01 May 1999
(2) Mortar Set Roofing. For tiles laid in mortar on lower slopes a nailed base sheet and
mopped mineral surfaced capsheet may serve as the underlayment. The roofing granules provide a
preferred surface texture to bond to the mortar/adhesive.
(3) Eave Protection. In the past, at eaves and other locations where ice dams could cause
temporary water backup, asphalt-saturated roofing felts were sealed together with asphalt mastic.
Today, self-adhering modified bituminous sheets (ASTM D1970) are often used. Self-adhesive
rubberized sheets have the added advantage of forming watertight seals where roofing nails penetrate
k. Vapor Retarders. Under most conditions with steep roof system designs the vapor retarder
(when present) will be on the attic floor slowing (but not completely stopping) moisture penetration into
the insulation. Unfortunately, most vapor retarders are not very effective at stopping air exfiltration as
little attempt is made to seal them at edges and penetrations such as light fixtures and vent pipes.
Much more attention to control air exfiltration can be justified considering the many problems caused
by moisture. Roof ventilation can remove some moisture with the result that a few defects are
generally not fatal. However, in more severe cases or for high humidity buildings, it is essential to
severely limit air exfiltration. This may necessitate installation of a separate air barrier. In order not to
create cold-side vapor traps, cold-side air barriers should have high vapor permeability.
l. Penetrations. Round pipe penetrations through steep roofing are handled with molded boots
which slide over the pipe providing a snug fit. The base of the boot is installed shingle fashion covering
the units of lower courses and with the up-slope end covered by later installed courses (figure 8-7).
m. Historical Roof Restoration. Since steep roofing is usually very visible efforts should be made
to retain historical accuracy. Where materials are no longer available, or no longer comply with fire or
other code requirements, refer to CEGS 02226 (Removal and Salvage of Historic Building Materials).
Look-alikes of metal or plastic may provide lightweight replicas of heavier, more combustible materials.
n. Aesthetics. Increased depth and shadow improves appearance. Some asphalt shingles
incorporate color blending of granules for enhanced appeal. Laminated asphalt shingles and thicker
wood shakes increase visual appeal as well as durability.
o. Flashings, Counterflashings and Closures. Flashings generally use a formed metal base and
step flashing. Each system has specific details for hip and ridge closures. See references or
p. Drainage, Valley and Intersection Details. Roof drainage systems are discussed in detail in
Chapter 1 of the SMACNA Sheet Metal Manual. Snow guards are necessary on most slate roofs in
sections of the country where snow and ice accumulate.
(1) Gutters. Roof gutters should be designed so that the front edge of the gutter is lower than
the back and so that any overflow will spill over the front of the gutter. The elevation difference should
be 1/12 of the gutter width, but not less then 25 mm (1 inch).
(2) Roof Slope. Minimum Slope should be 33% (4 in./ft.) for tile, slate, or shingles. Some
specifications permit lower slopes. In these situations more stringent underlayment (and/or
interlayment) specifications, reduced exposures, and careful consideration of roof layout, valley
details, and climatic conditions are necessary.
(3) Valley Details.
Valley details may consist of formed metal combined with an
underlayment. Open, Woven, and Closed Valleys may be selected. Open valleys are recommended
where falling leaves and debris can accumulate on the roof.