What is SBS-modified bitumen?

SBS bitumen is bitumen to which the rubber-like polymer styrene-butadiene-styrene (SBS) has been added. Ordinary bitumen becomes hard and brittle in the cold and soft in the heat. SBS modification makes the material elastic like rubber: it remains flexible even at low temperatures and can follow movements of the substrate without cracking. SBS bitumen is supplied as roofing rolls with a polyester or glass fibre reinforcement.

SBS versus APP

Alongside SBS, APP-modified bitumen (with atactic polypropylene) is the other widely used type.

• SBS: elastic, good resistance to cold and movement. Often used in two-layer systems.
• APP: more plastic than elastic, resistant to higher temperatures and UV, and easy to torch-apply.

Which suits best depends on the roof, the substrate and the manufacturer’s system.

Installation

• Torch-on: the roll is heated with a gas torch and bonded in place. This is work for a professional with fire safety precautions, especially on timber roofs and roof edges.
• Cold-bonded or self-adhesive: some SBS rolls are bonded cold or have a self-adhesive underside. Handy where you do not want an open flame.
• Mechanically fixed: the underlayer is screwed down, then the cap sheet is torched on top.

A common system consists of an underlayer and a cap sheet with a slate granule finish, which protects the bitumen from UV.

Applications

• Flat and low-pitched roofs of houses, extensions, garages and commercial buildings.
• Renovation of old bitumen roofs, often laid over the existing layer if it still adheres well.
• Foundations and basement walls as a waterproof layer.

Tips

• Have the roof inspected once a year, particularly seams, upstands, penetrations and the rainwater drainage.
• Keep the roof clean: leaves and moss hold water.
• Do not walk on the roof with sharp objects or heavy loads without protective boards.

Related terms

• Bitumen (mastic) roof
• Roof covering
• ECB (ethylene copolymer bitumen)
• Adhesion

Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.