APP-modified bitumen is a roofing and waterproofing material in which atactic polypropylene (APP) is blended into standard bitumen, significantly improving its resistance to ultraviolet radiation, high temperatures and ageing.
APP modification produces a plastic-like bitumen compound that remains flexible over a wide temperature range and can withstand prolonged sun exposure without cracking or becoming brittle.
How is APP-modified bitumen made?
During production, atactic polypropylene — a by-product of isotactic polypropylene manufacturing — is mixed with bitumen at high temperatures. The APP polymer dissolves into the bitumen matrix and changes its physical characteristics. The resulting compound is then coated onto a reinforcement carrier, typically polyester fleece or glass fibre, and finished with surface protection such as mineral granules, slate chips or a thin polyethylene film.
The finished product is supplied in rolls, commonly 1 metre wide and 5 to 10 metres long, ready for installation on site.
Installation method
APP-modified membranes are applied by torch-on technique. A gas burner melts the underside of the roll as it is unrolled across the substrate, forming a heat-welded bond. Because APP bitumen has a higher melting point than standard oxidised bitumen, the torch flame must be applied carefully to achieve proper adhesion without overheating.
Some manufacturers also offer self-adhesive or hot-air-weldable versions for situations where open flame is restricted.
Key properties
- UV resistance — APP membranes can be left exposed without additional protective coatings, making them suitable for flat roofs with mineral-finished top layers.
- High-temperature performance — softening point typically above 140 degrees Celsius.
- Elongation — good flexibility and ability to bridge hairline cracks in the substrate.
- Ageing resistance — slower degradation compared to unmodified bitumen.
APP versus SBS
The main alternative is SBS-modified bitumen (styrene-butadiene-styrene). SBS offers better low-temperature flexibility and elasticity, making it preferable in cold climates. APP excels in hot, sunny climates thanks to its superior UV and heat resistance.
Related terms
- SBS-modified bitumen
- Torch-on membrane
- Flat roof waterproofing
- Reinforcement carrier