01 May 1999
(c) Metals should not be used as base flashing, but are suitable counterflashing
(d) Metal gravel stops embedded in the roof membrane should be avoided, as they
require frequent maintenance. When used they must be attached securely. Use raised edges when
possible (figures 12-7a and 12-7b).
(3) Sealants. Sealants are used primarily for metal work. Asphalt mastics are used on
bituminous flashing details.
o. Drainage, Valley and Intersection Details.
(1) Drainage. Roof drains should be sized to meet or exceed the appropriate plumbing code.
Secondary (overflow) drains or scuppers should be designed in accordance with ANSI/ASCE-7-95.
Over-the-eaves drainage is acceptable in areas where ice dams are not expected. Interior drains are
best in order to avoid drain and leader freeze-up. Primary roof drains should be recessed below the
roof drainage line by using deck sumps (uninsulated decks) or by creating a sump out of roof
insulation in order to properly collect water.
(2) Valley and Intersection Details. Valleys and intersections are fabricated out of membrane
materials. Crickets and saddles help divert water to the drains. Refer to NRCA details for their design.
4-3. DESIGN CONSIDERATIONS AND ALERTS.
a. Attachment. Bituminous membranes require strong and uniform attachment to their substrate.
The use of the nail-one, mop-one technique to install thermal insulation is a good way to achieve that.
b. Phased Construction. All components from the top of the roof deck through the completed
membrane (excluding surfacing) should be completed in a single day. Just installing a base sheet to
protect the substrate then returning later to install the rest of the plies, is called phasing and is to be
avoided. Dirt and debris that collects on the base sheet creates a plane of weakness in the
c. Bitumen Compatibility. Use bitumen compatible with the reinforcements and appropriate for
the climate and slope.
d. Walkways. Protective walkways are needed if more than occasional roof traffic is expected.
e. The Equiviscous Temperature Concept.
EVT controls application temperatures. Follow
NRCA procedures (See entire text of NRCA Bulletin in Appendix 4-1) for EVT use.
Coal Tar Pitch Slope. Do not use coat tar pitch specifications above 2% (1/4 in./ft.) slope.
g. Flood Coat. For the flood coat at least, use asphalt with the lowest practicable softening point
suited to slope and climate as follows: