Garden Decking Design and Installation UK – Materials, Costs and How to Build It Right

A garden deck done well is one of the most valuable improvements you can make to an outdoor space – it extends a house into the garden, creates a defined area for eating and relaxing, and makes a damp or sloped plot usable in a way that a lawn or border bed simply cannot. Done badly, it becomes a source of ongoing maintenance, rot problems and structural remediation costs that make the original project look like a bargain in hindsight. The difference between the two outcomes comes down almost entirely to material choice and sub-frame construction – the foundation nobody sees but that determines everything about how the deck performs over its lifetime.

This guide covers permanent ground-level garden decking built on a timber or composite sub-frame – the most common form of domestic decking in the UK. Raised decks above 30cm from ground level involve different structural and planning considerations and are outside the scope of this article, though the material and design principles covered here apply equally. Ground-level decks up to 30cm high on a residential property do not normally require planning permission, but if your garden is in a conservation area or your house is listed, check with your local planning authority before starting work.

What You’ll Need

Choosing Your Decking Material

Three materials dominate the UK domestic decking market: pressure-treated softwood (typically pine or larch), hardwood (predominantly bangkirai, ipe or teak), and composite (wood-plastic blend). Each has a genuinely different service life, maintenance requirement and cost profile in UK conditions, and the right choice depends on your priorities rather than on any single factor. The table below compares the three across the criteria that matter most for longevity in the UK climate.

Pressure-treated softwood is by far the cheapest option upfront and is perfectly adequate when properly maintained and built on a well-ventilated frame. The problem in UK conditions is that softwood decking requires annual or biennial treatment to stay looking good and resist rot, and many homeowners simply do not do it – which is why so many softwood decks look tired and grey within five years and structurally compromised within ten. Larch is more naturally durable than pine and a better choice for softwood if available, as it weathers more gracefully and holds treatment better. If you are prepared to treat it every year, softwood is a reasonable choice for a budget build. If you are not, composite is the more honest and cost-effective option over its service life.

Composite decking has become significantly better in the last decade. Modern capped composites – where the wood-plastic core is wrapped in a protective polymer shell – have substantially better fade resistance, scratch resistance and moisture performance than earlier uncapped products. The surface temperature of composite in direct sun can be higher than timber, which is worth noting for decks in full south-facing sun, but for the vast majority of UK gardens this is not a practical issue. Composite requires no annual treatment, does not splinter, and maintains its appearance with nothing more than an occasional wash. The higher upfront cost relative to softwood is recovered within the first 5-7 years in avoided maintenance costs alone. When comparing composite products, check whether the boards are solid or hollow – solid boards are heavier but more rigid and better suited to wider joist spacings, while hollow boards are lighter and faster to install but may feel less substantial underfoot.

Design and Planning

The single most important design decision beyond material choice is the joist spacing in your sub-frame. For most 145mm softwood or composite boards, the maximum joist centres are 400mm for a standard 90-degree board run. If you want to lay boards at a 45-degree angle for a diagonal pattern – which looks striking and suits square deck shapes particularly well – drop the joist spacing to 300mm. Getting this right at frame stage costs nothing; discovering it after the boards are down means dismantling the deck. Also consider the orientation of the deck relative to the house: a ledger board fixed to the house wall is the most common starting point, but this means drilling into the masonry and using appropriate wall fixings – never skip this step or use inadequate fixings, as the ledger carries significant load. If you are not confident fixing to masonry, freestanding post-and-beam framing is a valid alternative that puts no load on the house structure at all.

Installation – the Build Process

Before any timber is cut, clear the ground beneath the deck area and lay a weed-suppressing membrane over the soil, topped with 50-75mm of gravel or shingle. This is not optional – an unventilated void with bare soil beneath a deck creates perfect conditions for rot in the sub-frame, regardless of treatment. The membrane and gravel allow water to drain, prevent vegetation from growing up into the structure and help maintain airflow. Allow at least 150mm of clear air gap between the ground surface and the underside of your joists at the lowest point of the deck. On a sloping site, this is measured at the high side of the slope – the lower side will have more clearance naturally. If the slope is significant, you may need to step the frame in height at intervals rather than running a single continuous beam level across the slope.

Post bases fixed to concrete pads are the most reliable way to support the deck frame at ground level – they hold the posts off the ground and prevent direct contact between timber and soil, which is where rot almost always begins. Set the pads in position using string lines and check for square using the 3-4-5 triangle method before the concrete sets. Joist hangers, not notched joints, are the correct fixing for attaching joists to the main beam – they are stronger, faster and allow the joint to be inspected in future. Use stainless steel fixings throughout. Zinc-plated or standard steel screws corrode and stain the surrounding timber within a few years in UK weather, and the cost difference is small. For composite decking, use the hidden clip system supplied or recommended by the board manufacturer – face screwing composite boards directly can cause cracking at the fixing point over time as the board expands and contracts with temperature.

Costs and What Affects Them

Decking costs vary significantly with material choice, size, height above ground and the complexity of the design. The table below gives typical installed costs for a straightforward 20m2 ground-level deck in each material, with boards supplied and fitted by a professional. DIY installation with purchased materials typically saves 30-40% on the total cost but requires the tools and skills to achieve a professional result. Bear in mind that quoted prices from decking companies typically cover boards and basic frame only – steps down from a higher level, balustrades, built-in lighting and any groundwork to level or prepare the site are priced separately and can add substantially to the total. Get at least three quotes and ask each contractor to specify exactly what is included in the price before comparing.