Historic Detailing and Replacing Roofs

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>>History of wood shingles in America >>Maintenance
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>>Specifications for the replacement roof  

By Sharon C. Park, AIA

wood roof
Replacementof particular historic details is important to the individual historic characterof a roof, such as this rounded butt wood shingle roof. In the restoration, thedrainage around a dormer was improved by the addition of carefully concealedmodern metal flashing. Photo: NPS files.

Modern wooden shingles, both sawn and split, continue to be made, but it isimportant to understand how these new products differ from the historic ones andto know how they can be modified for use on historic buildings. Today'scommercially available shakes are generally thicker than the historic hand splitcounterpart and are usually left "undressed" with a rough, corrugatedsurface. The rough surface shake, furthermore, is often promoted as suitable forhistoric preservation projects because of its rustic appearance. It is anerroneous assumption that the more irregular the shingle, the more authentic or"historic" it will appear.

While the size, shape and finish of the shingle determine the roof's textureand scale, the installation patterns and details give the roof its uniquecharacter. Many details reflect the craft practices of the builders and thearchitectural style prevalent at the time of construction.

Other details hadspecific purposes for reducing moisture penetration to the structure. Inaddition to the most visible aspects of a shingle roof, the details at the rakeboards, eaves, ridges, hips, dormers, cupolas, gables, and chimneys should notbe overlooked. The long, biaxially tapered hand split shingles are overlappedboth vertically and horizontally.

The way the shingles were laid was often based on functional and practicalneeds. Because a roof is the most vulnerable element of a building, many of theroofing details that have become distinctive features were first developedsimply to keep water out. Roof combs on the windward side of a roof protect theridge line. Wedges, or cant strips, at dormer cheeks roll the water away fromthe vertical wall. Swept valleys and fanned hips keep the grain of the wood inthe shingle parallel to the angle of the building joint to aid water runoff. Theslight projection of the shingles at the eaves directs the water runoff eitherinto a gutter or off the roof away from the exterior wall. These details variedfrom region to region and from style to style. They can be duplicated even withthe added protection of modern flashing.

In order to have a weather-tight roof, it was important to have adequatecoverage, proper spacing of shingles, and straight grain shingles. Many roofswere laid on open shingle lath or open sheathing boards. Roofers typically laidthree layers of shingles with approximately 1/3 of each shingle exposed to theweather. Spaces between shingles (1/8"1/2" depending on wood type)allowed the shingles to expand when wet. It was important to stagger eachoverlapping shingle by a minimum of 11/2" to avoid a direct path formoisture to penetrate a joint. Doubling or tripling the starter course at theeave gave added protection to this exposed surface.

In order for the roof to layas flat as possible, the thickness, taper and surface of the shingles wasrelatively uniform; any unevenness on hand-split shingles had already beensmoothed away with a draw-knife. To keep shingles from curling or cupping, theshingle width was generally limited to less than 10".

Not all shingles were laid in evenly spaced, overlapping, horizontal rows. Invarious regions of the country, there were distinct installation patterns; forexample, the biaxially-tapered long shingles occasionally found in areas settledby the Germans. These long shingles were overlapped on the side as well as ontop. This formed a ventilation channel under the shingles that aided drying.Because ventilation of the shingles can prolong their life, roofers paidattention to these details.

Early roofers believed that applied coatings would protect the wood andprolong the life of the roof. In many cases they did; but in many cases, theshingles were left to weather naturally and they, too, had a long life.Eighteenth-century coatings included a pine pitch coating not unlike turpentine,and boiled linseed oil or fish oil mixed with oxides, red lead, brick dust, orother minerals to produce colors such as yellow, Venetian red, Spanish brown,and slate gray.

In the 19th century, in addition to the earlier colors, shingleswere stained or painted to complement the building colors: Indian red, chocolatebrown, or brown-green. During the Greek Revival and later in the 20th centurywith other revival styles, green was also used. Untreated shingles age to asilver-gray or soft brown depending on the wood species.

The craft traditions of the builders often played an important role in thefinal appearance of the building. These elements, different on each building,should be preserved in a re-roofing project.

Replacing Deteriorated Roofs:Matching the Historic Appearance

Historic wooden roofs using straight edge-grain heartwood shingles have beenknown to last over sixty years. Fifteen to thirty years, however, is a morerealistic lifespan for most premium modern wooden shingle roofs.

Contributing factors to deterioration include:

  • thinness of the shingle;
  • durability of the wood species used;
  • exposure to the sun;
  • slope of the roof;
  • presence of lichens or moss growing on the shingle;
  • poor ventilation levels under the shingle or in the roof;
  • presence of overhanging tree limbs;
  • pollutants in the air;
  • original installation method;
  • and the history of the roof maintenance.

Erosion of the softer wood within the growth rings is caused byrainwater, wind, grit, fungus and the breakdown of cells by ultraviolet rays insunlight. If the shingles cannot adequately dry between rains, if moss andlichens are allowed to grow, or if debris is not removed from the roof, moisturewill be held in the wood and accelerate deterioration. Moisture trapped underthe shingle, condensation, or poorly ventilated attics will also acceleratedeterioration.

In addition to the eventual deterioration of wooden shingles, impact fromfalling branches and workmen walking on the roof can cause localized damage. If,however, over 20% of the shingles on any one surface appear eroded, cracked,cupped or split, or if there is evidence of pervasive moisture damage in theattic, replacement should be considered.

If only a few shingles are missing ordamaged, selective replacement may be possible. For limited replacement, the oldshingle is removed and a new shingle can be inserted and held in place with athin metal tab, or "babbie." This reduces disturbance to the soundshingles above. In instances where a few shingles have been cracked or the jointof overlapping shingles is aligned and thus forms a passage for waterpenetration, a metal flashing piece slipped under the shingle can stop moisturetemporarily. If moisture is getting into the attic, repairs must be made quicklyto prevent deterioration of the roof structural framing members.

When damage is extensive, replacement of the shingles will be necessary, butthe historic sheathing or shingle lath under the shingles may be in satisfactorycondition. Often, the historic sheathing or shingle laths, by their size,placement, location of early nail holes, and water stain marks, can giveimportant information regarding the early shingles used.

Before specifying a replacement roof, it is important to establish theoriginal shingle material, configuration, detailing and installation. If thehistoric shingles are still in place, it is best to remove several to determinethe size, shape, exposure length, and special features from the unweatheredportions. If there are already replacement shingles on the roof, it may benecessary to verify through photographic or other research whether the shinglescurrently on the roof were an accurate replacement of the historic shingles.

The following information is needed in order to develop accuratespecifications for a replacement shingle:

  • Original wood type (White Oak, Cypress, Eastern White Pine, Western, Red Cedar, etc.)
  • Size of shingle (length, width, butt thickness, taper)
  • Exposure length and nailing pattern (amount of exposure, placement and type of nails)
  • Type of fabrication (sawn, handsplit, dressed, beveled, etc.)
  • Distinctive details (hips, ridges, valleys, dormers, etc.)
  • Decorative elements (trimmed butts, variety of pattern, applied color coatings, exposed nails)
  • Type of substrate (open shingle lath or sheathing, closed sheathing, insulated attics, sleepers, etc.)

Replacement roofs must comply with local codes which may require, forexample, the use of shingles treated with chemicals or pressure-impregnatedsalts to retard fire. These requirements can usually be met without long-termvisual effects on the appearance of the replacement roof.

The accurate duplication of a wooden shingle roof will help ensure thepreservation of the building's architectural integrity. Unfortunately, thechoice of an inappropriate shingle or poor installation can severely detractfrom the building's historic appearance.

There are a number of commercially available wooden roofing products as wellas custom roofers who can supply specially-made shingles for historicpreservation projects. Unless restoration or reconstruction is being undertaken,shingles that match the visual appearance of the historic roof withoutreplicating every aspect of the original shingles will normally suffice. Forexample, if the historic wood species is no longer readily available, WesternRed Cedar or Eastern White Pine may be acceptable. Or, if the shingles arelocated high on a roof, sawn shingles or commercially available shakes with therustic faces factory-sawn off may adequately reproduce the appearance of anhistoric hand-split and dressed shingle.

There will always be certain features, however, that are so critical to thebuilding's character that they should be accurately reproduced. Following isguidance on matching the most important visual elements.

Highest Priority in Replacement Shingles:

  • Best quality wood with a similar surface texture
  • Matching size and shape: thickness, width, length
  • Matching installation pattern: exposure length, overlap, hips, ridges, valleys, etc.
  • Matching decorative features: fancy butts, color, exposed nails

Areas of Acceptable Differences:

  • Species of wood
  • Method of fabrication of shingle, if visual appearance matches
  • Use of fire retardants, or preservative treatments, if visual impact is minimal
  • Use of modern flashing, if sensitively installed
  • Use of small sleepers for ventilation, if the visual impact is minimal and rake boards are sensitively treated
  • Method of nailing, if the visual pattern matches

Treatments and Materials to Avoid:

  • Highly textured wood surfaces and irregular butt ends, unless documented
  • Standardized details (prefab hips, ridges, panels, etc.) unless documented
  • Too wide shingles or those with flat grain (which may curl), unless documented

Currently Available Types of Wood:

  • Western Red Cedar, Eastern White Pine, and White Oak are most readily available today.
  • For custom orders, cypress, red oak, and a number of other historically used woods may still be available.
  • Some experiments using nontraditional woods (such as yellow pine and hemlock) treated with preservative chemicals are being tested for the new construction market, but are generally too thick, curl too easily, or have too pronounced a grain for use on historic buildings.

Method of manufacture:

  • Commercially available modern shingles and shakes are for the most part machine-made. While commercially available shakes are promoted by the industry as hand-split, most are split by machine (this reduces the high cost of hand labor).
  • True hand-split shingles, made the traditional way with a froe and mallet, are substantially more expensive, but are more authentic in appearance than the rough, highly textured machine-split shakes. An experienced shingler can control the thickness of the handsplit shingle and keep the shingle surface grain relatively even. To have an even roof installation, it is important to have handsplit shingles of uniform taper and to have less than 1/8th variation across the surface of the shingle. For that reason, it is important to dress the shingles or to specify uniform butt thickness, taper, and surfaces.
  • Commercially available shakes are shipped with a range of butt sizes within a bundle (e.g., ", 5/8", 3/4" as a mix) unless otherwise specified. Commercially available shakes with the irregular surfaces sawn off are also available. In many cases, except for the residual circular saw marks, these products appear not unlike a dressed hand-split shingle.
  • Sawn shingles are still made much the same way as they were historically--using a circular saw. The circular saw marks are usually evident on the surface of most sawn shingles. There are a number of grooved, striated, or steamed shingles of the type used in the 20th century to effect a rustic or thatched appearance. Custom sawn shingles with fancy butts or of a specified thickness are still available through mill shops. In fact, shingles can be fabricated to the weathered thickness in order to be integrated into an existing historic roof. If sawn shingles are being used as a substitute for dressed handsplit shingles, it may be desirable to belt sand the surface of the sawn shingles to reduce the prominence of the circular saw marks.

Few of the commercially available shakes can be used without somemodification or careful specification. Some, such as heavy shakes with acorrugated face, should be avoided altogether. While length, width, and buttconfiguration can be specified, it is more difficult to ensure that thethickness and the texture will be correct. For that reason, whatever shingle orshake is desired, it is important to view samples, preferably an entire bundle,before specifying or ordering. If shingles are to be trimmed at the site forspecial conditions, such as fanned hips or swept valleys, additional shinglesshould be ordered.

Coatings and Treatments: Shingles are treated to obtain afire-retardant rating; to add a fungicide preservative (generally toxic); torevitalize the wood with a penetrating stain (oil as well as water based); andto give color. While shingles can be left untreated, local codes may requirethat only fire-retardant shingles be used. In those circumstances, there areseveral methods of obtaining rated shingles (generally class "B" or"C").

  • The most effective and longest lasting treatment is to have treated salts pressure-impregnated into the wood cells after the shingles have been cut.
  • Another method (which must be periodically renewed) is to apply chemicals to the surface of the shingles. If treated shingles need trimming at the site, it is important to check with the manufacturer to ensure that the fire-retardant qualities will not be lost. Pressure-impregnated shingles, however, may usually be trimmed without loss of fire-retardant properties.

The life of a shingle roof can be drastically shortened if moss, lichens,fungi or bacterial spores grow on the wood. Fungicides (such as chromated copperarsenate, CCA) have been found to be effective in inhibiting such fungal growth,but most are toxic. Red cedar has a natural fungicide in the wood cells andunless the shingles are used in unusually warm, moist environments, or wherecertain strains of spores are found, an applied fungicide is usually not needed.

For most woods, the Forest Products Laboratory of the U.S. Department ofAgriculture has found that fungicides do extend the life of the shingles byinhibiting growth on or in the wood. There are a variety available. Care shouldbe taken in applying these chemicals and meeting local code requirements forproper handling.

Penetrating stains and water repellent sealers are sometimes recommended torevitalize wood shingles subject to damage by ultraviolet rays. Some treatmentsare oil-borne, some are waterborne, and some are combined with a fungicide or awater repellent. If any of these treatments is to be used, they should beidentified as part of the specifications. Manufacturers should be consultedregarding the toxicity or other potential complications arising from the use ofa product or of several in combination. It is also important not to coat theshingles with vapor impermeable solutions that will trap moisture within theshingle and cause rotting from beneath.

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-- NPS Preservation Brief 19

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