Flat roofs, terraces and balconies

7.1.6Timber structure and deck

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

  • Structure and durability
  • Joist hangers, straps and strutting
  • Installing timber decks

Structure and durability

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 including their support
  • infill joists
  • cantilevered joists or decks

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.

The use of timber in balcony and terrace constructions may also be restricted by fire protection requirements set out in the National building regulations.

Timber, including solid and engineered joists, 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 but avoid sweating.
  • 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.
  • be temporarily covered to prevent wetting, unless the waterproofing is to be installed immediately.

Timber structure should:

  • be in accordance BSEN 1995-1-1 or appropriate load/span table 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 joists is exposed to external conditions.
  • have joists which are sized and spaced in accordance with the design and at maximum 600mm centres
  • 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)

Timber decks should be formed from one of the materials listed in Tables 3, 4 and 5.

Table 3: Materials used for deck to flexible membrane roof coverings

MaterialMinimum thickness of deck (mm) ¹ 450mm joist centresMinimum thickness of deck (mm) ¹ 600mm joist centres
Plywood board to BS EN 636, Use Class 3.2, ‘S’, ²15 ⁴18 ⁴
Oriented strand board, to BS EN 300 type OSB315
18
Pre-treated timber planking, tongue and grooved (close boarded timber). Maximum plank width 100mm ³1919

Notes

1 Thickness of deck may need to be thicker to resist pull-out forces on fixings.

2 All square board edges to be supported.

3 Moisture content between 16-20% at time of fixing.

4 For curved roofs two layers of thinner boards to achieve min thickness.

Table 4: Materials used for deck for fully supported traditional hard metal roof coverings

MaterialMinimum thickness of deck (mm)
Plywood board to BS EN 636, Class 3.2, ‘S’ ¹ square edged with 2-3mm gaps between boards ³18 ²
Pre-treated timber square edged sarking boards, width 100-125mm with 3-5mm gaps between ⁴18

Notes

1 avoid yellow and maritime pine or low-quality softwoods.

2 for curved roofs two layers of thinner boards to achieve min 18mm thickness

3 all board edges to be supported.

4 Moisture content between 16-20% at time of fixing.

Table 5: Materials used for deck to lead roof coverings

MaterialMinimum thickness of deck (mm)
Plywood board to BS EN 636, Class 3.2 ‘S’ ¹ square edged with 3-5mm gaps between boards ³18 ²
Pre-treated softwood timber square edged sarking boards, width 100-125mm with 2-3mm gaps between ⁴18

Notes

1 avoid oak, Douglas fir and western red cedar.

2 for curved roofs two layers of thinner boards to achieve min 18mm thickness

3 all board edges to be supported.

4 Moisture content between 16-20% at time of fixing.

Joists 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
  • 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 structural, they should be:

  • Spaced at a maximum of 2m centres at the perimeters.
  • Fixed with 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 or I-joist sections.
  • Not prevent cross ventilation in cold roofs.
  • Spaced in accordance with Table 6.

Table 6: 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 decks

When installing timber decks:

  • 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
  • materials that have been damaged or adversely affected by moisture should be discarded
  • tops of boards should be laid flush with no deviation greater than 2mm
  • boards should be laid in a staggered pattern
  • cut edges should be treated to prevent moisture ingress
  • should have a minimum ‘good one side’
  • should have all nails and screws punched or countersunk below the surface of the board.
  • Should be free of sharp arises on external angles

Plywood and oriented strand board (OSB) should:

  • have tongued and grooved boards installed with the long edges at right angles to the supporting firrings or joists, and short edges supported on a joist or nogging. Note: use square edge boards for support of hard metal and lead roofs.
  • have edges of square edged boards supported on joists or noggings with movement gaps of 3-5mm between boards.
  • have a minimum movement gap of 10mm (or 2mm per metre of boarding whichever is the greater) 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, and where square edged boards within the roof area do not coincide with a joist.
  • be fixed at a maximum of 100mm centres (unless the design specifies closer)
  • be fixed with flat-headed ring shank nails (minimum of 50mm long x 3mm for plywood, minimum of 3mm x 2.5 x board thickness for OSB) or screws of at least the same length.

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 8mm 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.

Cross Laminated Timber (CLT)

Due to the semi-impervious nature of CLT panels and the risk of trapping moisture in the panels during the construction period, CLT panels are not accepted for flat roof construction.

Structural insulated panels (SIPs) (used as self-supported deck)

Due to the difficulty in applying an effective AVCL to the warm side of a SIPs panel to prevent water vapour entering the panel, and the risk of trapping moisture in the panels during the construction period, SIPs panels are not accepted for warm flat roof construction.