01 May 1999
GENERAL SELECTION CONSIDERATIONS
STARTING POINTS FOR ROOF SYSTEM SELECTION. This Chapter is intended to
introduce the major considerations in selecting a roofing system. Figure 2-1 depicts the various
alternative roofing systems and how they relate. When commencing the selection process there
are two different starting points.
a. New vs. Reroofing. The roof may be part of a new building design; or, it may involve the
reroofing of an existing structure (replacement or re-cover). Today, approximately 75% of roofing
activity is reroofing.
b. Steep-Slope vs. Low-Slope. In new construction the designer is very likely to have a
preconceived notion as to whether a highly visible sloped-roof is wanted, or whether a less visible
low-slope roof design is acceptable. Appendix 2-1, extracted from the Decision Guide for Roof
Slope Selection, indicates the standard practice for use of today's common roofing systems,
touching on slope and fire requirements. Positive drainage is a very important design criterion.
In general, all low-slope roof systems should have a minimum of 2% (1/4 in. ft.) slope. When
reroofing, it may be feasible to improve drainage by using tapered insulation or sloped deck fills.
SELECTION CONSIDERATIONS FOR STEEP ROOFING SYSTEMS.
evaluates common steep roofing systems based upon some use criteria.
a. Aesthetics. Steep roof systems make a strong visible statement about a building. The
texture, shadow-line, and color are major factors in selection.
b. Minimum Slope Requirements. Steep roofs function by shedding water rather than by
being waterproof (figure 2-2). Minimum slopes as shown in Table 2-13, are required in order to
insure proper drainage.
c. Categories of Steep Roofing. Major categories of steep roofing include asphalt shingles,
wood shingles and shakes, tile, slate, architectural metal, asphalt roll roofing, and fabricated units
of metal or plastic intended to look like the others. Only asphalt roll roofing and asphalt or wood
shingles may be re-covered.
d. Snowshedding and Ventilation. Sloped roofs are effective snowshedders. In addition,
the attic space that accompanies steep roofing makes it easy to ventilate the roofing system.
e. Maintenance Requirements. Sloped roofs in general, require less maintenance than flat
f. Steep Roof Conversions. When considering reroofing a flat roof, it may be possible to
convert the low-slope roofing system to a steeply sloped roof. This may improve the appearance
of the building while resolving drainage problems as well. Steep roof conversions are a viable
option for relatively narrow buildings. Refer to Chapter 11 and CERL Technical Report M-85/05
for more information on steep roof conversions.