At a glance
A raised deck solves one of the most common UK garden problems – ground that slopes away from the house, making the outdoor space immediately adjacent to it effectively unusable for furniture, entertaining or children’s play. A well-built raised deck creates a level platform at or near house floor level, extending the living space outdoors and dramatically increasing the usability of a garden that was previously frustrating to use. It is one of the most impactful garden projects available to a UK homeowner in terms of lifestyle return on investment.
Building a raised deck is a substantial DIY project that requires careful planning, accurate setting out and a methodical approach to the structure. It is well within the capability of two competent DIYers with a long weekend and the right tools – including a good cordless drill, which will be heavily used throughout. The result, done correctly, is a structure that will last 20-25 years with basic maintenance.
Planning and permissions
In England, a raised deck requires planning permission if it is more than 30cm above natural ground level and the total area of decking plus other outbuildings exceeds 50% of the garden. A deck under 30cm high and covering less than 50% of the garden typically falls within permitted development. Always confirm with your local planning authority before starting – particularly in Conservation Areas or on listed properties where permitted development rights are more restricted.
Measure the height at the highest point of the deck, not the average. On a sloping garden the deck will be higher at one end than the other. The 30cm permitted development threshold is measured at the highest point above natural ground level. If any part of the deck exceeds 30cm, planning permission may be required for the whole structure regardless of the average height.
Before setting out the deck, mark the proposed footprint clearly and check the position of any underground services – gas, water, electricity, drainage – that may run beneath the garden. A cable avoidance tool (CAT scanner) can be hired to confirm the position of buried services before any post holes are dug.
Materials and timber choice
Setting the posts
The posts are the most critical element of a raised deck – they carry all the load and their position determines whether the finished deck is level, square and structurally sound. Take the time to set posts correctly. Errors at this stage are difficult and expensive to correct once the frame is built on top.
- 1Mark and excavate the post hole positions – post positions should be no more than 1.8m apart in any direction. Mark each position, check the overall layout is square by measuring diagonals and excavate holes at least 450mm deep – deeper in softer ground. A post hole borer from a tool hire shop makes this significantly faster than manual digging.
- 2Pour concrete and set post bases – pour a concrete mix (1:2:4) to within 75mm of the surface in each hole. Set a bolt-down post base into each pour while the concrete is wet, checking for level and alignment carefully. Adjustable post bases keep the timber out of ground contact and extend the structural life of the deck significantly.
- 3Allow concrete to cure fully before loading – allow a minimum of 48 hours before fitting posts and applying any load. In cold or wet weather allow longer. Post bases must be solidly set before the frame is constructed on top.
- 4Fit and brace posts to height – fit posts into the bases and cut to the correct height using a string line and spirit level across the top of all posts. Temporarily brace posts with diagonal timber braces while the frame is fitted.
Building the frame
The frame consists of perimeter beams bolted to the posts and internal joists spanning between them. All timber connections should use structural joist hangers or post caps rather than relying on screws alone for load-bearing joints. Screws are appropriate for secondary fixings and deck board attachment but structural connections need the correct metal hardware.
Fit 47x150mm perimeter beams around the outside, bolted to the posts with 10mm coach bolts – use two bolts per post connection at different heights to prevent rotation. Fit internal joists at 400mm centres spanning between the perimeter beams, supported in galvanised joist hangers. Check for level across the full frame using a long spirit level or laser level before fixing permanently. Fit noggins – short pieces of joist timber – between joists at mid-span to prevent twisting and lateral movement as the timber dries.
Laying the deck boards
Lay deck boards with the grooved face up, running at 90 degrees to the joists. Leave a 5-8mm gap between boards for drainage and seasonal movement – a piece of 6mm ply or a purpose-made deck spacer makes maintaining consistent gaps straightforward. Fix each board with two stainless steel screws per joist – pre-drilling avoids splitting and produces a neater finish. At the perimeter, boards overhang the outer joist by 25-50mm and are cut level after all boards are laid using a circular saw run along a straight edge – this produces a far cleaner edge than trying to cut each board individually before fixing.
Finishing and maintenance
Apply a quality decking oil or preservative treatment to all exposed timber surfaces within a few weeks of completing the build. New pressure treated timber needs time to weather slightly before oil penetrates effectively – applying immediately to very fresh sawn timber produces poor absorption. A good decking oil applied annually keeps the timber looking its best and significantly extends its life by preventing water ingress and UV degradation.
Check fixings and structural connections annually and re-tighten any that have worked loose as the timber dries and moves seasonally. Inspect post bases for signs of moisture accumulation and ensure drainage beneath the deck is not obstructed. A deck built to the specification above and maintained annually will serve reliably for 20-25 years.
Share on socials:
