How to build a raised deck

Video by Wickes. This walk-through is based on the video "How to Build a Raised Deck with Wickes" from the Wickes YouTube channel. It covers the full process from marking out and sinking posts through to laying boards and trimming the frame. Worth watching before you hire a post hole digger — the section on squaring the frame is particularly useful if you have never done it before.

1. Plan the deck and mark out the area

Before you buy a single plank, spend time on the layout. Measure the area carefully and sketch it to scale. Think about which way the boards will run, where the steps will go, and whether any side of the deck sits against the house. If it does, you will need to keep the deck surface at least 150 mm below the damp-proof course — worth checking before you start digging.

Mark the corners with wooden pegs and run a builder's line between them. Use a set square or the 3-4-5 triangle method to check the corners are square. It sounds like extra work, but a frame that is even slightly out of square will cause headaches when you get to laying boards. Spend ten minutes getting this right and save yourself a lot of faffing later.

2. Dig the post holes and set the posts

Raised decking needs vertical posts sunk into the ground, either set in concrete or in purpose-made post spikes. For a deck up to around 600 mm high, 100 mm square pressure-treated posts are the standard choice. Hire a post hole digger if you have more than four holes to do — digging by hand in Kent clay is hard going.

Each hole should be at least 450 mm deep — deeper if the posts are carrying a lot of load or the ground is soft. Mix Postcrete or a standard 1:6 concrete mix, pour it in around the post, and use a spirit level to get each post plumb before the concrete goes off. Check plumb on two adjacent faces, prop the post if needed, and leave it a full day before you continue. That said, if rain is forecast, cover the wet concrete or it will wash out.

3. Cut the posts to height and build the outer frame

Once the posts are set and the concrete has cured, cut them down to the finished deck height. Use a chalk line or laser level to mark the cut line across all posts at the same height — a tape measure and a pencil on each post individually is asking for variation. A circular saw on a straight guide makes short work of this.

Fix your outer frame timbers — typically 150 × 47 mm treated joists — to the tops of the posts using joist hangers or by bolting through. Use two coach bolts per joint rather than just screwing, especially on the corners. Check the frame is level as you go; packing under a post with a slate or treated offcut is fine if one corner is slightly low. Mind you, do not be tempted to pack more than about 20 mm — if it is more than that, recut the post.

4. Fit the internal joists

Internal joists run between the outer frame at right angles to the direction your deck boards will eventually lie. Space them at 400 mm centres for standard 32 mm softwood decking boards. Closer spacing — 300 mm — is needed for thinner boards or for composite decking. Use joist hangers rather than skew-nailing where you can; they are quicker, neater, and a lot stronger.

Add noggins between the joists anywhere the frame spans more than 1,200 mm. These short cross-pieces stop the joists from twisting over time, which matters in a structure that will see rain and sun for years. It only takes a few extra minutes and makes the deck feel noticeably more solid underfoot once it is finished. Cut each noggin to a snug fit and skew-nail or screw in from both sides.

5. Lay the decking boards

Use pressure-treated softwood decking boards or hardwood if the budget stretches. Lay the first board with the grain curving downward (cup down) so any water runs off rather than pooling. Pre-drill every screw hole to avoid splitting, and countersink slightly so the screw head sits flush or just below the surface.

Leave a 5–8 mm expansion gap between each board. Timber swells in wet weather and shrinks in a dry summer, and without that gap you will end up with a buckled deck within a season. A short offcut of 6 mm ply makes a perfect spacer. Work your way across the frame, checking alignment every few boards by measuring from the far edge. Trim the last board to width with a circular saw fitted with a fine-toothed blade.

6. Finish the edges and treat the timber

Fascia boards hide the frame and give the deck a tidy finished look. Cut them to length and fix them vertically to the outer joists with decking screws, keeping the top edge flush with the deck surface. Mitre the corners at 45° for a neater finish, though a simple butt joint with a trim piece over it is perfectly acceptable and a lot faster.

Apply a good quality decking stain or oil within a couple of weeks of laying, while the timber is still clean. A lot of people skip this step and then wonder why their deck looks grey and rough within two years. Two coats of a UV-resistant decking oil brushed well into the grain is the minimum. Pay particular attention to the end grain on cut boards — that is where moisture gets in fastest and rot starts.

When to call a handyman

Call Richard if the ground is sloping and you are not sure how to level the post heights, if the deck is larger than about 15 m² and you want it done in a weekend, or if you need steps built to connect the deck to the garden. Single-handed deck building is hard work — the outer frame timbers are heavy and awkward to hold in position while fixing. Having someone who has done it a few times makes the whole job considerably quicker and safer.