01 May 1999
MODIFIED BITUMEN ROOFING SYSTEMS
REFERENCE CEGS 07551
5-1. MB ROOFING SYSTEMS.
a. Overview. As contrasted to BURs which rely on field application of bitumen to make them
waterproof, polymer modified bituminous systems (MB) use reinforcing sheets that have been factory-
coated with rubberized bitumen. The sheets themselves are inherently waterproof. The use of
polymers mixed with the coating bitumen improves flexibility, toughness and low temperature
properties. A major advantage of MB systems over BUR is that they generally use fewer layers and
are therefore less labor and material intensive. Two categories of MB roofing are SBS (ASTM D6162,
D6163, D6164), and APP (ASTM D6222 and D6223).
(1) Three basic types of MB Systems.
(a) Heat fused systems use a torch or hot air to remelt the factory applied bitumen for use
as the adhesive.
(b) Mopped-in-place systems which, like BUR systems, still rely on field-applied hot
bitumen as the adhesive.
(c) In a few instances a solvent-based cold adhesive is used to adhere and seal the
sheets (sometimes only the field of the sheet is adhesively bonded while side and end laps are heat
(2) Polymer Modifiers.
Generally either thermoplastic (Atactic Polypropylene/APP) or
elastomeric (Styrene-Butadiene-Styrene/SBS) modifiers are used. Included in the thermoplastic
modifier group are APP, APO (Alpha-Polyolefin) and Isotactic Polypropylene (IPP). APP systems
usually are torch fused while SBS systems may be mop or torch applied. Both are occasionally
installed using cold adhesive.
(3) Reinforcements. Reinforcements of glass fiber, polyester or combinations may be either
woven (scrim) or non-woven. The reinforcement carries the sheet during manufacture and provides
tensile strength and stability to the finished membrane.
(4) MB Systems. The completed membrane is frequently only two layers, compared to three
or more in BUR. The top layer is surfaced with mineral granules, metal foil, or occasionally it is
smooth surfaced. The base layer may be polymer modified or a conventional, less expensive coated
BUR base sheet. Multiple plies of BUR ply felts are sometimes used below. When BUR components
are used with MB capsheets the roofing is referred to as a hybrid system.
(5) Climate/Weather. Coated sheets, even with polymer modification, are stiff in cold
weather. Rolls should be stored at temperatures above freezing for 24 hours before unrolling. SBS
sheets are more flexible in the cold than APP sheets and thus are preferred in cold regions. MB
sheets will blister if not fully adhered due to cold, wet or frozen substrates. In hot climates SBS sheets
are prone to slippage. Backnailing and careful use of approved mopping bitumen (special Type IV
asphalt) is necessary.
(6) Logistics. While material efficient as compared to BUR, MB systems require relatively
heavy rolls. Torch application uses propane which necessitates safe handling and storage. A fire
watch is needed each day for a period of time after the last torch is extinguished. Careless torching of