Guide for In-Place 

Treatment of Wood in 

Historic Covered 

and Modern Bridges 

Stan Lebow

Grant Kirker

Robert White

Terry Amburgey

H. Michael Barnes

Michael Sanders

Jeff Morrell

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical 

Report

FPL–GTR–205

250 mL

200 mL

150 mL

100 mL

50 mL

In cooperation 

with the

United States

Department of

Transportation

Federal 

Highway 

Administration
March 2012

Lebow, Stan; Kirker, Grant; White, Robert; Amburgey, Terry; Barnes, H. 

Michael; Sanders, Michael; Morrell, Jeff. 2012. Guide for In-Place Treatment of Wood in Historic Covered and Modern Bridges. General Technical 

Report FPL-GTR-205. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 

43 p.

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Abstract

Historic covered bridges and current timber bridges can be 

vulnerable to damage from biodeterioration or fire. This 

guide describes procedures for selecting and applying inplace treatments to prevent or arrest these forms of degradation. Vulnerable areas for biodeterioration in covered bridges include members contacting abutments, members near 

the ends of bridges subject to wetting from splashing and 

members below windows or other openings that allow entry 

of wind-blown precipitation. Pressure-treated timber bridge 

members can be vulnerable when untreated wood is exposed 

by field fabrication or by the development of drying checks. 

The objective of an in-place preservative treatment is to 

distribute preservative into areas of a structure that are 

vulnerable to moisture accumulation and/or not protected 

by the original pressure treatment. Types of field treatments 

range from finishes, to boron rods or pastes, to fumigants. 

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

forced deeply into the wood as is done in pressure-treatment 

processes. However, some can be applied into the center 

of large members via treatment holes. These preservatives 

may be available as liquids, rods or pastes. Bridge members 

can be treated with fire retardants to delay ignition, reduce 

heat release, and slow the spread of flames. In-place coating 

products are available to reduce surface flammability, but 

these coatings may need to be reapplied on a regular basis if 

exposed to weathering. For more integrated protection, fire 

retardant treatment of bridge members may be combined 

with other forms of protection such as lights, alarms, 

sprinklers and monitoring systems. 

Keywords: guide, covered bridge, timber bridge, 

deterioration, fire, wood preservatives, in-place treatment

This study is part of the Research, Technology and 

Education portion of the National Historic Covered 

Bridge Preservation (NHCBP) Program administered 

by the Federal Highway Administration. The NHCBP 

program includes preservation, rehabilitation and restoration of covered bridges that are listed or are eligible 

for listing on the National Register of Historic Places; 

research for better means of restoring, and protecting 

these bridges; development of educational aids; and 

technology transfer to disseminate information on covered bridges in order to preserve the Nation’s cultural 

heritage.

This study is conducted under a joint agreement between the Federal Highway Administration–Turner 

Fairbank Highway Research Center, and the Forest 

Service – Forest Products Laboratory.

Federal Highway Administration Program Manager– 

Sheila Rimal Duwadi, P.E.

Forest Products Laboratory Program Manager– 

Michael A. Ritter, P.E.
Contents

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

 Causes of Biodegradation ................................................................... 1

 Effects of Climate ............................................................................... 2

 Role of Wood Structure....................................................................... 3

 Problem Areas for Biodegradation in Wooden Bridges...................... 3

 Good Practices.................................................................................... 7

Types of Supplemental Treatments......................................................... 8

 Water-Diffusible Preservatives ........................................................... 9

 Non-Diffusible Liquid Treatments.................................................... 12

 Fumigants.......................................................................................... 13

Applying Supplemental Treatments...................................................... 14

 Internal Treatments ........................................................................... 14

 Water-Diffusible Internal Treatments................................................ 14

 Fumigants.......................................................................................... 15

 Non-Diffusible Liquids..................................................................... 16

 External Treatments.......................................................................... 16

Research on the Use of Supplemental Preservative Treatments for 

Covered Bridges.................................................................................... 17

Summary of Supplemental Treatment Concepts................................... 21

 Liquid Surface Treatments................................................................ 21

 Paste Surface Treatments.................................................................. 21

 Internal Treatments ........................................................................... 21

 Example Supplemental Treatment Applications............................... 22

Who Can Apply Supplemental Preservative Treatments? .................... 31

 The EPA Pesticide Label is the Law ................................................. 31

 Collection of Drill Savings............................................................... 32

Fire Prevention...................................................................................... 32

 Contributing Factors......................................................................... 32

 Research on the Use of Supplemental Fire-Retardant Treatments 

 for Covered Bridges.......................................................................... 34

 Fire Protection Technology............................................................... 35

Conclusions........................................................................................... 35

References............................................................................................. 37

Additional Sources................................................................................ 38

Appendix—Contact Information for State Offices Conducting the 

U.S. EPA’s Certified Pesticide Applicator (CPA) Program................... 39

Guide for In-Place Treatment of Wood in 

Historic Covered and Modern Bridges

Stan Lebow, Research Forest Products Technologist

Grant Kirker, Research Forest Products Technologist

Robert White, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin

Terry Amburgey, Professor Emeritus, Department of Forest Products

H. Michael Barnes, Professor, Department of Forest Products

Michael Sanders, Senior Research Associate, Department of Forest Products

Mississippi State University, Starkville, Mississippi

Jeff Morrell, Wood Science and Engineering Professor

Oregon State University, Corvallis, Oregon

Introduction

Wooden bridges have a long history of use throughout the 

world, and like bridges made of other materials, the service 

life of wooden bridges can be enhanced through proper 

construction, inspection, and maintenance. Wooden bridges, 

whether historic covered bridges or current highway timber 

bridges, can be vulnerable to damage from biodegradation. 

Because of the long history of using wooden bridges, the 

causes of wood biodeterioration are well documented, as are 

the means to mitigate their effects. Biodeterioration is minimized through design and construction practices, and in the 

case of modern timber bridges, through pressure treatment 

of the timbers with wood preservatives. However, the potential for degradation remains, and over time many bridges 

need maintenance that may include in-place treatment with 

preservatives. Fire is also a major threat, particularly for 

covered bridges. In this manual, we describe procedures 

for selecting and applying in-place treatments to bridges to 

prevent or arrest degradation. This guide focuses on preservative treatments to protect against biodeterioration, but also 

briefly discusses approaches for minimizing damage caused 

by fire.

Causes of Biodegradation

Some understanding of the causes of biodegradation is helpful when considering in-place treatment. Because there are 

many excellent sources of information on the organisms that 

damage wooden structures (see References), this guide provides only a brief summary. 

In most applications of wooden construction materials, decay fungi are the most destructive organisms. Fungi are microscopic thread-like organisms whose growth depends on 

mild temperatures, moisture, and oxygen. In part, the high 

degree of damage by wood-decay fungi reflects their 

ubiquitous presence in all locations. Given suitable 

conditions, attack by some type of wood-decay fungus is 

assured. Numerous species of fungi colonize wood, and 

they have a range of preferred environmental conditions. 

Decay fungi are often separated into three major groups: 

brown-rot fungi, white-rot fungi, and soft-rot fungi. Brown 

rot and white rot are usually the most destructive and are the 

fungi most likely to be found in wood above ground. These 

two groups of decay fungi have differences in wood species 

preferences and in the manner that they degrade the wood, 

but the optimal environmental conditions to cause wood 

decay are fairly similar for both groups. Both decay types 

can cause substantial damage in a relatively short amount of 

time. Soft-rot fungi, in contrast, generally prefer wetter, and 

sometimes warmer, environmental conditions. Damage by 

soft-rot fungi resembles that by brown-rot fungi, but typically occurs more slowly and nearer to the wood surface. 

Termites follow fungi in terms of the amount of damage 

to wood structures in the United States. Their damage can 

occur more rapidly than that caused by decay, but their geographic distribution is less uniform. Numerous termite species are native to the United States, and like decay fungi, the 

type and severity of attack varies by species. Termite species 

in the United States can be grouped into the categories of 

ground-inhabiting (subterranean) or wood inhabiting (nonsubterranean) termites. Most damage in the United States is 

caused by species of subterranean termites. The threat from 

subterranean termites has increased with the spread of the 

non-native Formosan subterranean termite (FST) in some 

areas of the southeastern United States. Non-subterranean 

termites are less damaging because they have a narrower 

geographic range and degrade wood more slowly due to 

smaller colony size. 

Other types of insects such as powderpost beetles and carpenter ants can cause notable damage in some situations, but 

their overall significance pales in comparison to decay fungi 
General Technical Report FPL–GTR–205

2

and termites. Other organisms, including bacteria and mold, 

can also cause damage in some situations. Several types of 

marine organisms degrade wood placed in seawater. On an 

economic basis, however, decay fungi and termites are by 

the far the most destructive pests of wood used in terrestrial 

applications.

Effect of Climate

We have long recognized that exposed wood deteriorates 

more rapidly in warm, wet climates than in cold and/or dry 

climates. Historically, use of wood as a construction material mirrored this effect, with greatest use occurring in northern latitudes. The two greatest factors influencing regional 

biodeterioration hazard are temperature and moisture. The 

growth of most decay fungi is negligible at temperatures 

below 2 °C (36 °F) and relatively slow at temperatures from 

2 to 10 °C (36–50 °F). The growth rate then increases rapidly, with most fungi having optimum growth rates at temperatures between 24 and 35 °C (75–95 °F). Soft-rot fungi 

typically tolerate warmer temperatures than brown- and 

white-rot fungi. Fungal growth rate declines steeply 

at higher temperatures, with little growth above 40 °C 

(104 °F) and no growth above 46 °C (115 °F). In most locations and applications in the United States, the lower end 

of this temperature range has the greatest effect on fungal 

growth. Northern regions of the United States may have several months of the year when temperatures are continuously 

too low for growth of decay fungi and other months when 

conditions for growth are only intermittently favorable. The 

result, as we see from practical experience, is that decay 

progresses more rapidly in warmer regions of the United 

States. Although temperatures on the surface of wood exposed to sunlight can exceed those favored by decay fungi, 

the inner portions of wood products are usually cooler. 

Decay tends to develop more rapidly in wood in shaded 

locations, but this is usually associated with a slower rate of 

drying rather than with protection from excessive heat. 

The role of moisture in biodeterioration, especially by decay 

fungi, cannot be overemphasized. 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. Decay fungi cannot colonize wood with 

a moisture content (MC) below the fiber saturation (average 

of 30% MC). Free water must be present. Most brown- and 

white-rot decay fungi prefer wood in the moisture content 

range of 40% to 80%. Growth at lower moisture contents is 

much slower, and typically wood with a moisture content 

of less than 25% cannot be attacked unless the fungus has 

another source of moisture nearby. Previously established 

fungi are not necessarily eliminated at even lower moisture 

contents. Some species of decay fungi produce thick-walled 

resting spores and have been reported to survive (without 

further growth) for 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, however, 

tolerate higher moisture contents and lower oxygen levels. 

As with temperature, the lower end of the moisture content 

limitations have the greatest effect on regional decay hazard. 

Humidity alone is not sufficient to raise wood moisture contents to levels needed by decay fungi, although equilibrium 

moisture contents of over 20% can occur in cool, moist 

climates (Fig. 1). Air is able to hold more moisture at warmer temperatures, lowering the relative humidity and equilibrium wood moisture content. Humidity does play a key 

role in slowing the drying of wood once it is wetted because 

wood will dry more quickly at lower relative humidity. 

The drying rate also depends on the length of dry periods 

between wetting and on construction details that affect the 

uptake of free water and the loss of water vapor from wood. 

Temperature affects not only the degree of activity, but also 

geographic distribution of termite species within the United 

States. The natural range of native subterranean termites is 

generally limited to areas where the average annual temperature exceeds 10 °C (50 °F), although they have been found 

farther north in areas where human activities create pockets 

of warmer temperatures. Within much of their range in the 

Midwestern and Eastern United States, termite activities 

gradually decline above ground, and little activity occurs in 

the winter. Termites cease their activities when temperatures 

fall below freezing, and in colder climates may burrow over 

1 m into the ground to avoid prolonged freezing temperatures. The net effect of temperature on termite degradation 

of wood is similar to that of decay fungi; conditions are 

most favorable in regions with warmer climates. The temperature effect may be more extreme for termites than fungi, 

however, as some regions of the northern United States have 

virtually no risk of termite attack. 

The effect of moisture on termite attack varies with termite species. To some extent, the type of termite and their 

dependence on moisture does vary with climate, but it is a 

loose correlation. Dampwood termites require wood with 

4

8

12

16

20

24

J F M A M J J A S O N D

Month

E

M

C (%)

Barrow (AK)

Hilo

Phoenix

Figure 1. Examples of equilibrium moisture content 

(EMC) of wood exposed outdoors and protected from precipitation in Barrow, Alaska; Hilo, Hawaii; and Phoenix, 

Arizona.
Guide for In-Place Treatment of Covered and Timber Bridges

3

high moisture levels and typically only attack wood that is 

in direct contact with the ground. As a result, their effect on 

wooden structures is relatively minor. Their high moisture 

requirements coincide with their preferred habitats in the 

northwestern United States and southern Florida, but they 

are found in the southwestern United States as well. Native 

subterranean termites require moisture to prevent desiccation, but can attack wood at moisture contents well below 

the fiber saturation point by building shelter tubes upward 

from their nests in the ground. Native subterranean termites 

are widely distributed in the southern two-thirds of the United States, although their distribution is less uniform along 

the Pacific Coast. 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. They are commonly found in southern California, 

Arizona, and in coastal areas from South Carolina to Texas. 

Role of Wood 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 concentric 

circles along the length of the tree (Fig. 2). 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 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. Because of this structure, fluids move 

much more readily along the grain than across it. 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 rather 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. The heartwood of some 

species contains toxic extractives that prevent attack by decay fungi or termites, and the heartwood of other species has 

water-repellent structural elements that limit water uptake 

and thus minimize decay. Many historic bridges were at 

least partially constructed with these durable wood species. 

Problem Areas for Biodegradation in 

Wooden Bridges

Significant decay can occur in any untreated portion of 

a bridge where oxygen is present and the wood moisture 

content is above 20% to 25% for sustained periods. Sufficient oxygen and moisture for decay are always present 

in members placed in contact with the ground, or near the 

waterline area of members placed in water. In most climates 

there is also sufficient moisture for decay in members that 

are not directly in contact with soil or water or protected 

by a covering. In general, larger members are most prone 

to developing decay because water becomes trapped inside 

the wood during precipitation and is slow to dry during 

subsequent dry weather. Liquid water is rapidly absorbed in 

end-grain during rain, 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 other means. 

Problem Areas in Covered Bridges

Because covered bridge members typically were not treated 

with wood preservatives before installation, they are vulnerable to biodeterioration in any areas with sustained exposure 

to moisture. One of the most common, and critical, areas of 

Figure 2. Typical structure of softwood species. 

Figure 3. The area where covered bridge members contact 

the abutment or supports at the ends of the bridge often 

provides conditions for deterioration.
General Technical Report FPL–GTR–205

4

deterioration in covered bridges is where the support members (bottom chord or bedding timbers) contact some form 

of an abutment (Fig. 3). Although the abutment area may be 

largely protected by the roof of the bridge, several factors 

combine to increase the risk of moisture accumulation: 

1) the stone or masonry used to construct abutments can 

wick and hold moisture, 2) the location near the end of the 

bridge increases the likelihood that water will enter through 

the bridge deck above, 3) high humidity and lack of air 

movement in this area retards drying. Similarly, all large 

members near the end of the bridge may be vulnerable to 

wind-blown or splashed precipitation. The deck members, 

the lower portions of the end posts, the ends of the bottom chords, and the ends of the diagonal bracing may all 

be exposed to wetting, depending on construction and site 

conditions. Wetting deck members near the ends of bridges 

is especially likely in bridges with vehicular traffic (Fig. 4). 

Areas below windows or other designed openings in the side 

of a bridge provide additional potential avenues for moisture 

intrusion (Fig. 5). Although these openings are typically 

placed relatively high on the side of a bridge, the roof overhang is not always sufficient to exclude moisture. 

Other areas of covered bridges become vulnerable to moisture as a result of leaks or vandalism. Sources of moisture 

from openings in the roof or cladding can occur almost anywhere in a bridge and are not always easily detected. However, water stains or general discoloration may be visible 

indicators of water leaks or concealed decay. The area where 

decay develops may not be immediately adjacent to where 

water enters the structure. As with other sources of moisture, 

problems are most likely to develop in larger members or at 

connections where wood is slow to dry. Vandalism is a frequent cause of water intrusion. Cladding may be repeatedly 

removed to allow access for fishing or swimming, exposing 

the bottom chords to precipitation (Fig. 6). 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.

Problem Areas in Modern Timber Bridges

The preservative treatments standardized for use in timber 

bridges are 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. The structure and chemistry of 

wood affect the ability of preservatives to penetrate into the 

wood, as well as the efficacy of some types of preservatives. 

The outer sapwood in most tree species is the part of the tree 

that is most easily treated with liquid preservatives, whereas 

the heartwood is much more resistant to preservative 

penetration. 

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 it 

may also be obtained in piles, ties, and structural timbers. 

Practically, however, the pressure treater cannot always 

ensure complete penetration of sapwood in every piece 

when treating large pieces of round material with thick 

sapwood. Therefore, specifications permit some tolerance. 

For instance, AWPA Processing and Treatment Standard 

T1 for Southern Pine Piles requires penetration of 75 mm 

(3 in.) or 85% of the sapwood thickness. The penetration 

requirements vary, depending on the species, size, class, and 

specified retention levels. The proportion of 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 wide sapwood 

zone that is readily penetrated by most types of preservatives. 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. The treatment standards 

recognize this, and require only penetration of 

19 mm (0.75 in.) and 85% of the sapwood in Douglas-fir 

piles. The proportion of heartwood varies in lumber and 

timbers. During sawmilling, larger dimension timbers tend 

Figure 4. Members at ends of covered bridge are vulnerable 

to wetting from wind-blown rain. The picture at the bottom 

shows decay in a stringer under the bridge decking shown 

on the top.
Guide for In-Place Treatment of Covered and Timber Bridges

5

to include the center of the tree and thus may have a 

substantial area of untreatable heartwood (Fig. 7).

Proper preservative treatment creates an excellent barrier 

against fungi and insects. However, this barrier can be compromised during on-site installation 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 cutting piles to height after driving. In almost all 

cases this practice exposes untreated heartwood in the center 

of the pile, and there is an increased probability that internal 

decay will develop if this exposed surface is left unprotected. Attempts to protect this cut surface may only be partially 

successful (Figs. 8–11). Cut-off piles that do not appear to 

have been adequately protected are among the most likely 

candidates for application of supplemental treatments. 

Check (crack) formation in both piles 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. Because wood does not 

shrink and swell equally in all directions, formation of some 

drying checks is not unexpected. Ideally, thoroughly drying 

the members before pressure treatment would encourage 

Figure 5. Windows and similar openings in covered bridges provide avenues for moisture entry. Larger and lower 

windows allow the most access.

Figure 6. Vandals removed sections of cladding on this 

bridge, exposing the support members to moisture and 

decay.

Figure 7. During pressure treatment, 

preservative typically penetrates only 

the sapwood. Round members have a uniform treated sapwood shell (upper photograph), but sawn members may have less 

penetration on one or more faces (lower 

photograph). 
General Technical Report FPL–GTR–205

6

these checks to form before treatment and allow them to be 

well protected with preservative. However, it is generally 

not feasible to dry large timbers or piles to their in-service 

moisture content prior to treatment. Small drying checks 

also may not be a concern if they do not penetrate past 

the treated shell. 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 (Figs. 12, 13). 

Another common source of breaks in the treated shell is 

field fabrication of treated members. Examples include 

cutting to length, notching, and boring holes for fasteners 

(Fig. 14). The extent of field fabrication during construction should be minimized by specifying as much fabrication 

as possible before treatment, but some field fabrication is 

usually necessary. The wood exposed during construction 

should be protected by application of a preservative such 

as copper naphthenate to the cut surface, but this practice is 

not always followed (Fig. 15). In some cases, construction 

personnel are concerned about the loss of excess liquid preservative into water beneath the structure. 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. 16). 

The placement of a member within a structure may also 

affect its susceptibility to decay (Fig. 17). Some evidence 

shows that members that are protected by the bridge deck 

and not placed into standing water are less likely to develop 

decay. This supposition is logical if these members are protected from wetting, but caution is needed in applying this 

finding categorically. Bridge designs and conditions vary, 

Figure 8. Tar-like coatings may not provide sufficient long-term protection for cut-off pile tops. 

These piles are likely candidates for development 

of internal decay.

Figure 9. 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 

shell may not be complete (see photograph on 

bottom). Fortunately, most pressure-treated members used in bridges have greater penetration than 

shown in these examples. 
Guide for In-Place Treatment of Covered and Timber Bridges

7

and in some cases members beneath the deck will have sufficient moisture for decay to develop. 

Good Practices

Although the purpose of this manual is to discuss protection of bridges with preservative treatments, it is essential 

to note that preservatives are not a substitute for other types 

of maintenance that can minimize the conditions favoring 

decay.

Because water is the key to biodegradation, maintenance 

of covered bridges to minimize water intrusion is essential. 

This includes prompt repair of leaks in roofs and replacement of sections of cladding that have been damaged or 

removed by vandals. An additional source of moisture that 

is sometimes overlooked is water splashed from puddles 

Figure 10. These piles are at risk of developing internal decay, and it is likely that the pile on the bottom 

already has substantial decay. Although cutting piles 

to height after installation is a common practice, the 

exposed untreated wood should be field treated and 

then protected with a durable cap. In this case, the 

tops are only partly protected, and organic matter is 

accumulating on the pile tops.

Figure 11. The older pile pictured on the bottom 

demonstrates how decay eventually develops in the 

untreated core of piles that have been cut to height 

on the job site. Cutting the piles at an angle with the 

intent of encouraging water to run off does not inhibit decay. 

Figure 12. Deep seasoning checks can be an indication of potential problem areas unless the checks 

formed prior to pressure-treatment.
General Technical Report FPL–GTR–205

8

that may form near the bridge entrance (Fig. 18). If possible, 

puddles in this area should be eliminated, especially for 

bridges that carry vehicle traffic. 

A more challenging maintenance task can be removal of 

dirt and organic debris that builds up in cracks and crevices 

over years of service. This organic material helps to trap 

moisture and provides a source of nutrients for decay fungi 

in both covered bridges and highway bridges (Figs. 19–22). 

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. 

Types of Supplemental Treatments

The objective of supplemental treatment is to distribute 

preservative into areas of a structure that are vulnerable to 

Figure 13. Checks allow ready access for moisture 

from rain and snow to reach the interior of large 

timbers. An accumulation of organic matter further 

increases the potential for decay.

Figure 14. Metal fasteners are sometimes associated with decay pockets if holes are drilled after 

treatment and expose untreated wood. Field-fabricated bolt holes should be field treated with a 

preservative such as copper naphthenate during 

construction. 

Figure 15. The upper bolt in this pile was either removed or never installed. The hole now serves as a 

ready access for fungi to the untreated interior of the 

pile. 

Figure 16. Unless it was pressure-treated after fabrication, this type of connection is a recipe for decay. It exposes untreated wood and creates an area that traps and 

holds moisture. 
Guide for In-Place Treatment of Covered and Timber Bridges

9

moisture accumulation and/or not protected by the original 

pressure treatment. A major limitation of supplemental treatments is that they cannot be forced deep into the wood under pressure, as is done in the pressure-treatment processes. 

Types of supplemental treatments range from finishes to 

boron rods to fumigants (Table 1).

Water-Diffusible Preservatives

Water-diffusible preservatives or diffusible components of 

preservatives move slowly through water within the wood 

structure. Water-diffusible preservatives do not “fix” in the 

wood and thus are able to diffuse through wood as long as 

sufficient moisture is present (Fig. 23). 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 surface treatments because the volume of actives applied to the 

Figure 17. The groundline area of piles represents a 

severe decay hazard. 

Figure 18. Puddles near bridge entrances cause lower 

members to be repeatedly wetted by splash from 

vehicle traffic.

Figure 19. Vegetation growing on a structure is an indication that conditions are favorable for decay. 

Figure 20. This type of accumulation of dirt and organic 

matter makes decay more likely and more rapid if moisture reaches this area.
General Technical Report FPL–GTR–205

10

surface is limited. The most commonly available diffusible 

preservatives are based on the use of some form of boron. 

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 a liquid or paste 

formulations. 

Boron-based supplemental treatments are widely used 

because they have several advantages. Boron has efficacy 

against both decay fungi and insects but has relatively low 

toxicity to humans. The sodium borate formulations used as 

supplemental 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. In addition, borates 

are corrosion inhibitors and have been shown to prevent fastener corrosion in some situations. 

Borate field treatment preservatives are available in a range 

of forms including powders, gels, thickened glycol solutions, solid rods, and as a 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 

Figure 21. Dense vegetation slows drying after rain and 

contributes to the deposition of organic matter under 

and in the bridge. 

Figure 22. Galvanized pile caps (top) can provide protection of cut surfaces if applied at the time of construction. However, pile caps alone do not stop decay that 

has already started, and they must be inspected to ensure that they have not been damaged (bottom). 

Figure 23. Examples of boron diffusion into lumber with 

20% moisture content over time. The red color is an indicator that reacts when boron is present. A 15% DOT solution 

was applied to the surface of the lumber. 

Figure 24. Powdered disodium octaborate tetrahydrate 

(DOT). 
Guide for In-Place Treatment of Covered and Timber Bridges

11

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. Whereas this moisture level is often surpassed for 

wood exposed outdoors, some members in a covered bridge 

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 typically contain 98% DOT and are often 

the least expensive product on the basis of active ingredient purchased (Fig. 24). 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 typically contain 40% 

DOT and polyethylene glycol, 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 (Fig. 25). Lower concentrations can also 

be prepared if desired. The glycol formulations allow a 

greater borate solution concentration than the powders, and 

the resulting dilutions tend to resist precipitation longer than 

those prepared from powders. Dilution by volume rather 

than weight can also be advantageous in some situations. 

The more viscous and more concentrated glycol–borate 

solutions are also thought to allow deposition of higher concentrations of boron on the wood surface during spray applications. This effect was recently evaluated with spray treatments of Southern Pine lumber specimens. Specimens were 

briefly sprayed with either a 15% DOT solution prepared 

from powder or 15% and 23% DOT solutions prepared from 

glycol–borate formulations. After spraying, the specimens 

were allowed to sit in humid conditions for 26 weeks, and 

then boron content was assayed at three depths from the 

wood surface. The specimens sprayed with the 23% DOT 

thickened solution had significantly greater boron in the 

outer 6 mm and slightly greater boron concentration within 

7–12 mm from the surface than specimens sprayed with 

either 15% DOT formulation. The 15% DOT glycol–borate 

solution also resulted in slightly higher boron concentrations than the 15% DOT solution prepared from powder. 

The glycol benefit appears to primarily be a function of 

increased surface loading, as there is some evidence that the 

glycol does not increase the rate of penetration of the boron 

through the wood. 

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 ships. 

Borate gels contain 40% DOT and are available in tubes for 

ease of application in standard caulking guns. An advantage of the gel formulation is that it can be applied to voids, 

cracks, and treatment holes that are oriented horizontally or 

downward and would not retain 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. 

The most common active ingredient is boron, although one 

product is composed of sodium fluoride (Fig. 26) and another contains small percentage of copper (Fig. 27). Fused borate rods are produced by heating DOT until it is molten and 

pouring this material into molds of various diameters. The 

boron cools into a glass-like rod with a high percentage of 

boron (Fig. 28). Both systems produce a maximum amount 

of boron per volume of rod. 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 

below a member. A disadvantage of the rods is that their application does not include water to assist the initial diffusion 

process. Because of this lack of moisture, some applicators 

will drill slightly over-sized treatment holes and fill the void 

space around the rod with a borate solution. This additional 

borate solution does appear to provide benefit in increasing 

boron concentrations in the wood around the treatment hole. 

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 

Figure 25. A 22% DOT glycol–borate solution was applied 

to the right side of this specimen and then allowed to dry 

before this picture was taken. 
General Technical Report FPL–GTR–205

12

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.

Sodium fluoride is a diffusible component of a liquid formulation that is primarily used for treatment of internal 

voids in poles and piles. The formulation also contains a 

water-based form of copper as a less mobile preservative 

component. It is applied by drilling a hole into the void and 

forcing the solution into the wood using air or mechanical 

pump pressure. Because the solution is applied under pressure, extra care must be taken to ensure that the void does 

not have other openings that will allow the formulation to 

exit the pile and into the surrounding environment. 

Non-Diffusible Liquid Treatments

The oldest and simplest method for applying supplemental 

preservative treatment during fabrication or routine maintenance involves brushing or spraying a preservative onto 

the untreated wood or suspected problem area (i.e., joints, 

fasteners, pile tops). Flooding of bolt holes and the tops of 

cut-off piles is particularly useful. Often the treated surface 

will be covered or closed during construction and will no 

longer be available for surface treatment. The 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 oilborne preservatives, which 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, (i.e., 

copper naphthenate or copper-8-quinolinolate) although 

zinc naphthenate is also available in some areas. Because 

of the limited volume solution applied and their superficial 

application, the efficacy of these treatments will gradually 

decline over time. One study found that a pentachlorophenol 

solution applied to bolt holes provided only 8 years of 

protection.

Oil-based copper naphthenate, the most common form of 

liquid field-treatment preservative, is available in copper 

concentrations ranging from 1% to 8% (as elemental copper). The solution is typically applied at 1% to 2% copper 

Figure 26. Sodium fluoride rod. 

Figure 27. Example of 19-mm- (0.75-in.-) diameter rod that 

contains both copper and boron. 

Figure 28. 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 29. A solvent-based copper naphthenate solution (1% 

copper) was applied to the right side of this specimen and 

allowed to dry before this photograph was taken. 
Guide for In-Place Treatment of Covered and Timber Bridges

13

concentration, and more concentrated solutions are diluted 

with mineral spirits, diesel, or a similar solvent (Fig. 29). 

Oil-based copper naphthenate is commonly used for treating areas of untreated wood exposed during fabrication of 

pressure-treated wood. These solutions impart an obvious 

green color to the wood, although some of the 1% copper 

solutions are tinted to dark brown or black. The green color 

weathers to brown with exposure (Fig. 30). Oil-based 

copper naphthenate solutions also have a noticeable odor.

Water-based copper naphthenate is currently less widely 

used than the oil-based formulations. It 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 slightly bluer (Fig. 31). 

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 does.

Oil-based copper-8-quinolinolate was recently standardized 

by the American Wood Protection Association for fieldtreatment 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, although zinc is less effective than copper in preventing decay 

from wood-destroying fungi or growth of mold fungi. However, an advantage of zinc naphthenate is that it is clear and 

does not impart the characteristic greenish color of copper 

naphthenate. It does, however, have a noticeable odor. Zinc 

naphthenate can be formulated in both water-based and 

solvent-based formulations. 

Fumigants

Fumigants are gases that are used to internally treat large 

piles or timbers. Like some water-diffusible formulations, 

fumigants are applied in liquid or solid form in predrilled 

holes. However, they then volatilize into gasses that move 

much greater distances through the wood than do the waterdiffusible 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. Fumigants tend to arrest fungal attack 

more quickly than water-diffusible systems and are not dependent on being applied to moist areas of the wood to function. 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 decomposes 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 metham 

sodium (sodium N-methyldithiocarbiamate), or the granular 

dazomet (tetrahydro-3, 5-dimethyl-2-H-1,3,5, thiodazine6-thione). One of the dazomet products is available in prepackage 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. Fumigant treatments are generally more 

toxic and more difficult to handle than the diffusible 

treatments. Some are considered to be Restricted Use 

Pesticides (RUPs) by the U.S. Environmental Protection 

Figure 30. 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 one year 

later. This wood was pressure-treated. 

Figure 31. A water-based copper naphthenate solution (1% 

copper) was applied to the right side of this specimen and 

allowed to dry before this photograph was taken. 
General Technical Report FPL–GTR–205

14

Agency (EPA), requiring extra precautions and application 

by specially trained personnel. 

Liquid fumigants are poured into pre-drilled treatment 

holes, necessitating that they be applied from above. The 

most commonly applied liquid fumigant is metham sodium 

(33% sodium N-methyldithiocarbamate). Like several fumigants, this liquid formulation decomposes to produce the 

active ingredient MITC. It tends to be less expensive than 

other sources of MITC, but also contains a lower proportion 

of the active ingredient. One of the oldest fumigants, chloropicrin, is only available in liquid form. It is the most effective, long-lasting fumigant but also difficult to handle safely 

because of its volatility. It is a RUP and its use 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-dimethyl-2-H-1,3,5, thiodazine-6-thione), which decomposes to produce MITC. The granular fumigant formulations 

offer relatively easy handling compared with liquid metham 

sodium and also contain a higher percentage of the active ingredient. However, they decompose to produce MITC more 

slowly than the liquids, and in some cases, liquid accelerants 

such as copper naphthenate (containing 1% copper) 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 particles while allowing MITC gas to escape (Fig. 32). Another 

encapsulated product consists of an aluminum tube filled 

with solid 97% MITC (Fig. 33). At the time of application, 

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

tube, and MITC vapors are released through this opening. 

A disadvantage of the encapsulated fumigants is their higher 

costs and that they require a minimum treatment-hole 

diameter and depth for application. 

Applying Supplemental Treatments

Internal Treatments

Internal decay in larger timbers is a function of their tendency to check, and for these checks to provide points of 

water ingress. Wood wets through sorption of liquid water 

but dries by evaporation of water vapor. As a result, wood 

almost always wets faster than it dries, particularly far from 

the surface. This creates elevated, relatively stable moisture conditions deeper in the wood. In most cases, bridge 

members are too thick to effectively treat the interior of the 

member with surface application of preservatives. Internal 

treatments are typically applied to these timbers by drilling 

holes into the wood, but there are many variations on this 

approach (Table 2).

Water-Diffusible Internal Treatments

Water-diffusible internal treatments generally do not move 

to the same extent as do fumigants, and so their application 

locations and spacing are critical. Although they could be 

used to treat the length of piles or beams, they may be better 

suited to protection of specific vulnerable areas such as near 

pile tops or the groundline, connections, and areas adjacent 

to 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. 

Research evaluating the mobility of boron from solid rods 

in above-ground softwood timbers suggests that rods would 

need to be placed no more than 50 mm (2 in.) apart across 

the grain and 300 mm (12 in.) apart along the grain. Somewhat tighter spacing may be needed for red oak. Substantial 

variability in boron mobility has been reported in timbers 

treated with combinations of liquid and solid internal 

treatments. However, the results indicate that spacing of 

approximately 75 mm (3 in.) across the grain and between 

75 and 125 mm (3–5 in.) along the grain would be needed to 

achieve overlapping boron penetration in southern pine timbers. The manufacturer of one of the boron rod products recommends parallel to the grain spacing of between 150 and 

380 mm (6–15 in.) depending on the size of the timber and 

the size of the rod installed. They recommend that the across 

Figure 32. A granular fumigant pre-packaged in a vaporpermeable membrane. 

Figure 33. A solid fumigant encapsulated in a metal tube. 

The cap is removed at installation.
Guide for In-Place Treatment of Covered and Timber Bridges

15

the grain distance between treatment holes not exceed 

150 mm (6 in.). 

Liquid borates may be applied in a manner similar 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 

do 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. The holes can be temporarily left 

unplugged to allow refilling as the liquid moves out of the 

treatment hole and into the wood in situations where the 

treatment holes are protected from precipitation and public 

access. 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 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 

was recommended, with 300 mm (12 in.) between holes 

along the grain and 100–150 mm (4–6 in.) across the grain. 

Likely penetration achieved using this approach would depend greatly on wood permeability. Risk of spillage into the 

area below the structure may be higher with this approach 

than with non-pressure applications because the treatment 

holes may cross seasoning checks.

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 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. 

In some instances, water-based external treatments that 

contain both non-diffusible 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. These products are typically grease-like in nature and will not run out of the wood as quickly or easily as 

non-diffusible liquids do.

There is less 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 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. Although fluoride is considered to be a 

diffusile preservative component, it may have slightly less 

mobility than boron. Fluoride tends to be a better fungicide 

than boron and would be expected to remain in the wood for 

a longer time if it is less mobile than boron.

Fumigants

To be most effective, a fumigant should be applied at locations where it will not leak away through checks 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 piles. Although these application instructions vary somewhat between products, they 

generally specify drilling holes of 19–22 mm (3/4–7/8 in.) 

diameter downward at angle of 45° to 60° through the 

center of the pile. The length of the hole is approximately 

2.5 times the radius of the pile. 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 piles. In terrestrial piles, the first hole is drilled at or 

slightly below the ground line. Subsequent holes are drilled 

higher on the pile, moving up and around the pile in a spiral 

pattern. Depending on the product and size of the pile, holes 

should be spaced at either 90° or 120° around the pile. 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 pile. Allowable uses of fumigants for aquatic piles are not always 

specified on the product labels, but at a minimum the lowest 

part of a treatment hole should be above the waterline, and 

considerable care should be taken, as most fumigants can be 

toxic to fish. 

There is much less information on application of fumigants 

to large timbers or glued-laminated beams. 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. The treatment holes can be 

drilled upward in a similar manner with the encapsulated 

solid fumigants. Solid fumigants provide a substantial 
General Technical Report FPL–GTR–205

16

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 liquid chemical out of the hole 

or impacting 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. 

Fumigants should not be applied into voids or when application holes intersect voids or checks, thus limiting the risk 

for accidental release of the product into the environment. 

Structures where fumigants have been applied should be 

marked to indicate its presence. Care should be taken in the 

removal of wood structures that have been treated with solid 

fumigants to ensure that the chemical has moved out of the 

treatment hole and into the surrounding wood. Some producers of solid fumigants have procedures for recovery of 

their tubes when a structure is removed. Consult the manufacturers of the formulation for specific information. 

Non-Diffusible Liquids

Non-diffusible liquid treatments, typically containing some 

form of copper, are sometimes used for internal treatments. 

Although these treatments do not diffuse in water within 

the wood, they can move for several inches parallel to the 

wood grain in permeable sapwood. 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 resistance to weathering 

is of greater importance than volume of wood protected. An 

example is bolt holes positioned in a manner that is likely 

to subject the hole to frequent wetting (in more sheltered 

locations, a concentrated water-diffusible treatment is likely 

to provide greater protection). Treatment holes can also be 

drilled above existing connectors, filled with preservative, 

and plugged. This type of treatment may be desirable if 

subsequent fabrication or construction activities will make 

that area difficult to access in the future. These preservative 

liquids may be used to flood internal voids such as decay 

pockets in poles and terrestrial piles, but the risk of spillage 

may make this type of application less suitable for aquatic 

applications. In addition, much of the chemical is absorbed 

by wood that is already decayed rather than adjacent sound 

wood. 

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 during fabrication. 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 gels, pastes, and 

water-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 the potential benefits from 

this approach must be weighed against the risks. In many 

cases, it may be more practical limit surface applications to 

localized areas. 

Water-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 tends to collect 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 preservative-based 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 

most beneficial in situations where penetration into the 

wood is less important than resistance to leaching. Perhaps 

the most obvious example is field-treatment of untreated 

wood exposed during fabrication of treated wood. Protection 

of pile tops is especially important, and in these situations a 

copper-containing solution should be applied to the exposed 

surface. Zinc naphthenate can also be used if a clear treatment is required. As discussed above, the non-diffusible 

liquids can also be applied after a diffusible treatment to 

slow leaching of the diffusible preservative and to provide 

long term protection. For example, a cut pile top can first be 

treated with a concentrated borate solution, and then treated 

with and oil-based copper after the borate solution has dried. 

At least one product uses pads soaked in a copper solution 

as part of a groundline wrap/bandage system. It is essential 

that any pile or pole top also be protected with a water shedding cap to prevent the wood from checking and allowing 

water and fungal spores to enter beyond the protected zone. 

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

protection of the ground-line area of support posts or piles 

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 
Guide for In-Place Treatment of Covered and Timber Bridges

17

formulation is brushed or troweled onto the exposed wood 

to form a 3–8-mm- (0.125–0.375-in.-) thick layer that extends 51–76 mm (2–3 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. Although these treatments are primarily used 

to supplement the groundline area of preservative-treated 

utility poles, they have also been shown to offer substantial 

protection to the groundline area of untreated wood. This 

type of system should 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 

simmilar barrier. For example, these products can be spread 

on to pile tops before flashing is applied, or on 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. 

Research on the Use of 

Supplemental Preservative 

Treatments for Covered Bridges

In 2001, Oregon State University conducted a study funded 

by the Federal Highway Administration (FHWA), “Identification of Preservative Treatments and Fumigants for 

Treating Historic Covered Bridges” (project DTFH61-

01-C-0059), which included both field and laboratory evaluations of remedial preservative treatments. The laboratory 

research compared the ability of numerous types of internal 

treatments to move through wood as a function of moisture content, wood species, and dosage. Movement of the 

water-diffusible preservatives BoraCare (boron, Nisus Corporation, Rockford, TN), ShellGuard (boron, Perma-Chink 

Systems, Inc., Knoxville, TN), Tim-bor (boron, Tim-bor 

Professional, Rockford, TN), CuRap 20 (boron, copper, ISK 

Biocides, Inc., Memphis,TN), Impel rods (boron, PRG, Inc., 

Rockville, MD) Cobra rods (boron, copper, Perma-Chink 

Systems, Inc., Knoxville, TN) and FluRods (fluoride, Osmose Utilities Services, Inc., Buffalo, NY) was evaluated, as 

were the fumigants dazomet (methylisothiocyanate), MITC 

(methylisothiocyanate) and chloropicrin (trichloronitromethane). Preservative mobility was compared for Douglas-fir, 

Southern Pine, eastern white pine, eastern hemlock, red oak, 

and white oak, and the diffusibles were evaluated at three 

wood moisture contents (30%, 60%, and 100%). As expected, movement of the water-diffusible preservatives was 

strongly related to moisture content, with relatively little 

diffusion noted at 30% MC. The study noted that because 

overall moisture contents in covered bridge members will be 

relatively low, care will be needed to place water-diffusible 

treatments where they are likely to be 

wetted. Diffusion was also positively correlated with 

concentration of chemical applied, both within and between 

types of preservatives. Wood species also affected diffusion, 

with the less permeable wood species having the lowest 

concentrations of actives at greater distances away from the 

treatment. This effect was particularly notable for impermeable white oak heartwood, which had only limited diffusion. 

Mobile concentrations of fluoride tended to be lower than 

those of boron, and diffusion of copper was very limited.

The laboratory evaluations revealed that fumigant moved 

quickly through blocks treated with MITC or chloropicrin, reaching maximum levels within one week and then 

declining as the volatile fumigant moved out of small test 

specimens. In contrast to the diffusible treatments, fumigant 

levels tended to be higher in the less permeable species such 

as Douglas-fir than in the highly permeable Southern Pine. 

This finding indicates that longer intervals between reapplication may be appropriate for covered bridges constructed 

with less permeable species such as Douglas-fir or white 

oak. Movement of fumigant from the blocks treated with 

dazomet was much slower, with only very low levels detected after one week, and slightly increased levels detected 

after 4 weeks. However, no fumigant was detected in some 

species treated with dazomet, and when concentrations were 

detected they were many times lower than the one-week 

concentrations observed for the MITC or chloropicrin-treated specimens.

The field portion of the research was conducted by installing two internal water-diffusible treatments (boron rods and 

fluoride rods) and two internal fumigant treatments (MITC 

and dazomet) in timbers in five covered bridges. The bridges 

were located in California, Vermont, Wisconsin, and Illinois 

and included timbers of white pine, spruce, Douglas-fir, and 

sugar pine. Mobility of the treatments was determined by 

assaying the treated timbers at 1 and 2 years after treatment. 

Sampling holes were drilled into the treated members at 

distances of 30, 60, and 90 cm (fumigant treatments) or 10, 

20, 30 cm (water-diffusible treatments) from each side of 

the treatment hole.

With few exceptions, no movement of boron and fluoride 

from the rods was detected in the field-treated bridges. 

Concentrations in assay samples were either not above 

background levels or not detected. The possible exceptions 

were low levels of fluoride detected in a few assay samples 

removed 10 cm from treatment holes after 2 years exposure 

in the California bridges. The general absence of boron and 

fluoride in the assay samples is in agreement with the lack 

of weight loss observed in the rods after 2 years exposure. 

The poor mobility observed in this study is probably attributable to the low moisture content of the bridge members. 

The highest moisture content detected in the members when 

the rods were placed in the bridge was 27%. Although the 

moisture content in the members likely fluctuates with precipitation events, it appears that moisture was never consistently elevated to the point that diffusion could occur from 

the rods.
General Technical Report FPL–GTR–205

18

In contrast to the water-diffusible treatments, MITC was 

detected in many of the samples removed from locations 

adjacent to the MITC treatments holes. Concentrations were 

generally greatest and most consistently elevated in samples 

removed from closest (30 cm) to the treatment holes, but 

elevated concentrations were also detected at distances of 

60 and 90 cm. Concentrations detected in samples removed 

from 4 of the 5 bridges were relatively similar. The highest concentrations after 1 year were detected in a California 

bridge located in hot, dry climate, while concentrations 

detected after 2 years were higher in the northern bridges. 

Sublimation of solid MITC is faster at higher temperatures 

and the higher temperatures at the warmer California location may have accelerated MITC release from the tubes. 

Weight losses measured after 2 years suggest that nearly all 

the MITC had been released from tubes at that bridge. Interestingly, MITC concentrations detected at the other California bridge were notably lower than for the other bridges. 

The reason for this is unclear, as the MITC weight loss from 

the tubes at this bridge after 2 years was similar to the other 

bridges.

None of the wood assay samples corresponding to the 

dazomet treatments contained detectable concentrations of 

MITC at any distance, bridge, or time point. Weight loss 

from the dazomet treatments was also minimal, indicating 

that little decomposition and release of MITC had occurred 

after 2 years. Dazomet requires moisture to decompose to 

MITC, and as with the water-diffusible treatments, the low 

wood moisture content may have limited the dazeomet’s 

mobility. Some suppliers recommend addition of accelerants 

to dazomet treatments to speed decomposition, which was 

not done in this study. It is possible that greater decomposition would have been observed with the use of these 

accelerants. 

The laboratory and field tests conducted in this study 

illustrate that movement of preservative away from solidrod water-diffusible treatments is highly dependent on 

moisture. Because the majority of covered bridge members 

are generally dry, the use of these solid water diffusibles 

would be most efficient if they were closely targeted to locations where moisture is suspected. The use of solid-waterdiffusible treatments in covered bridges may also be seen 

as a type of insurance against future moisture problems. In 

theory, if these moisture problems do occur, the preservative in the rods would become activated and spread into the 

moistened area. Movement from the rods could have also 

have been given an initial boost by adding water or a liquid 

borate solution (in the case of the boron rod) to the treatment hole. Of the two fumigants evaluated, the MITC tube 

was clearly the most effective at moving into the wood during this 2-year study. Because MITC and chloropicrin treatments do not rely on moisture for their mobility, they have 

greater potential for movement in dry bridge timbers. MITC 

from the MITC treatments in this study routinely moved 60 

cm (24 in.) from the treatment hole, suggesting that installation of this treatment with a spacing of 120 cm (48 in.) 

would provide for adequate protection of members. MITC 

was also detected at 90 cm (36 in.) from the treatment holes, 

but the concentrations detected were often below the 

20 ug/g thought to be needed to prevent growth by decay 

fungi. Average MITC concentrations increased during year 

2 of the study, suggesting that the treatments will be effective for at least 3 years. Research on utility poles indicates 

that MITC levels in wood decline gradually over time and 

fall below effective concentrations 5 to 7 years after treatment. In covered bridges, the longevity of the treatment will 

be less predictable because of the wide range of designs and 

member dimensions. However, the lack of soil contact for 

most covered bridge members should also slow fungal colonization once the fumigant has dissipated. 

In another study directly applicable to timber bridges, researchers at Mississippi State University (MSU) evaluated 

the efficacy of field treatments in protecting timber connections (joints) or pile sections. In the connections study the 

MSU researchers evaluated the installation of saturated felt 

pads within the joints, the application of preservatives to the 

joint surface, and the application of preservatives to holes 

drilled into the timber near the connection. The joint treatments were evaluated on southern pine, red oak, and yellow 

poplar wood species. The researchers found that pads saturated with a solvent based copper naphthenate solution were 

largely effective in protection the joint area from decay, 

while pads saturated with a solvent-based water repellent offered little protection. The efficacy of treatments applied to 

holes near the joint varied by wood species. Boron solution, 

boron rods, copper borate paste and liquid fumigant (33% 

sodium N-methyldithiocarbamate) all provided substantial 

protection for red oak, but the copper borate paste was less 

effective in protecting southern pine. None of the treatments 

applied to holes were effective in protecting yellow poplar 

specimens. Borate solution or copper borate paste applied 

directly to the outer surface of the connection was generally 

effective in protecting the joints, although the borate solution was much less effective in protecting the yellow poplar 

specimens. Copper borate paste applied to surface of the 

joint was the most effective overall treatment. 

The MSU researchers also evaluated the protection of untreated southern pine pile sections with both internal and external treatments. The internal treatments (liquid fumigant, 

boron rods or fluoride rods) were applied to holes drilled 

into the center of the piles, while the external treatments 

(copper borate paste, fluoride paste or a pentachlorophenol 

grease) were applied as groundline wraps. The exterior 

pastes applied as wraps were generally effective in protecting the groundline area of the piles. In contrast, the internal 

treatments were much less effective, although a combination 

of fluoride rods and fumigant provided moderate protection. 

The poor performance of the internal treatments probably 

resulted from their inability to protect the exterior of the 

pile, and it should be noted that internal treatments are typically applied to pressure-treated piles that have a protected 

exterior shell. 
Guide for In-Place Treatment of Covered and Timber Bridges

19

Table 1—Summary of supplemental preservative treatments properties and applications Applied as Actives 

Supplied

as Dilution 

EPA hazard

category Uses 

Mobility 

in wood 

Examples of 

trade name(s) 

Liquid 98% DOT Powder Dilute 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% DOT 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 water

based 

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% DOT, 

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

Water based RTU Caution Surface spray, brush, 

or foam, internal injection 

B high, Cu low Genics CuB 

Liquid Copper naphthenate, 

5% as Cu 

Water based 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 

Oil based 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 Zinc naphthenate, 

(1%–2% as Zn) 

Oil or water

based 

RTU Warning Surface spray or brush, 

poured into holes 

Low Jasco ZPW 

Liquid Copper-8-quinolinolate 

(0.675%) 

Oil based RTU Caution Surface spray or brush, 

poured into holes 

Low Outlast Q8 Log Oil 

Liquid 33% Sodium 

N-methyldithiocarbamate 

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% DOT, 

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 
General Technical Report FPL–GTR–205

20

Table 1—Summary of supplemental preservative treatments properties and applications—con. Applied as Actives 

Supplied

as Dilution 

EPA hazard

category Uses 

Mobility 

in wood 

Examples of 

trade name(s) 

Paper tube 98% Dazomet Paper tube 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 

Paste bandage 

70.6% Sodium fluoride Bandage RTU Danger Applied as self-contained 

bandage to groundline area of poles 

High Pole Wrap 

Saturated bandage 

17.7% Copper 

Naphthenate (2% Cu) 

Bandage RTU Caution Applied as self-contained 

bandage to groundline area of poles 

Low Cunap Wrap 

Bandage containing solid disks 

30%–60% DOT, 10%–30% Sodium fluoride 

Bandage RTU Caution Applied as self-contained 

bandage to groundline area of poles 

High BioGuard 

Gel 40% DOT Gel RTU Caution Internal, injected into holes High Jecta 
Guide for In-Place Treatment of Covered and Timber Bridges

21

Summary of Supplemental 

Treatment 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 to 

flood checks, exposed end-grain, bolt holes, etc. They may 

move several inches parallel to the grain of the wood if 

the member is allowed to soak in the solution.

•	Surface treatments with water-diffusible components will 

be leached 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 piles that are not usually 

exposed to standing water, but can also be applied to endgrain of connections or pile tops. 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 

Table 2—Application characteristics for internal preservative treatments

Type

of treatment 

Target retention 

in wood 

(oz/ft3

 or kg/m3)

Hole 

dimensions 

Spacing 

of treatment holes 

Diameter Length Posts/piles Timbers 

Boron rod 1.7–5, as DOT 5/16–13/16 in.

(8–21 mm) 

2.5–13 in. 

(64–330 mm)

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 DOT 1/4–3/4 in. 

(6–19 mm) 

1.5–5.5 in. 

(38–140 mm)

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 in.

(11–16 mm) 

3–5 in. 

(76–127 mm)

6 in. (152 mm) 

vertical, 

90–120° intervals 

Not described 

Borate, liquid glycol 1.1, as DOT 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 in. 

(13 mm) 

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 in. 

(25 mm) 

Variable Not described Not described 

Borax/CuNaph paste Not provided 3/4 in. 

(19 mm) 

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 DOT 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 in. 

(19–22 mm) 

Through 

center, 

12 in. 

(305 mm) 

minimum for 

MITC-fume 

6–12 in. 

(152–305 mm), 

90–120° intervals 

Maximum 

of 4 ft 

(1.23 m) 

along grain 
General Technical Report FPL–GTR–205

22

current or future concern. These treatments can be applied 

to smaller members in some situations.

•	Water-diffusible treatments move with moisture in the 

wood. They are generally easier to handle, but do not 

move for as great a distance as do fumigants and do not 

move in dry wood. The diffusion distance in moist wood 

is approximately 50–100 mm (2 to 4 in.) across the grain 

and 150–300 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 rod treatments provide a longer, slower 

release of chemical 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 terms of rate of release and 

longevity.

•	Fumigant treatments: Fumigants volatilize and 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.

•	Void-flooding treatments can be used where large decay 

pockets have been detected. Care must be used to avoid 

loss of these preservatives from the wood into the environment from checks or during the application process. 

Example Supplemental Treatment 

Applications 

Exposed End of Pile or Other Pressure-Treated 

Vertical Member 

The exposed end-grain of vertical members such as piles 

can be an entry point for precipitation or other sources of 

moisture. In some cases, these members are cut to height on 

site, potentially exposing untreated wood in the center of the 

member. These members can be supplemental treated using 

combinations of internal treatments and application of liquid 

or paste preservative to the top surface, along with a water 

shedding cap (Fig. 34). For relatively recent construction in 

which no decay is suspected, application of liquid or paste 

treatments to the top surface, followed by flashing or pile 

cap is probably sufficient. In older structures, supplemental 

treatments further down the member may be necessary, especially in areas of critical connections. If accessible, holes 

can be drilled vertically down from the top face and filled 

with some combination of liquids, rods, and/or pastes. Treatment holes may also be drilled downward at an angle from 

the vertical faces. Generally, treatment holes should extend 

through approximately 2/3 of thickness of the member. 

Ideally, internal treatments other than fumigants would be 

applied just above fasteners as the diffusion distance is typically greater downward than upward. Fumigant treatments 

can be applied above or below fasteners and at a greater 

Solution

Metal flashing/pile cap

Copper borate paste

250 mL

200 mL

150 mL

100 mL

50 mL

Figure 34. 

Example 

treatment 

of exposed 

end of pile 

or other 

pressuretreated 

vertical 

member. 
Guide for In-Place Treatment of Covered and Timber Bridges

23

spacing than diffusible treatments. Application of internal 

treatments can be continued down the pile to the water or 

soil line (Fig. 34). 

Covered Post/Pile

In situations where the top of the post/pile is capped or covered with another member (Fig. 35), treatment options are 

limited to internal treatments. Water-diffusible treatments 

can be placed into downward angled holes near the top of 

the post/pile cap, or rods can be applied to holes drilled horizontally under the cap. Fumigant treatments can be placed 

lower on the vertical support. 

A horizontal member supported on the pile can also be protected with internal treatments, but solid treatments may be 

the most practical option if limited access does not allow 

drilling of downward-angled holes. Rods can be installed 

in holes drilled horizontally into the member or into holes 

drilled upward from the underside the member. Solid fumigants can also be applied in this manner if the depth of the 

member allows it. 

Bridge Bents 

The piles and horizontal members associated with bents 

are areas of concern for application of supplemental treatments. Bridge bents are the substructure unit supporting 

each end of a bridge span. They are typically composed of 

two or more piles connected by a beam or girder that suppports the bridge. Typically the end of the horizontal support 

extends well beyond the bridge deck and is fully exposed to 

the precipitation, as is the end pile supporting that member. 

Ideally, the pile top was protected with a permanent cap, but 

this is not always the case, and sometimes the horizontal 

member is expected to provide sufficient protection. Access 

to the ends of bents is typically good, allowing flexibility 

in approaches to treatment. Internal treatments can be used 

to protect both the pile and the beam. Fumigants and/or a 

combination of rods, pastes, or liquids can be applied to 

treatment holes. If decay near the top of the pile is a concern, holes can be drilled horizontally into the pile near the 

top and treated with rods. The remainder of the pile can be 

treated down to the water line using holes drilled downward 

at an angle and filled with diffusible treatments. The remainder of the pile could also be treated with fumigants using 

fewer holes. Because fumigants have greater mobility, the 

first fumigant hole can be placed further from the top of the 

pile. The horizontal cap beam could also be protected using 

diffusible treatments and/or fumigants. For example, any 

of the forms of diffusible treatment could be applied to 

holes drilled down into the top of the beam approximately 

150 mm (6 in.) from the end. Fumigant could also be applied to a hole drilled into the top of the beam to provide 

protection that would extend back under the bridge deck. 

Glued-Laminated (Glulam) Member

In many ways a glulam stringer or beam can be viewed as 

a large timber, although the glueline can affect chemical 

250 mL

200 mL

150 mL

100 mL

50 mL

Figure 35. Example approach for supplemental 

treatment of piles and horizontal supports under the 

bridge deck at an abutment. 
General Technical Report FPL–GTR–205

24

movement. Areas of primary concern are at connections 

and fasteners such as at the bridge abutments (Fig. 36). If 

glulams are pressure-treated after lamination, they may have 

a sizable core of untreated wood in a manner similar to a 

large timber. However, glulam members are different from 

sawn timbers in some respects when applying supplemental 

preservative treatments. The depth of checking in gluedlaminated members in service is usually less than that in 

solid-sawn timbers, and there is typically less value in flooding checks or otherwise applying liquid preservatives to the 

surface. Although there is some evidence that preservatives 

can move across the glue lines, it is also reasonable to expect that the glue line will impede movement of preservative 

between laminates. Thus, even though the sides of gluedlaminated members are often accessible for drilling, drilling 

through the narrow faces and through as many laminates 

as possible is preferred. Unfortunately, this usually limits 

applications to the bottom face of the beam because the upper face is not accessible. Encapsulated fumigants are well 

suited for this situation. Multiple fumigant capsules can be 

used in one hole or void space can be left in the upper part 

of the hole because fumigant vapors can travel upward to fill 

this space. Drilling horizontal holes into the bottom laminate 

may be warranted if surface decay in the contact area of the 

abutment is suspected. These holes will be too small for fumigant application but could be treated with rods, pastes, or 

gels. A bead of paste or gel could also be applied along the 

edge of the beam, but it is uncertain how far chemical applied in this manner will move under the beam. 

Town Lattice Under Opening

Town lattice bridges present somewhat unique challenges 

for in-place treatment because lattice members have smaller 

(thinner) dimensions than many other types of covered 

bridge supports. Their narrow dimensions discourage water 

entrapment, but town lattice connections can trap moisture, 

especially in areas below bridge openings and in lower 

chords near the roadway. Internal treatments can be used 

to provide some protection for these connections. The diffusible internal treatments can be applied into the narrow 

face of each member on each side of the connection 

(Fig. 37). Rods can be purchased in various diameters allowing use of relatively small-diameter treatment holes. Liquids, pastes, and gels can also be applied to small-diameter 

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, and the high surface-to-volume ratio 

Figure 36. Example approach 

for supplemental treatment of 

glued-laminated beam contacting an abutment. 
Guide for In-Place Treatment of Covered and Timber Bridges

25

is likely to result in more rapid loss of fumigant from the 

member. 

Town Lattice Chord Connection

The bottom chord of a town lattice bridge near the road deck 

is in an area where wetting is possible, and the chord/lattice 

connection provides ample area for water entrapment. The 

large number of these connections and the fact that each 

connection involves six individual planks makes treatment 

a challenge (Fig. 38). One approach is to drill downwardsloping holes into each member to allow introduction of 

diffusible preservatives. Any combination of liquids, pastes, 

rods, or gels could be used. Treatment holes can be placed 

near the treenails, but not so close that they weaken the 

connection. In this situation, a surface-applied preservative 

liquid may also be beneficial. A clear solution such as a borate liquid could be used, with emphasis placed on working 

the liquid down in between the individual planks around the 

connection. Borate solutions are not fixed to the wood and 

may be washed off the wood surface with splashing from 

vehicular traffic. Depending on the configuration, the application of a more water-resistant preservative to the portion 

of the chord near the road deck may be a consideration. A 

clear zinc naphthenate solution is one possibility, or if the 

area is not highly visible, a copper-based solution could be 

used to provide additional protection. An especially vulnerable area of the chord nearest the road deck is the butt joint 

between two planks. The end-grain in this butt joint will 

readily absorb and hold water splashed from the roadway. 

Treatment solution can be worked into this joint where it 

will also be absorbed into the end-grain to provide protection from decay associated with wetting.

Members of a Burr Arch Bridge Below a Window 

Depending on the extent of roof overhang, configuration of 

the opening, and climate, areas below bridge openings may 

receive sufficient wetting to sustain decay. If the opening 

is low enough to serve as a viewpoint, the likelihood of the 

public viewing and touching these members is increased 

(Fig. 39). Typically, the water-trapping areas around connections are most likely to retain sufficient moisture to support 

decay. These areas can be treated using a combination of 

boron or copper boron internal treatments and a surface application of a liquid boron solution. Treatment hole plugs 

may attract attention and it may be desirable to use a type of 

plug, such as a driven wooden dowel that cannot be easily 

removed by vandals. 

250 mL

200 mL

150 mL

100 mL

50 mL

Figure 37. Example in-place treatment of 

town lattice members below a window 

opening. 
General Technical Report FPL–GTR–205

26

250 mL

200 mL

150 mL

100 mL

50 mL

Figure 38. Approach to in-place 

treatment of the connection 

of diagonal members to chord 

members at the bridge deck. 

250 mL

200 mL

150 mL

100 mL

50 mL

Figure 39. Example 

approach to in-place 

treating members of a 

Burr arch bridge exposed to wetting from 

a window opening. 
Guide for In-Place Treatment of Covered and Timber Bridges

27

Members Contacting Abutment

Areas where wooden bridge members contact stone or 

masonry abutments are among the most prone to decay or 

termites, and bridge designers often included sacrificial 

members in these areas that could be periodically replaced. 

In many cases, previous restorative work has addressed this 

issue by changing the contact point so that the untreated 

covered bridge timber rests on pressure-treated wood or 

some other type of support that is less conducive to moisture 

accumulation. However, untreated structural members in 

some bridges do rest on stone or masonry, and these can 

challenging but important areas to protect with field treatments (Fig. 40). Access is often limited, and unlike in most 

connections, the area of moisture accumulation is on an 

exterior surface that is inaccessible. 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 a water-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 (Fig. 41). 

Heel Connection of Diagonal Truss to Bottom Chord

Several covered bridge designs incorporate some type of 

heel connection where a diagonal support member rests on 

the bottom chord. There are many forms of this connection 

but often the diagonal sits in a notch in the bottom chord to 

prevent it from sliding along the chord (Fig. 42). These 

connections are well-suited to trapping and holding 

Figure 40. Example approach for inplace treatment of the area of a bridge 

chord contacting an abutment. 

Figure 41. Example of application of a copper-borate paste 

to the members contacting a bridge abutment. 
General Technical Report FPL–GTR–205

28

250 mL

200 mL

150 mL

100 mL

50 mL

Figure 42. Possible approaches to protection of heel connection with (a) fumigants only 

or (b) fumigant and borates 

targeted in the area of the 

connection.

(a)

(b)
Guide for In-Place Treatment of Covered and Timber Bridges

29

moisture if exposed to wetting. Of particular concern is 

the large area of end grain of the diagonal member where 

it rests in the notch. As is often the case, the wood in this 

connection could be treated using a range of approaches. 

Fumigants could be applied close to the connection in both 

the chord and diagonal members (Fig. 42a). Because the 

holes are oriented downward, liquid, granular, or encapsulated fumigants could be applied. Fumigants would have the 

advantage of protecting a large volume of wood, but may 

not be as efficient in targeting the notch area of the connection. If existing wetting/decay is suspected, application of 

diffusible preservatives in the areas of the connection may 

be beneficial (Fig. 42b). Holes can be drilled into the end of 

the diagonal member and flooded with borate solution with 

the objective of permeating the end-grain at the connection. 

Borate rods or gel could then be placed into the hole before 

plugging. Borate gel could also be applied around the perimeter of the connection.

Timber Frame Connection

Structural support timbers may be exposed to moisture either as result of the original design or loss of siding or 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 members, 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 inside the bridge (Fig. 43). 

Solid diffusible rods can be applied from beneath the large 

beams and angled upwards towards the connection. Downward sloping treatment holes can accommodate liquiddiffusible treatments or solid-diffusible treatments or both. 

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 do and are not dependent 

on localized moisture conditions for movement through the 

wood. However, their use may not be appropriate in many 

structures and particularly those with limited air exchange 

or human habitation. 

Immersion of Portions of Covered Bridge Substructure 

During Flooding 

Immersion of portions of the substructure during high water 

is an extraordinary circumstance. Brief immersions occurring rarely are unlikely to introduce sufficient water into 

a structure to sustain decay. However, prolonged or more 

frequent immersion could possibly be a concern if sufficient 

water is wicked into the end-grain of large support members 

at connections and around fasteners. Water absorbed into 

large members can be slow to dry and it is possible that sufficient moisture to support decay would exist if flooding occurred regularly. 

Because of the large volume of wood involved, potential 

treatments for this problem are complex. If the bridge is sufficiently damaged by the flooding that repair or rehabilitation is required, then serious consideration should be given 

to replacing the larger, more critical members with pressuretreated wood as part of this process. Members sent for 

pressure-treatment should be cut to length and pre-drilled 

prior to treatment to ensure that wood at the end grain and 

connector holes is treated. If no member replacement is warranted, existing members can be treated with borate products while still wet. The moisture in the wood will allow the 

borates to diffuse more deeply into the wood than might otherwise occur. Concentrated borate solutions can be brushed 

or sprayed onto the wood surface, with special emphasis on 

flooding checks as well as treating end-grain at joints and 

around connectors. Holes can be drilled near joints for installation of boron or fluoride rods and/or boron solutions.

Once the wood surface has dried, a penetrating waterrepellant finish can be applied to slow subsequent boron 

leaching.

Figure 43. Possible approach to treatment of a timber connection. 
General Technical Report FPL–GTR–205

30

Terrestrial Pile with Cross-Bracing

A common support configuration for the shoreline area of 

timber bridges is the placement of bents supported by two 

or more piles. Diagonal cross-bracing will often connect the 

piles (Fig. 44). In most cases, the piles will have been pressure-treated with preservative, resulting in a shell of protected wood surrounding a core of untreated wood. In this 

situation, the ground-contact area of the pile is most vulnerable especially if drying checks have penetrated through 

the treated zone to expose untreated wood. However, the 

area where the cross-bracing attachment penetrates through 

the treated shell can also be of concern if that area receives 

sufficient wetting. The large interior volume of wood associated with a pile can perhaps be most efficiently treated with 

fumigants because they move greater distances through the 

wood than water-diffusible treatments do. In this situation, 

fumigants can be applied to 19- to 22-mm- (3⁄4- to 7/8-in.-) 

diameter holes that are sloped downward at an angle of 

45° to 65°. The holes should extend through the center of 

the pile and to about 2/3 of the pile diameter. Treatment 

holes are started at or slightly below the groundline and 

continue up the pile in spiral pattern. The vertical distance 

between holes is typically 150–300 mm (6–12 in.) with the 

holes staggered by 90 to 120 degrees. The closer spacing 

is used for larger diameter piles. Because of the downward 

slope to the holes, liquid or granular fumigants can be 

applied. If exterior decay is a concern, the pile can also be 

protected with an external groundline treatment (see example “Groundline area of terrestrial pile” below).

Groundline Area of Terrestrial Pile

The external groundline area of terrestrial pots or piles 

can be treated using groundline wraps, or bandages, in a 

manner similar to that commonly used for utility poles 

(Fig. 45). Soil around the wood is excavated to depth of 

approximately 0.46 m (18 in.), and remaining soil and any 

decayed wood is scraped off the surface. The preservative 

paste is then applied from the bottom of the hole to slightly 

above the groundline using a stiff brush or trowel. 

A water-resistant wrap (often supplied with the paste) is 

then wrapped around the paste and stapled to the pile. The 

wrap serves the important purpose of trapping the paste 

against the wood, creating a reservoir of preservative that 

slowly diffuses into the wood. Alternatively, some products 

are supplied as prepared bandages with the preservative 

incorporated in a treated pad. The interior cover of the pad 

next to the wood is slit or otherwise opened immediately 

prior to installation. Although primarily used to replenish 

the groundline area of preservative-treated posts or piles, 

wraps can provide some level of protection to the groundline area of untreated wood. Decay will still occur above 

ground unless the upper portion of the member is protected 

from moisture. The labels of the paste products do not 

always explicitly provide their suitability for use relative to 

Figure 44. Example use of 

fumigants for internal treatment of bridge pile with 

cross-bracing. 
Guide for In-Place Treatment of Covered and Timber Bridges

31

the water levels of streams or rivers. However, they do state 

that the product should not be applied to areas with surface 

water or below the mean high water mark of intertidal areas. 

One product, MP400-EXT, also states that it should not be 

applied in aquatic environments. 

Who Can Apply Supplemental 

Preservative Treatments?

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

and thus are regulated by the EPA. The EPA regulations 

provide a minimum set of requirements, and each state may 

have additional requirements for use of a pesticide. The 

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. EPA regulations require that RUP applicators be 

certified as competent to apply RUPs 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, 

and 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.

States vary in their regulations about application of non-restricted 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 supplemental treatments 

to bridges by state, county, or local government employees 

can be somewhat of a grey area. Although these employees 

could be considered as 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 the state agency responsible for conducting 

the EPA’s pesticide applicator program. Contact information 

for each state can be found in Appendix. 

The EPA Pesticide Label is the Law

Pesticide product labels provide critical information about 

how to safely and legally handle and use pesticide products. 

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. Although all portions of the EPA label 

contain critical information, and the entire label should be 

read before use, some sections are particularly relevant for 

field application of preservatives:

Ingredient Statement

The ingredient statement identifies the name and the percentage by weight of each active ingredient and the percentage by weight (but generally not the names) of other/inert 

ingredients.

Net Contents/Net Weight

The net contents/net weight section identifies the weight or 

volume of pesticide in the container expressed in conventional U.S. units of measurement. It should not include any 

packaging materials.

Precautionary Statements

Precautionary statements are designed to provide the pesticide user with information regarding the toxicity, irritation, and sensitization hazards associated with the use of a 

pesticide, as well as treatment instructions and information 

to reduce exposure potential. Four kinds of precautionary 

statements may appear on a typical pesticide label.

1. Hazards to humans and domestic animals statement: 

Where a hazard exists to humans or domestic animals, 

Figure 45. Steps in application of an external groundline wrap treatment. 
General Technical Report FPL–GTR–205

32

precautionary statements that describe the particular hazard, route of exposure, and precautions to be taken must 

appear on the label.

2. First aid statement: This section of the label provides 

information to the pesticide user concerning appropriate 

first aid for the various routes of exposure associated with 

accidental exposure. If the first aid statement appears 

on the back panel, then there must be a statement on the 

front panel indicating that first aid information can be 

found on the back panel.

3. Environmental hazards statement: Where a hazard exists 

to non-target organisms, precautionary statements that 

identify the hazards and necessary precautions must appear on the label.

4. Physical or chemical hazards statement: Hazards such 

as flammability, explosive potential, or electric insulator breakdown, as well as the various precautions to be 

taken, must be identified, as applicable.

Directions for Use

Directions for use provide instructions to the user on how 

to use the product and identifies 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 (non-site-specific) precautions, restrictions, or limitations of the product 

are stated, as are any precautions and restrictions that apply 

to specific sites. An endangered species statement may also 

be included if applicable.

Worker Protection

The worker protection section provides information for 

the applicator to minimize their potential exposure to the 

supplemental treatment. The four types of worker protection 

statements that generally appear in the precautionary statements include the following:

1. Handler personal protective equipment (PPE) statement: 

Addresses handler PPE requirements such as gloves and 

safety glasses.

2. User safety requirement statements that address how 

to handle contaminated PPE: Provides instructions for 

cleaning and maintaining PPE, and sometimes for disposing of heavily contaminated PPE.

3. Engineering controls statement: Describes any reductions 

or modifications to handler PPE requirements that may be 

made in the presence of certain engineering controls (e.g., 

closed systems, enclosed cabs, lock and load containers).

4. User safety recommendations: Provides additional user 

safety information.

Storage and Disposal Instructions

Storage and disposal instructions provide instructions for 

storing the pesticide product and for disposing of any unused pesticide and the pesticide container.

Collection of Drill Shavings

When holes are drilled into pressure-treated wood to apply 

internal treatments, the resulting shavings contain preservative treatment. Because of their greater surface area to 

volume ratio, preservative leaching from these shavings 

is many times greater than from the treated wood itself. If 

the shavings generated during construction are allowed to 

enter the water below a treated wood structure, they make a 

disproportionately large contribution to the overall preservative release from that structure. However, this concern can 

be minimized if reasonable efforts are made to collect the 

shavings. Many approaches can minimize discharge of drill 

shavings into the environment. Tarps are commonly used 

and may be the most practical approach for collecting shavings from large numbers of treatment holes. However, the 

use of tarps becomes more difficult under windy or rainy 

conditions. Plastic bags, tubs, or trays are also useful collection devices for collecting shavings from individual treatment holes, and vacuum cleaners are also sometimes used. 

Regardless of the method used, it is inevitable that collection of construction debris will add some time and expense 

to a field treatment project. The importance of collection 

should be stressed in planning and budgeting for the project 

so that the application crew is clear that debris collection is 

an integral part of the job.

Fire Prevention

Fire is another serious threat for covered bridges and is a 

leading cause of loss and damage. Although some fires are 

accidental, many are set by vandals or arsonists. Fire is difficult to prevent, especially if intentionally set. The cause 

of the fire is relevant, as it affects the likelihood that steps 

taken to prevent fire damage will be successful.

Contributing Factors

All covered bridges are potentially vulnerable to fire, but 

several factors can increase the risk for damage.

Location

Location can influence vulnerability to fire in multiple ways. 

Bridges that are isolated or are on roads with light traffic can 

be more vulnerable to fire deliberately caused by arsonists. 

They can also become an attractive gathering place for parties, which can lead to accidental or purposeful fires. The 

remoteness of a location also increases the time before a 

fire is reported and the time required for responders to reach 

the bridge. Location in a hot, dry climate can also increase 

susceptibility to fire by lowering the moisture content of the 

wood and of any flammable material that has accumulated 

inside the bridge. It also increases the possibility of fire 

spreading to the bridge from external vegetation.

Bridge Design

Large heavy timbers are more difficult to ignite than smaller 

members with a higher proportion of surface area. Larger 

timbers also sustain less damage during brief exposure to 
Guide for In-Place Treatment of Covered and Timber Bridges

33

fire. Thus, in theory, bridge designs with larger members 

such as king post or Burr arch designs would be less vulnerable than would a bridge built with a town lattice design. 

However, the cladding on all bridges remains vulnerable 

and even the largest timbers can be ignited by a determined 

arsonist. Wood species can also play a role in both in ease of 

ignition and in the rate of flame spread. Among softwoods, 

some pine species tend to have greater flame spread than 

do species such as spruce, hemlock, or Douglas-fir. Among 

hardwoods, the higher density species such as oak tend to 

have lower flame spread than do less dense species such as 

yellow poplar. 

Accumulation of Organic Debris

The presence of large amounts of dry organic matter, such 

as leaves, can increase the susceptibility of bridges to accidental fire caused by cigarettes or sparks. Although it is 

not practical to keep a bridge completely free of this type of 

debris, obvious accumulations should be removed.

Selection of Roofing Material

In areas where wildfire is a concern, use of metal roofing 

or shakes/shingles protected with fire retardant (discussed 

below) can reduce the risk of ignition from wind-blown 

firebrands.

External Vegetation

Combustible vegetation should be cleared away from the 

bridge in areas where wild fire is a concern.

Protection with Fire-retardant Treatments

Wood products can be treated with fire retardants to improve 

their fire performance. Fire-retardant treatment results in delayed ignition, reduced heat release rate, and slower spread 

of flames, but it does not make the wood non-combustible. 

Fire-retardant treatment of wood generally improves the fire 

performance by reducing the amount of flammable volatiles 

released during fire exposure or by reducing the effective 

heat of combustion, or both. The wood may then self-extinguish when the primary heat source is removed. 

In terms of fire performance, fire-retardant treatments are 

marketed to improve the flame spread characteristics of the 

wood products as determined by standardized flammability 

tests. These tests are used to generate a flame-spread index 

(FSI), which is a measure of the overall rate of flame spreading in the direction of air flow. Lower FSI values correspond 

to a lower rate of flame spread. In the building codes, the 

classes for FSI are A (FSI of 0 to 25), B (FSI of 26 to 75), 

and C (FSI of 76 to 200). The classification labels of I, II, 

and III have sometimes been used instead of A, B, and C. 

The FSI for most domestic wood species is between 90 and 

160. Some domestic softwood species meet the Class B FSI 

without treatment. Other domestic softwood species have 

FSIs near the upper limit of 200 for Class C. All available 

data for domestic hardwoods are for Class C. Fire-retardant 

treatments are necessary when a Class A flame spread index 

is desired for a wood product.

A wood product labeled as “fire-treated wood” for purpose 

of compliance with applicable sections of the building code 

must meet certain performance requirements in the codes 

and related specifications (AWPA & NFPA). The fire performance requirement for FRT wood is that its FSI is 25 or 

less when tested according to the ASTM E 84 flame-spread 

test and that it shows no evidence of significant progressive 

combustion when this 10-min test is continued for an additional 20 min. In addition, it is required that the flame front 

in the test shall not progress more than 3.2 m beyond the 

centerline of the burner at any given time during the test.

Fire-retardant treated roof covering materials are designated 

Class A, B, or C based on their performance in a different 

type of test as well as their use with other building material. 

The roofing material protocol includes intermittent flame 

exposure, spread of flame, burning brand, flying brand, and 

rain tests. Each of the three classes has a different version 

of the pass–fail test. The Class A test is the most rigorous. 

Class C the least. FRT wood shingles and shakes are available that carry a Class B or C fire rating. The Class A rating 

can be achieved by placing FRT shingles over specific types 

of underlayment assemblies. 

Fire-retardant treatments that contain boron can potentially 

impart some degree of preservative protection. Some products make claims about preservative efficacy while others 

do not, even though they may contain boron. As with boronbased wood preservatives, the efficacy of these treatments 

against decay and insects depends on the concentration of 

boron delivered to the wood as well as the penetration of 

boron into the wood. Boron-containing FRTs applied by 

pressure treatment would be expected to provide the greater 

preservative efficacy than surface treatments. 

Pressure Treatment with Fire Retardants

To be most effective, wood members should be pressureimpregnated with fire-retardants in a manner similar to 

the processes used to produce preservative-treated wood. 

However, considerably heavier absorptions of chemicals are 

necessary for fire-retardant protection. Penetration of chemicals into the wood depends on species, wood structure, and 

moisture content. Because some species are difficult to treat, 

the degree of impregnation needed to meet the performance 

requirements for FRT wood may not be possible.

Inorganic salts are the most commonly used fire retardants 

for interior wood products, and their characteristics have 

been known for more than 50 years. These salts include 

monoammonium and diammonium phosphate, ammonium 

sulfate, zinc chloride, sodium tetraborate, and boric acid. 

Guanylurea phosphate is also used. Traditional fire-retardant 

salts are water soluble and are leached out in exterior applications or with repeated washings. However, water-insoluble organic fire retardants have been developed to meet 

the need for leach-resistant systems. These water-insoluble 

systems include (a) resins polymerized after impregnation 

into wood and (b) graft polymer fire retardants attached 
General Technical Report FPL–GTR–205

34

directly to cellulose. An amino resin system based on urea, 

melamine, dicyandiamide, and related compounds is of the 

first type. 

Wood pressure-treated with fire-retardants can be considered for replacement members in covered bridges. Pressure 

treatment will generally provide greater protection than the 

surface-applied treatments. Negatives associated with fireretardant pressure treatment include increased cost as well 

as some reduction in mechanical properties. Fire-retardanttreated wood is often more brash than untreated wood. For 

structural applications, information on mechanical properties of the FRT wood product needs to be obtained from 

the treater or chemical supplier. 

In-Place FRT Applications (Coatings)

Commercial coating products are available to reduce the 

surface flammability characteristics of wood. The two types 

of coatings are intumescent and nonintumescent. The widely 

used intumescent coatings “intumesce” to form an expanded 

low-density film upon exposure to fire. This multicellular 

carbonaceous film insulates the wood surface from high 

temperatures. Intumescent formulations include a dehydrating agent, a char former, and a blowing agent. One potential 

dehydrating agent is polyammonium phosphate. Ingredients 

for the char former include starch, glucose, and dipentaerythritol. Potential blowing agents for the intumescent coatings include urea, melamine, and chlorinate parafins. Nonintumescent coating products include formulations of the 

water-soluble salts such as diammonium phosphate, 

ammonium sulfate, and borax.

Because coatings are not pressure impregnated or incorporated during manufacture, fire-retardant coated wood is not 

FRT wood as defined in most codes or standards including 

National Fire Protection Association (NFPA) 703. In NFPA 

703, a fire-retardant coating is defined as a coating that reduces the flame spread of Douglas-fir and all other tested 

combustible surfaces to which it is applied by at least 50% 

or to a flame spread classification value of 75 or less, 

whichever is the lesser value, and has a smoke developed 

rating not exceeding 200 when tested in accordance with 

ASTM E 84 or UL 723. There is no requirement that the 

standard test be extended for an additional 20 min as required for FRT wood. NFPA 703 differentiates between a 

Class A coating 

as one that reduces flame spread index to 25 or less and a 

Class B coating as one that reduces flame spread index to 

75 or less.

Fire-retardant coatings for wood are tested and marketed 

to reduce flame spread. Clear intumescent coatings are 

available. Such coatings allow the exposed appearance of 

old structural wood members to be maintained while providing improved fire performance. This is often desirable in the 

renovation of existing structures such as covered bridges. 

Studies have indicated that coatings subjected to outdoor 

weathering are of limited durability and would need to be 

reapplied on a regular basis.

Although their use to improve the resistance ratings of wood 

products has been investigated, there is no general acceptance for using coatings to improve the fire resistance rating 

of a wood member. There is a lack of full-scale ASTM E 

119 test data to demonstrate their performance and validate 

a suitable calculation methodology for obtaining the rating.

Even pressure-applied fire-retardant treatments should not 

be considered an absolute solution for the threat of fire to 

covered bridges. A determined arsonist will still be able 

to cause substantial damage to a bridge treated with fire 

retardants. However, FRTs should lessen the risk or extent 

of damage from accidental fires or from less determined 

vandals. 

Research on the Use of Supplemental 

Fire-Retardant Treatments for Covered 

Bridges

In 2000, Mississippi State University conducted a study 

funded by the FHWA (Fire Retardant Treatments for Historic Covered Bridges, contract DTFH61-00-C-0017) investigating the potential of several types of pressure-treatment 

applied FRTs and an on-site borate spray for fire protection 

of wood used in covered bridges. One exterior and one interior commercial fire-retardant were used to pressure-treat 

specimens of Southern Pine, yellow poplar, and white oak. 

The treated specimens were then evaluated for strength 

properties and for relative fire performance using a cone 

calorimeter. Additional specimens were spray-treated with a 

borate solution to simulate potential in-place FRT treatment, 

and subsequently tested for fire-retardant properties. 

The mechanical properties of the specimens pressure-treated 

with fire retardants were reduced in comparison to the controls, especially with the exterior fire retardant. Work to 

maximum load (WML) suffered the greatest impact, with 

approximate 50% reduction noted for Southern Pine and 

yellow poplar and 24% reduction in white oak. Modulus 

of rupture (MOR) was reduced by about 25% in Southern 

Pine and yellow poplar but only slightly reduced in white 

oak. However, the FRT treatments had little effect on stiffness (modulus of elasticity). These reductions in mechanical 

properties are worthy of consideration, but do not exclude 

the use of pressure FRT in many applications. 

Cone calorimeter tests indicated that pressure applied fireretardant treatments substantially reduced heat release and 

substantially increased time to ignition, for both southern 

pine and yellow poplar. Estimates of the flame spread index 

(FSI) indicated that the pressure-treated Southern Pine and 

yellow poplar would meet the specifications for Class I fireretardant material (FSI below 25). The white oak specimens 

were less well protected by pressure treatment, probably 

because white oak is resistant to pressure treatment and 

absorbed less fire-retardant. The estimated FSI for white 
Guide for In-Place Treatment of Covered and Timber Bridges

35

oak was lowered from 75 for the untreated specimens to 

between 43 and 47 for the treated specimens. The results of 

this portion of the study indicate that when covered bridge 

members are to be replaced, pressure-treatment of the replacement members with a fire retardant may be justified.

The borate spray applications were much less effective in 

reducing heat release or increasing time to ignition, although 

yellow poplar showed some improvement. The average 

estimated FSI for yellow poplar was lowered from over 

130 for untreated specimens to between 77 and 91 for the 

spray-treated specimens. Average estimated FSI decreases 

for Southern Pine and white oak were less evident, perhaps 

because both species have lower estimated FSI values for 

untreated wood. It is difficult to determine if the marginal 

fire-retardant benefits seen with borate spray treatments 

would have practical benefits in protecting covered bridges 

from fire. It should be noted that the borate formulations 

evaluated in this MSU study were commercial wood preservative formulations, and not commercial fire-retardant 

formulations. The results of this portion of the study indicate 

that if fire retardancy is the primary goal, then in-place application of treatment intended for this purpose, such as an 

intumescent coating, may provide greater benefit than application of borates. 

The overall conclusions of the authors are that (1) replacement bridge members should be pressure-treated with fire 

retardant, and (2) spraying the interior of covered bridges 

with borates solutions is worthwhile. They based the latter recommendation on the ease of application, possible 

increase in fire retardancy, and additional benefits such as 

preservative protection and possible protection of metal 

fasteners from corrosion. 

Fire Protection Technology

For more integrated protection, FRT of bridge members may 

be combined with other forms of protection such as lights, 

alarms, sprinklers and monitoring systems. 

Lighting

Installation of lighting is the least expensive deterrent to 

vandalism. However, lighting may not be a great deterrent 

in remote locations, and could increase the use of the bridge 

as a gathering place.

Cameras

Cameras can be deterrents to vandalism, especially with 

accompanying warning signs. However, they can also be 

targets of vandalism, and only serve as a fire detection tool 

if the camera is being actively monitored.

Alarms

Alarms based on smoke or heat detection can be configured 

to alert a local fire department and/or activate warning sirens. Smoke detectors may require frequent maintenance 

to remove dust. Heat detectors must be broadly dispersed 

to ensure that heat is detected before substantial damage 

has occurred. Heat-detecting cables (linear heat detectors) 

can be used for this purpose. The value derived from the 

sensor-alarm combination depends largely on the potential 

responses to the alarm. Major damage can only be prevented 

if fire suppression crews can reach the bridge quickly.

Sprinkler Systems

Sprinkler systems that are automatically activated by heat or 

smoke sensors provide the most immediate fire suppression, 

but are also costly to install. Typically, pumping stations 

must be installed to service the sprinkler system.

Remote Monitoring

In another study funded by the FHWA, researchers at the 

FPL and Iowa State University evaluated the use of remote 

monitoring based on newer technologies using flame detectors, fiber optic sensors, and infra-red cameras. The type of 

flame detector evaluated was designed to detect fire based 

on the light wavelength spectra of a burning flame. Field 

tests found that the flame detector detected flame within a 

bridge within 5–7 s. Detection did not appear to be limited 

by distance between the flame and detector, but was limited 

by line of sight interference (i.e., a bridge member between 

the flame and detector). The fiber optic sensors also detected 

flame within 5–8 s, but only if the flame source was located 

within a few feet of the detector. The infra-red camera was 

mounted on a pole approximately 300 ft from the bridge 

and could only monitor one end of the bridge and the approach. It detected fire in those locations within 10 s, and 

most cases in less than 5 s. It could also be programmed to 

detect the heat signal of the human body, which may have 

value in alerting authorities to other types of vandalism. As 

was noted with other types of alarms, these new monitoring 

technologies will not prevent damage unless coupled with a 

sprinkler system or rapid response from a local fire station. 

Conclusions

Wooden bridges, whether historic covered bridges or current 

highway timber bridges, can be vulnerable to damage from 

biodegradation and fire. This manual describes procedures 

for selecting and applying in-place treatments to bridges to 

prevent or arrest degradation. Although the guide focuses on 

preservative treatments to protect against biodeterioration, 

approaches to minimizing damage caused by fire are also 

discussed.

Efficiently protecting bridges requires some understanding of the causes of deterioration. Decay fungi are the most 

common cause of deterioration and are commonly grouped 

into brown-rot, white-rot, and soft-rot fungi. Although these 

groups of fungi differ in their preferences for wood species 

and environmental conditions, they all require moisture to 

colonize wood. Insects, especially subterranean termites, 

can also be important causes of deterioration in warmer climates. Termites prefer moist wood, but can also degrade 

dry wood if a source of moisture is available. In general, 
General Technical Report FPL–GTR–205

36

emphasis should be placed on protecting wood from moisture, with use of preservative treatments focused on those 

areas where moisture cannot be controlled. Vulnerable areas 

in covered bridges include members contacting abutments, 

members near the ends of bridges subject to wetting from 

splashing, and members below windows or other openings 

that allow entry of wind-blown precipitation. Pressuretreated timber bridge members can be vulnerable when 

untreated wood beneath the treated zone is exposed by field 

fabrication or by the development of large drying checks. In 

older structures, the external ground contact area of treated 

members may need supplemental treatment. 

The objective of an in-place treatment is to distribute preservative into areas of a structure that are vulnerable to moisture accumulation and/or not protected by the original pressure treatment. Types of field treatments range from finishes 

(coatings), to boron rods, to fumigants. 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, some 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. Surface treatments 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. Pastes 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. Treatments can also be applied 

to smaller members in some situations. Water-diffusible 

internal treatments move through moisture in the wood. 

They are relatively easy to handle but do not move for as 

great a distance as do fumigants, nor do they 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.

Fire is another serious threat for covered bridges and is 

a leading cause of loss and damage. All covered bridges 

are potentially vulnerable to fire, but several factors can 

increase the risk for damage. Bridges that are in isolated 

areas can be more vulnerable to fire deliberately caused by 

arsonists. Bridges in dry climates are more vulnerable to 

wildfire and accumulation of dry organic matter, such as 

leaves, within the bridge. Dry vegetation near the bridge can 

increase fire vulnerability. 

Bridge members can be treated with fire retardants to improve their fire performance. Fire-retardant treatment (FRT) 

results in delayed ignition, reduced heat release rate, and 

slower spread of flames, but it does not make the wood 

noncombustible. Fire-retardant treatment of wood generally 

improves the fire performance by reducing the amount of 

flammable volatiles released during fire exposure or by reducing the effective heat of combustion, or both. The wood 

may then self-extinguish when the primary heat source is 

removed. 

To be most effective, covered bridge replacement members 

should be pressure-impregnated with fire retardants in a 

manner similar to preservative-treated wood. Negatives 

associated with fire-retardant pressure treatment include increased cost as well as some reduction in mechanical properties. Fire-retardant-treated wood is often more brash than 

untreated wood. For structural applications, information on 

mechanical properties of the FRT wood product needs to be 

obtained from the treater or chemical supplier. 

In-place coating products are available to reduce the surface flammability characteristics of wood. The two types of 

coatings are intumescent and nonintumescent. The widely 

used intumescent coatings intumesce (expand abnormally) 

to form an expanded low-density film upon exposure to 

fire. This multicellular carbonaceous film insulates the 

wood surface below from high temperatures. Intumescent 

formulations include a dehydrating agent, a char former, 

and a blowing agent. Clear intumescent coatings are available. Such coatings allow the exposed appearance of old 

structural wood members to be maintained while providing 

improved fire performance. However, studies have indicated 

that these systems would need to be reapplied on a regular 

basis if exposed to weathering.

Even pressure-applied fire-retardant treatments should not 

be considered an absolute solution for the threat of fire to 

covered bridges. A determined arsonist can cause substantial 

damage to a bridge treated with fire retardants. For more 

integrated protection, FRT of bridge members may be combined with other forms of protection such as lights, alarms, 

sprinklers, and monitoring systems. 
Guide for In-Place Treatment of Covered and Timber Bridges

37

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Militz, H. 1991. Diffusion of bifluorides and borates from 

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Morrell, J.J.; Corden, M.E. 1986. Controlling wood deterioration with fumigants: a review. Forest Products Journal. 

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Phares, Brent M.; LaViolette, Michael D.; Wipf, Terry J.; 

Ritter, Michael A. 2010. Remote monitoring of historic 

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inspection and maintenance. EM 7700–8. Washington, DC: 

U.S. Department of Agriculture, Forest Service. 944 p. 

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with Timbor and creosote or copper naphthenate. 20 years 

of exposure in AWPA hazard zone 4. Crossties. November/

December. p. 20, 22.

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wood decks. In: Proceedings, Annual Meeting American 

Wood Protection Association. Birmingham, AL. 104: 

30–43.

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crossties... a 20 year field test. In: Proceedings, Annual 

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the service life of wooden crossties by using pre- and supplemental preservative treatments. 15 year exposure report. 

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structures. In: Schultz, T.; Militz, H.; Freeman, M.; Goodell, 

B.; Nicholas, D. (eds.). Development of commercial wood 

preservatives: efficacy, environmental, and health issues. 

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Society, Chapter 3. 

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with a copper-borate groundline treatment for poles. Doc. 

no. IRG/WP30017. Stockholm, Sweden: International Research Group on Wood Preservation, IRG Secretariat.

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windows, doors, and adjacent framing. In: Proceedings, 

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Guide for In-Place Treatment of Covered and Timber Bridges

39

Alabama

http://www.agi.state.

al.us/

http://www.agi.

state.al.us/pesticide_

management

Department of Agriculture

Division of Plant Protection and 

Pesticides

PO BOX 3336

Montgomery, AL 36109-0336

(334) 240-7171

Alaska

http://www.state.

ak.us/dec/eh/index.

htm 

http://www.state.

ak.us/dec/eh/pest/

index.htm

Alaska Department of 

Environmental Conservation

Pesticide Program

1700 E. Bogard Rd. Building B 

Suite 202

Wasilla, AK 99654

(907) 376-1870 or 1-800-478-

2577 (In-State Only)

Arizona

http://www.azdeq.

gov/

http://www.azda.gov/

ESD/esd.htm

Arizona Department of 

Agriculture

Environmental Services

1688 W Adams

Phoenix, AZ 85007

(800) 223-0618 (602) 255-3664 

(602) 542-3579

Arizona Structural Pest Control

9545 East Double Tree Ranch 

Rd

Scottsdale, AZ 85258

Arkansas

http://www.

plantboard.org/

http://www.

plantboard.org/

pesticides_licensing.

html

Arkansas State Plant Board

Division of Feeds, Fertilizers 

and Pesticides

#1 Natural Resource Dr

PO BOX 1069

Little Rock, AR 72205

(501) 225-1598

California

http://www.cdpr.

ca.gov/ 

http://www.cdpr.

ca.gov/docs/license/

liccert.htm

California Department of 

Pesticide Regulation

1001 I Street 

Sacramento, CA 95812

Contact: County Agric. 

Commissioner

Colorado

http://www.ag.state.

co.us

http://www.colorado.

gov/cs/Satellite/

Agriculture

Colorado Department of 

Agriculture

Division Plant Industry

700 Kipling St Suite 4000

Lakewood, CO 80215-5894

(303) 239-4140

Connecticut

http://dep.state.ct.us/

http://dep.state.

ct.us/wst/prgactiv.

htm#Pesticides

Connecticut Department 

Environmental Protection

Pesticide Division

79 Elm St

Hartford, CT 06106

(860) 424-3369

Delaware

http://www.state.

de.us/deptagri/index.

htm

http://dda.delaware.

gov/pesticides/index.

shtml

Delaware Department of 

Agriculture

2320 South Dupont Hwy

Dover, DE 19901

(800) 282-8685 (302) 739-4811

Florida

http://www.

doacs.state.fl.us/

environment.html 

http://www.flaes.org/

pesticide/index.html

Florida Department of 

Agriculture & Consumer 

Services

Bureau of Entomology and 

Pesticides

644 Cesery Boulevard, Suite 

200

Jacksonville, FL 32211

(904) 727-6592

Georgia

http://rules.sos.

state.ga.us/pages/

GEORGIA

http://agr.georgia.

gov/portal/site/AGR/

menuitem.

http://agr.georgia.

gov/portal/site/AGR/

menuitem.

Georgia Department Agriculture

Pesticide Division

19 Martin Luther King Dr SW

Atlanta, GA 30334

(404) 656-9378

Appendix—Contact Information for State Offices Conducting the U.S. PA’s 

Certified Pesticide Applicator (CPA) Program
General Technical Report FPL–GTR–205

40

Hawaii

http://hawaii.gov/

hdoa

http://hawaii.gov/

hdoa/pi/pest 

Hawaii Department of 

Agriculture

Division of Plant Industry

Pesticides Branch

1428 S King St

Honolulu, HI 96814-2512

(808) 973-9401

Idaho

http://www.agri.state.

id.us/

http://www.agri.state.

id.us/Categories/

Pesticides/

Idaho Department of Agriculture

Division of Agricultural 

Resources

PO BOX 7723, Boise, ID 83707

2270 Old Penitentiary Rd.

Boise, ID 83712

(208) 332-8590

Illinois

http://www.agr.state.

il.us/ 

http://www.agr.state.

il.us/Environment/

Pesticide

http://www.idph.state.

il.us/

Illinois Department of 

Agriculture

Bureau of Environmental 

Programs

PO BOX 19281

Springfield, IL 62794-9281

(217) 785-2427 (800) 641-3934

Illinois Department of Public 

Health

Division of Environmental 

Health

Structural Pest Control Program

525 West Jefferson

Springfield, IL 62761

Indiana

http://www.isco.

purdue.edu/

http://www.isco.

purdue.edu/pesticide/

Office of Indiana State Chemist

Purdue University

175 S. University St.

West Lafayette, IN 47907-1154

(765) 494-1594

Iowa

http://www.

agriculture.state.ia.us/

http://www.

iowaagriculture.gov/

pesticides.asp

Iowa Department of Agriculture

Pesticide Bureau

Wallace State Office Bldg.

502 E. 9th Street

Des Moines, IA 50319

(515) 281-5321

Kansas

http://www.

accesskansas.org/kda/ 

http://www.ksda.gov/

pesticides_fertilizer/

Kansas State Board of 

Agriculture

Pesticide and Fertilizer Program

109 SW 9th Street, 3rd Floor

Topeka, KS 66612 (785) 296-

3786

Kentucky

http://www.kyagr.

com/ 

http://www.kyagr.

com/consumer/

envsvs/testing/index.

htm

Kentucky Department of 

Agriculture

Division of Pesticides

107 Corporate Dr.

Frankfort, KY 40601

(502) 573-0282

Louisiana

http://www.ldaf.state.

la.us/ 

http://www.ldaf.state.

la.us/portal/Offices/

Louisiana Department of 

Agriculture

Pesticide & Environmental 

Programs

PO BOX 3596

Baton Rouge, LA 70821-3596

(225) 925-3763

Maine

http://www.maine.

gov/agriculture/index.

shtml

http://www.state.

me.us/agriculture/

pesticides/

Maine Department of 

Agriculture

Pesticides Control

State House Station 28

Augusta, ME 04333

(207) 287-2731

Maryland

http://www.mda.state.

md.us/ 

http://www.mda.state.

md.us/plants-pests/

Maryland Department of 

Agriculture

Pesticide Regulation Section

50 Harry S Truman Parkway

Annapolis, MD 21401

(410) 841-5700

Massachusetts

http://www.state.

ma.us/dfa/ 

http://www.state.

ma.us/dfa/pesticides/

index.htm 

Massachusetts Department of 

Agriculture

Pesticides Bureau

251 Causeway St. Suite 500

Boston, MA 02114

(617) 626-1720
Guide for In-Place Treatment of Covered and Timber Bridges

41

Michigan

http://www.michigan.

gov/mda 

http://www.michigan.

gov/mda

Michigan Department of 

Agriculture

Pesticide and Plant Pest 

Management Division

PO BOX 30017

Lansing, MI 48909

(800) 292-3939 (517)-241-6666

Minnesota

http://www.mda.state.

mn.us/ 

http://www.mda.state.

mn.us/plants.aspx

Minnesota Department of 

Agriculture

Pesticide and Fertilizer 

Management Division

625 Robert St. N

St. Paul, MN 55155

(612) 201-6615 (800) 627-3529

Mississippi

http://www.mdac.

state.ms.us http://

http://www.mdac.

state.ms.us/n_library/

Mississippi Department of 

Agriculture

Bureau of Plant Industry 

PO BOX 5207

Mississippi State, MS 39762

(662) 325-7763

Missouri

http://mda.mo.gov/

http://mda.mo.gov/

plants/pesticides/

Missouri Department of 

Agriculture

Bureau of Pesticide Control

PO BOX 630, 1616 Missouri 

Blvd.

Jefferson City, MO 65102

(573) 751-5504

Montana

http://agr.state.mt.us/ 

http://agr.

mt.gov/pestfert/

pesticidePrograms.asp

Montana Department of 

Agriculture

Agricultural Sciences Division

302 North Roberts

Helena, MT 59620-0201

(406) 444-5400

Nebraska

http://www.agr.state.

ne.us/

http://www.agr.state.

ne.us/division/bpi/

pes/pest1.htm 

Nebraska Department of 

Agriculture

Bureau of Plant Industry

301 Centennial Mall

Lincoln, NE 68509

(402) 471-2394

(800) 831-0550

Nevada

http://agri.state.nv.us/ 

http://agri.nevada.

gov/PCO_index.htm 

Nevada Department of 

Agriculture

Plant Industry Division

405 South 21st Street

Sparks, NV 89431

(775) 353-3600

New Hampshire

http://agriculture.

nh.gov/ 

http://www.nh.gov/

agric/divisions/

pesticide

New Hampshire Department of 

Agriculture, Markets and Food

Division of Pesticide Control

PO BOX 2042, 25 Capitol St. 

2nd Floor

Concord, NH 03302-2042

(603) 271-3550

New Jersey

http://www.state.

nj.us/dep/

http://www.state.nj.us/

dep/enforcement/

New Jersey Department of 

Environmental Protection

Bureau of Pesticide Operations

22 South Clinton Ave. 

PO BOX 402

Trenton, NJ 08625-0411

(609) 530-4070

New Mexico

http://nmdaweb.nmsu.

edu/

http://nmdaweb.

nmsu.edu/pesticides/

applicators

New Mexico Department of 

Agriculture

Pesticide Bureau

MSC 3189 BOX 30005 

Las Cruces, NM 88003-8005

(575) 646-2134 (800) 222-1222

New York

http://www.dec.

ny.gov/

http://www.dec.

ny.gov/permits/209.

html

New York Department of 

Environmental Conservation

Bureau of Pest Management

625 Broadway

Albany, NY 12233-7254

(518) 402-8748

North Carolina

http://www.agr.state.

nc.us/division.htm

http://www.ncagr.

com/fooddrug/

pesticid/ 

North Carolina Department 

of Agriculture & Consumer 

Services

Structural Pest Control & 

Pesticide Divistion

1090 Mail Service Center

Raleigh, NC 27699-1090

(919) 733-3933
General Technical Report FPL–GTR–205

42

North Dakota

http://www.

agdepartment.com/ 

http://www.

agdepartment.com/

programs/

North Dakota Department of 

Agriculture

Pesticide, Feed and Fertilizer 

Division

600 E Boulevard Ave. Dept. 602

Bismark, ND 58505-0020

(701) 328-4922

Ohio

http://www.

ohioagriculture.gov/ 

http://www.agri.ohio.

gov/apps/odaprs/

Ohio Department of Agriculture

Pesticide and Fertilizer 

Regulation Section

8995 E Main St

Reynoldsburg, OH 43068

(800) 282-1955 (614) 728-6987

Oklahoma

http://www.state.

ok.us/~okag/ 

http://www.state.

ok.us/~okag/cps

Oklahoma Department of 

Agriculture

Division Plant Industry

2800 N Lincoln Blvd

Oklahoma City, OK 73105-4298

(405) 521-3864

Oregon

http://www.oregon.

gov/ODA/ 

http://www.oregon.

gov/ODA/PEST/

index.shtml 

Oregon Department of 

Agriculture

Pesticides Division

635 Capitol St NE

Salem, OR 97310-0110

(503) 986-4635

Pennsylvania

http://www.

agriculture.state.

pa.us/ 

http://www.

agriculture.state.

pa.us/portal/

Pennsylvania Department of 

Agriculture

Bureau of Plant Industry

2301 N Cameron St

Harrisburg, PA 17110-9408

(717) 787-4843

Rhode Island

http://www.dem.

ri.gov/

http://www.dem.

ri.gov/programs/

bnatres/agricult/

Rhode Island Department of 

Environmental Mgmt

Division of Agriculture

235 Promenade St.

Providence, RI 02908

(401) 222-2781

South Carolina

http://agriculture.

sc.gov/

http://dpr.clemson.

edu/

South Carolina Department of 

Agriculture

Pesticide Regulation

Clemson University

511 Westinghouse Rd.

Pendleton, SC 29670 (864) 646-

2150

South Dakota

http://sdda.sd.gov/

http://sdda.sd.gov/

Ag_Services/

Agronomy_Services

South Dakota Department of 

Agriculture

Division of Agricultural 

Services 

523 E Capitol, Foss Bldg

Pierre, SD 57501

(800) 228-5254 (605) 773-4432

Tennessee

http://www.state.

tn.us/agriculture/ 

http://www.state.

tn.us/agriculture/

regulatory/aip

Tennessee Department of 

Agriculture

Pesticide and Agricultural Inputs

440 Hogan Rd, Bruer Bldg

Nashville, TN 37204

(615) 837-5148

Texas

http://www.agr.state.

tx.us/agr/index

http://www.tda.state.

tx.us/spcs/consumer/

IPM

Texas Department of 

Agriculture - Pesticide Division

PO BOX 12847

Austin, TX 78711

(512) 463-7622 (800) 835-5832

Utah

http://ag.utah.gov/

http://ag.utah.gov/

divisions/plant/

pesticide/

Utah Department of Agriculture 

and Food

350 N Redwood Rd

PO BOX 146500

Salt Lake City, UT 84114-6500

(801) 538-7188

Vermont

http://www.state.

vt.us/agric/

http://www.

vermontagriculture.

com/ARMES/

Vermont Agency of Agriculture

Agrichemical Management

116 State St

Montpelier, VT 05620

(802) 828-3482
Guide for In-Place Treatment of Covered and Timber Bridges

43

Virginia

http://www.vdacs.

virginia.gov/

http://www.vdacs.

virginia.gov/

pesticides

Virginia Department of 

Agriculture

Office of Pesticide Services

102 Governor St. 1st Floor

Richmond, VA 23219

(804) 371-6558 (800) 552-9963

Washington

http://agr.wa.gov/ 

http://agr.wa.gov/

PestFert/default.htm 

Washington Department of 

Agriculture

Pesticide Management Division

PO BOX 42560

Olympia, WA 98504-2560

(360) 902-2010 (877) 301-4555

Washington, 

D.C.

http://ddoe.dc.gov/

ddoe/site/

http://ddoe.

dc.gov/ddoe/cwp/

iew,a,1209,q,494917.

asp

Environmental Regulation 

Administration

Department of Consumer and 

Regulatory Affairs

1104 4th St. SW

Washington, DC 20024

(202) 442-4307

West Virginia

http://www.

wvagriculture.org/ 

http://www.

wvagriculture.org/

Division 

West Virginia Department of 

Agriculture

Pesticide Regulatory Program

1900 Kanawha Blvd E

Charleston, WV 25305

(304) 558-2209

Wisconsin

http://datcp.state.

wi.us

http://www.datcp.

state.wi.us/core/

insectspesticides

Wisconsin Department of 

Agriculture, Trade and 

Consumer Protection

Agricultural Resources Mgmt 

Division

PO BOX 8911, 2811 Agriculture 

Dr

Madison, WI 53708-8911

(608) 224-4500

Wyoming

http://wyagric.state.

wy.us/

http://wyagric.state.

wy.us/divisions

Wyoming Department of 

Agriculture

Technical Services Division

2219 Carey Ave

Cheyenne, WY 82002

(307) 777-7321