01 May 1999
MB sheets has caused several very costly fires. Mop application avoids torch application by using hot
bitumen as the adhesive, similar to BUR applications. MB systems require somewhat hotter asphalt
(260C/500F+) at the point of application than most BURs do.
(7) Familiarity with the System and Site. Modified bitumens are well known to the commercial
roofing trade. Only Certified Torch Applicators should be permitted to use torches. The certification
(CERTA) consists of completing a minimum one-day safety-training course developed by the Midwest
Roofing Contractors Association.
(8) Life Expectancy. MB systems are estimated to have a useful life of 15 years or more.
Uncoated systems (where granules or foil have been omitted) require recoating at 5-8 year intervals.
(9) Costs. MB systems are competitive with BUR systems.
(10)Structural Considerations. As with BUR, most MB membranes are placed on thermal
insulation. Nailed or mopped base sheets are needed. Torching directly to thermal insulation is not
5-2. BUILDING ELEMENTS.
a. Slope. The minimum slope of 2% (1/4 in./ft) applies to MB systems. Maximum slope depends
upon technique of application and backnailing. MB is used as vertical flashings when adequately top
b. Low-Sloped Roofs. Place sheets so that head and sidelaps do not buck water flow. Since
thick sidelaps and tee joints are vulnerable to water penetration, minimize these features in valleys.
c. Steep Sloped Roofs. MB systems are suitable for folded plate and barrel roofs. Torched and
cold adhesive specifications may allow slopes to 50% (6:12) with backnailing.
d. Structural Considerations. Pallets of MB rolls are heavy and weight should be distributed on
the roof deck to avoid damage. In general, MB systems do not use ballast or aggregate and are
therefore lightweight systems.
e. Expansion Joints, Seismic Joints and Area Dividers.
(1) Expansion Joint Locations. Joints should be located at high points where practicable and
placed on curbs above the water line. Membrane expansion joints should be provided only at each
expansion joint in the structure. Structures typically need expansion joints at intervals not over 60 m
(200 ft) in length or width. Joints should also be used at changes in deck direction or membrane
(2) Area Dividers. Area dividers do not provide for movement but are useful in subdividing
large areas into isolated smaller areas, each of which can be roofed (and maintained) separately.
f. Re-entrant Corners. Expansion joints or area dividers should be used at most re-entrant
corners where stress concentrations are likely.
g. Roof Venting. Roof vents have been proven to be of no value in drying wet materials and their
efficacy for pressure relief is questionable. Roof vents may be required in some proprietary wet-fill
deck materials in conjunction with venting base sheets. When insulation and a MB membrane are
installed over an existing bituminous membrane roof edges should not be sealed. This will prevent