A flat roof is only as good as the carpentry beneath the covering. RPG Carpentry & Joinery builds the timber structure for flat roofs and decking across Devon and Cornwall — correctly sized joists, accurate firrings and falls, and solid decks that keep extensions, garages and garden spaces watertight in the wettest of South West weather.
Flat roofing carpentry is the structural first-fix work that the waterproof layer sits on top of. Get it right and the roof lasts for decades; get it wrong and no covering can save it. We size and install the joists, build in the falls with tapered firrings, lay and fix the deck, and form the upstands, edges and outlets ready for the weatherproofing. We also build timber garden decking that uses the same principles of solid framing and proper drainage.
The complete timber structure for extension, garage, dormer and outbuilding roofs — built to span correctly and carry the loads the South West weather throws at it.
Graded structural joists at the correct centres, topped with exterior-grade ply or OSB3 decking, screwed and sealed ready for an EPDM, GRP or felt finish.
Tapered firring strips that build the all-important fall into the roof, so rainwater drains away cleanly instead of ponding and causing leaks.
Raised and ground-level decks in softwood or hardwood, framed on proper bearers and posts for a level, long-lasting outdoor space.
The Devon and Cornwall climate is unforgiving on flat roofs. Driving rain off the Atlantic, long wet winters and exposed coastal sites mean that any weakness in the structure shows up quickly. We treat the carpentry as the foundation of a watertight roof: joists sized to span without sagging, firrings set out to a reliable minimum 1-in-40 fall, and a deck that is fully supported with no soft spots or movement.
Most of our flat roofing is part of a larger job — a single-storey extension, a kitchen or utility addition, a garage or a porch. Because we carry out the wider construction carpentry too, the roof integrates properly with the new walls, the existing roofline and the rainwater goods, rather than being a bolt-on afterthought. We coordinate with the roofing covering specialist so the deck is built to exactly suit their system.
We use graded C16 or C24 structural timber for joists and firrings, sized to the span and load of each roof. Decking is exterior-grade plywood or OSB3, chosen to suit the waterproof covering going over it. For the weatherproof layer itself we work alongside EPDM rubber, GRP fibreglass and modern torch-on felt systems — all proven performers against the wind-driven rain of the South West.
For garden decking, pressure-treated softwood offers excellent value and a long life, while hardwoods such as oak and balau give a premium finish and superb durability in coastal gardens. We frame every deck on solid bearers with adequate ventilation underneath, which is essential in damp Devon and Cornwall conditions to stop the timber rotting from below.
The figures below are a guide for flat roof carpentry and decking across Devon and Cornwall. The waterproof covering is usually quoted separately, and span, access and timber grade all affect the price. We give a fixed quote after seeing the project.
| Work | Price Range | Notes |
|---|---|---|
| Garage / small extension roof structure | £1,500 – £3,000 | Joists, firrings & deck |
| Larger extension flat roof | £3,000 – £4,000+ | Excludes waterproof covering |
| Softwood garden decking | £80 – £150/m² | Supplied & fitted |
| Hardwood garden decking | £150 – £250/m² | Oak or balau |
Properly set-out firrings and falls so water always drains away — the single biggest factor in a long-lasting flat roof.
We handle the wider carpentry too, so your flat roof ties cleanly into the extension, walls and rainwater system.
Structures and decks built to take everything the Atlantic-facing South West climate delivers.
Flat roof carpentry for a typical single-storey extension or garage usually costs between £1,500 and £4,000 for the timber structure and deck, before the waterproof covering. Garden decking is generally priced at £80 to £150 per square metre for softwood and £150 to £250 per square metre for hardwood, supplied and fitted. We provide a fixed quote once we have seen the project.
A flat roof is never truly flat. It needs a slight slope, called a fall, so that rainwater runs to the gutter or outlet rather than ponding on the surface. In the wet Devon and Cornwall climate this is critical: standing water shortens the life of any covering and is a leading cause of leaks. We create the fall using tapered timber firrings fixed over the joists, typically to a minimum 1 in 40 pitch.
The timber structure uses graded C16 or C24 joists with firrings to form the falls and a deck of exterior-grade plywood or OSB3. Over that, the waterproof layer is usually EPDM rubber, GRP fibreglass or high-performance felt, all of which perform well against the wind-driven rain common across the South West. We build the deck to suit whichever covering best fits your roof and budget.
Yes. We carry out flat roof carpentry and timber decking throughout Devon and Cornwall, including Plymouth, Exeter, Torquay, Barnstaple, Truro and Tavistock, and the surrounding areas. Flat roofing is often part of a wider extension or construction project, and we are equally happy to handle it as a standalone job.
Get in touch for a free, no-obligation quote on flat roof carpentry or timber decking anywhere in Devon or Cornwall.
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