Roofing Practices to Avoid
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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
Certain common roofing practices for modern installations should beavoided in re-roofing a historic building unless specifically approved inadvance by the architect. These practices interfere with the proper drying ofthe shingles or result in a sloppy installation that will acceleratedeterioration. They include improper coverage and spacing of shingles, use ofstaples to hold shingles, inadequate ventilation, particularly for heavilyinsulated attics, use of heavy building felts as an underlayment, improperapplication of surface coatings causing stress in the wood surfaces, and use ofinferior flashing that will fail while the shingles are still in good condition.
Avoid skimpy shingle coverage and heavy building papers. It has become acommon modern practice to lay impregnated roofing felts under new wooden shingleroofs. The practice is especially prevalent in roofs that do not achieve a fulltriple layering of shingles. Historically, approximately one third of eachsingle was exposed, thus making a three-ply or three-layered roof. This assuredadequate coverage. Due to the expense of wooden shingles today, some roofersexpose more of the shingle if the pitch of the roof allows, and compensate forless than three layers of shingles by using building felts interwoven at the topof each row of shingles. This absorptive material can hold moisture on theunderside of the shingles and accelerate deterioration. If a shingle roof hasproper coverage and proper flashing, such felts are unnecessary as a generalrule. However, the selective use of such felts or other reinforcements atridges, hips and valleys does appear to be beneficial.
Beware of heavily insulated attic rafters. Historically, the longest lastingshingle roofs were generally the ones with the best roof ventilation. Roofs withshingling set directly on solid sheathing and where there is insulation packedtightly between the wooden rafters without adequate ventilation run the risk ofcondensation-related moisture damage to wooden roofing components. This isparticularly true for air-conditioned structures. For that reason, if insulationmust be used, it is best to provide ventilation channels between the rafters andthe roof decking, to avoid heavy felt building papers, to consider the use ofvapor barriers, and perhaps to raise the shingles slightly by using"sleepers" over the roof deck. This practice was popular in the 1920sin what the industry called a "Hollywood" installation, and examplesof roofs lasting 60 years are partly due to this undershingle ventilation.
Avoid staples and inferior flashing. The common practice of using pneumaticstaple guns to affix shingles can result in shooting staples through theshingles, in crushing the wood fibers, or in cracking the shingle. Instead,corrosion resistant nails, generally with barked or deformed shanks long enoughto extend about 3/4" into the roof decking, should be specified. Many goodroofers have found that the pneumatic nail guns, fitted with the proper nailsand set at the correct pressure with the nails just at the shingle surface, haveworked well and reduced the stress on shingles from missed hammer blows.
If redcedar is used, copper nails should not be specified because a chemical reactionbetween the wood and the copper will reduce the life of the roof. Hot-dipped,zinc-coated, aluminum, or stainless steel nails should be used. In addition,copper flashing and gutters generally should not be used with red cedar shinglesas staining will occur, although there are some historic examples where veryheavy gauge copper was used which outlasted the roof shingles. Heavier weightflashing (2() oz.) holds up better than lighter flashing, which may deterioratefaster than the shingles. Some metals may react with salts or chemicals used totreat the shingles. This should be kept in mind when writing specifications.Terne-coated stainless steel and lead-coated copper are generally the top of theline if copper is not appropriate.
Avoid patching deteriorated roof lath or sheathing with plywood or compositematerials. Full size lumber may have to be custom ordered to match the size andconfiguration of the original sheathing in order to provide an even surface forthe new shingles. It is best to avoid plywood or other modern composition boardsthat may deteriorate or delaminate in the future if there is undetected moistureor leakage. If large quantities of shingle lath or sheathing must be removed andreplaced, the work should be done in sections to avoid possible shifting orcollapse of the roof structure.
Avoid spray painting raw shingles on a roof after installation. Rapidlydrying solvent in the paint will tend to warp the exposed surface of theshingles. Instead, it is best to dip new shingles prior to installation to keepall of the wood fibers in the same tension. Once the entire shingle has beentreated, however, later coats can be limited to the exposed surface.
-- NPS Preservation Brief 19
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