CEMP-ET
TI 809-53
01 May 1999
Table 2-12. Some Typical Slope Limitations for Low-Slope Roof Systems
%
In./ft.
% Max.
In./ft.
Min.
Min.
Max.
Ballasted Membranes
2
1/4
16.7
2
Gravel Surfaced BUR
1
1/8
25
3
Cap Sheets, Narrow Lap Widths
2
1/4
50
6
MB smooth or Cap Sheet
2
1/4
50
6
2-9. PRINCIPAL
CONSIDERATIONS
WHEN
SPECIFYING
STEEP-SLOPE
(WATERSHEDDING) ROOFING. This category covers systems that range from asphalt
shingles, wood shingles and shakes, clay and concrete tile, slate, and metal look-a-likes. Also
included are architectural metal panels with a variety of seams (figure 2-11). Slopes are
generally 25% (3:12) or greater. Most must be continuously supported on a solid deck (e.g.,
plywood or oriented strand board [OSB]). However, some varieties (e.g., clay and concrete tiles)
may be supported on spaced horizontal batten boards. Underlayments such as roofing felt, self-
adhering MB or plastic film are usually required over the entire roof to provide a secondary line of
defense against driving rain and blowing snow. In cold regions, a completely sealed MB
underlayment is needed along eaves, in valleys, and at dormers, skylights, chimneys and such to
resist leaks from water ponded behind ice dams.
a. Aesthetics. By their very nature steep roofing is highly visible. Appearance may be of
primary concern to the designer. Regional preferences exist. For example, red tile roofing is very
common and highly desirable in the Southwest, while light gray concrete tile is preferred in
Florida. Wood shakes give a textured natural look preferred in the Pacific Northwest.
b. Labor Intensity and Labor Skill.
(1) High Intensity. Heavy brittle units of clay, tile or slate.
(2) Medium Intensity. Architectural metal, wood shakes.
(3) Low Intensity. Shingles.
c. Watershedding. Steep roofs rely on gravity to cause water to flow away from headlaps.
Recommended minimum slopes are shown in Table 2-13. Lower slopes are sometimes
permissible by increasing overlap or enhancing the waterproofness of the underlayment.
2-17
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