Premier Mastic Asphalt

Roof Decks

Structural decking to comply with the recommendations of B.S. 6229:2003 – ( Flat roofs with continuously supported coverings) and be laid in accordance with relevant Codes of Practice.

All substrates to receive mastic asphalt roofing must be true, plane and even free from ridges, hollows and identations.

IN SITU CONCRETE

Concrete laid in situ to receive asphalt should have a surface free from ridges and indentations. To achieve this and to provide adequate falls it is usually necessary to apply a sand and cement screed. Construction water should be drained by forming temporary drainage holes through slab – Reference BS 6229.

PRECAST CONCRETE

A variety of precast deck units are available in dense or lightweight concrete and these should be used and fixed in accordance with the manufacturer’s instructions.

Precast concrete units usually require a screeded finish to take out any deck irregularity and provide drainage falls unless specifically excluded by the manufacturer.

Construction water should be drained by leaving the deck joints open or by forming temporary drainage holes through the deck – Reference BS 6229.

TIMBER

Where the sub-structure consists of timber board, plywood, woodwool slabs or decking units liable to slight leteral movement a free standing splayed timber kerb must be fixed to the sub-structure adjacent to walls and upstands (see detail finishes page 5).

TIMBER BOARD

Timber boards to receive asphalt should be tongued and grooved, well seasoned, at least 19mm nominal thickness, closely clamped together and well nailed.

PLYWOOD

Plywood roof decks should be designed to minimize the effects of warping, shrinkage or movement of the boards. Timber surrounds are necessary at eaves and verges. Decks of this type should be kept dry. They should not be fixed in position unless they can be covered the same day. For structural deck applications plywood should comply with the relevant requirements of BS EN 636-3:1997, be WBP bonded in accordance with BS EN 314-2:1993 Part 8, and be at least 19mm thick.

WOODWOOL SLABS

These must be of a roofing grade, complying with BS EN 13168:2001 Type SB fixed according to the manufacturer’s instructions to timber joists or steel purlins with all joints taped. Pre-felted or un-screeded types would only be used in conjunction with board insulation in a warm roof specification.

METAL DECKING

Steel or aluminium decking with a vapour control layer, insulation boards and asphalt can be laid to form a complete roof structure. When asphalt is used in conjunction with metal decking, the deflection should not be more than 1/325. The decking units are fixed in accordance with the manufacturer’s instructions. The vapour control layer is bonded to the top surface of the metal decking, followed by the insulation board which is bonded in hot bitumen. Timber facings to free standing metal upstands are required for the application of the asphalt finishes. For high humidity situations aluminium decks are recommended, but if steel decks are used a protective coating to both sides of the deck should be specified.

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