Built-up Roofing Felt ~ this consists of three layers of bitumen
roofing felt to BS EN 13707, and should be laid to the
recommendations of BS 8217. The layers of felt are bonded
together with hot bitumen and should have staggered laps of
50 mm minimum for side laps and 75 mm minimum for end laps † for
typical details and references see pages 448 & 449.
Other felt materials which could be used are the two layer
polyester based roofing felts which use a non-woven polyester
base instead of the woven rag fibre base used in traditional felts.
Mastic Asphalt ~ this consists of two layers of mastic asphalt laid
breaking joints and built up to a minimum thickness of 20 mm and
should be laid to the recommendations of BS 8218. The mastic
asphalt is laid over an isolating membrane of black sheathing felt
which should be laid loose with 50 mm minimum overlaps.
Typical Datails ~
Ref. BS 8218: Code of practice for mastic asphalt roofing.
Typical Timber Flat Roof Coverings