A bitumen roof — called mastiek in Dutch — is a waterproofing layer of bitumen laid on a flat or shallow-pitched roof. Originally it was a hot, filler-thickened bitumen mass that was poured and worked flat. Today the covering is almost always made from factory-produced reinforced bitumen membranes with a polyester or glass fibre carrier.
How the covering is built up
A bitumen roof is normally laid in two layers. The base layer bonds to the deck and spreads stresses; the cap sheet provides the waterproofing and protects the bitumen from UV light. That is why the cap sheet usually carries mineral granules.
Two types of modified bitumen are common:
- APP (polypropylene modified) — high heat resistance, torch applied
- SBS (rubber modified) — stays flexible in cold weather, less prone to cracking
Sheets are torched, bonded (cold adhesive or hot bitumen) or mechanically fixed. On a timber deck and at upstands, torching is risky; self-adhesive or mechanically fixed systems are usual there now.
Points to watch
- Fall. Provide enough fall to the outlets; around 1 in 60 (roughly 1.6 per cent) is a sensible working minimum. Standing water shortens the life of the roof
- Laps. Keep generous laps — in the order of 80 to 100 mm on side laps and more on end laps — and look for a small bead of bitumen squeezing out: that is the proof the lap has taken
- Junctions. Almost every leak is at a detail rather than in the field: edges, upstands, penetrations and the angle between roof and wall. Use a cant strip there and take the upstand at least 100 to 150 mm above the finished roof
- Overflow. Fit an emergency overflow alongside the outlet so a blockage cannot pond the roof
- Life expectancy. A well-executed two-layer bitumen roof typically lasts 20 to 30 years; inspect laps, granules and outlets annually
- Fire safety. Where a torch is used, keep a fire watch and extinguisher to hand. Roof fires nearly always start during or shortly after the work
Related terms
- Bitumen
- Flat roof
- Roof covering
- Cant strip
- APP modified bitumen
- Roof upstand
- Emergency overflow
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