01 May 1999
e. Expansion and Seismic Joints. Flush joints are a high-risk feature. Joints should be at least
200 mm (8 in.) up on curbs. Use short legs at crossover corners and terminations to allow for differential
movement (figure 12-8).
f. Re-entrant Corners. On fully adhered membrane systems re-entrant corners can be high stress
points likely to suffer splitting. Permanent repairs may require the installation of a curbed expansion joint
or area divider. Flexible single ply systems are more capable of handling stress concentrations and
differential movement than BUR systems.
g. Roof Access. Roofs won't get inspected if they can't be accessed. Interior roof hatches or
permanent ladders are best.
Permanent ladders are sometimes installed to accommodate climbing on very
(2) Walkways. Walkways may help protect the roof against foot traffic especially at access
points and around mechanical equipment that must be serviced. These are available for metal roofs as
well as for membrane systems.
(3) Controlled Access. Gates, locks, or ladders that can only be reached by other ladders must
control unauthorized roof access.
h. Roof Venting. Attic ventilation reduces the accumulation of moisture and the formation of icings
at the eaves of steep roofing systems. Inadequate ventilation may result in growth of mold and mildew
and cause roof decking to buckle resulting in an uneven roof appearance. New soffit and ridge vents
may be needed if inspection indicates there are moisture problems. It is also very important to reduce air
exfiltration up into the roof by installing or improving air barriers and vapor retarders. Topside venting of
compact roofing systems is of dubious benefit.
i. Roof Decks and Nailers. Secure attachment of the roof deck to structural members is critical.
Refer to references on roof decks for details. For steel decks screw fastening is preferred over rewelding
as a method of repairing failed welds. Anchorage of nailers is very important. Additional fastening,
especially at perimeters, may be needed if evidence of deck movement or wind damage is found. Nailers
should be installed to meet Factory Mutual Data Sheet 1-49, even if FM Approval is not specified (CEGS
j. Underlayment. In some long life steep roofing systems such as slate and tile the valley metal
and underlayment may deteriorate before the primary covering. In this case removal of the roofing,
replacement of the flashing and underlay, and reinstalling the roofing may be appropriate. However,
localized leakage might indicate that only underlay repairs are needed.
k. Vapor Retarders.
(1) Sealed Retarders. When steep roof system require vapor retarders, the retarders (or air
barriers) will not be effective unless serious attempts are made to seal them at penetrations, edges, and
ends of batts. Unsealed penetrations at light fixtures and vent pipes can cause serious moisture
(2) Low-Slope System Retarders. In adhered low-slope membrane systems air exfiltration is
near zero. Since moisture problems are the result of air exfiltrations these systems have few moisture
problems. However, some may still require vapor retarders.