- Design guides
- Flat roof design considerations
Deck types for flat roofs
Materials used for the roof deck should be selected from those listed in the latest version of BS 6229:2003. It is important to refer to this document for design guidance, but, for convenience, the various materials are outlined below:
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.
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. The trough span should not exceed 150mm, otherwise the vapour barrier may sag at the laps and adequate support will not be provided or 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.
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 barriers, 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 suitable for hot melt or cold liquid applied systems.
Plywood used for roof decks is generally 18mm thick, but it is possible to specify a minimum 15mm 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 300:1997. Thickness and fixing recommendations are similar to those for plywood. Suitable for use with all Bauder systems.
When using this type of decking, slabs must conform to BS EN 13168 and be at least 50mm thick. They should have a prescreeded top surface and have channel reinforced sides. Please note that this type of decking is now classed as ‘fragile’ in the UK and therefore not recommended by Bauder for new build construction.
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.