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The Repair, Replacement, and Maintenance of Historic Slate Roofs

By The Old House Web

Home page logo: Ongong slate roof repair. Photo: Jeffrey S. Levine

Jeffrey S. Levine

����History of SlateUse
����Character and Detailing
����Where Does Slate Come From?
����Deterioration ofSlate and Slate Roofs
����Repairing Slate Roofs
����The Replacement ofDeteriorated Roofs

Slate is one of the most aesthetically pleasing and durable of all roofingmaterials. It is indicative at once of the awesome powers of nature whichhave formed it and the expertise and skill of the craftsman in handshaping 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.

Arlington House, VA
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. Photo: NPS files.

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 U.S. National Park Service 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 slateduring the late nineteenth and early twentieth centuries means that manyslate roofs, and the 60 to 125 year life span of the slates most commonlyused, may be nearing the end of their serviceable lives at the end of thetwentieth century. Too often, these roofs are being improperly repairedor replaced with alternative roofing materials, to the detriment of thehistoric integrity and appearance of the structure. Increased knowledgeof the characteristics of slate and its detailing and installation on theroof can lead to more sensitive interventions in which original materialis preserved and the building's historic character maintained. Every effortshould be made to replace deteriorated slate roofs with new slate and todevelop an effective maintenance and repair program for slate roofs thatcan be retained.

History ofSlate Use in the United States

Although slate quarrying was not common in the United States until thelatter half of the nineteenth century, slate roofing is known to have beenused prior to the Revolution. Archeological excavations at Jamestown, Virginia,haveunearthed roofing slate in strata dating from 1625-1650 and 1640-1670. Slateroofs were introduced in Boston as early as 1654 and Philadelphia in 1699.Seventeenth century building ordinances of New York and Boston recommendedthe use of slate or tile roofs to ensure fireproof construction.

French Roof House in A.J. Downing's Victorian Cottage Residences
Architectural pattern books of the mid-19th century awakened Americans to the availability and quality of slate for roofing purposes. Drawing: Design XX, "A French Roof House," in A.J. Downing's Victorian Cottage Residences.

In the early years of the Colonies, nearly all roofing slate was importedfrom North Wales. It was not until 1785 that the first commercial slatequarry was opened in the United States, by William Docher in Peach BottomTownship, Pennsylvania. Production was limited to that which could be consumedin local markets until the middle of the nineteenth century. Knowledgeof the nation's abundant stone resources was given commercial impetus atthis time by several forces, including a rapidly growing population thatdemanded housing, advances in quarrying technology, and extension of therailroad system to previously inaccessible markets. Two additional factorshelped push the slate industry to maturity: the immigration of Welsh slateworkers to the United States and the introduction of architectural patternand style books. Slate production increased dramatically inthe years following the Civil War as quarries were opened in Vermont, NewYork, Virginia, and Lehigh and Northampton Counties, Pennsylvania. By 1876,roofing slate imports had all but dried up and the United States becamea net exporter of the commodity.

The U.S. roofing slate industry reached its highest point in both quantityand value of output in the period from 1897 to 1914. In 1899, there wereover 200 slate quarries operating in 13 states, Pennsylvania historicallybeing the largest producer of all. The decline of the U.S. roofing slateindustry began c.1915 and resulted from several factors, including a declinein skilled labor for both the fabrication and installation of slate andcompetition from substitute materials, such as asphalt shingles, whichcould be mass produced, transported and installed at a lower cost thanslate. Only recently, with the increasing popularity of historic preservationand the recognition of the superiority of slate over other roofing materials,has slate usage begun to increase.

TheCharacter and Detailing of Historic Slate Roofs

During some periods of architectural history, roof design has gone farbeyond the merely functional and contributed much to the character of buildings.Roofs, by their compelling forms, have defined styles and, by their decorativepatterns and colors, have imparted both dignity and beauty to buildings.The architectural styles prevalent during the latter half of the nineteenthand early twentieth centuries placed strong emphasis on prominent rooflines and greatly influenced the demand for slate. Slate, laid in multicoloreddecorative patterns, was particularly well suited to the Mansard roofsof the Second Empire style, the steeply pitch roofs of the Gothic Revivaland High Victorian Gothic styles, and the many prominent roof planes andturrets associated with the Queen Anne style. The Tudor style imitatedthe quaint appearance of some English slates which, because of their granularcleavage, are thick and irregular. These slates were often laid in a graduatedpattern, with the largest slates at the eaves and the courses diminishingin size up the roof slope, or a textural pattern. CollegiateGothic style buildings, found on many university campuses, were often roofedwith slate laid in a graduated pattern.

graduated slate roof
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. Photo: Jeffrey S. Levine.

The configuration, massing, and style of historic slate roofs are importantdesign elements that should be preserved. In addition, several types ofhistoric detailing were often employed to add visual interest to the roofessentially elevating the roof to the level of an ornamental architecturalelement. When repairing or replacing a slate roof, original details affectingits visual character should be retained.

Before repairing or replacing an existing slate roof, it is importantto document the existing conditions and detailing of the roof using written,visual, and physical evidence so that original features can be identifiedand preserved. Documentation should continue through the repair or replacementprocess as significant details, long obscured, are often rediscovered whilecarrying out these activities. Local histories, building records, old receiptsand ledgers, historic photographs, sketches, and paintings, shadow linesand nail hole patterns on the roof deck, and bits of historic materialleft over from previous interventions (often found in eave cavities) areall useful sources of information which can be of help in piecing togetherthe original appearance of the roof. Size, shape, color, texture, exposure,and coursing are among the most important characteristics of the originalslates which should be documented and matched when repairing or replacingan historic slate roof.

Historically, three types of slate roofing--standard, textural, and graduated-wereavailable according to the architectural effect desired. Standard gradeslate roofs were most common. These are characterized by their uniformappearance, being composed of slates approximately 3/16" (0.5cm) thick,of consistent length and width, and having a smooth cleavage surface. Thirtydifferent standard sizes were available, ranging from 10"(25cm) x 6" to 24" x 14" (15cm x 61cm x 35cm). The slateswere laid to break joints and typically had square ends and uniform colorand exposure. Patterned and polychromatic roofs were created by layingstandard slates of different colors and shapes on the roof in such a wayas to create sunbursts, flowers, sawtooth and geometric designs, and eveninitials and dates. On utilitarian structures, such as barnsand sheds, large gaps were sometimes left between each slate within a givencourse to reduce material and installation costs and provide added ventilationfor the interior.

Textural slate roofs incorporate slates of different thicknesses, uneventails, and a rougher texture than standard slates. Textural slate roofsare perhaps most often associated with Tudor style buildings where slatesof different colors are used to enhance the effect.

Graduated slate roofs were frequently installed on large institutionaland ecclesiastical structures. The slates were graduated accordingto thickness, size, and exposure, the thickest and largest slates beinglaid at the eaves and the thinnest and smallest at the ridge. Pleasingarchitectural effects were achieved by blending sizes and colors.

Detailing at the hips, ridges and valleys provided added opportunityto ornament a slate roof. Hips and ridges can be fashioned out of slateaccording to various traditional schemes whereby the slates are cut andoverlapped to produce a watertight joint of the desired artistic effect.Traditional slate ridge details are the saddle ridge, strip saddle rid~eand comb ridge, and for hips, the saddle hip, mitered hip Boston hip, andfantail hip. A more linear effect was achieved by covering theridges and hips with flashing called "cresting" or "ridgeroll" formed out of sheet metal, terra cotta, or even slate. Snow guards, snow boards, and various types of gutter and rake treatmentsalso contributed to the character of historic slate roofs.

Two types of valleys were traditionally employed, the open valley andthe closed valley. The open valley is lined with metal over which slateslap only at the sides. Closed valleys are covered with slate and have eithera continuous metal lining or metal flashing built in with each course.Open valleys are easier to install and maintain, and are generally morewatertight than closed valleys. Round valleys are a type of closed valleywith a concave rather than Vshaped section. Given the broadersweep of the round valley, it was not uncommon for roofers to interweaveasphalt saturated felts rather than copper sheet in the coursing in orderto cut costs.

Although principally associated with graduated and textural slate roofs,round valleys were infrequently employed due to the difficulty and expenseof their installation.

Common types of sheathing used include wood boards, wood battens, and,for fireproof construction on institutional and government buildings, concreteor steel. Solid wood sheathing was typically constructed oftongue and groove, square edged, or shiplapped pine boards of 1" (2.5cm) 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 preferredas they were less likely to warp and curl.

Wood battens, or open wood sheathing, consisted of wood strips, measuringfrom 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 equaledthe exposure. Slates were nailed to the batten that transected its midsection.The upper end of the slate rested at least ½" (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 buildingsin the South where the insulating value of solid wood sheathing was nota strict requirement. To help keep out dust and wind driven rain on residentialbuildings, mortar was often placed along the top and bottom edge of eachbatten, a practice sometimes referred to as torching.

snow guards
Eave details include snow guards, snow boards, and gutter treatments. Snow guards are generally used in areas where the ice and snow accumulate to avoid dangerous slides from the roof. Photo: Jeffrey S. Levine.

Steel angles substituted for the wood battens in fireproof construction.The slates were secured using wire wrapped around the steel angle, whereit was twistedoff tight. Alternately, any of a variety of special fastenerspatented over the years could have been used to attach the slate to thesteel angle. On roofs with concrete decks, slates were typicallynailed to wood nailing strips embedded in the concrete.

Beginning in the late nineteenth century, asphalt saturated roofingfelt was installed atop solid wood sheathing. The felt provided a temporary,watertight roof until the slate could be installed atop it. Felt also servedto cushion the slates, exclude wind driven rain and dust, and ease slightunevenness between the sheathing boards.

Slate was typically laid in horizontal courses starting at the eaveswith a standard headlap of 3" (7.5 cm) (Figure 10). Headlap was generallyreduced to 2" (5 cm) on Ma

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