APP Modified Bitumen

APP modified bitumen is roofing membrane made from asphalt blended with atactic polypropylene, a plastic polymer. The blend is often called "plastomeric" bitumen, as opposed to the elastomeric SBS type. APP modification was first reported in Italy by Romolo Gorgato in 1967 and was available in the Netherlands and Great Britain by the mid-1970s.

Composition

APP compounds use a relatively high polymer content, roughly 25-30% polypropylene blended into asphalt over several hours. The result is a stiffer compound with good resistance to high temperatures and ultraviolet light. APP sheets are most often reinforced with heavy spunbonded polyester.

Installation

APP sheets are designed to be torch-applied: the installer heats the underside until the compound flows, then rolls the sheet into place so it bonds to the substrate and seals the laps. APP needs higher torching temperatures than SBS. Some products have special backings for cold adhesive, but APP sheets are generally not compatible with hot-mopped asphalt.

Torching is a fire risk, and insurers have placed increasing scrutiny on torch-applied systems. Safe practice includes fire watches, keeping flames away from combustible substrates and edges, and using flame-free details at vulnerable locations.

Uses

APP membranes are used as cap sheets, base sheets, flashing membranes and below-grade waterproofing. Many APP cap sheets are mineral-surfaced; some smooth sheets are coated after installation.

Comparison with SBS

APP is more heat- and UV-tolerant but less flexible in cold weather than SBS, and offers fewer installation choices.

Standards

In North America APP sheets are covered by ASTM product specifications for atactic polypropylene modified bituminous sheet materials, and repair guidance for both APP and SBS membranes appears in the ARMA/NRCA/SPRI repair manual for low-slope membrane roof systems.