What modified bitumen actually is
A modified bitumen membrane is composed primarily of polymer-modified bitumen reinforced with one or more plies of fabric — polyester, fiberglass, or a combination. Factory surfacing, where applied, includes mineral granules, slag, aluminum or copper.
The waterproofing medium is traditional asphalt, modified with atactic polypropylene (APP), styrene-butadiene-styrene (SBS), synthetic rubber or other agents that enhance the asphalt's properties. APP and SBS are the most common modifiers.
SBS versus APP
SBS — Styrene-Butadiene-Styrene
SBS forms a polymer network within the bitumen, giving it rubber-like characteristics and improved resistance to aging and weathering. SBS sheets are set in hot mopping asphalt, torch-applied, or adhered with cold-process adhesives. Without factory granule or foil surfacing, they need a field-applied UV protective coating.
APP — Atactic Polypropylene
A thermoplastic polymer that forms a uniform matrix within the asphalt. It increases UV resistance, improves flexibility at low temperatures, and improves flow resistance at high temperatures. APP sheets are generally applied with a propane torch: the heat softens the underside into a molten adhesive that flows onto the substrate and cools into a waterproof bond.
The base sheet still matters
Modified bitumen systems are technically classed as single-ply, but Lloyd Roofing still installs a base sheet or underlayment beneath them — a 2-ply fiberglass platform minimum, or a 3-ply base, using standard glass-fiber felt, coated glass-fiber felt or coated organic felt.
That base protects the system from water penetration and protects the insulation from the heat of the torch. For that reason we treat MBS as a built-up roof.