7.1.6 Timber 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.
7.1.6.1 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 (TDCA), 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 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)
- re-treated 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 BS EN 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
Note(s)
1. 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 minimum thickness.
Table 4: Decking materials for fully supported traditional hard metal roof coverings
| Material | Minimum thickness of deck (mm) |
|---|---|
| Plywood board to BS EN 636, Class 3.2, ‘S’ ¹ square edged with 2-3mm gaps between boards(3) | 18(2) |
| Pre-treated timber square edged sarking boards, width 100-125mm with 3-5mm gaps between(4) | 18 |
Note(s)
1. Avoid yellow and maritime pine or low-quality softwoods.
2. For curved roofs, two layers of thinner boards to achieve minimum 18mm thickness.
3. All board edges to be supported.
4. Moisture content between 16-20% at time of fixing.
Table 5: Decking materials for lead roof coverings
| Material | Minimum thickness of deck (mm) |
|---|---|
| Plywood board to BS EN 636, Class 3.2 ‘S’ ¹ square edged with 3-5mm gaps between boards(3) | 18(2) |
| Pre-treated softwood timber square edged sarking boards, width 100-125mm with 2-3mm gaps between(4) | 18 |
Note(s)
1. Avoid oak, Douglas fir and Western red cedar.
2. For curved roofs, two layers of thinner boards to achieve minimum 18mm thickness.
3. All board edges to be supported.
4. Moisture content between 16-20% at time of fixing.
7.1.6.2 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
- be spaced in accordance with Table 6.
Table 6: Spacing for strutting
| Joist span (m) | Rows of strutting |
|---|---|
| Up to 2.5 | None needed |
| 2.5 – 4.5 | One (at centre of span) |
| Over 4.5 | Equally spaced along the span at maximum 2.5m centres |
7.1.6.3 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
- boards should have a minimum ‘good one side’
- boards should have all nails and screws punched or countersunk below the surface of the board.
- boards 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 noggin (always use square edge boards for support of hard metal and lead roofs)
- have edges of square edged boards supported on joists or noggins 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 noggin 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, or for OSB, a 3mm gauge nail size of minimum length 2.5 x board thickness. For woodscrew fixings, the same minimum lengths will apply.
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, they are not accepted for flat roof construction.
Structural insulated panels systems (SIPS) (used as self-supported deck)
Due to the difficulty in achieving effective continuity of an AVCL across the warm side of a SIPS deck construction, and the risk of trapping moisture in these panel assemblies during the construction period, SIPS elements are not accepted for warm flat roof construction.
Last updated: 2nd January 2025