The rules

This all changed last year when the robust details accompanying the new Part L of the Building Regulations were published: they did not contain a design detail for built-in joists. The feeling was that they are more likely to be susceptible to air leakage than when floor joists are supported by hangers.

However, in its April technical newsletter Standards Extra, the NHBC pointed out that the robust details in Part L were only guidance, and that alternatives were acceptable if they met performance standards. After considering various proposals for built-in joists, it published "acceptable alternatives" for solid timber joists and timber engineered joists.

Solid timber joists
The NHBC states that solid timber joists can be built into the inner leaf of a cavity wall, provided that:

• The mortar joints around each joist perimeter are recessed or struck (pressed in at the lower edge)
  
• The joint between the masonry and the timber is carefully pointed with silicone mastic.

Timber-engineered joists
According to the NHBC, timber-engineered joists may be built into the inner leaf of a cavity wall provided that:

• Proprietary filler pieces are fitted on both sides of the web between the top and bottom flanges. The depth of the filler pieces should be slightly less than the dimension between the joist flanges to achieve a loose fit
  
• The mortar joints around each joist perimeter are recessed or struck
  
• The joint between the masonry and the timber and any other air paths are carefully pointed with silicone mastic.

The NHBC has listed a few points to watch out for when using these details:

• The sealant should be applied after the home is roofed so that the joists and masonry are reasonably dry
  
• Masonry blocks should be cut accurately (sawn where possible) and a struck or recessed mortar joint formed to allow a good silicone mastic bead to allow for shrinkage both of timber and masonry
  
• Expanding polyurethane should not be used for sealing around the joint ends
  
  
• Guidance accompanying new Part E acoustic Building Regulations only show joists supported on hangers for separating walls between dwellings. If designs are used that show built-in joists in separating walls then the ends should be sealed to limit air leakage.

Specification of floor hangers
The Heath and Safety Executive recently expressed concern that incorrect specification of joist hangers could potentially lead to a floor collapse during construction. As a result, the NHBC has reissued advice on the correct use of hangers, a number of which apply to the use of hangers with masonry walls.

l Use joist hangers that comply with BS 6178 Part 1, 1999 l Ensure that hangers and blocks are compatible. Do not use hangers marked for use with a particular block strength on blocks of a lower strength – so for example, do not use 3.5 N/mm² hangers on 2.8 N/mm² blocks

• Ensure that the hanger back-plates are flush to the wall and that the hangers are vertical
  
• Do not load the joists until the required number of courses have been laid above the masonry flanges (usually three block courses, but consult the hanger manufacturer) and the mortar joints have achieved the required strength. Alternatively install an engineer-designed support system under the joists and have it checked before loading the floor.

NHBC www.nhbc.co.uk