How to build a composite decking frame

How is a composite decking frame built?

Like any deck frame, with tighter joist spacing and more attention to air. A UK builder building for composite starts with the outer frame, gets it level, then fills in the middle, like laying the edge pieces of a jigsaw first. Decide which way the boards run before you start, because the joists run across them.

What clearance and fall does the frame need?

One builder gives the frame a slight fall rather than dead level and sets posts in concrete with the top sloped away from the post so water doesn’t collect at the ground line. He checks the frame is square by measuring both diagonals, and braces it until the concrete sets.

What materials can the frame use?

Pressure-treated structural timber is the usual choice. Treat every cut end, and put joist tape on the joist tops. Aluminium and recycled plastic joists avoid the rot; see the joists guide for sizes and prices. Millboard’s DuoLift pedestals raise a frame from 15 mm up to 350 mm on a solid base. On very low sites, ask the maker about minimum build height, because most makers need airflow underneath.

What about the frame near doors and drains?

Add bracing where people step out of the door, because a bouncy deck shows most there. Leave an access hatch over drainage covers rather than boarding over them. If the deck must sit level with the door, work out the board thickness first: composite boards run 21 to 32 mm, so swapping boards on an existing frame can raise the surface above the threshold, as one UK owner notes.

Which sub frame kit should I buy?

See frame kits for what a kit holds. Whatever you choose, confirm that the board maker accepts the frame; some warranties require the maker’s own subframe. See how to build decking for the full order of work.

How do you build a decking subframe from timber?

TDCA says deck boards should be pressure treated to at least Use Class 3 for a 15 year service life, while joists, posts and beams should be treated to Use Class 4 whether or not they touch the ground. Use Class 3 covers timber exposed to frequent wetting but out of the ground, and Use Class 4 covers ground or fresh water contact. Its raised-deck code sets C16 as the minimum strength class above 600 mm.

Maker and retailer guidance for the frame:

• Ecoscape specifies pressure-treated C16 softwood joists, or composite or aluminium ones, fixed to a post set in a concrete foundation. It recommends weatherproofing tape along the joist tops and around cut noggin ends.
• Wickes’ raised deck guide sets internal joists no more than 400 mm apart, noggins no more than 1,200 mm apart and perimeter posts no more than 1,500 mm apart in holes about 300 by 300 mm and 700 mm deep with a concrete block at the base.
• BSW says each composite board needs three joists; Ecoscape wants at least 25 mm clearance beneath for drainage and ventilation.
• Whatever the timber, cut ends, notches and bore holes get two brush coats of end-grain preservative, according to TDCA’s code.

Those raised-deck figures apply to decks above 600 mm on new homes. For a ground-level garden deck, follow your board maker for centres and see joists.