Flat roofs and balconies

7.1.4Timber and timber decks

Timber flat roofs and balconies shall be of adequate strength and durability, and be installed to form a satisfactory substrate for the waterproofing system. Issues to be taken into account include:

  1. structure and durability
  2. joist hangers, straps and strutting
  3. installing timber substrates.

Structure and durability

Timber should be:

  • checked for conformity with the design upon delivery
  • rejected where excessively wet, damaged or not of a suitable quality or shape
  • stored under cover to prevent wetting
  • preservative treated or naturally durable, in accordance with Chapter 3.3 ‘Timber preservation (natural solid timber)’
  • retreated along the cut edges with a coloured preservative, where preservative treated timber has been cut.

Timber decks should:

  • be in accordance BS EN 1995-1-1 or appropriate load/span tables published by TRADA in support of building regulations
  • be from regularised timber, dry graded to BS 4978 and marked ‘DRY’ or ‘KD’ where softwood is used internally
  • have I-joists or metal web joists specified in accordance with the manufacturer’s recommendations and not used where any part of the joist is exposed to external conditions
  • have joists which are sized and spaced in accordance with the design and at a maximum of 600mm centres
  • be temporarily covered to prevent wetting, unless the waterproofing is to be installed immediately
  • be level and, where necessary, using hard packing such as tiles or slates bedded in mortar to adjust joists (loose or soft packing, including timber, should not be used)
  • formed with one of the materials listed in Table 1.

Table 1: Materials used for decks

MaterialThickness of deck (mm): 450mm joist centresThickness of deck (mm): 600mm joist centres
Plywood to BS EN 636, Class 31518
Oriented strand board, type OSB31518
Pretreated timber planking, tongue and grooved (close boarded timber)
Maximum board width 100mm

Structural elements of balconies should have a service life of at least 60 years.

Timber in balconies should be limited to elements which are supported by materials other than timber. Timber should not be used for:

  • gallows brackets supporting a balcony
  • posts or columns supporting a balcony
  • guardrails or their support
  • cantilevered decks or joists
  • infill joists.

Decking boards should be specified and fixed in accordance with:

  • guidance from the Timber Decking and Cladding Association, or
  • an engineer’s design, in accordance with Technical Requirement R5.

Joist hangers, straps and strutting

Masonry carrying joist hangers should be level and at the correct height.

Mild steel straps and fixings should be protected against corrosion in accordance with BS EN 845-1.

Joist hangers should be:

  • in accordance with BS EN 845-1
  • the correct size for the timber joist or trimmer
  • fixed in accordance with the design.

Where holding-down straps are required to prevent the roof from lifting from the supporting structure, they should be:

  • spaced at a maximum of 2m centres
  • fixed with a minimum of four hardened nails 4mm in diameter x 75mm long, or No 12. wood screws x 50mm long, into plugs (where fixed to masonry)
  • fixed with the lowest fixing secured within 150mm of the bottom of the vertical strap
  • 30mm x 2.5mm and 1m long
  • predrilled for fixings.

Strutting should be provided to prevent excessive movement, and:

  • be either herringbone type (timber 38mm x 38mm), solid blocking (38mm thick timber x 0.75 depth of joist) or proprietary steel strutting
  • not prevent cross ventilation in cold deck roofs.

Table 2: Spacing for strutting

Joist span (m)Rows of strutting
Up to 2.5None needed
2.5-4.5One (at centre of span)
Over 4.5Equally spaced along the span at maximum 2.5m centres

Installing timber substrates

When installing timber substrates:

  • conditions should be dry, and materials protected from wetting until the roof is complete
  • the area of deck installed should be of a size which can be quickly covered in the event of rain
  • joints in sheet materials which are precovered or coated should be sealed immediately after fixing
  • materials that have been damaged or adversely affected by moisture should be discarded.

Plywood and oriented strand board should:

  • have tongued and grooved boards installed with the long edge at right angles to the joists, and short edges supported on a joist or nogging
  • have a maximum movement gap between boards of 3mm for square edge boards
  • have a minimum movement gap of 10mm where boards abut a rigid upstand
  • be supported on noggings where the edges of boards situated along the roof perimeter do not coincide with joists
  • be fixed at a maximum of 100mm centres (unless the design specifies closer)
  • be fixed with flat-headed ring shank nails (50mm long x 3mm for plywood, 3mm x 2.5 x board thickness for OSB).

OSB should be:

  • installed over supports in the direction indicated on the boards, with the stronger axis installed at right angles to the supporting joists
  • fixed a minimum of 9mm from the edge of the board.

Softwood tongued and grooved boarding should be:

  • closely clamped together with end joints staggered
  • fixed with two ring shank nails to each joist or firring, with nail heads punched below the timber surface.