Composite decking subframe: timber, composite battens or plastic joists

The boards are the part of a deck you see, and the subframe is the part that decides how long it lasts. A composite board carries a ten-year warranty and will outlive it; a treated softwood frame sitting on damp soil will not. This guide covers the three frame materials we see used under composite decking in the UK, the joist centres and clearances our boards need, and what a frame costs for a typical deck.

The short answer

On a patio or concrete base, screw 40 by 30mm composite battens straight down at 350mm centres: cheapest, rot-proof, done in an afternoon. On soil or where the deck is raised, use 50 by 100mm recycled plastic joists on plastic posts; they cost about twice timber and never rot, so the frame lasts as long as the boards. Use timber only if it is ground-contact treated, kept off the soil on pads or posts, and you accept re-doing it in ten to fifteen years.

What our boards need from a frame

• Joist centres of 350mm or less. 300mm if the boards run at 45 degrees. Our boards are hollow, which keeps them light and affordable, and hollow boards flex more between supports than solid ones. Wider centres mean bounce, creak and clips working loose.
• A double joist under every board end and under a picture-frame border. Each board end needs its own support and its own clip; two boards cannot share one clip on one joist.
• 40mm of clear air under the frame. Air movement dries the underside of the boards and the frame after rain and stops algae taking hold. Do not build the frame tight to the ground.
• A fall of about 1 in 100 away from the house, so rain runs off rather than sitting in the board grooves.
• Level, flat and square. Composite follows the frame exactly. A dip in a joist is a dip in the deck forever.

The full sequence, with expansion gaps and clip spacing, is in the installation guide.

Option 1: composite battens on a hard base

If the deck is going over an existing patio, slabs or a concrete pad, you do not need joists at all. Our 40 by 30mm composite battens (3m, £16.19 and 3.6m, £17.19) lay flat on the base at 350mm centres, fixed with frame fixings or adhesive, and give the boards the 40mm of air they need. They cannot rot, they do not need treating, and a 3.6m by 3m deck needs nine of them, about £155.

Limits: the base must already be flat and drain; battens will not correct a sloping patio. The deck sits only 70mm above the base, so this is for ground-level decks, not raised ones.

Option 2: recycled plastic joists and posts

For a deck on soil, over a lawn, or raised to meet a door threshold, the frame is joists on posts, exactly as it would be in timber, but in recycled plastic. Our 50 by 100mm joists are £44.95 a length, 80 by 80mm posts £56.99 and 100 by 100mm posts £77.99, all 3m. Plastic does not rot, does not absorb water, is not eaten by anything, and can be cut, drilled and screwed with wood tools.

Span: keep 50 by 100mm plastic joists to spans of 1.2m between posts or bearers, closer than you would with timber, because plastic is less stiff. For a big raised deck that means more posts, not bigger joists.

Cost: a 4.8m by 4m deck needs about 55 linear metres of joist and eight to ten posts; allow £900 to £1,000 for a frame that will still be there when the boards are replaced in twenty years. The cost guide works it through.

Option 3: treated timber

Timber is the cheapest frame and the one most decks are built on. It is fine under composite boards if you treat it as the weak link and protect it: C16 or C24 joists with a UC4 (ground contact) treatment, joist tape along the top edge so water does not sit under each board, posts set in post shoes or concrete rather than in the ground, and the 40mm air gap. Expect ten to fifteen years from a well-built timber frame and five to eight from a bad one. A 3.6m by 3m deck frame in timber is roughly £150 to £200 in materials.

The honest comparison: a timber frame saves £300 to £500 on a mid-sized deck today and costs the whole deck when it fails, because the boards have to come up to replace it.

Mixing them

A common and sensible build is plastic posts and bearers where the frame touches the ground, with timber or composite joists above, off the ground and out of the wet. It puts the rot-proof material where the rot happens.

Frame checklist before the first board goes down

1. Joists at 350mm centres or less, 300mm for diagonal boards.
2. Double joists at board ends and under any border.
3. 40mm clearance under the frame, weed membrane and gravel on soil.
4. Slight fall away from the house.
5. Frame square (measure the diagonals) and level along every joist.
6. Outer joist positioned so the fascia board will sit flush with the board ends.

Send us the deck size and whether it is going on slabs, soil or posts, and we will list the frame, boards, clips and trims as one order. Call 0161 399 1792 or reply to any order email.