How to deck a sloping garden

Video by Wickes. This guide is based on the video "How to Build a Raised Deck with Wickes" from the Wickes YouTube channel. The video specifically covers decking a sloping or uneven garden, which makes it directly relevant to this guide. The sections on post setting and frame construction are particularly worth watching before you pick up a saw.

1. Plan the deck size and check for permissions

Before buying a single length of timber, work out the deck footprint and how high it will be at its tallest point. In most gardens a deck up to 30 cm above ground level does not require planning permission in England, but a raised deck on a slope can easily creep beyond that at the downhill end. Check with Swale or Dover Borough Council if you are in any doubt — it is a quick conversation and saves a lot of grief later.

Think about access to the space underneath the deck too. Ventilation is important to prevent the timber rotting from below. If the deck will be more than about 600 mm off the ground at any point, you will also need a balustrade for safety — Building Regulations require one where the drop is significant. It depends on the garden, but for most East Kent plots the sweet spot is a deck that sits 300–600 mm above the lower ground level.

2. Mark out and dig the post positions

Mark out the deck footprint with pegs and string lines, keeping everything square by checking the diagonal measurements — they should be equal. Post positions typically go at each corner and at no more than 1.8 m centres along the beams, more on any deck above 600 mm high.

Dig each post hole to a depth of at least 450 mm — deeper if the ground is soft or the post will be carrying significant height. You can use post spikes on firmer ground, but for a raised deck on a slope, concrete-set posts are more reliable. Mix a dry post-fix concrete or use a standard mix and pour it around the post, checking plumb on two faces before it sets. Leave it to cure for at least 24 hours before loading the frame.

3. Set the post heights to get a level frame

This is the critical stage on a sloping garden. Using a builder’s line level or a long spirit level rested on a straight piece of timber, transfer the finished deck height from your highest post to each of the others in turn. Mark each post clearly at the correct height with a pencil line all the way round.

The posts on the downhill side will be taller — sometimes significantly so on a steep slope. Use 100×100 mm treated timber posts for any deck where the post height exceeds around 600 mm. Cut the posts off at your marked lines using a circular saw or a hand saw guided by a speed square to keep the cut square. Double-check the levels before you move on. A 5 mm error on a post translates directly into a visible dip or hump in the finished deck surface.

4. Build and fix the frame

The frame consists of outer rim joists bolted or screwed to the tops of the posts, with internal joists fixed between them at 400 mm centres. Use 47×150 mm C16 or C24 treated softwood for the joists on most domestic decks. Fix rim joists to posts using galvanised joist hanger brackets or coach bolts — screws alone are not reliable enough here.

Run the internal joists perpendicular to the direction you will lay the deck boards. Use joist hangers to fix internal joists to the rim joists for a clean and structurally sound connection. Check the whole frame is level in both directions before laying a single board. Mind you, a slight fall of around 1:80 away from the house is actually desirable for drainage — just do not let it run towards a door threshold.

5. Lay the deck boards

Use 145×28 mm pressure-treated softwood deck boards or composite decking. Lay boards at 90 degrees to the joists, starting from the most visible edge. Leave a gap of around 5–6 mm between boards to allow for expansion and drainage — a 6 mm spacer made from a piece of off-cut works perfectly. Fix each board with two 75 mm decking screws at every joist, pre-drilling to avoid splitting.

On the final boards, measure carefully and cut to length. Run a chalk line along the outer edge of the last full board and trim any slight variations with a circular saw guided by a batten. The finished edge should be straight and consistent. If you are adding a fascia board to hide the joist ends, fix that last, flush with the top surface of the deck boards.

6. Fit handrails, steps, and finish the timber

If the deck is more than 600 mm above the lower ground level at any point, fit a balustrade. Standard height is 1,000 mm above the deck surface, and the infill gaps should be no more than 100 mm so children cannot pass through. Proprietary deck rail kits from Wickes or Jewsons make this straightforward — the posts bolt through the rim joist and the rails slot in between.

Steps are a job in themselves but the basic principle is simple: work out the total rise from the garden level to the deck surface, divide by 175 mm per riser to get the number of steps, and build accordingly. Once the structure is complete, treat all exposed end grain with a timber preservative and finish the whole deck with a good quality decking oil or stain. Leave it bare and untreated and the greyness sets in within one season — especially in the damp East Kent winters.

When to call a handyman

Call Richard if the slope is steep and you are not confident about getting the post heights right, if the deck will be high enough to need a balustrade and you want it done to Building Regulations standards, or if you simply want the structural work completed properly before you tackle the boards and finishing yourself. Building a raised deck on a slope is a satisfying project but the framing stage in particular benefits from someone who has done it before. Richard covers decking and garden projects across Sandwich and East Kent.