01 May 1999
(8) Steep Roofing. For most steep roofing systems, additional fastening is required for
high wind areas (e.g., six fasteners per asphalt shingle instead of four, addition of nose clips for
b. Ice and Hail. The formation of ice can cause the roof membrane to split. Ice can also
affect roofing performance by scrubbing the membrane and eroding the surface. This can be
especially detrimental to materials with a relatively high glass transition temperature (Tg).
Bituminous materials have a Tg of approximately 0C (32F). Modified bituminous materials with
an SBS modifier can have a Tg as low as minus 34C (-30F). EPDM membranes report a Tg
less than -40C (-40F). The Tg of thermoplastics may increase with age (i.e., loss of plasticizer
in PVC). Ponding promotes ice damage; drainage avoids it.
(1) Impact Damage. Falling ice, such as from overhead towers, causes impact damage.
Ballasted EPDM provides some protection. Protected membrane roofs in which both polystyrene
insulation and ballast are placed over the finished roof membrane provide excellent impact
(2) Perimeter Icing. Ice formation at eaves, scuppers, and gutters is a major design
concern. For low-slope roofing selection of internal drainage where building heat keeps the drain
lines unfrozen is recommended.
(3) Minimizing Icing Problems. For metal and steep roofing heating cables are
sometimes necessary but not especially reliable. In cold regions use of a cold roof in which the
roof is ventilated to prevent formation of icicles and ice dams is preferred. Self-adhering
waterproof membranes are needed to avoid leaks from ice damming (figure 2-10).
(4) Hail Damage. Weather maps are available that generally divide the U.S. into regions
that require resistance to severe hail (50 mm [2 in. dia.]), moderate hail (38 mm [1-1/2 in. dia.]),
substrate, thickness of the membrane, tensile strength, and age/brittleness of the material.
c. Snow. Snow removal operations in which shovels or snow blowers are used can
cause severe damage especially to cold, brittle membranes. Smooth single-ply membranes and
metal roofing are extremely slippery when wet or when a thin ice film covers melt water. Roof
walkways consisting of compatible materials are essential when it is necessary to walk on wet or
(1) Metal Roofs. TI 809-52 recommends that SSSMRs should have a minimum slope of
8.3% in cold regions.
(2) Snow Loads. Snow load information is available in ANSI/ASCE 7-95, TI 809-01, and
d. Slope. Drainage is essential on all roofing systems. (See ORNL-6520 Decision Guide for
Roof Slope Selection
.) For hydrokinetic roofing the drainage must
be positive and rapid. Shingles,
tiles, and the like, generally have industry minimum recommended slopes of 33% to 42%.
Sometimes a lower slope option is available if waterproof underlayments are used.
(1) Metal Roofs. Minimum slopes for metal roofs vary from 4% to 33%, depending upon
roof type as indicated in Table 9-1.
(2) Membrane Slope. Low-slope membranes should also comply with a minimum slope
of 2% (1/4 in./ft.). Where ponding is unavoidable such as in spray ponds, a BUR with double
poured aggregate and bitumen is sometimes used. Coal tar pitch membranes are used at slopes
as low as dead level and to a maximum slope of 2% (1/4 in/ft.).