Timber Roof Decking Installation

Decking stiffens the roof, helps spread loads and creates an accurate base for the layers above it. It is most useful in these situations:

• Renovations where the old roof structure is slightly uneven and needs stabilising
• Roof coverings that need a solid base, such as asphalt shingles or standing seam metal
• Insulated loft rooms, where the build-up has to be accurate and airtight
• Areas with heavy snow or strong wind, where a stiffer roof pays off

Not every roof needs it. On a new, accurate roof structure with ordinary tiles, a good underlay with counter-battens and battens is often enough. If the design does not call for decking, ask your carpenter to explain what it would actually do for your roof.

One more important point: decking does not fix a failing roof structure. If rafters are sagging, rotten or attacked by wood-boring insects, a new layer only hides the problem. Damaged timber has to be repaired or replaced first.

Timber board decking

Traditional decking uses planed or sawn boards, often tongue and groove. Boards adapt more easily to an uneven roof structure, unusual shapes and details around chimneys. That makes them a good fit for renovations, cottages and older houses. The timber must be dry and straight. Damp or warped boards twist after fixing, open up gaps and may creak later.

OSB decking

OSB comes in large sheets, so it goes down faster and gives a flat, solid surface. For roofs, use boards rated for humid conditions, usually OSB/3, with a thickness that suits the rafter spacing and the manufacturer's guidance. OSB is sensitive to moisture, though. It needs gaps between sheets, proper fixing, and it must not stay wet and unprotected for long.

The right choice depends on the type of roof, the condition of the structure, the planned covering and the whole roof build-up, including ventilation. That decision belongs to the design or an experienced carpenter, not simply to whichever option is quicker to fit.

1. Inspect the roof structure. The carpenter checks rafters, purlins, wall plates and joints, measures how flat the roof plane is and looks for signs of damp, rot or insects.
2. Repair and level. Damaged members are replaced or strengthened, and sagging rafters are packed out or levelled.
3. Prepare the material. Boards or OSB are stored dry and under cover, never straight on the ground.
4. Lay the decking. Work usually runs from the eaves up to the ridge. Board joints must always land on a rafter and be staggered between rows. OSB goes with its long edge across the rafters.
5. Fix it down. Every board is fixed to every rafter with nails or screws at the spacing the manufacturer recommends. With OSB, a small expansion gap is left between sheets, usually around 2 to 3 mm.
6. Form the details. Around chimneys, roof windows, valleys, hips and verges, the decking is cut and joined accurately.
7. Protect it and move on. The finished decking is covered with underlay as soon as possible so rain does not damage it.

This work is done at height on a sloping surface, so it should be carried out by a roofing or carpentry contractor with fall protection and proper scaffolding. It is not a safe DIY job.

The simplest case is a regular pitched roof with a gentle slope, a straight structure and good access for materials. More time and precision are needed with:

• A steep pitch where moving around the structure is difficult
• A complex roof with dormers, valleys and hips
• Many penetrations, chimneys and roof windows
• Stripping the old covering, battens and existing boarding
• Repairs to the structure that only show up once the roof is opened
• Poor access to the house and awkward lifting of materials onto the roof

In renovations, the real condition of the roof structure often only becomes clear once the roof is stripped. Agree with your carpenter in advance how unexpected repairs will be handled. If the work changes the roof shape, the load-bearing structure or the height of the house, check with your local building authority what approval you need under local building regulations. Do not assume it is automatically allowed. In a block of flats, any work on the roof goes through the building management or the owners' association.

In practice, a fault in the decking often does not show straight away. It appears during battening, when the covering goes on, or after the first winter. Typical mistakes include:

• Fixing decking to a sagging or damaged structure without repairing it
• Damp, warped or too thin boards
• OSB sheets butted tight with no gaps, which then buckle
• Joints that miss the rafters, or all joints in one line
• Too few fixings or unsuitable screws
• Inaccurate details at chimneys and valleys that leak later
• No ventilation in the roof build-up, so the timber stays damp

Why does ventilation matter so much? Decking is a continuous surface, and moisture from the rooms below or from leaks cannot always escape through it easily. If the build-up has no proper ventilation gaps, the decking, the insulation and the roof structure itself all suffer.

Before the underlay and covering go on, look over the decking yourself or ask the carpenter to show you:

• The surface is flat, with no obvious waves or dips
• Boards do not flex, creak or move when walked on
• Joints sit on rafters and are staggered between rows
• Gaps between OSB sheets are even
• Nails or screws are in every rafter and do not stand proud of the surface
• Details at chimneys, windows and valleys are cut accurately
• Edges at the eaves and verges are straight and ready for flashings
• The decking is dry and shows no signs of having been soaked for long
• The site is tidy and ready for the next layers