How to Install Decking — Step-by-Step UK Guide

What You'll Need

Before You Start

• Check for underground services before digging for post supports.
• Ensure the finished deck is below the DPC of the house. If attaching a ledger board, it must be below the DPC line.
• Use our Decking Calculator to work out board quantities and our Composite Decking Calculator if using composite boards.
• Clear the area of vegetation. Lay weed membrane and a 50mm gravel bed beneath the deck area to prevent weed growth and improve drainage.
• Allow timber to acclimatise on site for 48 hours before construction.

Step-by-Step Instructions

Step 1: Plan the Layout

Decide on the deck size, shape and board direction. Mark out the area with string lines. Check that it is square by measuring diagonals — they should be equal. Plan your joist layout at 400mm centres (or 300mm for composite boards).

Step 2: Install the Support Posts or Blocks

For a low-level deck (under 300mm), use concrete deck blocks placed at 1.2m centres to support the bearers. For raised decks, dig post holes at least 450mm deep, set 100mm x 100mm treated posts in concrete and allow to cure for 48 hours. Use post base brackets bolted to concrete pads as an alternative.

Step 3: Fit the Bearer Beams

Bearers run perpendicular to the joists and sit on the posts or blocks. Use doubled 47 x 150mm C16 timbers bolted together with M10 coach bolts at 400mm centres. Check each bearer is level and at the correct height. For a ledger board against the house, use M12 expanding anchors into the masonry (not into the mortar joints).

Step 4: Install the Joists

Fix joists at 400mm centres using joist hangers or skew-nailed to the bearers. Ensure all joists are level across the deck. Apply a slight fall (1:80) away from the house for drainage. Use DPM tape on top of each joist where it contacts the decking boards — this extends the life of both the joists and the boards.

Step 5: Fit the Decking Boards

Start at the house end with the first board parallel to the wall. Leave a 10mm expansion gap between the wall and the first board. Pre-drill and fix each board with two stainless steel screws at every joist crossing. Leave a 5-8mm gap between boards for drainage and expansion. Use spacers for consistent gaps.

Step 6: Trim the Edges

Once all boards are fixed, snap a chalk line along the overhanging edges and trim with a circular saw for a clean, straight edge. Fit a fascia board around the perimeter to conceal the subframe.

Step 7: Finish and Treat

For timber decking, apply two coats of decking oil or stain. Treat the end grain of all cut boards with end-grain preservative. Composite decking requires no treatment. Fit any steps, balustrades or lighting as needed.

Common Mistakes to Avoid

• Insufficient ventilation: Low-level decks must have airflow underneath. Leave gaps or install ventilation grilles to prevent timber rot.
• Wrong screw type: Never use mild steel screws — they corrode within months. Use stainless steel A2 or A4 grade screws only.
• No expansion gaps: Both timber and composite boards expand. Leave 5-8mm between boards and 10mm at walls. Composite boards can expand up to 6mm per 3.6m length.
• Joist centres too wide: 600mm centres (common in floor joists) causes bounce and flex in decking. Stick to 400mm maximum for timber, 300mm for composite.
• No ground preparation: Vegetation growing up through the deck looks terrible and holds moisture against the timber.

Cost Estimate (2026 UK Prices)

Use our Decking Calculator and Composite Decking Calculator for exact material quantities.