How to Build Garden Decking – UK Step by Step Guide

Garden decking is one of the most transformative outdoor DIY projects available to a UK homeowner. A well-built deck extends the usable living space, creates a defined outdoor room and adds significant visual appeal to a garden. It is also one of the more achievable large DIY projects – the skills required are basic carpentry rather than anything specialist, and the materials are straightforward to work with for anyone comfortable using a circular saw and drill.

The key to decking that lasts – rather than the soft, slimy, warping deck that gives UK timber decking a bad reputation – is in the timber choice, the frame construction and the annual treatment. A deck built correctly on a solid frame from good pressure-treated timber and treated annually will look good for 15-20 years. A deck built from cheap untreated boards on a poorly ventilated frame will be a problem within 3-5 years. If you are also planning garden lighting to make the most of the deck in the evenings, our guide on how to install garden lighting UK covers the full range of low-voltage and solar options.

Planning and regulations

A ground-level deck (under 300mm above ground) in a rear garden generally does not require planning permission in England under permitted development rights, provided it covers less than 50% of the total garden area. Decks over 300mm high, those in Conservation Areas or those on listed buildings require a planning application. Always check with your local planning authority if in doubt – our article on garden offices and planning permission UK covers the broader permitted development rules that also apply to decking.

Choosing materials

Building the frame

The frame (or subframe) is the most important part of the deck and receives the least attention. A poorly built frame causes the deck boards to move, flex, warp and rot far faster than they otherwise would. A solid, well-ventilated frame built from 100x47mm pressure-treated joists is the foundation of a long-lasting deck.

1. 1
   Clear the ground and treat for weedsRemove all vegetation from the deck area. Lay weed control membrane across the entire ground area beneath the proposed deck. This prevents plant growth pushing up through the deck and retains some moisture in the soil rather than creating a wet, stagnant environment beneath the boards.
2. 2
   Set out the outer frame and check for squareLay out the outer perimeter frame of 100x47mm joists. Check for square by measuring diagonals – both measurements must be equal. A deck that is not square looks wrong and causes problems when fitting boards. Use 3-4-5 triangles or the diagonal measurement method to confirm.
3. 3
   Add internal joists at 400mm centresFix internal joists across the frame at 400mm centres for 32mm boards, or 600mm centres for thicker boards. All joist ends and any cut ends must be treated with end-grain preservative before assembly. Support joists on concrete pads or adjustable deck feet – never bury timber directly in soil.
4. 4
   Ensure a slight fall away from the houseThe frame must slope very slightly away from any adjacent house wall – a fall of 1:100 (10mm per metre) prevents water pooling on the deck surface and draining toward the building.

Laying the deck boards

• Start from the most visible edge – always start with a full board at the most visible side and work toward the house or a less visible edge where any cut board will fall.
• Leave a 5-6mm gap between boards – this gap allows water to drain and the wood to expand in wet weather. Use offcuts of timber or purpose-made spacers to maintain consistent gaps throughout. Boards that are touching will lift and cup as they absorb moisture.
• Pre-drill near board ends – drilling pilot holes near board ends before screwing prevents splitting. Use stainless steel or galvanised decking screws – never zinc-plated screws which corrode quickly outdoors.
• Fix with two screws per joist – two screws at each joist crossing prevents boards twisting and cupping as they weather. Drive screws slightly below the surface to avoid raised screw heads that are a trip hazard.
• Cut final boards to a consistent overhang – snap a chalk line across all board ends and cut to give a neat, consistent overhang of 20-30mm beyond the outer frame joist. A circular saw guided by a straight batten gives the cleanest result.

Finishing and treatment

Apply a decking oil, stain or treatment to all new softwood boards before or immediately after installation. New pressure-treated timber benefits from an initial treatment that seals the surface and begins the UV protection that prevents greying and weathering. Thereafter treat annually in spring – clean thoroughly first, allow to dry, then apply the treatment. A deck that is treated every year is a completely different product from one that is left untreated.

Fit fascia boards to the outer edges of the frame to give a neat, finished appearance and prevent the structural joists being visible. Sand any rough edges and splinters on board surfaces, particularly around areas where children or bare feet will be present.

Typical decking costs UK 2025

A well-built deck adds genuine usable space and real value to a UK home. Take the frame seriously, use good timber, treat it annually and it will look excellent for years. For more outdoor structure guide on how to install garden lighting UK for the finishing touches.