Timber and concrete upper floors

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6.4.10Construction of timber floors

Upper floors shall be constructed in a workmanlike manner and provide satisfactory performance. Issues to be taken into account include:

  1. levelling
  2. joist spacing and clearance
  3. support.


Bearings for joists should be level. The floor should be levelled:

  • from the staircase trimmer and trimming joist
  • in accordance with the manufacturer’s recommendations
  • using hard packing; loose or soft packing should not be used.

Joist spacing and clearance

Joist spacing should:

  • be in accordance with the design and not increased
  • account for the decking material to be used
  • be a maximum of 600mm
  • have a clearance of 25-75mm between the first joist and the wall face to aid the installation of services and the fixing of floor decking.


The floor should have an adequate bearing on the supporting structure.

Timber joists should normally have a minimum bearing as shown in Table 3.

Table 3: Support of joists

Type of timber joistMinimum bearing (mm)
End supportIntermediate support
Solid joist on masonry walls90 (75)90 (75)
Solid joist on timber wall plate7575
I-joist90 (45)90
Metal web joist90 (75)90

The figures in brackets should only be used when the joist is not providing restraint to the wall.

Joists may be:

  • supported on joist hangers or on internal load-bearing walls
  • built into the inner leaf of an external cavity wall, with care taken to ensure air-tightness.

Where joists are built into separating walls, fire- and sound-resisting performance, in accordance with building regulations, should be taken into account.

Solid timber joists

Where built into solid external walls, joists should be treated with preservative.

I-joists and metal web joists

I-joists and metal web joists should not be built into solid external walls.

The support reaction, due to dead and imposed loads on the floor, should not exceed the recommended value specified by the manufacturer.

Where there are concentrated loads:

  • web stiffeners should be used for I-joists
  • uprights between the flanges, held in place by punched metal plate fasteners or bottom chord (flange) support, should be used for metal web joists
  • the manufacturer’s recommendations should be followed.

Where joists are supported on walls, noggings may be required at the top flange along the wall to support the floor decking, and at the bottom flange to support the plasterboard ceiling. Where joists are not built into brickwork or blockwork, blocking should be provided at the joist bearing. The blocking may be used for fixing plasterboard and floor decking.