What foundations does a deck need?
Pads, screw piles, concrete footings and paving supports compared on cost, effort, ground conditions and how long the timber above them lasts.
- Difficulty
- intermediate
- Time
- 30 min
- Read
- 5 min
- Safety
- caution
Short answer
For a low deck on firm ground, concrete pads or paving slabs on compacted sub-base under bearers are enough. For a raised deck, posts on concrete footings dug to firm bearing, or ground screws. In all cases the aim is the same: spread the load onto ground that will not move, and keep every piece of timber out of contact with soil.
A deck's foundations are doing two separate jobs — carrying the load down to ground that will not settle, and holding the timber up out of the wet. Most decking foundation failures are one or the other: a pad bearing on topsoil that compresses, or a post buried in soil that rots. The options below all solve both, at different costs and different amounts of digging.
Safety
Step by step
- Find out what your ground is.Dig a small hole at the lowest and softest-looking point. Topsoil is dark, loose and full of roots and it is not a bearing material. Below it you want firm subsoil. On made ground, fill, or soft clay, the foundation has to reach through to something solid, which changes the answer.
- Work out the load path.Boards to joists, joists to bearers, bearers to posts or pads, pads to ground. Each support carries the area of deck around it. Corners carry less than intermediate supports; anything under a hot tub carries far more.
- Choose the foundation to the deck height.A ground-hugging deck can sit on pads under bearers with no posts at all. A raised deck needs posts, and posts need footings that resist both the vertical load and, through the bracing, sideways forces.
- Keep every piece of timber clear of the soil.Whatever the foundation, the timber sits on top of it with an air gap, on a post anchor, a pad with DPC, or a bracket. The single biggest determinant of how long a deck frame lasts is whether any of it touches the ground.
- Prepare the ground under the whole deck, not just the supports.Strip vegetation, lay a permeable membrane, and cover with gravel. It stops growth pushing up through the deck, reduces the humidity under it and makes the whole void less hospitable to everything you do not want living there.
- Check the falls before you set the levels.The deck falls away from the building at about 1 in 100, and the ground under it should drain away too. Set your foundation heights knowing which way everything is going.
Common mistakes
- Bearing a pad on topsoil — Topsoil is compressible and its organic content decomposes. Anything on it settles, unevenly, for years. Dig through to firm subsoil however tempting it is not to.
- Spacing the supports by eye — The support spacing sets the bearer span, and an over-long bearer span produces exactly the same bounce as over-long joists. Work out the spacing from the timber section you are using.
- Setting posts in the ground because it is quicker — It is quicker, and it puts the timber in the one condition guaranteed to rot it. An anchor on a pad takes an extra ten minutes per post and adds a decade or more.
- Building on made ground without investigating it — Rubble, old foundations and buried fill are common in gardens and they settle unpredictably. Find out what is under the deck before you decide what it stands on.
Alternatives
- Paving slabs or concrete pads under bearers: The simplest option for a low deck. Excavate to firm ground, compact 100mm of MOT type 1, bed a slab on mortar or sharp sand, and stand the bearer on it over a DPC strip. Cheap, quick, no concrete mixing, and entirely adequate for a ground-level deck on firm ground.
- Concrete pad footings with post anchors: The conventional answer for a raised deck. A hole dug to firm bearing, filled with concrete, with a galvanised post anchor set in while it is wet so the post stands clear of the surface. Strong, permanent, and the digging is the whole cost.
- Ground screws or screw piles: Galvanised steel screws driven into the ground with a machine or a bar, with a bracket on top for the post or bearer. No excavation, no concrete, no curing time, minimal spoil, and they work well in ground where digging is unpleasant. Higher material cost and they need reasonably obstruction-free ground.
- Adjustable paving pedestals: Screw-adjustable plastic supports designed to sit on an existing hard surface. Excellent over an existing patio or slab, useless on soil. They take out unevenness quickly and keep the frame ventilated.
- Posts concreted directly into the ground: The traditional method and the one that fails first. If it is used at all, the timber must be treated for ground contact and the concrete domed well above the surface to shed water. A post anchor on top of the same concrete is better in every respect for very little extra cost.
Questions people ask
How deep should a deck footing be?
Deep enough to reach firm undisturbed ground and to be below the depth that seasonal shrinkage and frost affect — in practice usually at least 450 to 600mm in a normal UK garden, and deeper on shrinkable clay or near trees. Dig until you are into material that a spade struggles with, and let that decide.
How many supports do I need?
It is set by the timber sections and the load, not by a rule of thumb. Work backwards: joist centres from the board, joist depth from the joist span, bearer span from the bearer section, and the supports go at the bearer span. Closing the supports up always costs less than a deck that moves.
Are ground screws as strong as concrete?
For domestic decking, properly specified screws in suitable ground are comparable and are sold with load ratings. They are less suitable in very stony ground, over buried obstructions, or where the load is unusually concentrated. The rating from the supplier is the thing to work from.