How to Lay Decking – DIY Guide
Lay decking by fixing timber joists to a frame on concrete or post supports, then screwing decking boards across them with even gaps. Use pressure-treated timber throughout, keep boards 3–5 mm apart for drainage, and apply a protective finish once complete.
Prepare the Ground and Install Post Supports
Clear the area of vegetation and lay weed-suppressing membrane across the entire footprint to prevent growth pushing up through the structure. Dig holes or position pre-cast concrete pads at 1.2–1.5 m intervals to support the bearer frame — the frame must sit at least 150 mm above soil level to allow airflow and prevent rot. Use a spirit level to ensure all support points sit at the same height before the concrete sets.
Build and Level the Bearer Frame
Fix the outer bearer timbers first to form the perimeter rectangle, then add internal bearers at regular intervals across the frame. Check the frame is level in all directions and introduce a slight fall of around 1 in 100 away from the house to assist water run-off. A solid, level frame is the most critical part of the job — time spent here prevents problems later. If you are also planning a patio adjacent to the deck, our guide on how to lay a patio covers the ground preparation process in detail.
Fix the Joists Across the Frame
Attach timber joists across the bearer frame at 400 mm centres using joist hanger brackets for a clean, strong connection — do not just toe-screw them. Joists should run perpendicular to the direction the decking boards will travel, and all joist tops must sit flush and level. Double up joists at any edges where boards will end mid-span to prevent movement.
Lay and Fix the Decking Boards
Start from the straightest, most visible edge and work inwards, using 3–5 mm spacers between each board to allow for drainage and natural timber movement. Fix each board with two stainless steel screws per joist, pre-drilling to prevent splitting, and countersink slightly so the screw head sits flush. Building a shed base uses the same framing principles if you want to extend your outdoor structures. Cut the final boards to a consistent overhang — 25–50 mm beyond the outer joist — and trim the far edge to a clean line with a circular saw using a guide batten.
Finish, Trim, and Apply Protective Treatment
Sand any rough board ends smooth and apply a penetrating decking oil or wood preservative to all exposed surfaces, including the cut ends, for maximum protection against weathering and fungal decay. Re-treat annually to maintain the finish and extend the life of the deck. For inspiration on complementary garden projects, explore our Garden Guide for a full range of outdoor how-tos.
Frequently Asked Questions
Do I need planning permission to lay decking in my garden?
In most cases, no — but decking that raises the area more than 300 mm above ground level, covers more than 50% of the garden, or is attached to a listed building will likely need permission. Always check with your local planning authority before you start; gov.uk has the definitive guidance on permitted development rights.
What is the best timber to use for decking boards?
Pressure-treated softwood (usually pine or spruce) is the most cost-effective and widely available option in the UK. Hardwoods such as oak or iroko last longer but cost significantly more and require different fixings — avoid tropical hardwoods unless they carry FSC certification.
How far apart should decking joists be?
For standard 32 mm or 38 mm decking boards, space joists at 400 mm centres. If your boards are thinner or you expect heavy loads, reduce to 300 mm centres to prevent flex.
How do I stop decking becoming slippery?
Lay boards with the ridged face upwards, keep the surface clean of algae and debris, and apply an anti-slip decking treatment each year. A regular cleaning regime similar to patio maintenance will significantly reduce the build-up of slippery growth.
Can I lay decking directly on soil or grass?
No — direct contact with soil causes rapid rot and allows moisture to wick into the timber. Always use concrete pad supports or posts to raise the frame off the ground, and lay weed-suppressing membrane beneath to stop vegetation growth. See our guide on building a garden path for similar ground preparation techniques.
Before screwing any board down, sight along the face to check for bow, and always lay the board crown-up (arched face upwards). This way, as the timber dries and the crown flattens, the board stays flat rather than cupping and trapping water in the middle.
Sources
- RHS — Decking: practical advice — rhs.org.uk
- HSE — Construction work: slips, trips and falls — external surfaces — hse.gov.uk
- Planning Portal — Outbuildings and garden features permitted development — planningportal.co.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
