01 May 1999
embedment of the polystyrene. This technique is very vulnerable to workmanship error, too cool--no
adhesion; too hot--insulation damage. Solvents used in cold process systems also damage
(5) Thermal Bridging and Nail Popping. When mechanical fasteners are used to secure
thermal insulation, it is recommended that the nail-one, mop-one technique be used. This minimizes
thermal bridging and avoids nail popping of the membrane. When multiple layers of insulation are
used, joints should be offset to reduce heat losses. Straight-through joints can take away 10% of a
roof's insulating ability.
Membranes and Surfacings.
(1) Reinforcements. Reinforcements are internal to the MB sheets. They consist of woven
and non-woven glass fiber or synthetic fibers. The MB family of products includes:
(a) Conventional and polymer-modified base sheets.
(b) Polymer-modified capsheets with mineral surfacing or metal foil.
(c) Unsurfaced MB sheets which must be field coated for heat reflection and UV
(d) Glass ply felts (used in hybrid systems and as fire protective underlayments).
(2) Base Sheets. Base sheets form the bottom ply in many MB specifications. The base
sheet can be nailed to nailable substrates, or mopped in place on others. Both unmodified coated
base sheets and polymer-modified base sheets are used. In addition to bridging the substrate, the
base sheet may serve as a fire barrier when installing torched-on MB systems. Conventional BUR
base sheets should meet ASTM D4601 (glass fiber base sheet), D2626 (coated organic base sheet),
or D4897 (glass fiber venting base sheet).
(3) Ply Sheets. Ply sheets, if used, should meet ASTM D2178 Type IV or VI (glass fiber ply
(4) MB Capsheets. Capsheets are the heart of most MB systems. They contain polymer-
modified bitumen and may be unsurfaced or surfaced with mineral granules or metal foil. Torching
grade materials have a thicker bottom side coating to serve as hot melt adhesive. Current ASTM
standards include the following with additional standards under development:
SBS sheets with combination of polyester and glass fiber reinforcements
SBS sheets with glass fiber reinforcement
SBS sheets with polyester reinforcement
APP sheets with polyester reinforcement
APP sheets with glass fiber reinforcement
(5) Mopping Asphalt. Asphalt meeting ASTM D312 Type IV (or occasionally Type III) is used.
(6) Cold Adhesives. Solvent-based adhesives are also used to install MB systems. They may
be ordinary cold process bituminous adhesives or polymer-modified bituminous adhesives depending
upon the MB system used. Adhesive and MB sheets should be from a single manufacturer.
(7) Surfacing Options.
(a) Surfacing is part of the capsheet such as in mineral granule and metal foil systems.