Deck Types for Flat Roofs

Materials used for the roof deck should be selected from those listed in BS:6229 2018 It is important to refer to this document for design guidance, but for convenience, the various materials are outlined below.

Reinforced concrete

Suitable for all Bauder waterproofing systems, this includes both in-situ and pre-cast forms, which can be covered with a cementitious screed if required. The surface should be wood float finished and the slab allowed to cure thoroughly and dry out to provide a suitable surface to which the membranes can be applied.

Profiled metal

Metal decking constructed to support built-up waterproofing systems should be of a suitable profile, i.e. featuring crowns that are wider than the respective troughs to achieve a minimum 45% bond area. The trough span should not exceed 150mm, otherwise the air and vapour control layer layer may sag at the laps and adequate support will not be provided for the insulation.

Suitably profiled decking is available in either aluminium or galvanised steel, however consideration should be given to the fact that many systems are applied using intense heat, in which case aluminium is best avoided as it is more likely to distort at high temperatures. Similarly, plastic or painted soffit types of decking may become damaged by the heat used when installing torch-applied membranes. This type of decking is not suitable for cold roof design unless it is subsequently overlaid with plywood or OSB.

Close boarded timber

Timber boarding suitable for roof decks should be a minimum 19mm nominal thickness, planed and closely clamped together, tongued and grooved or closely butted. When using this type of deck in conjunction with bonded vapour control layers, it will be necessary to first install a random nailed isolating layer prior to the installation of the main roofing system to cover the joints between adjacent boards. This decking is not recommended for hot melt or cold applied liquid systems


Plywood used for roof decks is generally 18mm thick, but it is possible to specify a minimum 15 mm if the supporting structure is more closely spaced. The plywood is normally square-edged. Longitudinal joints should occur on the centre line of supporting joists. Cross joints should be staggered and will require additional support, e.g. by noggins. A joint gap of 1 mm per metre of panel should be allowed at all edges. The plywood should comply with BS EN1995 to be suitable for structural use. Materials suitable for flat roofing would be marked BS EN 636-2 or BS EN 636-3 and must be suitable for exterior use. Plywood is suitable for all Bauder waterproofing systems.

Oriented strand board (OSB)

OSB used for flat roof decking should be type OSB/3 or OSB/4, conforming to BS EN 300: Thickness and fixing recommendations are similar to those for plywood. Suitable for use with all Bauder systems

Wood particle board (chipboard)

This type of deck is no longer recommended for use in flat roof applications due to its long term structural instability, especially if contaminated by moisture.

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