3m x 3m Composite Decking Project

Ground-level frame, in the corner against the fence. Your build reference — updates and saves automatically.

Preparation

Get this sorted before any timber gets cut.

Site checks

  1. Clear the old decking and subframe fully. Lift boards, unscrew and remove every old joist and perimeter timber. Check for any rot that's spread into the ground contact points.
  2. Inspect your existing support blocks. You've confirmed these are still sound and slightly raised — good, that's your levelling work basically done. Just check each one is stable and not rocking.
  3. Fill any gaps in the pad layout. Pick up plastic adjustable deck support pads for any spot the old blocks don't cover, aiming for roughly a 750mm grid under the frame.
  4. Weed membrane. If it's not already down under the old blocks, lay heavy-duty membrane across the full 3x3m area (with a bit of overlap) before the frame goes down.
  5. Check the fall. Confirm the pads give you a slight fall away from the house — a few mm across the 3m is plenty, just enough that water doesn't sit.

Tools you'll want on hand

  • Tape measure
  • Spirit level (long one, or a straight edge + short level)
  • Chalk line
  • Set square / speed square
  • Circular saw or mitre saw
  • Drill driver + bits
  • Pencil / marker
  • Safety glasses, gloves, ear defenders
  • Clamps (handy for holding joists square while fixing)
  • Wood preservative + small brush (for cut ends)

Notes

Anything you want to jot down as you go — this saves automatically.

What to Buy

Tick items off as you buy them, and fill in quantity + price paid — it's all saved on this device automatically.

Item Spec / needed Qty bought Price each (£) Line total Notes
Total spent so far £0.00

Build Order

Boards run away from the house, joists run parallel to the house wall. Fence sides: far edge and right. Open sides: near (house) edge and left edge.

  1. 1. Strip out. Remove all old decking boards and the old subframe completely. Dispose of rotten timber.
  2. 2. Ground check. Confirm existing support blocks are solid, level to a slight fall away from the house, add plastic pads to fill any gaps (~750mm grid). Lay membrane if needed.
  3. 3. Build the perimeter frame. 47x150mm treated timber: two 3000mm lengths and two 2906mm lengths, screwed/bracketed into a square. Check it's square by measuring both diagonals — equal diagonals means square.
  4. 4. Fit internal joists. 47x100mm treated timber, 9 pieces at 2906mm, running parallel to the house wall (perpendicular to where the boards will run), spaced at 300mm centres.
  5. 5. Fit the noggins. 47x100mm, one row at 750mm, one at 1500mm, one at 2250mm from the near edge. All noggins run parallel to each other and to the perimeter — no angles. One per bay in every row. Alternate which side you screw in from, bay to bay.
  6. 6. Blocking for fascia. Double up (or add a flush blocking piece) behind the two open sides — near/house edge and left edge — so there's solid timber to fix the fascia trim into later.
  7. 7. Level and square check. Before boards go down, re-check the whole frame is level, square, and sitting firm on every pad.
  8. 8. Lay the boards. Start with a full board on the open left edge (factory edge on show). Run boards away from the house, 19 boards total, ~8mm gap between each (check your manufacturer's tolerance — usually 5-10mm). Screw into every joist crossing. Last board finishes a few mm short of the fence — no cutting needed.
  9. 9. Fit the fascia trim. Cover the two open sides only — one trim over the board ends (near/house side), one over the long edge (left side). Fence sides need nothing.
  10. 10. Final check. Sweep off, check screws are all seated, check the fall still drains properly, stand back and admire it.