SBS or APP? Material science of modern roof membranes

The word felt roofing comes from when bituminous felt was impregnated with coal tar. Today the material is something quite different: a membrane of bitumen — a byproduct of oil refining — that is reinforced with armature and additives of polymers that improve properties. If you are to work professionally with the material, you must know the two major polymer families: SBS and APP.

Pure bitumen has a fundamental problem: it becomes brittle and stiff in frost and soft and flowing in strong summer heat. A roof in Denmark must be able to handle both – from hard night frost in February to a black roof surface that gets 70-80 degrees warm in July sun. By mixing polymers into bitumen you move the material's working range so the membrane remains functional throughout the temperature range.

SBS stands for styrene-butadiene-styrene a synthetic rubber type. SBS-modified bitumen is called elastomer bitumen because the material behaves like rubber: it can stretch and return to its original shape. The great strength is cold flexibility — the membrane remains elastic at low temperatures which according to manufacturers like SOPREMA makes SBS the safe choice in a climate like Denmark where roof movements and frost effects are everyday. In return SBS is less UV-resistant in itself and is therefore typically protected by slate chippings on the surface.

APP stands for atactic polypropylene, a thermoplastic. APP-modified bitumen is called plastomer bitumen and forms a uniform plastic matrix in the bitumen. The advantages are high UV resistance and good shape stability at high temperatures — APP membranes do not flow as easily in heat. Therefore APP is more often seen in southern European countries and on very sun-exposed surfaces, while in Nordic cold it is stiffer than SBS.

Inside every modern membrane is a reinforcement — typically polyester felt, glass felt or a combination. The reinforcement provides tensile strength and dimensional stability, so the membrane can absorb building movements without cracking. Outside the reinforcement is the modified bitumen, and on top of the upper sheet is a layer of slate granules that protects the bitumen from the sun's UV rays and mechanical wear.

• 01Base layer: the bottom layer in a two-layer system—mechanically fastened bonded or welded to the substrate and forms the first waterproofing.
• 02Top ply: the uppermost, slate-covered layer — welded on top of the underlay with offset joints so there is never a joint on a joint.
• 03Single-ply solutions: available for e.g. sheds and carports, but on large and low-slope roofs, two ply is the robust standard solution.
• 04Overlap: the sheets are always laid with overlap, which is welded homogeneously — it is the joints, not the surfaces, that determine whether the roof becomes tight.

For you as a roofer the material choice means something very concrete: SBS membranes are welded at lower temperatures and are more forgiving to work with in cold weather while APP requires more heat and sets faster. Mixing the products incorrectly — for example welding an APP membrane onto an SBS surface without knowing compatibility — risks joints that look tight at handover but fail after a few frosty winters. Material science is not theory for its own sake; it is the prerequisite for a roof that lasts the 20-30 years a properly executed tar paper roof is expected to last.