29 PRESERVATION 

BRIEFS 

The Repair, Replacement, and 

Maintenance of Historic Slate Roofs 

Jeffrey S. Levine 

Introduction 

U.S. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

Slate is one of the most aesthetically pleasing and durable 

of all roofing materials. It is indicative at once of the 

awesome powers of nature which have formed it and the 

expertise and skill of the craftsman in hand-shaping and 

laying it on the roof. Installed properly, slate roofs require 

relatively little maintenance and will last 60 to 125 years 

or longer depending on the type of slate employed, roof 

configuration, and the geographical location of the 

property. Some slates have been known to last over 200 

years. Found on virtually every class of structure, slate 

roofs are perhaps most often associated with institutional, 

ecclesiastical, and government buildings, where longevity 

is an especially important consideration in material 

choices. In the slate quarrying regions of the country, 

where supply is abundant, slate was often used on farm 

and agricultural buildings as well. 

Because the pattern, detailing, and craftsmanship of slate 

roofs are important design elements of historic buildings, 

they should be repaired rather than replaced whenever 

possible. The purpose of this Preservation Brief is to assist 

property owners, architects, preservationists, and building 

managers in understanding the causes of slate roof 

failures and undertaking the repair and replacement of 

slate roofs. Details contributing to the character of historic 

slate roofs are described and guidance is offered on 

maintenance and the degree of intervention required at 

various levels of deterioration. 

The relatively large percentage of historic buildings roofed 

with slate during the late nineteenth and early twentieth 

centuries means that many slate roofs, and the 60 to 125 

year life span of the slates most commonly used, may be 

nearing the end of their serviceable lives at the end of 

the twentieth century. Too often, these roofs are being 

improperly repaired or replaced with alternative roofing 

materials, to the detriment of the historic integrity and 

appearance of the structure. Increased knowledge of the 

characteristics of slate and its detailing and installation on 

the roof can lead to more sensitive interventions in which 

original material is preserved and the building'S historic 

character maintained. Every effort should be made to 

replace deteriorated slate roofs with new slate and to 

develop an effective maintenance and repair program for 

slate roofs that can be retained. 

History of Slate Use in the United States 

Although slate quarrying was not common in the United 

States until the latter half of the nineteenth century, slate 

roofing is known to have been used prior to the Revolution. 

Archeological excavations at Jamestown, Virginia,have 

unearthed roofing slate in strata dating from 1625-1650 

and 1640-1670. Slate roofs were introduced in Boston as 

early as 1654 and Philadelphia in 1699. Seventeenth 

century building ordinances of New York and Boston 

recommended the use of slate or tile roofs to ensure 

fireproof construction. 

In the early years of the Colonies, nearly all roofing slate 

was imported from North Wales. It was not until 1785 

that the first commercial slate quarry was opened· in the 

United States, by William Docher in Peach Bottom 

Township, Pennsylvania. Production was limited to that 

which could be consumed in local markets until the 

middle of the nineteenth century. Knowledge of the 

nation's abundant stone resources was given commercial 

impetus at this time by several forces, including a rapidly 

growing population that demanded housing, advances in 

quarrying technology, and extension of the railroad system 

to previously inaccessible markets. Two additional factors 

helped push the slate industry to maturity: the immigration 

of Welsh slate workers to the United States and the 

introduction of architectural pattern and style books 

(Figure 1). Slate production increased dramatically in the 

years following the Civil War as quarries were opened in 

Vermont, New York, Virginia, and Lehigh and 

Northampton Counties, Pennsylvania. By 1876, roofing 

slate imports had all but dried up and the United States 

became a net exporter of the commodity. 

1 
Figure 1. Architectural pattern books of the mid-nineteenth centun) 

awakened Americans to the availability and quality of slate for roofing 

pLi rposes In) incorporating slate roofs ill their designs. Design XX, 

'/\ French Roof House," in A. J. Downing's Victorian Cottage 

Residences is shown. 

The U.S. roofing slate industry reached its highest point in 

both quantity and value of output in the period from 1897 

to 1914. In 1899, there were over 200 slate quarries 

operating in 13 states, Pennsylvania historically being the 

largest producer of all. The decline of the U.S. r~ofing 

slate industry began c.191S and resulted from several 

factors, including a decline in skilled labor for both the 

fabrication and installation of slate and competition from 

substitute materials, such as asphalt shingles, which could 

be mass produced, transported and installed at a lower 

cost than slate. Only recently, with the increasing 

popularity of historic preservation and the recognition of 

the superiority of slate over other roofing materials, has 

slate usage begun to increase. 

The Character and Detailing of Historic 

Slate Roofs 

During some periods of architectural history, roof design 

has gone far beyond the merely functional and contributed 

much to the character of buildings. Roofs, by their 

compelling forms, have defined styles and, by their 

decorative patterns and colors, have imparted both dignity 

and beauty to buildings. The architectural styles prevalent 

during the latter half of the nineteenth and early twentieth 

centuries placed strong emphasis on prominent roof lines 

and greatly influenced the demand for slate. Slate, laid in 

multi-colored decorative patterns, was particularly well 

suited to the Mansard roofs of the Second Empire style, 

the steeply pitch roofs of the Gothic Revival and High 

Victorian Gothic styles, and the many prominent roof 

planes and turrets associated with the Queen Anne style. 

The Tudor style imitated the quaint appearance of some 

English slates which, because of their granular cleavage, 

are thick and irregular. These slates were often laid in a 

graduated pattern, with the largest slates at the eaves and 

the courses diminishing in size up the roof slope, or a 

textural pattern (Figure 2). Collegiate Gothic style 

buildings, found on many university campuses, were 

often roofed with slate laid in a graduated pattern. 

The configuration, massing, and style of historic slate 

roofs are important design elements that should be 

preserved. In addition, several types of historic detailing 

2 

were often employed to add visual interest to the roof, 

essentially elevating the roof to the level of an ornamental 

architectural element. When repairing or replacing a slate 

roof, original details affecting its visual character should 

be retained. 

Before repairing or replacing an existing slate roof, it is 

important to document the existing conditions and 

detailing of the roof using written, visual, and physical 

evidence so that original features can be identified and 

preserved. Documentation should continue through the 

repair or replacement process as significant details, long 

obscured, are often rediscovered while carrying out these 

activities. Local histories, building records, old receipts 

and ledgers, historic photographs, sketches, and paintings, 

shadow lines and nail hole patterns on the roof deck, and 

bits of historic material left over from previous interventions 

(often found in eave cavities) are all useful sources of 

information which can be of help in piecing together the 

original appearance of the roof. Size, shape, color, texture, 

exposure, and coursing are among the most important 

characteristics of the original slates which should be 

documented and matched when repairing or replacing an 

historic slate roof. 

Historically, three types of slate roofing-standard, 

textural, and graduated-were available according to the 

architectural effect desired. St~ndard grade slate roofs 

were most common. These are characterized by their 

uniform appearance, being composed of slates 

approximately 3/16" (0. Scm) thick, of consistent length 

and width, and having a smooth cleavage surface. Thirty 

different standard sizes were available, ranging from 10" 

(2Scm) x 6" to 24" x 14" (lScm x 61cm x 3Scm). The 

slates were laid to break joints and typically had square 

ends and uniform color and exposure. Patterned and 

polychromatic roofs were created by laying standard slates 

of different colors and shapes on the roof in such a way as 

to create sunbursts, flowers, sawtooth and geometric 

designs, and even initials and dates (Figure 3). On 

utilitarian structures, such as barns and sheds, large gaps 

were sometimes left between each slate within a given 

course to reduce material and installation costs and 

provide added ventilation for the interior (Figure 4). 

Figure 2. The quaint character of this Tudor style residence is derived, 

ill part, from its textural roof. 
Figure 3. A sawtooth geometric design composed of red, green, and 

black slates makes this roof the most visually importallt feature of the 

buildillg. 

Textural slate roofs incorporate slates of different 

thicknesses, uneven tails, and a rougher texture than 

standard slates. Textural slate roofs are perhaps most often 

associated with Tudor style buildings where slates of 

different colors are used to enhance the effect. 

Graduated slate roofs were frequently installed on large 

institutional and ecclesiastical structures (Figure 5). The 

slates were graduated according to thickness, size, and 

exposure, the thickest and largest slates being laid at the 

eaves and the thinnest and smallest at the ridge. Pleasing 

architectural effects were achieved by blending sizes and 

colors. 

Detailing at the hips, ridges and valleys provided added 

opportunity to ornament a slate roof. Hips and ridges can 

be fashioned out of slate according to various traditional 

schemes whereby the slates are cut and overlapped to 

produce a watertight joint of the desired artistic effect. 

Traditional slate ridge details are the saddle ridge, strip 

saddle ridge, and comb ridge, and for hips, the saddle 

hip, mitered hip, Boston hip, and fantail hip (Figure 6). 

A more linear effect was achieved by cpvering the ridges 

and hips with flashing called "cresting" or "ridge roll" 

formed out of sheet metal, terra cotta, or even slate 

(Figure 7). Snow guards, snow boards, and various types 

of gutter and rake treatments also contributed to the 

character of historic slate roofs (Figure 8). 

Two types of valleys were traditionally employed, the 

open valley and the closed valley. The open valley is lined 

with metal over which slates lap only at the sides. Closed 

valleys are covered with slate and have either a continuous 

metal lining or metal flashing built in with each course. 

Open valleys are easier to install and maintain, and are 

generally more watertight than closed valleys. Round 

valleys are a type of closed valley with a concave rather 

than V-shaped section (Figure 9). Given the broader 

sweep of the round valley, it was not uncommon for 

roofers to interweave asphalt saturated felts rather than 

copper sheet in the coursing in order to cut costs. 

Figure 4. Widely spaced opell slating was often used on utilitariall 

structures where ventilation was desirable. It provided an interesting 

textu re and visual pattern to often plain structit res. 

Figure 5. This graduated slate roof is composed of large, thick slates at 

the eave which are reduced in size and thickness as the slating 

progresses to the ridge. 

Although principally associated with graduated and 

textural slate roofs, round valleys were infrequently 

employed due to the difficulty and expense of their 

installation. 

Common types of sheathing used include wood boards, 

wood battens, and, for fireproof construction on 

institutional and government buildings, concrete or steel 

(Figure 10). Solid wood sheathing was typically 

constructed of tongue and groove, square edged, or 

ship lapped pine boards of 1" (2.5 cm) or 1 1/4" (3 cm) 

nominal thickness. Boards from 6" (15 cm) to 8" (20 cm) 

wide and tongue and groove boards were generally 

preferred as they were less likely to warp and curl. 

Wood battens, or open wood sheathing, consisted of wood 

strips, measuring from 2" (5 cm) to 3" (7.5 cm) in width, 

nailed to the roof rafters. Spacing of the battens depended 

on the length of the slate and equaled the exposure. Slates ' 

were nailed to the batten that transected its mid-section. 

The upper end of the slate rested at least 1/2" (1.25 cm) on 

the batten next above. Open wood sheathing was employed 

primarily on utilitarian, farm, and agricultural structures 

in the North and on residential buildings in the South 

where the insulating value of solid wood sheathing was 

not a strict requirement. To help keep out dust and wind 

driven rain on residential buildings, mortar was often 

placed along the top and bottom edge of each batten, a 

practice sometimes referred to as torching. 

3 
4 

Various Roofing Details 

Figure 6. Hips are formed at the external angle of two roofing slopes. In (a), the hips at both the roof and the dormer are covered with metal. Note also the 

open valley and the built-in gutter. In (b), the dormer hip slates have been laid in a fantail pattern to help shed water. Note also the metal ridge cap. 

Figure 7. Ridge caps and cresting can be elaborate. Ridges are formed at the long horizontal juncture of two roofing slopes and capping protects this joint 

from moisture. In (a), a terra cotta capping with a decorative profile complements the finial over the various roof peaks. In (b), the mansard roof has a decorative iron cresting at the break between the lower and the upper roof slopes. 

Figure 8. Eave details include snow guards, snow boards, gutter treatments. Snow guards are generally used in areas where ice and snow accumulate to 

avoid dangerous slides from the roof. In (a) the snow guards are set in two staggered rows above a pole gutter. In (b), the copper wire snow guards are set 

more frequently up a very steep gable. 
Figure 9. Valleys are formed at the internal angle of two roofing slopes. Flashing is often placed under the slate to increase moisture protect at this 

vulnerable joint. Shown in (a) is a closed valley where the slates are held tight to the valley line. (b) illustrates of a round valley where the transition 

between the two slopes is a continuous curve. It requires careful workmanship and an experienced roofer. 

(a) TYPICAL SLATE 

WIDTH 

] LAP 

+ HOLES + 

LENGTH 

(b) TYPICAL SLATE ON OPEN LATH 

WEDGE 

(d) SLATE ON IRON PURLIN (NORCROSS) 

SCREW 

Fig ure 10. (a) shows a typical slate; exposure may be calculated btJ 

subtracting the headlap from the total length of the slate and dividing by 

two. Slates were typically nailed either to closed wooden decking or to open laths (b). In the late 19th century, with the concern for fireproof 

construction, special fa steners were developed to sew re slate to steel 

pu r!ins (c) 1881 patent to James G. Hill; (d) 1889 patent to Orla ndo W 

Norcross). 

Steel angles substituted for the wood battens in fireproof 

construction. The slates were secured using wire wrapped 

around the steel angle, where it was twisted-off tight. 

Alternately, any of a variety of special fasteners patented 

over the years could have been used to attach the slate to 

the steel angle (Figure 10). On roofs with concrete decks, 

slates were typically nailed to wood nailing strips 

embedded in the concrete. 

Beginning in the late nineteenth century, asphalt saturated 

roofing felt was installed atop solid wood sheathing. The 

felt provided a temporary, watertight roof until the slate 

could be installed atop it. Felt also served to cushion the 

slates, exclude wind driven rain and dust, and ease slight 

unevenness between the sheathing boards. 

Slate was typically laid in horizontal courses starting at the 

eaves with a standard headlap of 3" (7.5 cm) (Figure 10). 

Headlap was generally reduced to 2" (5 cm) on Mansard 

roofs and on particularly steep slopes with more than 20" 

(50 cm) of rise per 12" (30 cm) of run. Conversely, headlap 

was increased to 4" (10 cm) or more on low pitched roofs 

with a rise of 8" (20 cm) or less per 12" (30 cm) of 

horizontal run. The minimum roof slope necessary for a 

slate roof was 4" (10 cm) of rise per 12" (30 cm) of run. 

Where Does Slate Come From? 

Slate is a fine grained, crystalline rock derived from 

sediments of clay and fine silt which were deposited on 

ancient sea bottoms. Superimposed materials gradually 

consolidated the sedimentary particles into bedded 

deposits of shale. Mountain building forces subsequently 

folded, crumpled, and compressed the shale. At the same 

time, intense heat and pressure changed the original clays 

into new minerals such as mica, chlorite, and quartz. By 

such mechanical and chemical processes bedded clays 

were transformed, or metamorphosed, into slate, whole 

5 
CLEAVAGE PLANE 

Cutting 

Original block is crosscut with a wet diamond blade saw 

A hammer and chisel are used to split the block to the proper dimension along the grain 

Splitting 

Shingles or tiles are created with a hammer and chisel by systematically halving and 

splitting the smaller block along the cleavage plane 

Figure 11. Slate roofing tiles or shingles are still manufactured using 

traditional methods brought to this country by Welsh immigrants in 

the nineteenth century. Shown above are the first 3 steps of cutting, 

sculping and splitting. Once the rough slate tile is made it is trimmed 

and punched for holes. 

geologic ages being consumed in the process. Slates vary 

in composition, structure, and durability because the 

degree to which their determinant minerals have been 

altered is neither uniform nor consistent. 

The adaptation of slate for roofing purposes is inextricably 

linked to its genesis. The manufacturing processes of 

nature have endowed slate with certain commercially 

amenable properties which have had a profound influence 

on the methods by which slate is quarried and fabricated 

(Figure 11), as well as its suitability for use as a roofing tile. 

Slate roofing tiles are still manufactured by hand using 

traditional methods in a five step process: cutting, 

sculping, splitting, trimming, and hole punching. In the 

manufacturing process, large, irregular blocks taken from 

the quarry are first cut with a saw across the grain in 

sections slightly longer than the length of the finished 

roofing slate. The blocks are next sculped, or split along 

the grain of the slate, to widths slightly larger than the 

widths of finished slates. Sculping is generally 

accomplished with a mallet and a broad-faced chisel, 

6 

although some types of slate must be cut along their 

grain. In the splitting area, the slightly oversized blocks 

are split along their cleavage planes to the desired shingle 

thickness. The splitter's tools consist of a wooden mallet 

and two splitting chisels used for prying the block into 

halves and repeating this process until the desired 

thinness is reached (Figure 12). The last two steps involve 

trimming the tile to the desired size and then punching 

two nail holes toward the top of the slate using a formula 

based on the size and exposure of the slate. 

Minerals, the building blocks of rocks, through their 

characteristic crystalline structures define the physical 

properties of the rocks which they compose. Slate consists 

of minerals that are stable and resistant to weathering and 

is, therefore, generally of high strength, low porosity, and 

low absorption. The low porosity and low absorption of 

slate mitigate the deleterious action of frost on the stone 

and make it well adapted for roofing purposes. The two 

most important structural properties of slate are cleavage 

and grain. 

The metamorphic processes of geologic change necessary 

to produce slate are dependent upon movements in the 

earth's crust and the heat and pressure generated thereby. 

For this reason, slate is found only in certain mountainous 

regions. The most economically important slate deposits 

in this country lie in the Mid-Atlantic and Northeastern 

states transversed by or bordering on the Appalachian 

Mountain chain. Variations in local chemistry and 

conditions under which the slate was formed have 

Figure 12. III the splitting area, the slightly oversized blocks are split 

along their cleavage planes. The splitter's tools consist of a woodell 

mallet alld splitting chisels. The process of halvillg the split portiolls 

is repeated until a tile or shingle of appropriate thilll1ess is obtailled. 
Figure 13. Paper thin lamination can be seen flaking off of this weathered, 120 year piece of Pennsylvania Hard-Vein slate. 

produced a wide range of colors and qualities and 

ultimately determine the character of the slate found in 

these areas. 

Slate is available in a variety of colors. The most common 

are grey, blue-grey, black, various shades of green, deep 

purple, brick red, and mottled varieties. The presence of 

carbonaceous matter, derived from the decay of marine 

organisms on ancient sea floors, gives rise to the black 

colored slates. Compounds of iron generate the red, 

purple, and green colored slates. 

Generally, the slates of Maine, Virginia, and the Peach 

Bottom district of York County, Pennsylvania are deep 

blue-black in color. Those of Virginia have a distinctive 

lustrous appearance as well due to their high mica 

content. The slates of Lehigh and Northampton Counties, 

Pennsylvania, are grayish-black in color. Green, red, 

purple, and mottled slates derive from the New YorkVermont district. The slate producing region of New York, 

which centers around Granville and Middle Granville, is 

particularly important because it contains one of the few 

commercial deposits of red slate in the world. 

Slates are also classified as fading or unfading according 

to their color stability. Fading slates change to new shades 

or may streak within a short time after exposure to the 

atmosphere due to the presence of fine-grained 

disseminated pyrite. For example, the "weathering green" 

or "sea-green" slates of New York and Vermont are grayish 

green when freshly quarried. Upon exposure, from 20% to 

60% of the slates typically weather to soft tones of orangebrown, buff, and gray while the others retain their original 

shade. Slates designated as unfading maintain their 

original colors for many years. 

Color permanence generally provides no indication of 

the durability of slate. Rather, time has shown that the 

Vermont and New York slates will last about 125 years; 

Buckingham Virginia slates 175 years or more; and 

Pennsylvania Soft-Vein slates in excess of 60 years; 

Pennsylvania Hard-Vein slates and Peach Bottom slates, 

neither of which is still quarried, had life spans of roughly 

100 and at least 200 years respectively. The life spans 

provided should be used only as a general guide in 

determining whether or not an existing slate roof is 

nearing the end of its serviceable life. 

Ribbons are visible as bands on the cleavage face of slate 

and represent geologic periods during which greater 

amounts of carbonaceous matter, calcite, or coarse quartz 

particles were present in the sediment from which the 

slate was formed. Ribbons typically weather more and 

were most common in Pennsylvania slate quarries. As 

they were not as durable as clear slates, ribbon slate is no 

longer manufactured for roofing purposes. Mottled grey 

slates from Vermont are the closest match for Pennsylvania 

ribbon slate available today. 

In recent years, slates from China, Africa, Spain and other 

countries have begun to be imported into the United 

States, primarily for distribution on the West Coast. The 

use of imported slates should probably be limited to new 

construction since their colors and textures often do not 

match those of U.S. slate. 

Deterioration of Slate and Slate Roofs 

The durability of a slate roof depends primarily on four 

factors: the physical and mineralogical properties of the 

slate; the way in which it is fabricated; installation 

techniques employed; and, regular and timely 

maintenance. The first three of these factors are examined 

below. The maintenance and repair of slate roofs are 

discussed in later sections of this Brief. 

The natural weathering of roofing slate manifests itself as 

a slow process of chipping and scaling along the cleavage 

planes (Figure 13). Paper thin laminations flake off the 

surface of the slate and the slate becomes soft and spongy 

as the inner layers begin to come apart, or delaminate. 

The nature of the sound given off by a slate when tapped 

with one's knuckles or slating hammer is a fair indication 

of its condition. High-grade slate, when poised upon the 

fingertips and struck, will emit a clear, solid sound. 

Severely weathered slates are much less sonorous, and 

give off a dull thud when tapped. 

The weathering of slate is chiefly due to mineral 

impurities (primarily calcite and iron sulfides) in the slate 

which, in concert with alternating wet/dry and hot/cold 

cycles, react to form gypsum (Figure 14). Because gypsum 

Figure 14. Th e white blotches on these Pennsylvania Soft-Vein slates 

indicate areas where gypsum is leaching out Ol1to the surface of the slates. 

7 
8 

Figure 15. View of the underside of slates laid on open sheathing shows 

that delamination and flaking is just as bad or worse on the underside of 

slates as on the exposed surface. This is why most slates cannot be 

flipped over for reuse. 

molecules take up about twice as much volume as calcite 

molecules, internal stresses result from the reaction, 

causing the slate to delaminate. This type of deterioration 

is as prominent on the underside of the roof as on the 

exposed surface due to the leaching and subsequent 

concentration of gypsum in this area (Figure 15). 

Consequently, deteriorated roofing slates typically cannot 

be flipped over and re-used. 

The chemical and physical changes which accompany 

slate weathering cause an increase in absorption and a 

decrease in both strength and toughness. The tendency of 

old, weathered slates to absorb and hold moisture can 

lead to rot in underlying areas of wood sheathing. Such 

rot can go undetected for long periods of time since, often, 

there is no accompanying leak. Due to their loss of 

strength, weathered slates are more prone to breakage, 

loss of corners, and cracking. 

Slates with low calcite content tend to weather slowly. 

Dense slates, with low porosity, likewise decay slower 

than slates with equal calcite, but with a greater porosity. 

The pitch of a roof can also affect its longevity. The steeper 

the pitch, the longer the slate can be expected to last as 

water will run off faster and will be less likely to be drawn 

under the slates by capillary action or driven under by 

wind forces. Spires and the steep slopes of Mansard roofs 

often retain their original slate long after other portions of 

the roof have been replaced. Areas of a roof subject to 

concentrated water flows and ice damming, such as along 

eaves and valleys, also tend to deteriorate more rapidly 

than other areas of the roof. 

Mechanical agents, such as thermal expansion and 

contraction and the action of frost, are subordinate in the 

weathering of slate, coming into play only after the slate 

has been materially altered from its original state by the 

chemical transformation of calcite to gypsum. The more 

rapid deterioration of slates found on roof slopes with the 

most severe exposure to the sun, wind, and rain (typically, 

but not always, a southern exposure) may be attributable 

to the combined result of the deleterious effects of impurities 

in the slate and mechanical agents. Atmospheric acids 

produce only negligible deterioration in roofing slate. 

It is difficult to assess the procedures by which a piece of 

slate has been fabricated without visiting the quarry and 

observing the process first hand. The location and size of 

nail holes, grain orientation, the condition of corners, and 

the number of broken pieces are all things which may be 

observed in a shipment of slate to judge the quality of its 

fabrication. Nail holes should be clean and with a shallow 

countersink on the face of the slate for the nail head; grain 

oriented along the length of the slate; and, corners left 

whole. An allowance for 10% breakage in shipment is 

typically provided for by the quarry. 

Installation problems often involve the improper nailing 

and lapping of slates. The nailing of slates differs from 

that of other roofing materials. Slate nails should not be 

driven tight as is the case with asphalt and wood shingles. 

Rather, they should be set such that the slate is permitted 

to hang freely on the nail shank. Nails driven too far will 

crack the slate and those left projecting will puncture the 

overlying slate (Figure 16). Nail heads left exposed accelerate 

roof deterioration by providing a point for water entry. 

Non-ferrous slater's nails, such as solid copper or stainless 

steel, should always be used since plain steel and galvanized 

nails will usually rust out long before the slate itself begins 

to deteriorate. The rusting of nineteenth century cut nails 

is a common cause of slate loss on historic roofs. 

When joints are improperly broken (i.e., when slates lap 

the joints in the course below by less than 3" [7.5 cm]), it 

is possible for water to pass between the joints, through 

the nail holes and ultimately to the underlying felt, where 

it will cause deterioration and leaks to develop. Insufficient 

headlap can also result in leaks as water entering the joints 

between slates may have a greater tendency to be wind 

blown beyond the heads of the slates in the course below. 

Occasionally, individual slates are damaged. This may be 

caused by falling tree limbs, ice dams in gutters, valleys, 

and chimney crickets, the weight of a workman walking 

on the roof, or a naturally occurring fault in the slate unit. 

Whatever the form of damage, if it is caught soon enough, 

the roof can usually be repaired or selectively replaced 

and deterioration mitigated. 

The ability to lay slate properly so as to produce a watertight and aesthetically pleasing roof requires training, 

much practice, and the right tools (Figure 17). The 

Figure 16. Detail view of a slate which has been punctured by the head 

of a nail used to secure the slate il1 the course below. Likely, the nail was not hammered in far enough whel1 origil1ally installed. 
Figure 17. Slater's Tools. The cutter (a) is used to trim slate edges; the slate hammer (b) is used for hammering nails, trimming, cutting and punching 

holes in slates, and pulling roofing nails; the steak (c) is a T-shaped piece of iron upon which the slater places the edge of a slate to be trimmed; and the 

ripper (d) is slid under the slates to pull out the nails. 

,,;1 

I I 

I , 

J , 

I 

, 

I 

I 

Figure 18. A method of securing replacement slates is shown. The copper strip is l1ailed to the roof sheathil1g just above the head of the slate ill the course below. The replacement slate is then inserted and the copper tab folded over its tail. This type of repair is not recommended for northern climates 

where SI10W and ice can cause the copper tab to fold over, aI/owing the replacement slate to slide out of position as occurred here on the left. 

9 
Repair Sequence 

(e) The bib is cut along its edges and bent into a concave shape to create a 

friction fit. 

Figure 19. Above is a repair sequence for replacing a damaged slate. 

10 

(f) The slate hammer is used to push the bib in place over the nail head. 
The installation and repair of slate roofs should be entrusted 

only to experienced slaters. 

Repairing Slate Roofs 

Broken, cracked, and missing slates should be repaired 

promptly by an experienced slater in order to prevent 

water damage to interior finishes, accelerated deterioration 

of the roof and roof sheathing, and possible structural 

degradation to framing members (Figures 18 and 19). 

The damaged slate is first removed by cutting or pulling 

out its nails with a ripper. If steel cut nails, rather than 

copper nails, were used in laying the roof, adjacent slates 

may be inadvertently damaged or displaced in the ripping 

process, and these, too, will have to be repaired. If the 

slate does not slide out by itself, the pointed end of the 

slate hammer can be punched into the slate and the slate 

dragged out. A new slate, or salvaged slate, which should 

match the size, shape, texture, and weathered color of the 

old slate, is then slid into place and held in position by 

one nail inserted through the vertical joint between the 

slates in the course above and approximately one inch 

below the tail of the slate two courses above. To prevent 

water penetration through the newly created nail hole, a 

piece of copper with a friction fit, measuring roughly 3" 

(7.5 cm) in width and 8 n (20 cm) in length, is slid lengthwise 

under the joint between the two slates located directly 

above the new slate and over the nail. Alternate methods 

for securing the replacement slate include the use of metal 

hooks, clips, and straps that are bent over the tail end of 

the slate. The application of roofing mastic or sealants to 

damaged slates should not be considered a viable repair 

alternative because these materials, though effective at first, 

will eventually harden and crack, thereby allowing water to 

enter (Figure 20). Mastic also makes future repairs more 

difficult to execute, is unsightly, and, when applied to 

metal flashings, accelerates their corrosion. 

When two or more broken slates lie adjacent to each 

other in the same course, or when replacing leaky valley 

flashings, it is best to form pyramids (i.e., to remove a 

diminishing number of slates from higher courses) to keep 

the number of bibs required to a minimum. When 

re-installing the slates, only the top slate in each pyramid 

will need a bib. Slates along the sides of the pyramid will 

receive two nails, one above the other, along the upper 

part of its exposed edge. 

When many slates must be removed to effect a repair, the 

sheathing should be checked for rotted areas and projecting 

nails. Plywood is generally not a good replacement 

material for deteriorated wood sheathing due to the 

relative difficulty of driving a nail through it (the bounce 

produced can loosen adjacent slates). Instead, new wood 

boards of similar width and thickness to those being 

replaced should be used. Because the nominal thickness 

of today's dimension lumber is slightly thinner than that 

produced in the past, it may be necessary to shim the new 

wood boards so that they lie flush with the top surface of 

adjacent existing sheathing boards. Pressure treated 

lumber is not recommended due to its tendency to shrink. 

This can cause the slates to crack and become displaced . 

To permit proper re-laying of the slate, the new roof 

sheathing must be of smooth and solid construction. At 

least two nails should be placed through the new boards 

at every rafter and joints between the ends of the boards 

should occur over rafters. Insufficient nailing will cause 

the boards to be springy, making nailing of the slates 

difficult and causing adjacent slates to loosen in the 

process. Unevenness in the sheathing will show in the 

finished roof surface and may cause premature cracking of 

the slate. Roof sheathing in valleys and along hips, ridges, 

and eaves may be covered with waterproof membrane 

underlayment rather than roofing felt for added protection 

against leakage. 

In emergency situations, such as when severe hurricanes 

or tornadoes blow numerous slates off the roof, a 

temporary roof covering should be installed immediately 

after the storm to prevent further water damage to the 

interior of the building and to permit the drying out 

process to begin. Heavy gauge plastic and vinyl tarpaulins 

are often used for this purpose, though they are difficult 

to secure in place and can be blown off in high winds. Roll 

roofing, carefully stitched in to areas of the remaining 

roof, is a somewhat more functional solution that will 

allow sufficient time to document the existing roof 

conditions, plan repairs, and order materials (Figure 21). 

Slate roof repair is viable for localized problems and 

damaged roofs with reasonably long serviceable lives 

remaining. If 20% or more of the slates on a roof or roof 

slope are broken, cracked, missing, or sliding out of 

position, it is usually less expensive to replace the roof 

than to execute individual repairs. This is especially true 

of older roofs nearing the end of their serviceable lives 

Figure 20. This roof has been poorly repaired numerous times in the 

past. The installation of mastic to seal out moisture has only exacerbated 

the problem. A timely repair and good maintenal1ce could have extended 

the life of this roof. 

11 
Figure 21. As a result of hurricanes and other disasters, it may be necessary to temporarily stabilize a roof until materials can be obtained and a qualified 

roofins. contractor hired. Heavy roofing felt was stitched into this slate roof to stop moisture penetration until matching slate was obtained for repair. 

Significant slate roofs should not be stripped off and replaced with asphalt shingles. Photograph courtesy of the National Park Seroice. 

because even the most experienced slater will likely 

damage additional slates while attempting repairs. 

Depending on the age of the slate, its expected serviceable 

life, and the cause(s) of deterioration, it mayor may not be 

cost effective to salvage slates. Where deteriorated nails or 

flashings are the cause of the roof failure, salvage of at 

least some slates should be possible for use in repairs. 

When salvaging slates, each must be sounded to discover 

cracks and faults and the degree to which it has 

weathered. It is usually wise to salvage slates when only 

a portion of the roof is to be replaced. In this way, the 

salvaged slates may be used for future repairs to the 

remaining sections of the roof. 

12 

The Replacement of Deteriorated Roofs 

Historic slate roofs should be repaired rather than 

replaced whenever possible. Before replacing a slate roof, 

check for isolated damage, corroded and worn flashings, 

leaky gutters, poor ventilation in the attic, and other 

possible sources of moisture. All too often slate roofs are 

mistakenly replaced when, in fact, they could have been 

effectively repaired. Deciding whether an historic slate 

roof should be repaired or replaced can be difficult and 

each roof must be judged separately (see guidance in 

shaded box on page 16). 

If repair is not possible and a new slate roof must be 

installed, it is important to remember that more than just 

the replacement of the slate is involved (Figure 22). The 

old slate should be removed to prevent overloading of the 

roof timbers. Stripping should be done in sections, with 

felt installed, to avoid exposing the entire sub-roof to the 

weather. In the process, rotted wood sheathing should be 

replaced and the roof timbers checked for signs of stress, 

including deflection, cracking, and twisting. If such 

conditions are found, a structural engineer experienced in 

working with older buildings should be consulted. Other 

repairs, such as chimney rep ointing, which may require 

access to the roof should be completed before the new 

roof is put on. 

Drawings and specifications for a new slate roof should be 

prepared by a restoration architect, especially if the project 

is going to be competitively bid or if the roof is particularly 

complex. Standard specifications, like those published in 

1926 by the National Slate Association may be used as a 

basis for developing specifications appropriate for a 

particular project. The specifications and drawings should 

contain all the information necessary to replicate the 

original appearance of the roof as closely as possible. 

Certain changes may have to be accepted, however, since 

several types of slate once prominent in this country, such 

as ribbon slate, are no longer quarried. It is wise to 
(d) Deteriorated sheathing was replaced and rafter tails were reinforced. 

(e) Roofing felt was installed over decking; a rubberized membrane was used (/) Lead coated copper flashing was installed throu~hout. Seen is the 

selectively at the eaves and under some flashing. offset between the portico roof and the main roof in progress. 

Figure 22. Installing a new roof involves more than just slate. Above is a sequence of views of the roof replacement at Arlington House. Photographs 

courtesy of the National Park Service. (Sequence continued on page 14.) 13 
14 

(g) Masonry chimneys were repaired and metal crickets were fabricated 

at the chimneys. 

(h) This installation pattern allows the slates to be laid in courses leaving a temporary path of travel to avoid stepping on installed slates. 

(i) Although the gutters and snow boards were the last elements 

installed, their support brackets were installed as the slates were laid. 

anticipate the replacement of older roofs so that proper 

planning can be undertaken and financ ial resources set 

aside, thereby, reducing the likelihood of rash last minute 

decisions. 

Roofing slate is sold by the square in the United States. 

One square is enough to cover 100 square feet (13.3 square 

meters) of plain roof surface when laid with a standard 

headlap of 3" (7.5cm). When ordering slate, considerable 

lead time should be allowed as delivery may take 

anywhere from 4 to 12 weeks and even as long as 1 year 

for special orders. Orders for random widths of a 

particular slate can generally be filled more quickly than 

orders for fixed widths. Once on site, slates should be 

stored on edge, under cover on pallets. 

A roof and its associated flashings, gutters, and 

downspouts function as a system to shed water. Material 

choices should be made with this in mind. For example, 

use a single type of metal for all flashings and the 

rainwater conductor system to avoid galvanic action. 

Choose materials with life spans comparable to that of 

the slate, such as non-ferrous nails. Use heavier gauge 

flashings or sacrificial flashings in areas that are difficult 

to access or subject to concentrated water flows. 

Flashings are the weakest point in any roof. Given the 

permanence of slate, it is poor economy to use anything 

but the most durable of metals and the best workmanship 

for installing flashings. Copper is one of the best flashing 

materials, and along with terne, is most often associated 

with historic slate roofs. Copper is extremely durable, easily 

worked and soldered, and requires little maintenance. 

Sixteen-ounce copper sheet is the minimum weight 

recommended for flashings. Lighter weights will not 

endure the erosive action of dust and grit carried over the 

roof by rain water. Heavier weight, 20 oz. (565 grams) or 

24 oz. (680 grams), copper should be used in gutters, 

valleys, and areas with limited accessibility. Lead coated 

copper has properties similar to copper and is even more 

durable due to its additional lead coating. Lead coated 

copper is often used in restoration work. 

Terne is a less desirable flashing material since it must be 

painted periodically. Terne coated stainless steel (TCS) is a 

modern-day substitute for terne. Although more difficult 

to work than terne, TCS will not corrode if left unpainted; 

a great advantage, especially in areas that are difficult to 

access. 

Once a metal is chosen, it is important to use it throughout 

for all flashings, gutters, downspouts, and metal roofs. 

Mixing of dissimilar metals can lead to rapid corrosion of 

the more electronegative metal by galvanic action. Where 

flashings turn up a vertical surface, they should be covered 

with a cap flashing. Slates which overlap metal flashings 

should be nailed in such a manner as to avoid puncturing 

the metal. This may be accomplished by punching a 

second hole about 2" (5cm) above the existing hole on the 

side of the slate not overlapping the metal flashing. It is 

important that holes be punched from the back side of the 

slate. In this way, a shallow countersink is created on the 

face of the slate in which the head of the nail may sit. 

.... Continuation of the roof repai r sequence of A rlington House from page 13. 
Figure 23. Slate roofs should not be walked on if at all possible. For 

large projects, lifts can be used to inspect roofs periodically to assess lIIaintenance /leeds. Photograph courtesy of the National Park Service. 

The use of artificial, mineral fiber slate is not recommended 

for restoration work since its rigid appearance is that of a 

man-made material and not one of nature. Artificial slates 

may also have a tendency to fade over time. And, although 

artificial slate costs less than natural slate, the total initial 

cost of an artificial slate roof is only marginally less than a 

natural slate roof. This is because all the other costs 

associated with replacing a slate roof, such as the cost of 

labor, flashings, and tearing-off the old roof, are equal in 

both cases. Over the long term, natural slate tends to be a 

better investment because several artificial slate roofs will 

have to be installed during the life span of one natural 

slate roof. 

Clear roof expanses can be covered by an experienced 

slater and one helper at the rate of about two to three 

squares per day. More complex roofs and the presence of 

chimneys, dormers, and valleys can bring this rate down 

to below one square per day. One square per day is a 

good average rate to use in figuring how long a job will 

take to complete. This takes into account the installation of 

flashings and gutters and the set-up and break-down of 

scaffolding. Tear-off of the existing roof will require 

additional time. 

Maintenance 

Given the relatively high initial cost of installing a new 

slate roof, it pays to inspect its overall condition annually 

and after severe storms. For safety reasons, it is 

recommended that building owners and maintenance 

personnel carry out roof surveys from the ground using 

binoculars or from a cherry picker (Figure 23). Cracked, 

broken, misaligned, and missing slates and the degree to 

which delamination has occurred should be noted, along 

with failed flashings (pin holes, open seams, loose and 

misaligned elements, etc.) and broken or clogged 

downspouts. A roof plan or sketch and a camera can aid 

in recording problems and discussing them with 

contractors. In the attic, wood rafters and sheathing 

should be checked for water stains and rot. Critical areas 

are typically near the roof plate and at the intersection of 

roof planes, such as at valleys and hips. Regular 

maintenance should include cleaning gutters at least twice 

during the fall and once in early spring, and replacing 

damaged slates promptly. Every five to seven years 

inspections should be conducted by professionals 

experienced in working with slate and steep slopes. Good 

record keeping, in the form of a log book and the 

systematic filing of all bills and samples, can help in 

piecing together a roof's repair history and is an important 

part of maintenance. 

As part of regular maintenance, an attempt should be 

made to keep foot traffic off the roof. If maintenance 

personnel, chimney sweeps, painters, or others must walk 

on the roof, it is recommended that ladders be hooked 

over the ridge and that the workmen walk on the ladders 

to better distribute their weight. If slates are to be walked 

on, it is best to wear soft soled shoes and to step on the 

lower-middle of the exposed portion of the slate unit. 

Conclusion 

Slate roofs are a critical design feature of many historic 

buildings that cannot be duplicated using substitute 

materials (Figure 24). Slate roofs can, and should be, 

maintained and repaired to effectively extend their 

serviceable lives. When replacement is necessary, details 

contributing to the appearance of the roof should be 

retained. High quality slate is still available from reputable 

quarries and, while a significant investment, can be a cost 

effective solution over the long term. 

Further Reading 

Copper And Brass Research Association. Copper Flashings. 2nd ed. 

New York: Copper And Brass Research Association, 1925. 

Dale, T. Nelson, and others. Slate In The United States, Bulletin 

586. Washington, D.C. : U.S. Department of the Interior, United 

States Geological Survey, 1914. 

Heim, David. "Roofing With Slate:' Fine Homebuilding, No. 20 

(April/May 1984): 38-43. 

Levine, Jeffrey S. "Slate Roofs For Historic Religious Buildings:' 

Inspired. Philadelphia: Philadelphia Historic Preservation 

Corporation, 1987. 

"Slate Quarrying and Shingle Manufacture:' Fine Homebuilding, 

No.71 (Jan. 1992): 64-68. 

McKee, Harley J. "Slate Roofing:' APT Bulletin, Vol. 2, Nos. 1-2 

(1970): 77-84. 

National Slate Association. Slate Roofs. 1925. Reprint. Fair Haven, 

Vermont: Vermont Structural Slate Co., Inc., 1977. 

Pierpont, Robert N. "Slate Roofing." APT Bulletin, Vol. 19, No.2 

(1987): 10-23. 

Sweetser, Sarah M. "Roofing for Historic Buildings." Preservation 

Briefs, No.4. Washington, D.C.: U.S. Department of the 

Interior, Technical Preservation Services Division, 1975. 

15 
16 

Repair/Replacement Guideline 

The following guideline is provided to assist in the 

repair/replace decision making process: 

1. Consider the age and condition of the roof versus 

its expected serviceable life given the type of slate 

employed. 

2. Calculate the number of damaged and missing 

slates. Is the number less than about 20%? Is the 

roof generally in good condition? If so, the roof 

should be evaluated for repair rather than 

replacement. Also, keep in mind that the older a 

roof becomes, the more maintenance it will likely 

require. 

3. Determine if there are active leaks and what their 

source may be. Do not assume the slates are 

leaking. Gutters, valleys and flashings are more 

likely candidates. "False leaks" can be caused by 

moisture condensation in the attic due to improper 

ventilation. 

4. Check the roof rafters and sheathing for moisture 

stains. Poke an awl into the wood to determine if it 

is rotted. Remember that very old, delaminating 

slates will hold moisture and cause adjacent wood 

members to deteriorate even if there are no 

apparent leaks. 

5. Are many slates sliding out of position? If so, it 

may be that ferrous metal fasteners were used and 

that these are corroding, while the slates are still in 

good condition. Salvage the slates and re-Iay them 

on the roof. If the slates have worn around the 

nails holes, it may be necessary to punch new 

holes before re-Iaying them. 

6. Consider the condition of the roofs flashings. 

Because slate is so durable, metal flashings often 

wear out before the slate does. Examine the 

flashings carefully. Even the smallest pinhole can 

pennit large quantities of water to enter the 

building. 

7. Is the deterioration of the slate uniform? Often this 

is not the case. It may be that only one slope 

needs replacement and the other slopes can be 

repaired. In this way, the cost of replacement can 

be spread over many years. 

8. Press down hard on the slates with your hand. 

Sound slates will be unaffected by the pressure. 

Deteriorated slates will feel brittle and will crack. 

Tap on slates that have fallen out or been removed. 

A full, deep sound indicates a slate in good 

condition, while a dull thud suggests a slate in 

poor condition. 

9. Are new slates readily available? Even if replacement 

is determined to be necessary, the existing roof 

may have to be repaired to allow time for 

documentation and the ordering of appropriate 

replacement slates. 

Figure 24. Although slate replacement roofs are expensive, the superiority 

of materials and craftsmanship will give years of continued service. If 

amortized over the life of the roof, the replacement cost can be very 

reasonable. Photograph courtesy of the National Park Service. 

Note: measurements in this publication are given in both 

u.s. Customary System and International (Metric) System 

for comparative purposes. Metric conversions are in some 

cases approximate and should not be relied upon in 

preparing technical specifications. 

Acknowledgements 

The author, Jeffrey S. Levine, is an Architectural Conservator with John Milner 

Associates, Inc., and gratefully acknowledges the technical review of this 

publication by the following: Russel Watsky, Watsky Associates; Kenton Lerch, 

The Structural Slate Company; Matt Millen, Millen Roofing Co.; Alex 

Echeguren, Echeguren Slate Company; Bill Markcrow, Vermont Structural Slate 

Company; and Dick Naslund, Department of Geological Sciences, State 

University of New York at Binghamton. In addition, invaluable comments were 

provided by Sharon Park, Doug Hicks and Michael J. Auer, National Park 

Service; Suzanna Barucco, Martin Jay Rosenblum, R.A. & Associates; and F~ed 

Walters, John Milner Associates, Inc. All photographs are by the author unless 

otherwise noted . 

Sharon C. Park, AlA, Senior Historical Architect, Preservation Assistance 

Division, National Park Service, is credited with directing the development of 

this publication and with its technical editorship. This publication has been 

prepared pursuant to the National Preservation Act of 1966, as amended, which 

directs the Secretary of the Interior to develop and make available information 

concerning historic properties. Comments on the usefulness of this publication 

may be directed to H. Ward Jand!, Chief, Technical Preservation Services 

Branch, Preservation Assistance Division, National Park Service, PO. Box 37127, 

Washington, D.C. 20013-7127. Drawings for this publication were prepared by 

Karin Murr Link. This publication is not copyrighted and can be reproduced 

without penalty. Normal procedures for credit to the author and the National 

Park Service are appreciated. 

ISSN: 0885-7016 Spring 1993 

Cover Photograph: A portiol1 of all advertisement for Slatil1gtol1-Bal1gor 

Slate SYl1dicate (Slatil1gtol1, PAY which appeared il1 the July 1910 issue 

of Building Age (Vo1.32 No.7).