Decking jobs rarely fail because someone chose the wrong deck board. They fail at the frame underneath: the subframe, the ventilation and the fixings. The board you walk on gets all the attention and causes the least trouble.
Timber choices for decking
Most UK decks are pressure-treated softwood, and that makes a perfectly good deck if it’s treated to Use Class 4. These species are the main alternatives for the boards. The best timber for decking guide compares every option in depth, including hardwoods and Accoya.
| Timber | What to know |
|---|---|
| Western Red Cedar | Naturally durable and stable. Imported heartwood is rated durable (EN 350 class 2), while UK-grown cedar is only moderately durable. It weathers to a silver-grey and doesn’t need preservative. It is soft, though, so it dents and scuffs more easily than other deck timbers, and its acidic extractives attack ordinary steel, so it needs stainless fixings. |
| Larch | Durable enough in the heartwood to use untreated if it’s well ventilated, and usually cheaper than cedar. It moves more than cedar or thermowood, so it needs more care in fixing. It is knotty and resinous, so check boards for loose knots and splits. |
| Douglas Fir | Strong for its weight, with a warm colour and straight grain. Imported heartwood is moderately durable and home-grown Douglas fir is rated lower, so ask where it was grown and ask for mostly heartwood. It is hard to push preservative into, so don’t count on treatment to make up for poor durability, and keep it out of ground contact. |
Board width and the subframe question
This is the distinction that actually decides a deck.
Narrower boards (around 120mm) move and cup less than wide ones, and they feel more traditional underfoot. Wider boards cover ground faster and look more contemporary, but they move more across their width, which means more allowance and more care with fixing.
So the width isn’t only a style choice, it decides how the board behaves. A wide board on a frame that wasn’t detailed for movement is more likely to cup.
The subframe matters more than the deck board. Joists, bearers and posts stay damp longer than the boards and are harder to replace, so build them from timber treated to Use Class 4, the class for timber in contact with the ground, whatever goes on top. Spaced and ventilated properly, on adequate bearers, that is what keeps a deck flat and dry. A deck built on a solid, unventilated base will rot from the underside, and you won’t see it happening.
Practical notes
Grooved, smooth or reversible
- Grooved: mostly a look. The grooves don’t reliably improve grip when wet, and they’re directional, so they show the line of the deck.
- Smooth: cleaner look, and it’s what most people mean by a modern deck. For grip when wet, look at boards with an anti-slip insert and keep the deck clean.
- Reversible boards: different faces on each side. Useful, but check which face you’re specifying, because you can’t change your mind after fixing.
Spacing changes with the weather
Boards expand and contract across their width with the seasons. If you butt them tight, they’ll push against each other in a wet spell and you’ll get cupping and fixing damage. Leave a consistent gap between boards (follow the board maker’s figure; for treated softwood boards it’s often 6 to 9mm). Fix each board at every joist, and let the gap take the movement.
Fixing and ventilation
- Corrosion-resistant fixings. Copper-based preservatives, and acidic timbers such as cedar and oak, attack plain steel. Use the coated or stainless screws the board maker recommends for treated softwood, and stainless steel for cedar, hardwoods and modified wood.
- Ventilation underneath is essential. A deck needs airflow below it to dry out after rain. Boards fixed to a solid surface trap moisture and rot from below.
- Don’t fix too close to board ends. Predrill and keep screws at least 25mm back from ends and edges, because a screw near an end splits the board.
- Treat every cut. On treated timber, brush end-grain preservative onto cut ends and drilled holes, and never put cut ends in the ground.
- Hidden fixings give a cleaner surface. Most clip systems set the board gap for you, so check the system is made for your board size and timber before you choose it on looks alone.
Finishing and weathering
Untreated exterior decking greys, and most timber decks are meant to. Oil slows the change and helps the board shed water, but it needs reapplying, so a deck is a maintenance commitment if you want it to stay a colour. Slipperiness matters more than colour on a deck: whatever you apply, make sure it doesn’t leave a slick film when wet.
Work out the boards before you buy
Deck quantities catch people out because board width and the gap together decide coverage, and a gap you can’t see in the arithmetic is a real number of boards over a large deck. Work out the area, then the board count with an allowance for wastage. The decking calculator does that, free to use, and the coverage calculator handles wastage allowances.
Sources
- NHBC Standards 10.2.10, Timber decking (decking and its supports naturally durable or treated to Use Class 4)
- GreenSpec, Timber cladding: durability, quality and movement (EN 350 classes for cedar, larch and Douglas fir by origin)
- Metsä Wood, Understanding Use Class 4 (Douglas fir and larch resistant to treatment, end-grain treatment of cuts)
- BSW Timber, How to guide: decking (6 to 9mm board gap, two fixings per joist, 25mm from ends, predrilling)
- Thermory, Installation guide for decking boards (stainless fixings, hidden clips setting the board gap, clearance below boards)
- BRANZ, Why timber eats metal (acidic cedar and oak, and copper preservatives, corroding steel and zinc)
- Real Cedar, Finishing choices (fixing metals that avoid staining on cedar)