A deck can make an uneven, muddy or underused corner of the garden properly usable. But the boards are only the visible finish. When learning how to lay garden decking, the work that determines whether it stays level, drains well and feels firm underfoot happens below the surface.
This is a manageable project for a confident DIYer, provided the area is modest and the ground conditions are straightforward. Larger raised decks, structures close to a house, or decks supporting heavy features such as hot tubs will need professional help and will probably be subject to planning or building regulations. Check the current local requirements before starting, particularly where a proposed deck is large or elevated.
Plan how to lay garden decking before ordering materials
Measure the proposed deck carefully, then decide which direction the boards will run. Boards usually look best running away from the house or along the longest visible line of the garden. This choice also determines the direction of the joists: joists must run at right angles to the deck boards.
Allow for a slight fall so surface water runs off the boards. A fall of around 1:100 is generally sufficient - that is 10mm over every metre. If the deck is adjacent to a building or wall, the fall should be away from them.
Timber decking needs airflow around the deck support frame as well as drainage below it, so any frame support posts are not standing in puddles of water. Avoid laying it directly onto soil, grass or a solid surface that traps water.
Consider the finished height of the deck early in the planning process. Factor in the thickness of the decking boards, joists, bearers and supports, rather than measuring only from ground level to the top of the frame. Check the decking will not interfere with door thresholds, air bricks and damp-proof courses. Decking that is partially supported by a sole plate fixed to a house wall should not bridge the damp-proof course or obstruct air-bricks for ventilation around the base of the property.
Draw a plan of the frame and decking, then calculate what materials will be required. You will normally need treated structural timber for the frame, decking boards, weed-control membrane, gravel, concrete deck support posts, exterior-rated screws, galvanised or stainless-steel fixings, and suitable decking frame joist hangers or brackets where needed. Buying a few extra decking boards is sensible, especially if you want to select boards with fewer knots.
Prepare a firm, free-draining base
Clear the working area of turf, roots, loose soil and old paving that is no longer stable. Mark the deck perimeter with string lines and pegs, checking the diagonals if the deck is rectangular. Equal diagonal measurements indicate square corners, or make up a 3,4-5 square.
The frame needs solid, evenly spaced support points. On a low-level deck, these may be concrete pads, paving slabs laid on a properly compacted bed, or purpose-made concrete deck support posts. Do not rest bearers on loose blocks, timber offcuts or bare soil.
Space support points according to the size and span of the timber you are using. There is no single measurement that suits every deck because joist dimensions, loading and soil conditions vary. As a general rule, the maximum distance between your decking supports should be less than 1.8m. Shorter spans and more, closer, support points create a more stable deck. Follow the recommendations for your selected structural timber and do not assume deck boards themselves will add enough strength to a weak frame.
If there are any weeds or grass left growing in what will be the area under the deck, it is a good idea to treat the area with weed killer a week or so before you plan to start work. When the weeds have died, and the deck supports, or posts, have been set in place. Lay a permeable weed-control membrane across the cleared ground and around the supports. Make sure there is a good overlap on any joints. This will reduce weed growth without preventing water from draining into the soil. Cover it with a thin layer of shingle or gravel to hold it in place, putting a thicker layer on the edges and overlaps to stop the wind from lifting the fabric.
Build the subframe level and secure
Construct the frame using treated joists of an appropriate section for your deck height and size. Set the frame to the planned fall using a long spirit level or straightedge. Take time here. A frame that is only slightly out becomes more obvious once the deck boards are fitted across it.
Fit internal joists at the spacing recommended for the decking board profile. Standard timber deck boards often use joist centres of around 400mm, but thinner boards, diagonal layouts and composite products can require closer spacing. Always use the manufacturer’s guidance for the boards being installed.
Fit the decking boards with room to move
Start with the straightest, most visible edge, then work away from that point across the deck in whatever pattern you have settled on. Position the first board neatly, allowing a small perimeter gap beside walls, fences and other fixed structures. Timber expands and contracts as moisture levels change, while composite boards can move with temperature. Allowing the correct gaps is very important - it gives water a route away and prevents boards from pushing against each other.
Use consistent spacing between boards. For many timber boards, a gap of roughly 5-8mm works well; many tradespeople use their carpenter's pencil, but the correct gap depends on the moisture content and profile of the product. Boards fitted very dry may close up in wet weather; freshly treated boards may shrink as they dry. Composite decking must be installed to its specific expansion guidance.
Fix each board with two decking/exterior screws into every joist it crosses. Screws need to be positioned evenly about 20mm in from each edge. Choose corrosion-resistant screws made for exterior decking. Cheap screws can stain the board surface, snap during installation or corrode long before the timber reaches the end of its life.
Predrilling helps prevent splitting, particularly near board ends and with hardwood or dense treated timber. Keep the screw heads flush with the surface, not buried deep enough to collect water. A spacer cut to the required gap will keep the rows uniform and save considerable time.
Stagger board joints so several joints do not land on the same joist line. Every board end must have proper support beneath it. If boards meet over a joist, it may be necessary to use doubled joists so both board ends can be securely fixed without moving the screws too close to the board ends.
Finish the edges and protect cut timber
Once the main boards are installed, trim uneven ends with a straight guide and a suitable saw. Add a fascia board or picture-frame border if you want a cleaner, more finished edge. A border can also conceal the ends of the joists, although it should not block the airflow beneath the deck.
Any cut end of pressure-treated timber needs additional protection. Apply a suitable end-grain preservative as soon as practical, paying particular attention to cuts close to the ground or in exposed positions. Factory treatment protects the original timber, but cutting exposes untreated end grain.
Sweep the deck clean and check every fixing before using it. Look underneath where possible to make sure the weed membrane, gravel and airflow have not been disturbed during the building phase. If the deck is raised, consider fitting skirting with a fine mesh that still allows ventilation but prevents animals or debris collecting below.
Common decking problems are usually base problems
A deck that moves, holds puddles or develops uneven gaps is rarely fixed by replacing a few boards. The usual causes are inadequate support, joists spaced too far apart, poor drainage or unsuitable fixings. Spending more care on ground preparation and the subframe will always give a better result than rushing to the visible stage.
Material choice matters too. Pressure-treated softwood decking is a practical, traditional option and can be maintained with a suitable finish. Composite decking offers a different appearance and lower routine maintenance, but it has its own requirements for joist spacing, expansion gaps and ventilation. Neither is automatically right for every garden, budget or location.
For homeowners and tradespeople across the South East, Crestala Fencing Centre can help match treated timber, boards, fixings and landscaping materials to the scale of the job. Bring your measurements, planned height and a simple sketch of the frame when choosing materials - sound advice before the first cut is one of the best ways to keep a garden deck level for years to come.
