treatments may not be 

desirable in narrower members.

Timber Frame Structure

Structural support timbers may be exposed to moisture either as result of the original design or loss of siding/roofing 

materials. As in other structures, areas around fasteners and 

connections are most likely to warrant preservative treatment. Because moisture conditions conducive to decay are 

likely to be inside the large timbers, surface treatments alone 

may not be particularly effective. However, application of 

concentrated solutions of a diffusible preservative to the 

end-grain areas may have value because subsequent wetting and wicking may draw the preservative a considerable 

distance into the wood. Drilling the holes needed to apply 

internal treatments may not always be acceptable, but in this 

example it is assumed that the holes can be drilled as long as 

they are not visible from the exterior (Fig. 40). Solid diffusible rods can be applied from beneath the large beams and 

angled upwards towards the connection. Downward sloping 

treatment holes can accommodate liquid diffusible treatments or solid diffusible treatments. Some beams may be 

large enough for application of a solid fumigant, which can 

also be applied to an upward-angled treatment hole. Fumigants protect a much larger volume of wood than diffusible 

treatments and are not dependent on localized moisture conditions for movement through the wood. However, their use 

may not be appropriate for use in many structures, particularly those with limited air exchange or human habitation.

Support Members Contacting Stone or Masonry

Areas where support members contact stone or masonry 

are among the most prone to decay. In many cases, previous restorative work has addressed this issue by changing the contact point so that the untreated timber rests on 

pressure-treated wood or some other type of support that is 

less conducive to moisture accumulation. However, in some 

structures untreated structural members do rest on stone or 

masonry, and these can be challenging, but important, areas 

to protect with field treatments (Fig. 41). Access is often 

limited, and unlike in most connections, the area of moisture 

accumulation is on an exterior surface that is inaccessible. 

Figure 37—Example approach to treatment of cabin corner at porch interface. 

(a) Application of concentrated borate 

solution to checks in upper surfaces and 

connection areas. (b) Brush application of 

concentrated borate solution to log ends 

and checks in top of logs. (c) Copperboron or boron rods angled upward into 

logs.

Dimension Lumber Exposed Below a Window Opening

Lumber presents somewhat unique challenges for in-place 

treatment because of the narrow dimensions. Although the 

narrow dimensions do discourage checking and subsequent 

water entrapment, connections can trap moisture if exposed 

to occasional wetting. Internal treatments can be used to 

provide some protection for these connections. The 
General Technical Report FPL–GTR–217

30

However, depending on the situation, substantially increased 

protection may be possible. Fumigants or other internal 

treatments can be used to protect the bulk of the interior, and 

rods containing diffusible preservatives can be placed in a 

series of horizontal holes just above the bearing surface. In 

some cases, it may be possible to inject preservative liquid, 

paste, or gel between the bearing surface and masonry, or a 

caulking gun can be used to deposit paste or gel of diffusible 

preservative along the edge of the member where it meets 

the masonry. However, this latter approach requires discretion as it does leave the preservative deposit exposed.

Who Can Apply In-Place Preservative 

Treatments?

Wood preservatives are defined as pesticides under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), 

and thus are regulated by the U.S. EPA. The EPA and each 

state have regulations about who can apply pesticides. The 

EPA regulations provide a minimum set of requirements, 

and each state may have additional requirements. The U.S. 

EPA is most concerned with the Restricted Use Pesticides 

(RUPs). Two of the fumigants discussed in this publication 

(chloropicrin and methylisothiocyanate) fall into this category. U.S. EPA regulations require that RUP applicators be 

certified as competent to apply restricted use pesticides in 

accordance with national standards. Certification programs 

are conducted by states, territories, and tribes in accordance 

with these national standards. Training of certified applicators covers safe pesticide use as well as environmental 

issues such as endangered species and water quality protection. Certified applicators are classified as either private or 

commercial. There are separate standards for each. All states 

require commercial applicators to be recertified, generally 

every 3 to 5 years. Some states also require recertification or 

other training for private applicators.

Figure 38—Example approaches to treatment of window woodwork. (a) Application of concentrated borate 

solution to connection areas. (b) Application of borate solution and small boron rods prior to painting. (c) Boron 

rods applied from below. These treatments would be of little value once deterioration reaches the stage shown in 

the upper left photo.
Guide for Use of Wood Preservatives in Historic Structures

31

Figure 39—Example in-place treatment of members below a window opening. 

(a) Boron rod. (b) Concentrated borate solution applied to treatment hole and 

then boron rod. (c) Brush application of concentrated boron solution.

Figure 40—Example approach to treatment of a timber frame structure. (a) Boron or boron-copper rod. 

(b) Application of concentrated borate solution to connection surfaces. (c) Encapsulated fumigant. (d) Boroncopper or boron rod.
General Technical Report FPL–GTR–217

32

States vary in their regulations about application of nonrestricted use pesticides. Most states require that commercial 

applicators become licensed to apply these products. However, a private applicator (property owner) can purchase and 

apply these pesticides on their own property without any 

type of licensing. Application of field treatments by state, 

county, or local government employees can be somewhat of 

a grey area. Although technically these workers are applying 

the treatments to their own property, the property itself is 

public. Thus, many states do require that government workers be trained and licensed as pesticide applicators. The best 

source of information for applicator licensing requirements 

is that state’s agency responsible for conducting the U.S. 

EPA’s pesticide applicator program.

Importance of the EPA Label

Pesticide product labels provide critical information for 

safely and legally handling and using pesticide products. 

The directions for use provide instructions and identify the 

pest(s) to be controlled, the application sites, application 

rates, and any required application equipment. Just as importantly, this section also includes a use restrictions statement. General (nonsite-specific) precautions, restrictions, or 

limitations of the product are stated, as are any precautions 

and restrictions that apply to specific sites. Unlike most 

other types of product labels, pesticide labels are legally 

enforceable, and all of them carry the statement: “It is a 

violation of Federal law to use this product in a manner inconsistent with its labeling.” Labeling can also include material to which the label (or other labeling material) refers. 

For example, if a label refers to a manual on how to conduct 

a procedure, that manual is also labeling that the user must 

follow.

Using Pressure-Treated Wood

When to Consider Using Pressure-Treated 

Wood

The Secretary of Interior’s Standards for the Treatment of 

Historic Properties are intended to aid in the preservation 

of the historic materials, features, and spatial relationships 

of a property or structure. Typically, if historic materials 

Figure 41—Example approach for in-place treatment of the area of a structural 

member contacting stone or masonry. (a) Spray application of concentrated borate 

solution. (b) Fluoride rod. (c) Small boron rod. (d) Borate gel or boron-copper paste.
Guide for Use of Wood Preservatives in Historic Structures

33

need to be repaired or replaced, the standards require the 

replacement of material in kind. For wooden elements, this 

typically means using the same species and cut of wood. 

For most historic structures, the wood used in the original 

construction and/or historic repair campaigns is not pressure 

treated, making repair or replacement of historic materials with pressure-treated lumber or timber an incompatible 

solution. However, there may be situations where the use of 

pressure-treated wood is warranted. Use of pressure-treated 

lumber in historic structures is sometimes warranted for the 

repair or replacement of wood members where moisture 

intrusion issues cannot be mitigated, such as areas where 

wooden elements are in contact with the ground or are located below grade. If, after careful evaluation of existing 

conditions, it is determined that moisture mitigation efforts 

such as improving drainage, increasing air circulation, or 

redirecting water flows will not effectively manage moisture 

conditions and continued exposure to moisture is expected, 

the use of preservative-treated wood may be warranted. Sill 

plates, sill beams, and sill logs of historic structures that rest 

directly on the ground are common examples of elements 

where moisture mitigation may not be enough to preserve 

the timber and where the judicious repair or replacement of 

elements with pressure-treated wood may help to preserve 

the life of the structure. Basement framing members such 

as columns or joists that are below grade are also potential 

structural elements where it may be appropriate to make 

repairs or replacements with pressure-treated wood. Historic 

timber and covered bridges also have wood elements where 

moisture issues cannot be fully mitigated and exposure to 

moisture is expected to continue on a cyclic basis. Use of 

pressure-treated timber in the repair or replacement of elements with this type of exposure to moisture can extend the 

service life of the structure and should therefore be considered as a viable alternative to repair or replacement with 

concrete, metal, or other materials that may alter the structure much more significantly than the use of pressure-treated 

lumber or timber.

Preservative Penetration

The goal of pressure treatment is to force preservative 

deeply in the wood, thus protecting a larger proportion of 

the wood volume. Although pressure preservative treatments 

are generally effective in protecting the treated wood, in 

many cases, and especially with larger members, the preservative does not penetrate all the way to the center of each 

piece. The proportion of treatable sapwood varies greatly 

with wood species, and this becomes an important factor in 

obtaining adequate penetration. Species within the Southern 

Pine group are characterized by a large sapwood zone that 

is readily penetrated by most types of preservatives. In part 

because of their large proportion of treatable sapwood, 

these pine species are used for the vast majority of treated 

products in the United States. Other important lumber 

species, such as Douglas-fir, have a narrower sapwood band 

in the living tree, and as a result products manufactured 

from Douglas-fir have a lower proportion of treatable sapwood. In lumber and timbers the proportion of heartwood 

varies. During sawmilling, larger dimension timbers tend to 

be cut from the center of the tree and thus may have a substantial area of untreated heartwood (Fig. 42).

Proper preservative treatment creates an excellent barrier 

against fungi and insects. However, this treated zone can 

be compromised during on-site installation or as a result 

of checks and cracks from normal weathering and moisture changes. Drying checks allow water to collect and be 

trapped within the wood. Because wood does not shrink and 

swell equally in all directions, formation of some drying 

checks is not unexpected. Ideally, thorough drying of the 

members would cause these checks to form before treatment and allow them to be well protected with preservative. 

Small drying checks also may not be a concern if they do 

not penetrate past the treated zone. However, the appearance 

of large drying checks in timbers or piles can be an indication of conditions favorable for internal decay, and these are 

areas that warrant closer inspection and possible field treatment (Fig. 43).

Another common source of breaks in the treated zone is 

field fabrication of treated members. Examples include 

cutting to length, notching, and boring of holes for fasteners (Fig. 44). Ideally the extent of field fabrication during 

construction is minimized by specifying as much fabrication 

as possible before construction, but some field fabrication 

is usually necessary (Fig. 45). Again, ideally the wood exFigure 42—During pressure treatment, preservative 

typically penetrates only the sapwood. Round members 

have a uniform treated sapwood zone, but sawn members 

may have less penetration on one or more faces.
General Technical Report FPL–GTR–217

34

posed during construction should have been protected by 

application of a preservative such as copper naphthenate 

to the cut surface, but this practice is not always followed. 

In some cases, construction personnel are concerned about 

the loss of excess liquid preservative into the environment. 

When inspecting an existing structure, it is often difficult to 

determine if cuts were made in the field and whether or not 

a preservative was applied to the cut surfaces (Fig. 46).

Treated Wood Use Category System

The type of preservative applied is often dependent on the 

requirements of the specific application. For example, direct 

contact with soil or water is considered a severe deterioration hazard, and preservatives used in these applications 

must have a high degree of leach resistance and efficacy 

against a broad spectrum of organisms. These same preservatives may also be used at lower retentions to protect wood 

exposed in lower deterioration hazards, such as above the 

ground. The exposure is less severe for wood that is partially protected from the weather, and preservatives that lack 

the permanence or toxicity to withstand continued exposure 

to precipitation may be effective in those applications. Other 

formulations may be so readily leachable that they can only 

be used indoors (see Table 3.)

Standardization

Before a wood preservative can be approved for pressure 

treatment of structural members, it must be evaluated to 

ensure that it provides the necessary durability and that it 

does not greatly reduce the strength properties of the wood. 

The EPA typically does not evaluate how well a wood preservative protects the wood. Traditionally, this evaluation 

has been conducted through the standardization process of 

the AWPA. The AWPA Book of Standards lists a series of 

laboratory and field exposure tests that must be conducted 

when evaluating new wood preservatives. The durability of 

Figure 43—Deep seasoning checks can be an indication of 

potential problem areas unless the checks formed prior to 

pressure treatment.

Figure 44—Examples of internal decay in 

vertical members that were cut to height after 

installation. Only the preservative-treated 

zone remains sound. In some sawn members 

with heartwood faces, the treated zone may 

not be complete (see photograph on bottom). 

Fortunately, most pressure-treated members 

have greater penetration than shown in these 

examples.

test products are compared with those of established durable 

products and nondurable controls. The results of those tests 

are then presented to the appropriate AWPA subcommittees 

for review. AWPA subcommittees are composed of represen-
Guide for Use of Wood Preservatives in Historic Structures

35

tatives from industry, academia, and government agencies 

who have familiarity with conducting and interpreting durability evaluations. Preservative standardization by AWPA is 

a two-step process. If the performance of a new preservative 

is considered appropriate, it is first listed as a potential preservative. Secondary committee action is needed to have the 

new preservative listed for specific commodities and to set 

the required treatment level.

More recently, the International Code Commission–

Evaluation Service (ICC–ES) has evolved as an additional 

route for gaining building code acceptance of new types 

of pressure-treated wood. In contrast to AWPA, the ICC–

ES does not standardize preservatives. Instead, it issues 

evaluation reports that provide evidence that a building 

product complies with building codes. The data and other 

information needed to obtain an evaluation report are first 

established as acceptance criteria (AC). AC326, which sets 

the performance criteria used by ICC–ES to evaluate proprietary wood preservatives, requires submittal of documentation from accredited third party agencies in accordance 

with AWPA, American Society for Testing and Materials 

(ASTM), and European Norm (EN) standard test methods. 

The results of those tests are then reviewed by an evaluation 

committee to determine if the preservative has met the appropriate acceptance criteria.

Treatment Specifications

In the United States, the AWPA is the primary standardsetting body, but there is also overlap with standards developed by ASTM, the Window and Door Manufacturers Association (WDMA), and other organizations. Specifications 

on the treatment of various wood products by pressure 

processes have been developed by AWPA. These 

specifications limit pressures, temperatures, and time of conditioning and treatment to avoid conditions that will cause 

serious injury to the wood. The specifications also contain 

minimum requirements for preservative penetration, retention levels, and recommendations for handling wood after 

treatment to provide a quality product. 

Penetration and retention requirements are equally important in determining the quality of preservative treatment. 

Penetration levels vary widely, even in pressure-treated material. Experience has shown that even slight penetration has 

some value, although deeper penetration is highly desirable 

to avoid exposing untreated wood when checks occur, particularly for important members that are costly to replace. 

The heartwood of coastal Douglas-fir, southern pines, and 

various hardwoods, although resistant, will frequently show 

transverse penetrations of 6 to 12 mm (0.25 to 0.5 in.) and 

sometimes considerably more. Complete penetration of the 

sapwood should be the goal in all pressure treatments. It can 

often be accomplished in small-size timbers of various commercial woods, and is sometimes obtained in piles, ties, and 

structural timbers. Practically, however, the operator cannot 

always ensure complete penetration of sapwood in every 

piece when treating large pieces of round material with thick 

sapwood (such as poles and piles). Therefore, specifications 

permit some tolerance. For instance, AWPA Processing and 

Treatment Standard T1 for Southern Pine poles requires that 

89 mm (3.5 in.) or 90% of the sapwood thickness be penetrated for waterborne preservatives. The requirements vary, 

depending on the species, size, class, and specified retention 

levels.

Figure 45—Unless it was pressure treated after fabrication, 

this type of connection creates a favorable environment 

for decay. It exposes untreated wood and creates an area 

that traps and holds moisture.

Figure 46—Metal fasteners are sometimes associated 

with decay pockets if holes are drilled after treatment and 

expose untreated wood.
General Technical Report FPL–GTR–217

36

Preservative retentions are typically expressed on the basis 

of the mass of preservative per unit volume of wood within 

a prescribed assay zone. The retention calculation is not 

based on the volume of the entire pole or piece of lumber. 

For example, the assay zone for southern pine poles is between 13 and 51 mm (0.5 and 2.0 in.) from the surface. To 

determine the retention, a boring is removed from the assay 

zone and analyzed for preservative concentration 

(Fig. 47). The preservatives and retention levels are listed in 

the AWPA Commodity Standards and ICC–ES evaluation 

reports. The current issues of these specifications should be 

referenced for up-to-date recommendations and other details. In many cases, the retention level is different depending on species and assay zone. Higher preservative retention 

levels are specified for products to be installed under 

severe climatic or exposure conditions. Heavy-duty 

Table 3—Summary of UCSa

 developed by the AWPAb

Use

category 

Service 

conditions 

Use

environment

Common agents

of deterioration 

Typical 

applications

UC1 Interior construction, 

above ground, dry 

Continuously protected

from weather or other 

sources of moisture 

Insects only Interior construction 

and furnishings 

UC2 Interior construction, 

above ground, damp 

Protected from weather,

but may be subject 

to sources of moisture 

Decay fungi 

and insects 

Interior construction 

UC3A Exterior construction 

above ground, 

coated and rapid 

water runoff 

Exposed to all weather 

cycles, not exposed 

to prolonged wetting 

Decay fungi 

and insects 

Coated millwork, 

siding, and trim 

UC3B Ground contact 

or fresh water, 

non-critical components 

Exposed to all weather 

cycles, normal exposure

conditions 

Decay fungi 

and insects 

Fence, deck, guardrail 

posts, crossties, 

and utility poles 

(low decay areas) 

UC4A Ground contact 

or fresh water, 

non-critical components 

Exposed to all weather 

cycles, normal 

exposure conditions 

Decay fungi 

and insects 

Fence, deck, guardrail 

posts, crossties, 

and utility poles 

(low decay areas) 

UC4B Ground contact 

or fresh water 

critical components 

or difficult replacement 

Exposed to all weather 

cycles, high decay 

potential includes 

salt water splash 

Decay fungi 

and insects 

with increased 

potential for 

biodeterioration 

Permanent wood 

foundations,

building poles, 

horticultural posts, 

crossties, 

and utility poles 

(high decay areas) 

UC4C Ground contact 

or fresh water 

critical structural 

components

Exposed to all weather 

cycles, severe 

environments,

extreme decay potential 

Decay fungi 

and insects 

with extreme 

potential for 

biodeterioration 

Land and freshwater 

piling, foundation piling,

crossties,and utility poles

(severe decay areas) 

UC5A Salt or brackish water 

and adjacent mud zone, 

northern waters 

Continuous marine 

exposure (salt water) 

Salt water 

organisms

including 

marine borers 

Piling, bulkheads, 

bracing 

UC5B Salt or brackish water 

and adjacent mud zone, 

New Jersey to Georgia, 

south of San Francisco 

Continuous marine 

exposure (salt water) 

Salt water 

organisms

including creosote

tolerant Limnoria

tripunctata

Piling, bulkheads, 

bracing 

UC5C Salt or brackish water 

and adjacent mud zone, 

south of Georgia, 

Gulf Coast, Hawaii, 

and Puerto Rico 

Continuous marine 

exposure (salt water) 

Salt water 

organisms

including Martesia

and Sphaeroma

Piling, bulkheads, 

bracing 

a

Use Category System. 

b

American Wood Protection Association. 
Guide for Use of Wood Preservatives in Historic Structures

37

transmission poles and items with a high replacement cost, 

such as structural timbers and house foundations, are required to be treated to higher retention levels.

Fortunately, the end-user does not need to become an 

expert in treated wood specifications. The UCS standards 

developed by the AWPA simplify the process of finding 

appropriate preservatives and preservative retentions for 

specific end uses. To use the UCS standards, one needs only 

to know the intended end-use of the treated wood. Another 

table in the UCS standards lists most types of applications 

for treated wood and gives the reader the appropriate Use 

Category and User Specification. The User Specification 

lists all the preservatives that are standardized for that Use 

Category, as well as the appropriate preservative retention 

and penetration requirements. The user needs only specify 

that the product be treated according to the appropriate Use 

Category.

As the treating industry adapts to the use of new wood 

preservatives, it is more important than ever to ensure that 

wood is being treated to standard specifications. In the 

United States, the U.S. Department of Commerce, American Lumber Standard Committee (ALSC) accredits third 

party inspection agencies for treated wood products. Quality 

control overview by ALSC-accredited agencies is preferable to simple treating plant certificates or other claims of 

conformance made by the producer without inspection by 

an independent agency. Updated lists of accredited agencies 

can be obtained from the ALSC website at http://www.alsc.

org. Wood that is treated in accordance with these quality assurance programs will have a quality mark or stamp 

of an accredited inspection agency on the wood. The use 

of treated wood with such third party certification may be 

mandated by applicable building code regulations. In addition to identifying information about the producer, the stamp 

indicates the type of preservative, the retention level of the 

preservatives, and the intended exposure conditions. Retention levels are specific to the type of preservative, species, 

and intended exposure conditions. Detailed specifications on 

the different treatments can be found in the applicable standards of the AWPA.

Environmental Considerations for 

Pressure-Treated Wood

Concerns are sometimes expressed about environmental 

impacts from preservative-treated wood, especially if used 

in aquatic environments. Preservatives intended for outdoor 

use have mechanisms that are intended to keep the active 

ingredients in the wood and minimize leaching. However, 

studies indicate that a small percentage of the active ingredients of all types of wood preservatives leach out of the 

wood over time. The amount of leaching depends on factors 

such as fixation conditions, the preservative’s retention in 

the wood, the product’s size and shape, the type of exposure, 

and the years in service. Ingredients in all preservatives are 

potentially toxic to a variety of organisms at high concentrations, but laboratory studies indicate that the levels of preservatives leached from treated wood generally are too low 

to create a biological hazard.

The recent studies of the environmental impact of treated 

wood reveal several key points. All types of treated wood 

evaluated release small amounts of preservative components 

into the environment. These components can sometimes be 

detected in soil or sediment samples. Shortly after construction, elevated levels of preservative components can sometimes be detected in the water column. Detectable increases 

in soil and sediment concentrations of preservative components generally are limited to areas close to the structure. 

The leached preservative components either have low water 

solubility or react with components of the soil or sediment, 

limiting their mobility and limiting the range of environmental contamination. The levels of these components in the 

soil immediately adjacent to treated structures can increase 

gradually over the years, while levels in sediments tended to 

decline over time. The research indicates that environmental 

releases from treated wood do not cause measurable impacts on the abundance or diversity of aquatic invertebrates 

adjacent to the structures. In most cases, levels of preservative components were below concentrations that might be 

expected to affect aquatic life. Samples with elevated levels 

of preservative components tended to be limited to fine sediments beneath stagnant or slow-moving water where the 

invertebrate community is particularly tolerant of pollutants.

Conditions with a high potential for leaching and a high 

potential for metals to accumulate are the most likely to affect the environment. These conditions are most likely to be 

found in boggy or marshy areas with little water exchange. 

Water at these sites has low pH and high organic acid content, increasing the likelihood that preservatives will be 

leached from the wood. In addition, the stagnant water prevents dispersal of any leached components of preservatives, 

allowing them to accumulate in soil, sediments, and organisms near the treated wood. Simple screening criteria 

and more detailed models have been developed to help 

users assess whether specific projects pose a risk to 

the environment. These models are available at 

http://www.wwpinstitute.org/.

Figure 47—Core samples are removed from pieces in each 

charge to measure penetration and assay for chemical 

retention.
General Technical Report FPL–GTR–217

38

It is worth noting that all construction materials, including 

the alternatives to treated wood, have some type of environmental impact. In addition to environmental releases from 

leaching and maintenance activities, the alternatives may 

have greater impacts and require greater energy consumption during production.

Best Management Practices (BMPs)

A preservative’s resistance to leaching is a result of chemical stabilization reactions that render the toxic ingredients 

insoluble in water. The mechanism and requirements for the 

stabilization reactions differ, depending on the type of wood 

preservative. For each type of preservative, some reactions 

occur very rapidly during pressure treatment, whereas others may take days or even weeks, depending on storage and 

processing after treatment. If the treated wood is placed in 

service before these fixation reactions have been completed, 

the initial release of preservative into the environment may 

be much greater than when the wood has been conditioned 

properly.

With oil-type preservatives such as creosote or pentachlorophenol in heavy oil, preservative bleeding or oozing out 

of the treated wood is a more visible concern. This problem 

may be apparent immediately after treatment. Such members should not be used in bridges or other aquatic applications. In other cases, the problem may not become obvious 

until after the product has been exposed to heating by direct 

sunlight. This problem can be minimized by using treatment 

practices that remove excess preservative from the wood.

BMP standards have been developed to ensure that treated 

wood is produced in a way that will minimize environmental concerns. The Western Wood Preservers Institute 

(WWPI) has developed guidelines for treated wood used 

in aquatic environments. Although these practices have not 

yet been adopted by the industry in all areas of the United 

States, purchasers can require that these practices be followed. Commercial wood treatment firms are responsible 

for meeting conditions that ensure stabilization and minimize bleeding of preservatives, but persons buying treated 

wood should make sure that the firms have done so.

Consumers can take steps to ensure that wood will be 

treated according to the BMPs. It is important to specify 

standards that allow the treater to produce a more environmentally friendly product. Asking the treater to increase the 

preservative retention beyond the standard requirements 

increases the amount of leachable chemicals in the wood 

without a noticeable improvement in service life. Similarly, 

retreating wood that has failed to meet AWPA standards 

for initial retention increases the leachable material present. Proper fixation may take time, and material should be 

ordered well before it is needed so that the treater can hold 

the wood while it stabilizes. If consumers order wood in 

advance, they may also be able to store it under cover, allowing further drying and fixation. In general, allowing the 

material to air dry before it is used is a good practice for 

ensuring fixation, minimizing leaching, and reducing risk 

to construction personnel. With all preservatives, the wood 

should be inspected for surface residue, and wood with excessive residue should not be placed in service.

Alternatives to Pressure-Treated Wood

There are several alternatives to pressure-treated wood that 

may be relevant for some applications in historic structures.

Naturally Durable Species

Naturally durable tree species native to North America include old growth baldcypress, catalpa, cedars, chestnut, junipers, black locust, mesquite, redwood, red mulberry, several species of oak, Osage orange, sassafras, black walnut, 

Pacific yew, and old growth southern yellow pine. A number 

of imported tropical hardwoods are also known for their 

natural durability. Naturally durable species produce chemicals that are toxic to wood-decay fungi. These chemicals 

(extractives) are produced as the wood cells transition from 

sapwood cells to heartwood cells. Extractives are found only 

in heartwood and serve to protect the tree from fungal, and 

in some cases, insect attack. The extractives remain in the 

wood when a tree is cut into lumber or timber and can serve 

to inhibit deterioration if the wood is used in applications 

where deterioration from decay is a possibility.

When considering the use of naturally durable wood species 

for repair or replacement material, it is important to consider the compatibility of the selected species with existing 

structural members, both in terms of aesthetic properties and 

movement in service. Naturally durable species also have 

vulnerabilities when used in conditions that favor biodeterioration. One widely recognized limitation is that only the 

heartwood is durable. Untreated sapwood of all wood species has low resistance to decay and usually has a short service life under decay-producing conditions. Therefore, it is 

important to specify 100% heartwood for repair or replacement material. Although the vulnerability of sapwood is understood, it can be difficult and expensive to find sufficient 

material in which all pieces are completely free of sapwood. 

The presence of sapwood can be both an aesthetic and a 

structural concern for large timbers in moisture-prone areas.

A less-recognized characteristic of many naturally durable 

species is the high degree of variability in durability. The 

properties that make a wood naturally resistant to decay 

and insects can vary considerably from tree to tree and even 

within the same tree; therefore, predicting performance 

based on durability can be difficult. The decay resistance of 

heartwood is greatly affected by differences in the preservative qualities of the wood extractives, the attacking fungus 

or insect, and the conditions of exposure. Considerable difference in service life can be obtained from pieces of wood 

cut from the same species, even from the same tree, and 

used under apparently similar conditions.
Guide for Use of Wood Preservatives in Historic Structures

39

Naturally durable species also seem to be more affected by 

the severity of the decay environment than wood treated 

with preservatives. Woods that provide adequate performance above ground may sometimes decay nearly as rapidly as nondurable species when placed into ground contact. 

These differences appear to be a function of wood permeability; less permeable woods absorb less moisture during 

wetting events and thus are less likely to be sufficiently 

moist to sustain growth of decay. By contrast, many pine 

species commonly used for construction contain a large proportion of highly permeable sapwood that can lead to rapid 

moisture uptake.

Thermal Modification (Heat Treatment)

Heat- or thermally treated wood is sometimes confused 

with surface charring, or with the heat treatment used to 

sterilize wood products for import and export. Neither of 

those processes imparts significant durability, but heating 

wood at high temperatures for extended periods can cause 

chemical changes that affect a range of wood properties, 

including decay resistance. Several thermal treatment processes are in commercial use in Europe, and to a lesser 

extent in North America. In these processes wood is heated 

to temperatures ranging from 160 to 260 °C (320 to 500 °F) 

in specially constructed kilns under controlled conditions. 

The processes may use steam, nitrogen, or vacuum to minimize degradation and lower the availability of oxygen. One 

process heats the wood in oil. Thermally treated wood has 

only moderate decay resistance, and most applications are 

confined to above-ground use. Decay resistance increases 

at higher processing temperatures, but losses in mechanical 

properties and especially impact bending, also increase. An 

advantage of heat treatment is that it can be used with wood 

species that are difficult to penetrate with preservatives. It 

also can lessen the tendency of wood to absorb moisture, 

and thus help to reduce problems associated with shrinking 

and swelling. It also retains a natural appearance, although 

the color is initially darkened somewhat and the wood does 

weather to gray when exposed to sunlight. Because of its 

qualities, thermally treated wood is sometimes used in nonload-bearing above-ground applications such as siding. The 

resistance of thermally modified wood to mold growth and 

termite attack has not been thoroughly evaluated.

Chemically Modified Wood

Chemical modification is a general term applied for treatments that attempt to modify the wood so that it is a less 

attractive nutrient source for decay and insects. Currently 

the two most prevalent processes are acetylation and furfurylation. In the acetylation process, wood is treated with 

acetic anhydride, which replaces hydroxyl groups (OH-

) 

groups within the wood structure. This process causes the 

wood to absorb less moisture. In the furfurylation process, 

the wood is treated with furfuryl alcohol that is catalyzed to 

form polymers in the wood. It is also thought to react with 

the wood structure, and especially with lignin. Furfurylation 

also causes the wood to absorb less water than untreated 

wood. To achieve significant durability, both processes 

require the use of much more chemical than is used in conventional wood preservatives. Weight gains of at least 20% 

are need for acetylation, and even greater weight gains are 

needed in the furfurylation process. In addition to decay 

resistance, the treated wood is harder, heavier, and more dimensionally stable. Protection against attack by mold fungi 

and termites has not been as thoroughly evaluated as decay 

resistance.

Summary

Historic preservation presents unique considerations for use 

of preservatives to protect wood from biodeterioration. In 

historic structures the Secretary of Interior’s Standards for 

the Treatment of Historic Properties place emphasis on retaining the historic character, including distinctive materials, 

features, esthetics, and spatial relationships. Accordingly, 

a careful evaluation of existing conditions should be conducted to determine the appropriated level of intervention. 

For distinctive features with severe deterioration, repair or 

limited replacement is preferred over full replacement. For 

all treatment options, new material should match the old 

in design, composition, color, and texture as much as possible, but compatible substitute materials may be acceptable. 

Overall, the preservation approach should use the gentlest 

means possible.

It is also important to note that the Guidelines are intended 

to provide general parameters of acceptable and unacceptable work techniques and treatments. Each historic building 

is unique, and decisions concerning the use of wood preservatives or pressure-treated wood must be reached by considering the historical significance of the material to be treated, 

repaired, or replaced, as well as the parameters outlined by 

the Standards and Guidelines. In some structures, the Standards and Guidelines must be balanced against the need for 

safety and functionality.

The potential benefit associated with the use of wood preservatives or pressure-treated wood must also be considered for 

each project. It is important to have some understanding of 

the organism responsible for the observed biodeterioration. 

Some forms of biodeterioration such as mold may cause 

relatively little structural damage, whereas decay fungi and 

termites can cause severe deterioration with relatively little 

outward evidence. Most wood preservatives are not intended to provide long-term protection against some types of 

organisms (such as mold or algae); they were typically developed to prevent attack by decay fungi and termites. The 

deterioration hazard of a particular application should also 

be considered. Wood in ground contact or above ground in 

moist climates will obtain a greater durability benefit from 

preservative treatment than wood used above ground in an 

arid climate. In general, preservatives are not needed for 
General Technical Report FPL–GTR–217

40

wood that is consistently protected from moisture, but wood 

that is moist (over 20% moisture for sustained periods) is 

vulnerable to colonization by decay fungi and possibly other 

organisms. Even when conditions are favorable to deterioration, one must consider whether the treatment options 

available will be effective. Surface-applied treatments may 

not be effective in reaching decay-prone areas within large 

timbers, and if the circumstances do not allow replacement 

of that member with a pressure-treated member or drilling 

of holes to apply internal treatments, then there may not be 

sufficient benefit to using preservatives. In this type of situation, other options, such as protecting the member from 

moisture or replacing the member with a naturally durable 

wood, may be preferable. One must also consider whether 

the choice of preservatives allowed for a project will be effective in that application.

For most historic structures, use of wood preservatives or 

pressure-treated wood typically becomes a consideration 

when deterioration has been identified and when there are 

concerns about the long-term serviceability of the wooden 

elements. If moisture problems and subsequent deterioration 

were caused by a lack of maintenance, there is generally no 

need to apply wood preservatives or repair materials with 

pressure-treated wood unless the maintenance issues cannot 

be addressed or the project is to be mothballed for a significant period of time. If the building has poor drainage conditions that cannot be mitigated, or if there are construction or 

design flaws that have led to deterioration, the application 

of preservatives and the use of pressure-treated wood for 

repairs may be warranted.

Wood preservative treatments are generally grouped into 

two categories. In-place, field treatments, or nonpressure 

preservatives include all types of preservative applications 

other than pressure treatments. Examples range from finishes to boron rods to fumigants. The objective of all these 

treatments is to distribute preservative into areas of a structure that are vulnerable to moisture accumulation or not protected by the original pressure treatment. A major limitation 

of in-place treatments is that they cannot be forced deep into 

the wood under pressure as is done in pressure-treatment 

processes. However, in-place treatments can be applied into 

the center of large members via treatment holes. These preservatives may be available as liquids, rods, or pastes.

Surface-applied liquid treatments should not be expected 

to penetrate more than a few millimeters across the grain 

of the wood, although those containing boron can diffuse 

more deeply under certain moisture conditions. Liquid surface treatments are most efficiently used to flood checks, 

exposed end-grain, and bolt holes. They may move several 

centimeters parallel to the grain of the wood if the member 

is allowed to soak in the solution. Surface treatments with 

diffusible components will be washed away by precipitation 

if used in exposed members. However, their loss can 

be slowed if a water-repellent finish is applied after the 

diffusible treatment has dried. They will not effectively 

protect the interior of large piles or timbers.

Paste surface treatments can provide a greater reservoir of 

active ingredients than liquids. When used in conjunction 

with a wrap or similar surface barrier, these treatments can 

result in several centimeters of diffusion across the grain 

into moist wood over time. They are typically used for the 

groundline area of posts or piles that are not usually exposed 

to standing water, but can also be applied to end-grain of 

connections or pile tops.

Internal treatments are typically applied to the interior of 

larger members where trapped moisture is thought to be a 

current or future concern. They can be applied to smaller 

members in some situations.

Diffusible internal treatments move through moisture in the 

wood. They are relatively easy to handle but do not move 

as great a distance as fumigants do and do not move in dry 

wood. Diffusible treatments may be best suited for focusing 

on specific problem areas such as near exposed end-grain, 

connections, or fasteners. In contrast, fumigant internal 

treatments move as a gas through the wood. They have the 

potential to move several feet along the grain of the wood 

but have greater handling and application concerns.

Preservatives used for pressure treatment represent the 

second broad category of wood preservatives. In these treatment plants, bundles of wood products are placed into large 

pressure cylinders and combinations of vacuum, pressure, 

(and sometimes heat) are used to force the preservative 

deeply into the wood. Pressure-treated wood and the pressure-treatment preservatives differ from nonpressure preservatives in three important ways. (1) Pressure-treated wood 

has much deeper and more uniform preservative penetration 

than wood treated in other manners. (2) Most preservatives 

used in pressure treatment are not available for application 

by the public. (3) Pressure-treatment preservatives and pressure-treated wood undergo review by standard-setting organizations to ensure that the resulting product will be sufficiently durable in the intended end-use. In contrast, nonpressure preservatives may undergo relatively little review, 

other than the U.S. EPA evaluation of pesticide toxicity.

The type of pressure treatment applied is often dependent on 

the requirements of the specific application. For example, 

direct contact with soil or water is considered a severe 

deterioration hazard, and preservatives used in these applications must have a high degree of leach resistance and 

efficacy against a broad spectrum of organisms. These same 

preservatives may also be used at lower retentions to protect 

wood exposed in lower deterioration hazards, such as above 

the ground. The exposure is less severe for wood that is partially protected from the weather, and preservatives that lack 

the permanence or toxicity to withstand continued exposure 

to precipitation but may be effective in those applications. 

To guide selection of the types of preservatives and 
Guide for Use of Wood Preservatives in Historic Structures

41

loadings appropriate to a specific end-use, the AWPA developed UCS standards. The UCS standards categorize treated 

wood applications by the severity of the deterioration hazard, and list the preservatives and preservative retentions 

that will protect wood under those conditions.

Although some pressure treatment preservatives are effective in almost all environments, they may not be well-suited 

for applications involving frequent human or animal contact 

or for exposures that present only low to moderate biodeterioration hazards. Additional considerations include cost, 

potential odor, surface dryness, adhesive bonding, and ease 

of finish application. Preservatives dissolved in medium to 

heavy oils may have an odor and a somewhat oily surface. 

Water-based preservatives are often used when cleanliness 

and paintability of the treated wood are required. Wood 

treated with water-based preservatives also typically has 

lower odor than wood treated with some types of oil-based 

preservatives. However, unless supplemented with a water 

repellent, the water-based systems do not confer dimensional stability to the treated wood.

Concerns are sometimes expressed about environmental 

effects from pressure-treated wood, especially if used in 

aquatic environments. Preservatives intended for use outdoors have mechanisms that are intended to keep the active 

ingredients in the wood, but studies indicate that a small 

percentage of the active ingredients does leach out of the 

wood over time. The amount of leaching depends on factors 

such as fixation conditions, the preservative’s retention in 

the wood, the product’s size and shape, the type of exposure, and the years in service. Research indicates that environmental releases from treated wood are usually too low 

to cause impacts on the abundance or diversity of aquatic 

invertebrates adjacent to the structures. Concerns may arise 

when large volumes of wood are placed into relatively small 

volumes of stagnant or slow-moving water. Simple screening criteria and more detailed models have been developed 

to help users assess whether specific projects pose a risk to 

the environment. These models are available free of charge 

from WWPI at http://www.wwpinstitute.org/.

BMP standards have been developed to ensure that pressuretreated wood is produced in a way that will minimize environmental concerns. WWPI has developed guidelines for 

treated wood used in aquatic environments. Although these 

practices have not yet been adopted by the industry in all 

areas of the United States, purchasers can require that these 

practices be followed. Commercial wood treatment firms are 

responsible for meeting conditions that ensure stabilization 

and minimize bleeding of preservatives, but persons buying 

treated wood should make sure that the firms have done so.

Sources of Information

Anthony, R.W.;Dugan, K.D. 2009. Wooden artifacts in cemeteries, a reference manual. NCPTT Grant No. MT-2210-

07-NC-04. Prepared for the National Park Service, National 

Center for Technology and Training. http://ncptt.nps.gov/

wp-content/uploads/2007-10.pdf. 81 p.

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Guide for Use of Wood Preservatives in Historic Structures

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476 p.
General Technical Report FPL–GTR–217

44

Appendix A—Pressure-Treated 

Wood Dichotomous Key

This appendix provides a key intended to help the reader 

determine which wood species/wood preservative combinations might be best suited for a particular application. It is 

based on the standards developed by the AWPA but does not 

provide the detail and level of specificity provided in the 

AWPA standards. The AWPA standards are the authoritative 

listing of standardized preservatives and should be consulted 

prior to finalizing preservative/wood species selection.

In addition to the AWPA standard listings, the key provides 

somewhat subjective distinctions based on treated wood 

appearance (color change versus no color change) as well 

as the potential for surface oiliness and odor. These characteristics vary with treatment processes and may not apply in 

every situation.

Descriptions, and in some cases links, to sources of supply 

for each of the preservatives listed in this key can be found 

in Appendix B. Preservative retentions vary depending on 

the exposure scenario, or Use Category. Not all preservatives have the same retention for the same Use Category. 

When specifying, refer to the Use Category descriptions 

table (Table 3) and in Appendix B.

This dichotomous key does not encompass preservative 

formulations that have International Code Commission–

Evaluation Service (ICC–ES) Evaluation Reports but not 

AWPA listings. For more information on formulations 

with ICC–ES Evaluation Reports, refer to Appendix B or 

http://www.icc-es.org/reports/index.cfm?csi_num=06%20

05%2073.13&view_details.

A. Sawn Lumber or Timbers

Sawn lumber and timbers encompass the most widely used 

treated wood products. Examples include decking, joists, 

stringers, and timbers. Lumber and timbers used in highway construction or permanent wood foundations are not 

included here.

Which level of exposure best describes your application 

for the treated wood?

1. Indoors or almost completely protected from moisture: Go to A1.

2. Outdoors but above ground or fresh water: Go to A2.

3. In contact with ground or fresh water, general use: 

Go to A3.

4. Critical contact with ground or fresh water (high 

decay hazard or critical members): Go to A4.

A1. Does the treatment need to impart little or no color to 

the wood?

YES: Go to A1-a.

NO: Go to A1-b.

A1-a. Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Pine, Douglas-fir, or Hem-fir: EL2, SBX, PTI, 

and penta-light oil1

2. Spruce-pine-fir mix (SPF): SBX

3. Western spruces: SBX, penta-light oil1

4. Oaks and gums: penta-light oil1

A1-b. Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Pine, Douglas-fir, or Hem-fir: ACC2, ACQA, ACQ-B, ACQ-C, ACQ-D, ACZA, CA-B, 

CA-C, CXA, SBX, EL2, KDS and PTI, Cu8, 

CuNaph-water, CuNaph-oil1, penta-light oil1, 

penta-heavy oil1, and creosote1

2. Spruce–pine–fir mix (SPF): ACQ-C, SBX

3. Western spruces: ACC2, ACQ-B, ACZA2, SBX

4. Oaks and gums: ACC2, ACZA2, penta-light oil1, 

penta-heavy oil1, creosote1

5. Maple: creosote1

A2. Does the treatment need to impart little or no color to 

the wood?

YES: Go to A2-a

NO: Go to A2-b

A2-a. Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Pine, Douglas-fir, or Hem-fir: EL2, PTI, pentalight oil

2. Western spruces, oak, or gum: penta-light oil

A2-b. Is odor or an oily surface a concern?

YES: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1 Although standardized for interior applications, these preservatives 

may have characteristics such as odor or surface oiliness that are inappropriate for enclosed areas. They should not be used in residential 

structures or other structures where human contact or indoor air quality 

is a concern.

2 Although standardized for interior applications, these preservatives 

contain either chromium (ACC) or arsenic (ACZA). The use of a more 

common preservative containing chromium and arsenic (CCA) has 

been limited to applications that lessen the likelihood of human contact. See Table B1 for a list of allowable uses of CCA.
Guide for Use of Wood Preservatives in Historic Structures

45

1. Pine, Douglas-fir, or Hem-fir: ACC, ACQA, ACQ-B, ACQ-C, ACQ-D, CA-B, CAC, CCA, CuN-W, CXA, EL2, KDS, PTI, 

penta-light oil

2. Spruce-pine-fir mix (SPF): ACQ-C, 

ACZA, CCA

3. Western spruces: ACC, ACQ-B, ACZA, 

CCA, penta-light oil

4. Oak or gum: ACC, ACZA, CCA, or pentalight oil

NO: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Pine, Douglas-fir, or Hem-fir: ACC, ACQA, ACQ-B, ACQ-C, ACQ-D, CA-B, 

CA-C, CCA, CuNaph-water, CuNaph-oil, 

CXA, EL2, KDS, PTI, penta-light oil, 

penta-heavy oil, creosote, Cu8

2. Spruce-pine-fir mix (SPF): ACQ-C, 

ACZA, CCA

3. Western spruces: ACC, ACQ-B, ACZA, 

CCA, penta-light oil, penta heavy oil, 

CuNaph-oil, creosote

4. Oak or gum: ACC, CCA, ACZA, pentalight oil, penta-heavy oil, creosote

5. Maple: creosote

A3. Does the treatment need to impart little or no color to 

the wood?

YES: The only clear treatment for these applications is penta-light oil. It is standardized for treatment of the following species groups:

1. Pine, Douglas-fir, or Hem-fir

2. Western spruces

3. Oaks and gums

NO: Go to A3-a

A3-a. Is odor or an oily surface a concern?

YES: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Pine, Douglas-fir, or Hem-fir: ACC, ACQA, ACQ-B, ACQ-C, ACQ-D, ACZA, CAB, CA-C, CCA, CuNaph-water, KDS, and 

penta-light oil

2. Spruce-pine-fir mix (SPF): ACQ-C, 

ACZA, CCA

3. Western spruces: ACC, ACQ-A, ACQ-B, 

ACQ-D, ACZA, CCA, penta-light oil

4. Oak or gum: ACC, ACZA, CCA, or pentalight oil

NO: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Pine, Douglas-fir, or Hem-fir: ACC, ACQA, ACQ-B, ACQ-C, ACQ-D, ACZA, CAB, CA-C, CCA, CuNaph-water, CuNaphoil, KDS, penta-light oil, penta-heavy oil, 

creosote

2. Spruce-pine-fir mix (SPF): ACQ-C, 

ACZA, CCA

3. Western spruces: ACC, ACQ-A, ACQ-B, 

ACQ-D, ACZA, CCA, penta-light oil, 

penta-heavy oil, CuNaph-oil, creosote

4. Oak or gum: ACC, CCA, ACZA, pentalight oil, penta-heavy oil, creosote

5. Maple: creosote

A4. Does the treatment need to impart little or no color to 

the wood?

YES: The only clear treatment for these applications 

is penta-light oil. It is standardized for treatment of the 

following species groups:

1. Pine, Douglas-fir, or Hem-fir

2. Western spruces

NO: Go to A4a

A4a. Is odor or an oily surface a concern?

YES: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Pine, Douglas-fir, or Hem-fir: ACQ-B, 

ACQ-C, ACQ-D, ACZA, CA-B, CA-C, 

CCA, penta-light oil

2. Spruce-pine-fir mix (SPF): ACQ-C, ACQD, ACZA, CCA

3. Western spruces: ACQ-B, ACQ-C, ACQD, ACZA, CCA, penta-light oil

4. Hardwoods: no preservatives standardized

NO: Standardized preservatives vary based on the wood 

species or species group that will be pressure treated:

1. Pine, Douglas-fir, or Hem-fir: ACQ-B, 

ACQ-C, ACQ-D, ACZA, CA-B, CA-C, 

CCA, penta-light oil, penta-heavy oil, 

CuNaph-oil, creosote
General Technical Report FPL–GTR–217

46

2. Spruce-pine-fir mix (SPF): ACQ-C, ACQD, ACZA, CCA

3. Western spruces: ACQ-B, ACQ-C, ACQD, ACZA, CCA, penta-light oil, pentaheavy oil, CuNaph-oil, creosote

4. Hardwoods: no preservatives standardized

B. Sawn Lumber or Timbers for Highway 

Construction

Highway construction is considered a structurally critical 

application, and preservative/wood species combinations are 

more limited than for general construction.

B1. Does the treatment need to impart little or no color to 

the wood?

YES: The only standardized preservative is penta-light 

oil. It is standardized for treatment of southern pines, 

coastal Douglas-fir, and western hemlock.

NO: Go to B2

B2. Is odor or an oily surface a concern?

YES: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Southern pines and western hemlock: ACQ-B, 

ACQ-C, ACZA, CA-B, CA-C, CCA, pentalight oil

2. Douglas-fir: ACQ-B, ACQ-C, ACQ-D, ACZA, 

CA-B, CA-C, CCA, penta-light oil

3. Hem-fir group: ACQ-C, CA-B, CA-C

NO: Standardized preservatives vary based on the wood 

species or species group that will be pressure treated:

1. Southern pines and western hemlock: ACQ-B, 

ACQ-C, ACZA, CA-B, CA-C, CCA, pentalight oil, penta-heavy oil, CuNaph-oil, creosote

2. Douglas-fir: ACQ-B, ACQ-C, ACQ-D, ACZA, 

CA-B, CA-C, CCA, penta-light oil, pentaheavy oil, CuNaph-oil, creosote

3. Hem-fir group: ACQ-C, CA-B, CA-C

C. Round Piles

This listing is for piles used in contact with soil or fresh 

water. For piles placed in seawater, refer to the “Wood in 

Seawater” section.

C1. Does the treatment need to impart little or no color to 

the wood?

YES: The only standardized preservative is penta-light 

oil. It is standardized for treatment of the following 

wood species: southern pines, red pine, jack pine, 

ponderosa pine, Douglas-fir, western larch, lodgepole 

pine, and oak.

NO: Go to C2.

C2. Is odor or an oily surface a concern?

YES: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Southern pine: ACZA, CCA, ACQ-C, CA-B, 

CA-C, penta-light oil

2. Red, jack, ponderosa pine: ACZA, CCA, 

penta-light oil

3. Douglas-fir: Coastal Douglas-fir: ACZA, CCA, 

penta-light oil. Interior Douglas-fir: ACZA, 

penta-light oil

4. Western larch: ACZA, CCA, penta-light oil

5. Oak: penta-light oil

NO: Standardized preservatives vary based on the wood 

species or species group that will be pressure treated:

1. Southern pine: ACZA, CCA, ACQ-C, CA-B, 

CA-C, penta-light oil, penta-heavy oil, CuNaph-oil, creosote

2. Red, jack, and ponderosa pine: ACZA, CCA, 

penta-light oil, penta-heavy oil, creosote

3. Douglas-fir: Coastal Douglas-fir: ACZA, CCA, 

penta-light oil, penta-heavy oil, CuNaph-oil, 

creosote. Interior Douglas-fir: ACZA, pentalight oil, penta-heavy oil, creosote

4. Western larch: ACZA, CCA, penta-light oil, 

penta-heavy oil, creosote

5. Oak: penta-light oil, penta-heavy oil, creosote

D. Round Posts

This listing is for round posts. For sawn posts, refer to the 

“Sawn Lumber or Timbers” section.

D1. Does the treatment need to impart little or no color to 

the wood?

YES: The only standardized preservative is penta-light 

oil. It is standardized for treatment of native pine species, Douglas-fir, western hemlock, and western larch.

NO: Go to D2

D2. Which level of exposure best describes your application 

for the treated wood?

1. General use, including fence posts: Go to D2-a

2. Structurally critical or severe decay hazard: 

Go to D2-b
Guide for Use of Wood Preservatives in Historic Structures

47

D2-a. Is odor or an oily surface a concern?

YES: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Southern pine: ACC, ACQ-B, ACQ-C, 

ACQ-D, ACZA, CA-B, CA-C, CCA, CuNaph-water, penta-light oil

2. Douglas-fir, western hemlock, western 

larch: ACC, ACZA, CCA, penta-light oil

3. Ponderosa pine: ACC, ACQ-B, ACZA, 

CCA, KDS, penta-light oil

4. Jack pine: ACC, ACZA, CCA, KDS, pentalight oil

5. Red and lodgepole pine: ACC, ACQ-C, 

ACZA, CCA, KDS, CA-B, CA-C, pentalight oil

6. Radiata pine: CCA, ACQ-C

NO: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Southern pine: ACC, ACQ-B, ACQ-C, 

ACQ-D, ACZA, CA-B, CA-C, CCA, 

CuNaph-water, CuNaph-oil, penta-light oil, 

penta-heavy oil, creosote

2. Douglas-fir: ACC, ACZA, CCA, pentalight oil, penta-heavy oil, CuNaph-oil, 

creosote

3. Western hemlock and western larch: ACC, 

ACZA, CCA, penta-light oil, penta-heavy 

oil, creosote

4. Ponderosa pine: ACC, ACQ-B, ACZA, 

CCA, KDS, penta-light oil, penta-heavy 

oil, CuNaph-oil, creosote

5. Jack pine: ACC, ACZA, CCA, KDS, pentalight oil, penta-heavy oil, creosote

6. Red pine: ACC, ACQ-C, ACZA, CCA, 

KDS, CA-B, CA-C, penta-light oil, pentaheavy oil, creosote

7. Lodgepole pine: ACC, ACQ-C, ACZA, 

CCA, KDS, CA-B, CA-C, penta-light oil, 

penta-heavy oil, CuNaph-oil, creosote

8. Radiata pine: CCA, ACQ-C

D2-b. Is odor or an oily surface a concern?

YES: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Southern pine: ACQ-B, ACQ-C, ACQ-D, 

ACZA, CA-B, CA-C, CCA, penta-light oil

2. Douglas-fir, western hemlock, and western 

larch: ACC, ACQ-B, ACZA, CCA, pentalight oil

3. Ponderosa pine: ACQ-B, ACZA, CCA, 

penta-light oil

4. Jack pine: ACZA, CCA, penta-light oil

5. Red and lodgepole pine: ACQ-C, ACZA, 

CCA, CA-B, CA-C, penta-light oil

6. Radiata pine: CCA, ACQ-C

NO: Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Southern pine: ACQ-B, ACQ-C, ACQ-D, 

ACZA, CA-B, CA-C, CCA, CuNaph-oil, 

penta-light oil, penta-heavy oil, creosote

2. Douglas-fir: ACQ-B, ACZA, CCA, pentalight oil, penta-heavy oil, CuNaph-oil, 

creosote

3. Western hemlock and western larch: ACQB, ACZA, CCA, penta-light oil, pentaheavy oil, creosote

4. Ponderosa pine: ACQ-B, ACZA, CCA, 

penta-light oil, penta-heavy oil, CuNaphoil, creosote

6. Jack pine: ACZA, CCA, penta-light oil, 

penta-heavy oil, creosote

7. Red pine: ACQ-C, ACZA, CCA, CA-B, 

CA-C, penta-light oil, penta-heavy oil, 

creosote

8. Lodgepole pine: ACQ-C, ACZA, CCA, 

CA-B, CA-C, penta-light oil, penta-heavy 

oil, CuNaph-oil, creosote

9. Radiata pine: CCA, ACQ-C

E. Poles

Which type of pole best matches your application?

1. Utility pole: Go to E1.

2. Glued-laminated pole: Go to E2.

3. Building pole: Go to E3.

E1. Utility poles: Does the treatment need to impart little or 

no color to the wood?

YES: The only standardized preservative is penta-light 

oil. It is standardized for treatment of southern pine, 

jack pine, red pine, lodgepole pine, western redcedar, 
General Technical Report FPL–GTR–217

48

Alaska yellow-cedar, western larch, and ponderosa pine 

utility poles.

NO: Go to E1-a

E1-a. Which situation hazard best matches your 

application?

1. All decay hazards, any wood species: 

Go to E1-a1.

2. General or moderate decay hazard, 

southern pine, and western redcedar 

only: Go to E1-a2.

E1-a1: Is odor or an oily surface a concern?

YES: Standardized preservatives vary with 

wood species treated:

1. Southern and other native pine species, coastal Douglas-fir, western 

larch, western redcedar, and Alaska 

yellow cedar: ACQ-B, ACZA, CCA, 

and penta-light oil.

2. Radiata pine: CCA

NO: Standardized preservatives vary with 

wood species treated:

1. Southern and other native pine species, coastal Douglas-fir, western 

redcedar, and Alaska yellow cedar: 

ACQ-B, ACZA, CCA, penta-light 

oil, penta-heavy oil, CuNaph-oil, and 

creosote.

2. Western larch: ACQ-B, ACZA, CCA, 

penta-light oil, penta-heavy oil, and 

creosote.

3. Radiata pine: CCA

E1-a2: Southern pine and western redcedar, general to moderate decay hazard: Additional standardized preservatives are CA-B, CA-C.

E2. Glued-laminated poles: Only two wood species or species groups, Southern pine species and Douglas-fir, are standardized for use in glued-laminated poles.

Does the treatment need to impart little or no color to 

the wood?

YES: The only standardized preservative is penta-light 

oil.

NO: Go to E2-a.

E2-a. Is odor or an oily surface a concern?

YES: The only standardized preservative is pentalight oil.

NO: Standardized preservatives are penta light-oil, 

penta-heavy oil, CuNaph-oil, and creosote.

E3. Building poles: Does the treatment need to impart little 

or no color to the wood?

YES: The only standardized preservative is penta-light 

oil.

NO: Go to E3-a.

E3-a. Is odor or an oily surface a concern?

YES: Standardized preservatives vary with wood 

species treated:

1. Southern pine and red pine: ACZA, CCA, 

CA-B, CA-C, penta-light oil

2. Douglas-fir and ponderosa pine: ACZA, 

CCA, penta-light oil

3. Radiata pine: CCA

NO: Standardized preservatives vary with wood 

species treated:

1. Southern pine and red pine: ACZA, CCA, 

CA-B, CA-C, penta-light oil, penta-heavy 

oil, creosote

2. Douglas-fir and ponderosa pine: ACZA, 

CCA, penta-light oil, penta-heavy oil, 

creosote

3. Radiata pine: CCA

F. Glued-Laminated Members 

This listing applies to horizontal glued-laminated members 

(beams) and other members with the exception of poles. For 

glued-laminated poles, refer to the “Poles” section.

How will the glued-laminated member be treated with 

preservative?

1. It will be treated after the laminates have been 

glued together: Go to F1.

2. Individual laminates will be treated before the 

beam is glued together: Go to F2.

F1. Does the treatment need to impart little or no color to 

the wood?

YES: The only standardized preservative is penta-light 

oil. It is standardized for above-ground uses as well 

as for uses in contact with ground or fresh water. The 

standardized wood species are southern pine, coastal 

Douglas-fir, western hemlock, and Hem-fir. Standardized exposures and applications are the following:

1. Above ground or above water

2. In contact with ground or fresh water 

(general use)
Guide for Use of Wood Preservatives in Historic Structures

49

3. In contact with ground or fresh water, severe decay hazard, or critical member

NO: Select the exposure or situation that best matches 

the end-use:

1. Above ground or above water, or in contact 

with ground or fresh water (general use):

 Go to F1-a.

2. In contact with ground or fresh water, severe 

decay hazard, or critical member: Go to F1-b.

F1-a. Is odor or an oily surface a concern?

YES: Standardized preservatives vary 

with the wood species or species group 

treated:

1. Southern pine, western hemlock, 

or hem-fir: penta-light oil

2. Coastal Douglas-fir: ACZA, 

penta-light oil

NO: Standardized preservatives vary with 

the wood species or species group treated:

1. Southern pine, western hemlock, 

or hem-fir: penta-light oil, pentaheavy oil, CuNaph-oil, creosote, 

Cu8

2. Coastal Douglas-fir: ACZA, 

penta-light oil, penta-heavy oil, 

creosote

3. Red oak, red maple, yellowpoplar: creosote

F1-b. Is odor or an oily surface a concern?

YES: The only standardized preservative is penta-light oil. It is standardized 

for treatment of southern pine or coastal 

Douglas-fir.

NO: Standardized treatments vary by 

wood species:

1. Southern pine: penta-light oil, 

penta-heavy oil, CuNaph-oil, creosote

2. Coastal Douglas-fir: ACZA, 

penta-light oil, penta-heavy oil, 

CuNaph-oil, creosote

F2. The wood species standardized for laminates treated 

before gluing are the following:

1. Southern pine

2. Coastal Douglas-fir

3. Western hemlock and Hem-fir

Each of the preservatives listed under section F2 is standardized for use with these species.

F2-a. To choose standardized preservatives, select the 

exposure situation that best matches the end-use:

1. Above ground or above water: Go to F2-a1.

2. In contact with soil, concrete, or fresh water: 

Go to F2-a2.

F2-a1. Does the treatment need to impart little or 

no color to the wood?

YES: Standardized preservatives are PTI and 

penta-light oil.

NO: Is odor or an oily surface a concern?

YES: Standardized preservatives are 

ACC, ACQ-A, ACQ-C, ACZA, CA-C, 

CCA, KDS, PTI, and penta-light oil.

NO: Standardized preservatives are ACC, 

ACQ-A, ACQ-C, ACZA, CA-C, CCA, 

KDS, PTI, penta-light oil, penta-heavy oil, 

CuNaph-oil, Cu8, and creosote.

F2-a2. Does the treatment need to impart little or 

no color to the wood?

YES: The only standardized preservative is 

penta-light oil.

NO: Is odor or an oily surface a concern?

YES: Standardized preservatives are 

ACC, ACQ-A, ACQ-C, ACZA, CA-C, 

CCA, and penta-light oil.

NO: Standardized preservatives are ACC, 

ACQ-A, ACQ-C, ACZA, CA-C, CCA, 

penta-light oil, penta-heavy oil, CuNaphoil, and creosote.

G. Plywood

For plywood used in permanent wood foundations, refer to 

the “Permanent Wood Foundations” section.

Plywood standards do not specify wood softwood species; 

however, hardwood plywood and softwood plywood containing hardwood veneers are excluded.

Select the exposure situation that best matches the end-use:

1. Indoors or otherwise protected from liquid water: 

Go to G1

2. Outdoors, above ground, or above water: Go to G2

3. In contact with the ground or fresh water, general 

use: Go to G3

4. In contact with the ground or fresh water, severe 

decay hazard, or structurally critical: Go to G4
General Technical Report FPL–GTR–217

50

G1. Does the treatment need to impart little or no color to 

the wood?

YES: The standardized preservatives are EL2, PTI, 

penta-light oil3 and SBX.

NO: Go to G1-a.

G1-a. Is odor or an oily surface a concern?

YES: The standardized preservatives are the 

following: ACC, ACQ-A, ACQ-B, ACQ-C, 

ACQ-D, ACZA4, CA-B, CA-C, CCA4, CX-A, 

EL2, KDS, PTI, penta-light oil3, and SBX.

NO: The standardized preservatives are the 

following: ACC, ACQ-A, ACQ-B, ACQ-C, 

ACQ-D, ACZA4, CA-B, CA-C, CCA4, CX-A, 

EL2, KDS, PTI, SBX, penta-light oil3, pentaheavy oil3, CuNaph-oil3, Cu83, and creosote3.

G2. Does the treatment need to impart little or no color to 

the wood?

YES: The standardized preservatives are EL2, PTI, and 

penta-light oil.

NO: Go to G2-a.

G2-a. Is odor or an oily surface a concern?

YES: The standardized preservatives are ACC, 

ACQ-A, ACQ-C, ACQ-D, ACZA, CA-B, CA-C, 

CCA, EL2, KDS, PTI, and penta-light oil.

NO: The standardized preservatives are ACC, 

ACQ-A, ACQ-C, ACQ-D, ACZA, CA-B, CA-C, 

CCA, EL2 , KDS, PTI, penta-light oil, penta-heavy 

oil, CuNaph-oil, Cu8, and creosote.

G3. Does the treatment need to impart little or no color to 

the wood?

YES: The standardized preservative is penta-light oil.

NO: Go to G3-a.

G3-a. Is odor or an oily surface a concern?

YES: The standardized preservatives are ACC, 

ACQ-A, ACQ-B, ACQ-C, ACQ-D, ACZA, CA-B, 

3 Although standardized for interior applications, these preservatives 

may have characteristics such as odor or surface oiliness that are inappropriate for enclosed areas. They should not be used in residential 

structures or other structures where human contact or indoor air quality 

is a concern.

4 Although standardized for interior applications, these preservatives 

contain either chromium (ACC) or arsenic (ACZA). The use of a more 

common preservative containing chromium and arsenic (CCA) has 

been limited to applications that lessen the likelihood of human contact. See Table B1 for a list of allowable uses of CCA.

CA-C, CCA, and penta-light oil.

NO: The standardized preservatives are ACC, 

ACQ-A, ACQ-B, ACQ-C, ACQ-D, ACZA, CA-B, 

CA-C, CCA, penta-light oil, penta-heavy oil, and 

creosote.

G4. Does the treatment need to impart little or no color to 

the wood?

YES: The standardized preservative is penta-light oil.

NO: Go to G4-a

G4-a. Is odor or an oily surface a concern?

YES: The standardized preservatives are ACQ-B, 

ACQ-D, ACZA, CA-B, CA-C, CCA, and pentalight oil.

NO: The standardized preservatives are ACQ-B, 

ACQ-D, ACZA, CA-B, CA-C, CCA, penta-light 

oil, penta-heavy oil, and creosote.

H. Permanent Wood Foundations

This listing applies to the lumber and plywood used in permanent wood foundations. Because of the structurally critical nature and lengthy expected service life of this application, the wood species/preservative combinations are more 

limited than for general construction.

H1. Select a material and species grouping:

1. Lumber: Go to H1-a.

2. Plywood: Go to H1-b.

H1-a. Standardized preservatives vary based on the 

wood species or species group that will be pressure 

treated:

1. Southern pine, red pine, ponderosa pine, 

western hemlock, or Hem-fir. Standardized preservatives are ACA-B, ACQ-C, 

ACQ-D, ACZA, CA-B, CA-C, and CCA.

2. Coastal Douglas-fir. Standardized preservatives are ACA-B, ACQ-C, ACQ-D, 

ACZA, CA-B, CA-C, and CCA.

3. Alpine fir. Standardized preservative is 

CCA.

4. Scots pine or patula pine: Standardized 

preservatives are ACQ-D, CA-B, and CAC.

H1-b. Plywood: ACA-B, ACQ-C, ACQ-D, ACZA, 

CA-B, CA-C, and CCA.
Guide for Use of Wood Preservatives in Historic Structures

51

I. Wood in Seawater

This listing covers treated wood products that are either 

completely immersed or frequently immersed in seawater. 

For these applications, a preservative must be effective in 

protecting the wood from marine borers.

Choose a type of wood product and number of treatments:

1. Lumber and timbers: Go to I1.

2. Dual-treated lumber and timbers: Go to I2.

3. Piles: Go to I3.

4. Dual-treated piles: Go to I4.

5. Plywood: Go to I5.

I1. Lumber and timbers. Standardized preservatives vary by 

species or species grouping:

1. Southern pine, red pine, ponderosa pine, and 

Douglas-fir: ACZA, CCA, and creosote.

2. Western hemlock and Hem-fir: ACZA, and 

creosote.

3. Oak and gum: creosote.

I2. Dual-treated lumber and timbers. The standardized 

preservatives and species follow:

First treatment: CCA or ACZA

Second treatment: creosote

Wood species standardized for those preservatives: 

Southern pine, Douglas-fir, Hem-fir

I3. Piles

Standardized preservatives are ACZA, CCA, and 

creosote.

Wood species standardized for those preservatives: 

Southern pine, Douglas-fir, red pine

I4. Dual-treated piles. The standardized preservatives and 

species are the following:

First treatment: CCA or ACZA

Second treatment: creosote

Wood species standardized for those preservatives: 

Southern pine, Douglas-fir

I5. Plywood. Standardized preservatives are: ACZA, CCA, 

creosote
General Technical Report FPL–GTR–217

52

Appendix B—Description of 

Pressure Treatment Preservatives 

(Grouped by Exposure Hazard)

Applications Protected from Moisture

An example of this type of application is framing lumber 

used in areas of high termite hazard. Often the primary 

threat in these applications is insect attack, but protection 

against mold fungi or decay fungi may also be desirable. 

The preservatives listed in this section are water-based preservatives that do not fix in the wood, and thus are readily 

leachable. They provide adequate protection as long as the 

wood is not sufficiently wetted to leach the preservative out 

of the wood.

Borates (SBX) 

Borate compounds are the most commonly used unfixed 

water-based preservatives. They include formulations prepared from sodium tetraborate, sodium pentaborate, and boric acid, but the most common form is disodium octaborate 

tetrahydrate (DOT). DOT has higher water solubility than 

many other forms of borate, allowing the use of higher solution concentrations and increasing the mobility of the borate 

through the wood. Glycol is also used to increase solubility in some formulations. With the use of heated solutions, 

extended pressure periods, and diffusion periods after treatment, DOT is able to penetrate relatively refractory species 

such as spruce. Borates are used for pressure treatment of 

framing lumber used in areas of high termite hazard, such 

as in Hawaii, and as surface treatments for a wide range of 

wood products such as log cabins and the interiors of wood 

structures. They are also applied as supplemental internal 

treatments via rods or pastes. At higher retentions, borates 

are also used as fire-retardant treatments for wood. Boron 

has some important advantages, including low mammalian 

toxicity, activity against both fungi and insects, and low 

cost. Another advantage of boron is its ability to move and 

diffuse with water into wood that normally resists traditional 

pressure treatment. 

In addition, wood treated with borates has no color (Fig. B1) 

or odor, is non-corrosive, and can be finished. Whereas boron has many potential applications in framing, it is not suitable for applications where it is exposed to frequent wetting 

unless the boron can be somehow protected from liquid water. In some countries such as New Zealand, it can be used 

in applications where occasional wetting is possible, and 

there is interest in use of borates in slightly more exposed 

applications with coating requirements. There is also interest 

in dual treatments, in which a borate treatment is followed 

by pressure treatment with a water-repellent oil type preservative. Research continues to develop borate formulations 

that have increased resistance to leaching while maintaining 

biocidal efficacy. Various combinations of silica and boron 

have been developed that appear to somewhat retard boron 

depletion, but the degree of permanence and applicability of the treated wood to outdoor exposures has not been 

well defined. For more information or sources of supply, 

see the following websites: http://www.osmosewood.com/

advanceguard/ http://www.archchemicals.com/Fed/WOLW/

Products/Preservative/sillbor/default.htm http://treatedwood.

com/products/timbersaver/.

Applications Above Ground with Partial 

Protection

This use category is characterized by wood that is above 

ground and occasionally exposed to wetting. Wood used 

in this manner typically has some type of surface finish. 

The most common examples of this type of application are 

millwork and siding. Some above-ground applications that 

retain moisture or collect organic debris may present a more 

severe deterioration hazard, and a preservative from one 

of the following sections may then be more appropriate. 

Although preservatives used for millwork treatments were 

traditionally carried in light solvents to prevent dimensional 

changes, there is an increasing trend to move away from use 

of light solvents because of economic and environmental 

concerns. In this category, the distinction between oil- and 

water-based preservatives blurs, as many of these components can be delivered either with solvents or as microemulsions. The azole fungicides, such as tebuconaozle 

and propiconazole, are becoming more widely used. Other 

azoles, including cypraconazole and azaconazole, are also 

used in more limited quantities.

Currently all pressure-treatment preservatives listed in this 

category are also listed for applications fully exposed to the 

weather (see “Applications Above Ground but Fully Exposed to the Weather”) and are described in that section.

Figure B1—Appearance of borate-treated wood.
Guide for Use of Wood Preservatives in Historic Structures

53

Applications Above Ground but Fully Exposed 

to the Weather

The preservatives listed in this section generally may not 

provide long-term protection for wood used in direct contact 

with soil or standing water, but are effective in preventing attack in wood exposed above the ground, even if it is 

directly exposed to rainfall. A typical example of this type 

of application is decking. The preservatives listed in the following section also perform well in these applications and 

can be used at their lower, above-ground retentions. Some 

above-ground applications that retain moisture or collect 

organic debris may present a deterioration hazard similar 

to ground contact. Preservatives discussed in the following 

section may be more appropriate for such applications, 

especially in critical structural members.

Copper HDO (CX-A)

Copper HDO (CX-A, also referred to as copper xyligen) 

is an amine copper water-based preservative that has been 

used in Europe and was recently standardized in the United 

States. The active ingredients are copper oxide, boric acid, 

and copper-HDO (Bis-(N-cyclohexyldiazeniumdioxy copper). The appearance (Fig. B2) and handling characteristics 

of wood treated with copper HDO are similar to the other 

amine copper-based treatments. Currently, copper HDO is 

only standardized for applications that are not in direct contact with soil or water. For more information or sources of 

supply, see the following: http://www2.basf.us/woodpreservatives/index.htm.

EL2

EL2 is a waterborne preservative composed of the fungicide 

4,5-dichloro-2-N-octyl-4-isothiazolin-3-one (DCOI), the 

insecticide imidacloprid, and a moisture control stabilizer 

(MCS). The ratio of actives is 98% DCOI and 2% imidacloprid, but MCS is also considered to be a necessary component to ensure preservative efficacy. EL2 is currently listed 

in AWPA standards for above-ground applications only. The 

treatment is essentially colorless (Fig. B3) and the treated 

wood has little odor. For more information or sources of 

supply, see http://treatedwood.com/products/ecolife/.

ESR–2067

ESR–2067 is an organic waterborne preservative with an 

active composition of 98% tebuconazole (fungicide) and 

2% imidacloprid (insecticide). The treatment does not impart any color to the wood. It is currently listed only for 

treatment of commodities that are not in direct contact with 

soil or standing water.

Oxine Copper (Copper-8-quinolinolate)

Oxine copper is an organometallic preservative comprised 

of 10% copper-8-quinolinolate and 10% nickel-2-ethylhexoate. It is characterized by its low mammalian toxicity 

and is permitted by the U.S. Food and Drug Administration 

for treatment of wood used in direct contact with food (e.g., 

pallets). The treated wood has a greenish brown color, and 

little or no odor. It can be dissolved in a range of hydrocarbon solvents but provides longer protection when delivered 

in heavy oil. Oxine copper solutions are somewhat heat 

sensitive, which limits the use of heat to increase preservative penetration. However, adequate penetration of difficult 

to treat species can still be achieved, and oxine copper is 

sometimes used for treatment of the above-ground portions 

of wooden bridges and deck railings. Oil-borne oxine copper does not accelerate corrosion of metal fasteners relative 

to untreated wood.

Pentachlorophenol (Light Solvent)

The performance of pentachlorophenol and the properties 

of the treated wood are influenced by the properties of the 

solvent. Pentachlorophenol is most effective when applied 

with a heavy solvent, but it performs well in lighter solvents 

for above-ground applications. Lighter solvents also provide 

the advantage of a less oily surface, lighter color (Fig. B4), 

Figure B2—Appearance of CX-A-treated wood. Figure B3—Appearance of EL2-treated wood with 

incorporated water repellent.
General Technical Report FPL–GTR–217

54

and improved paintability. Pentachlorophenol in light oil 

can be used to treat relatively refractory wood species and 

does not accelerate corrosion. However, one disadvantage 

of the lighter oil is that less water repellency is imparted to 

the wood. Although pentachlorophenol in light oil provides 

a dryer surface, the same active ingredient is present and 

this treatment may not be appropriate for applications where 

exposure to humans is likely.

Propiconazole-Tebuconazole-Imidacloprid (PTI)

PTI is a waterborne preservative solution composed of two 

fungicides (propiconazole and tebuconazole) and the insecticide imidacloprid. PTI is currently listed in AWPA standards 

for above-ground applications only. The efficacy of PTI is 

enhanced by the incorporation of a water-repellent stabilizer 

in the treatment solutions, and lower retentions are allowed 

with the stabilizer. The treatment is essentially colorless and 

has little odor. For more information or sources of supply, 

see http://www.wolmanizedwood.com/Products/Preservative/Authentic/default.htm.

Applications in Direct Contact with the 

Ground or Fresh Water

These preservatives exhibit sufficient toxicity and leach 

resistance to protect wood in contact with the ground, fresh 

water, or in other high-moisture, high-deterioration hazard 

applications. Preservatives listed in this section are also effective in preventing decay in other, less severe exposures 

but may not be well suited to those applications because of 

cost, color, toxicity, odor, or other characteristics.

Acid Copper Chromate (ACC)

ACC is an acidic water-based preservative that has been 

used in Europe and the United States since the 1920s. ACC 

contains 31.8% copper oxide and 68.2% chromium trioxide. 

The treated wood has a light greenish-brown color and little 

noticeable odor. Tests on stakes and posts exposed to decay 

and termite attack indicate that wood well-impregnated with 

ACC gives acceptable service. However, it may be susceptible to attack by some species of copper-tolerant fungi, and 

because of this its use is sometimes limited to above-ground 

applications. It may be difficult to obtain adequate penetration of ACC in some of the more refractory wood species 

such as white oak or Douglas-fir. This is because ACC must 

be used at relatively low treating temperatures and because 

rapid reactions of chromium in the wood can hinder further 

penetration during longer pressure periods. The high chromium content of ACC, however, has the benefit of preventing much of the corrosion that might otherwise occur with 

an acidic copper preservative. The treatment solution does 

use hexavalent chromium, but the chromium is converted 

to the more benign trivalent state during treatment and 

subsequent storage of the wood. This process of chromium 

reduction is the basis for fixation in ACC and is dependent 

on time, temperature, and moisture. For information and 

sources of supply, see http://www.fprl.com/products.html.

Alkaline Copper Quat (ACQ)

Alkaline copper quat (ACQ) has an active composition of 

67% copper oxide and 33% quaternary ammonium compound (quat). Multiple variations of ACQ have been standardized. ACQ Type B (ACQ-B) is an ammoniacal copper 

formulation, ACQ Type D (ACQ-D) is an amine copper 

formulation, and ACQ Type C (ACQ-C) is a combined 

ammoniacal-amine formulation with a slightly different quat 

compound. The multiple formulations of ACQ allow some 

flexibility in achieving compatibility with a specific wood 

species and application. When ammonia is used as the carrier, ACQ has improved ability to penetrate into difficult to 

treat wood species. However, if the wood species is readily 

treatable, such as southern pine sapwood, an amine carrier 

can be used to provide a more uniform surface appearance 

(Fig. B5). For information and sources of supply, see 

http://treatedwood.com/locator.

Figure B4—Appearance of wood treated with 

pentachlorophenol in light oil. Figure B5—Appearance of wood treated with ACQ-D.
Guide for Use of Wood Preservatives in Historic Structures

55

Ammoniacal Copper Zinc Arsenate (ACZA)

ACZA is a water-based preservative that contains copper oxide (50%), zinc oxide (25%), and arsenic pentoxide 

(25%). It is a refinement of an earlier formulation, ACA, 

that is no longer in use. The color of the treated wood varies 

from olive to bluish green (Fig. B6). The wood may have a 

slight ammonia odor until it is thoroughly dried after treatment. The ammonia in the treating solution, in combination 

with processing techniques such as steaming and extended 

pressure periods, allows ACZA to obtain better penetration 

of difficult to treat wood species than many other waterbased wood preservatives. ACZA has been commonly used 

for treatment of Douglas-fir poles, piles, and large timbers. 

For more information and sources of supply, see 

http://www.archchemicals.com/Fed/WOLW/Products/

Preservative/Chemonite/default.htm.

Chromated Copper Arsenate (CCA)

Wood treated with CCA (commonly called green treated) 

dominated the treated wood market from the late 1970s until 

2004. However, as the result of the voluntary label changes 

submitted by the CCA registrants, the EPA labeling of CCA 

currently permits the product to be used for primarily industrial applications (Fig. B7), and CCA-treated products are 

generally not available at retail lumber yards. CCA can no 

longer be used for treatment of lumber intended for use in 

residential decks or playground equipment. It is important to 

note that existing structures are not affected by this labeling 

change, and that the EPA has not recommended removing 

structures built with CCA-treated lumber. These changes 

were made as part of the ongoing CCA registration process 

and in light of the current and anticipated market demand 

for alternative preservatives for non-industrial applications. 

The allowable uses for CCA are based on specific commodity standards listed in the 2001 edition of the AWPA standards. The most important of these allowable uses are based 

on the standards for poles, piles, and wood used in highway 

construction. A list of the most common allowable uses is 

shown in Table B1.

While several formulations of CCA have been used in the 

past, CCA Type C has been the primary formulation and is 

currently the only formulation listed in AWPA standards. 

CCA-C was found to have the optimum combination of efficacy and resistance to leaching, but the earlier formulations 

(CCA-A and CCA-B) have also provided long-term protection for treated stakes exposed in Mississippi. CCA-C has 

an active composition of 47.5% chromium trioxide, 34.0% 

arsenic pentoxide, and 18.5% copper oxide. AWPA Standard 

P5 permits substitution of potassium or sodium dichromate 

for chromium trioxide; copper sulfate, basic copper carbonate, or copper hydroxide for copper oxide; and arsenic acid, 

sodium arsenate, or pyroarsenate for arsenic pentoxide. For 

Table B1—Allowable uses of CCAa for wood pressure treated after 2003 

Type of end-use 

2001 AWPA 

Standard 

Lumber and timbers used in seawater C2

Land, fresh water, and marine piles C3

Utility poles C4

Plywood C9

Wood for highway construction C14

Round, half-round, and quarter-round fence posts C16

Poles, piles, and posts used as structural members on farms C16

Members immersed in or frequently splashed by seawater C18

Lumber and plywood for permanent wood foundations C22

Round poles and posts used in building construction C23

Sawn timbers (at least 5 in. thick) used to support residential and commercial structures C24

Sawn cross-arms C25

Structural glued-laminated members C28

Structural composite lumber (parallel strand or laminated veneer lumber) C33

Shakes and shingles C34

a

Chromated copper arsenate. 

Figure B6—Appearance of wood treated with ACZA.
General Technical Report FPL–GTR–217

56

more information, see http://www.wolmanizedwood.com/

Products/Preservative/original/default.htm or 

http://treatedwood.com/locator.

Copper Azole (CA-B, CA-C, and CBA-A)

Copper azole (CA-B) is a formulation composed of amine 

copper (96%) and tebuconazole (4%). Copper azole (CA-C) 

is very similar to CA-B, but one-half of the tebuconazole 

is replaced with propiconazole. The active ingredients in 

CA-C are in the ratio of 96% amine copper, 2% tebuconazole, and 2% propiconazole. An earlier formulation (CBAA) also contained boric acid. The appearance of copperazole-treated wood is similar to that of wood treated with 

other waterborne copper formulations (Fig. B8). Although 

listed as an amine formulation, copper azole may also be 

formulated with an amine-ammonia formulation. The ammonia may be included when the copper azole formulations 

are used to treat refractory species, and the ability of such a 

formulation to adequately treat Douglas-fir has been demonstrated. The inclusion of the ammonia, however, is likely to 

have slight effects on the surface appearance and initial odor 

of the treated wood. For more information, see 

http://www.wolmanizedwood.com/Products/Preservative/

genuine/default.htm.

Coal–tar Creosote

Coal–tar creosote is the oldest wood preservative still in 

commercial use, and remains the primary preservative used 

to protect wood used in railroad construction. It is made by 

distilling the coal tar that is obtained after high-temperature 

carbonization of coal. Unlike the other oil-type preservatives, creosote is not usually dissolved in oil, but it does 

have properties that make it look and feel oily. Creosote 

contains a chemically complex mixture of organic molecules, most of which are polycyclic aromatic hydrocarbons (PAHs). The composition of creosote depends on the 

method of distillation, and is somewhat variable. However, 

the small differences in composition within modern creosotes do not significantly affect its performance as a wood 

preservative. Creosote-treated wood has a dark brown to 

black color (Fig. B9) and a noticeable odor that some people 

consider unpleasant. It is very difficult to paint creosotetreated wood. 

Workers sometimes object to creosote-treated wood because 

it soils their clothes and photosensitizes the skin upon contact. The treated wood sometimes also has an oily surface, 

and patches of creosote sometimes accumulate, creating a 

skin contact hazard. Because of these concerns, creosotetreated wood is often not the first choice for applications 

where there is a high probability of human contact. This is 

a serious consideration for treated members that are readily 

accessible to the public. However, creosote-treated wood 

has advantages to offset concerns with its appearance and 

odor. It has lengthy record of satisfactory use in a wide 

range of applications and a relatively low cost. Creosote is 

also effective in protecting both hardwoods and softwoods 

and is often thought to improve the dimensional stability 

of the treated wood. With the use of heated solutions and 

lengthy pressure periods, creosote can be fairly effective 

at penetrating even fairly difficult to treat wood species. 

Creosote treatment also does not accelerate, and may even 

inhibit, the rate of corrosion of metal fasteners relative to 

untreated wood. Three formulations of creosote are listed 

in AWPA standards. Straight-run creosote (CR) is straight 

coal–tar distillate, CR-S may be a mixture of coal tar and 

coal–tar distillate, and CR-PS may contain up to 50% petroleum solvent. The listings in Appendix A were based on CR. 

In some but not all cases, CR-S and CR-PS are standardized 

as well. Consult AWPA standards for additional details.

Pentachlorophenol (Heavy Oil)

Pentachlorophenol has been widely used as a pressure treatment since the 1940s. The active ingredients, chlorinated 

phenols, are crystalline solids that can be dissolved in different types of organic solvents. The performance of pentachlorophenol and the properties of the treated wood are influenced by the properties of the solvent. The heavy oil solvent 

Figure B7—CCA-treated dock.

Figure B8—Appearance of copper-azole-treated wood.
Guide for Use of Wood Preservatives in Historic Structures

57

is preferable when the treated wood is to be used in ground 

contact because wood treated with lighter solvents is not as 

durable in such exposures. Wood treated with pentachlorphenol in heavy oil typically has a brown color and may 

have a slightly oily surface that is difficult to paint. It also 

has some odor, which is associated with the solvent. Like 

creosote, pentachlorophenol in heavy oil should not be used 

in applications where there is likely to be frequent contact 

with skin (i.e., hand rails). Pentachlorophenol in heavy oil 

has long been a popular choice for treatment of utility poles, 

bridge timbers, glued-laminated beams (Fig. B10) and foundation piling. Like creosote, it is effective in protecting both 

hardwoods and softwoods, and is often thought to improve 

the dimensional stability of the treated wood. With the use 

of heated solutions and extended pressure periods, pentaFigure B9—Appearance of creosote-treated wood.

chlorphenol is fairly effective at penetrating difficult to treat 

species. It does not accelerate corrosion of metal fasteners 

relative to untreated wood, and the heavy oil solvent helps 

to impart some water-repellency to the treated wood.

Copper Naphthenate (Heavy Oil)

The preservative efficacy of copper naphthenate has been 

known since the early 1900s, and various formulations have 

been used commercially since the 1940s. It is an organometallic compound formed as a reaction product of copper salts 

and petroleum-derived naphthenic acids. It is often recommended for field treatment of cut ends and drill holes made 

during construction with pressure-treated wood. Copper 

naphthenate treated wood initially has a green color 

(Fig. B11) that weathers to light brown. The treated wood 

also has an odor that dissipates somewhat over time. Depending on the solvent used and treatment procedures, it 

may be possible to paint copper-naphthenate-treated wood 

after it has been allowed to weather for a few weeks. Like 

pentachlorophenol, copper naphthenate can be dissolved 

in a variety of solvents but has greater efficacy when dissolved in heavy oil. Although not as widely used as creosote 

and pentachlorophenol treatments, copper naphthenate is 

increasingly used in the treatment of utility poles. Copper naphthenate has also been formulated as a water-based 

system, and is sold commercially in this form for consumer 

use. The water-based formulation helps to minimize concerns with odor and surface oils but is not currently used for 

pressure treatment.

Copper Naphthenate (Waterborne)

Waterborne copper naphthenate (CuN–W) has an actives 

composition similar to oil-borne copper naphthenate, but the 

actives are carried in a solution of ethanolamine and water 

instead of petroleum solvent. Wood treated with the waterborne formulation has a drier surface and less odor than the 

oil-borne formulation. The waterborne formulation has been 

standardized for above ground and some ground-contact 

applications.

Figure B10—Glued-laminated beam treated with pentachlorophenol in heavy oil.

Figure B11—Salt storage shed treated with copper naphthenate in heavy oil.
General Technical Report FPL–GTR–217

58

KDS

KDS and KDS Type B (KDS–B) use copper and polymeric 

betaine as the primary active ingredients. The KDS formulation also contains boron and has an actives composition 

of 47% copper oxide, 23% polymeric betaine, and 30% 

boric acid. KDS–B does not contain boron and has an actives composition of 68% copper oxide and 32% polymeric 

betaine. KDS is listed for treatment of commodities used 

above ground and for general use in contact with soil. The 

listing includes treatment of common pine species as well 

as Douglas-fir and western hemlock. Although AWPA standards would allow use in fresh water, the manufacturer does 

not recommend using KDS in aquatic applications. AWPA 

standards do not list KDS or KDS–B for severe exposures 

or critical applications, but they are listed for these uses under ICC–ESR 2500. The appearance of KDS-treated wood 

is similar to that of wood treated with other alkaline copper 

formulations (light green–brown), but KDS may also be formulated with incorporated pigments to produce other shades 

(Fig. B12). It has some odor initially after treatment, but this 

odor dissipates as the wood dries. For more information and 

sources of supply, see http://www.ruetgers-organics.com/

index.php?FOLDERID=461&PHPSESSID=d722c92681e4

10981b3dc89f75ca0def.

ESR–1721

ESR–1721 recognizes three preservative formulations. 

Two are the same formulations of copper azole (CA-B and 

CA-C) also listed in AWPA standards. The other (referred to 

here as ESR–1721) uses a particulate copper that is ground 

to submicron dimensions and dispersed in the treatment 

solution. Wood treated with ESR–1721 has a lighter green 

color than the CA-B or CA-C formulations because the 

copper is not dissolved in the treatment solution. All three 

formulations are listed for treatment of commodities used in 

a range of residential lumber applications, including contact 

with soil or fresh water. Use of ESR–1721 (particulate copper) is currently limited to easily treated pine species.

ESR–1980

ESR–1980 includes a listing for both the AWPA standardized formulation of ACQ-D and a waterborne, micronized 

copper version of alkaline copper quat (referred to here as 

ESR–1980). The formulation is similar to ACQ in that the 

active ingredients are 67% copper oxide and 33% quaternary ammonium compound. However, in ESR–1980 the 

copper is ground to sub-micron dimensions and suspended 

in the treatment solution instead of being dissolved in ethanolamine. The treated wood has little green color because 

the copper is not dissolved in the treatment solution. The 

use of the particulate form of copper is currently limited to 

the more easily penetrated pine species, but efforts are under way to adapt the formulation for treatment of a broader 

range of wood species. ESR–1980 is listed for treatment of 

commodities used in both above ground and ground-contact 

applications.

ESR–2240

ESR–2240 is a waterborne formulation that uses finely 

ground (sub-micron) copper in combination with tebuconazole in an actives ratio of 25:1. It is listed for above ground 

and ground-contact applications. In addition to wood products cut from pine species, ESR–2240 can be used for treatment of hem–fir lumber and Douglas-fir plywood.

ESR–2325

ESR–2325 is another waterborne preservative that uses finely ground (sub-micron) copper particles and tebuconazole as 

actives. The ratio of copper to tebuconazole in the treatment 

solution is 25:1. Its use is currently limited to more readily 

treated species such as the Southern Pine species group, but 

Figure B12—Examples of wood treated with pigmented 

KDS formulations.
Guide for Use of Wood Preservatives in Historic Structures

59

Douglas-fir plywood is also listed. ESR–2315 is listed for 

treatment of wood used above ground and in contact with 

soil or fresh water.

ESR–2711

ESR–2711 combines copper solubilized in ethanolamine 

with the fungicide 4,5-dichloro-2-N-octyl-4-isothiazolin3-one (DCOI). The ratio of copper (as CuO) to DCOIT 

ranges from 10:1 to 25:1. The ESR listing provides for both 

above ground and ground-contact applications. The appear-

ance of the treated wood is similar to that of wood treated 

with other formulations using soluble copper, such as ACQ. 

It is currently only listed for treatment of pine species.

Marine (Seawater) Applications

Marine borers present a severe challenge to preservative 

treatments. Some preservatives that are very effective 

against decay fungi and insects do not provide protection in 

seawater. The preservatives that are most commonly used to 

protect wood in marine environments are forms of creosote 

as well as the water-based preservatives containing cop-

per and/or arsenic. Properly applied, creosote effectively 

prevents attack by all marine borers except the gribble (Lim-

noria tripunctata). Water-based preservatives such as CCA 

or ACZA effectively protect against attack by shipworms (Teredo and Bankia spp.) and gribbles (Limnoria spp.) but 

do not protect against pholads (Martesia spp.). Because no 

single preservative is effective against all marine borers, 

dual treatments may be required in some locations. Dual 

treatments involve an initial treatment with a water-based 

preservative followed by a conventional creosote treatment. 

Dual treatments are more expensive than single treatments. 

For both creosote and water-based treatments, much higher 

preservative retentions are required to protect against ma-

rine borers than are needed to protect wood in terrestrial or 

freshwater applications. Physical barriers such as plastic 

sleeves or wraps have also been used to protect piling, but 

these systems are vulnerable to breaches from mechanical 

damage. They are most effective when applied to piles that 

have been pressure treated with preservatives.