The short answer
Decking needs some form of stable support, but not always deep concrete foundations. A low ground-level deck on firm, level ground can often rest on paving slabs or a compacted, free-draining base, which spread the load and keep the frame off the soil. A raised deck, a sloping site, or a large deck, however, usually needs posts set into concrete footings dug below ground, so the structure is firmly anchored and will not sink, lean or settle unevenly. What decides it is the deck's height, size and the ground: the more load the supports carry and the softer or more uneven the ground, the more substantial the foundations need to be.
"Foundations" means different things for different decks. A low platform and a raised deck have very different support needs — here is what each requires and why.
Decking support at a glance
- Low deck, firm groundSlabs or compacted base
- Raised deckPosts in concrete footings
- Sloping groundPosts and footings
- Soft / clay groundDeeper, firmer footings
- PurposeStop sinking, leaning, rot
What 'foundations' means for a deck
Decking does not usually need a continuous concrete foundation like a building. Instead it needs stable, load-spreading support points that keep the sub-frame firm, level and clear of the ground. These range from simple to substantial:
- Paving slabs or pads: the frame rests on slabs laid on firm ground, spreading the load and lifting the timber off the soil. Suitable for low, ground-level decks on firm, level ground.
- Compacted base: a layer of compacted hardcore or gravel under the support points, giving a firm, free-draining bearing.
- Concrete footings with posts: holes dug below ground, filled with concrete, with posts set in to carry a raised or sloping frame. This is the proper foundation for decks that carry real load.
The common thread is that the support must not move, settle unevenly or let the timber sit in damp ground.
When slabs or a compacted base are enough
A low, ground-level deck on firm, level ground often does not need concrete footings. Resting the sub-frame on well-bedded paving slabs or pads over a compacted, free-draining base can be enough, because the deck is close to the ground, the loads are modest, and there is little leverage to lift or tip the frame.
For this to work the ground must genuinely be firm and stable — slabs on soft, freshly dug or poorly draining soil can settle and let the deck go out of level. A weed-control membrane is usually laid first, and the pads are spaced to suit the frame so no part of it sags between supports. This lighter approach is one reason a small flat deck is a realistic DIY job, but it relies on the ground beneath being sound.
When you need concrete footings and posts
Proper concrete footings with posts are needed when the deck has to carry more load or resist movement:
- Raised decks: a deck lifted off the ground relies on posts to carry the frame, and those posts must be anchored in concrete so they cannot sink or lean.
- Sloping or uneven ground: taller supports on the low side carry significant load and leverage, so they need firm footings below ground.
- Large decks and long spans: more weight and more frame to support call for more, and firmer, support points.
- Soft or clay soils: weak ground needs footings taken down to firmer bearing, and dug deeper or wider, so the deck does not settle.
Footings are dug, filled with concrete and given time to cure before the frame is loaded onto them, which is part of why a raised deck takes longer to build. The principle is that the support has to match the load and the ground — undersized footings on a raised deck are exactly where structural problems begin.
Drainage, ground clearance and getting the support right
Whatever the support method, two things protect a deck's life. The first is keeping timber off wet ground: the frame sitting directly on soil traps damp and rots, so slabs, pads or posts that lift the frame clear and allow air to circulate underneath are important. The second is drainage — a free-draining base or footings that shed water, rather than ground that holds it against the timber and concrete.
Getting the support right is the single most consequential decision in a deck build, because almost every serious deck problem — sinking, leaning, bounce, a frame that drifts out of level — comes back to foundations that were too light for the job. If in doubt about a raised deck, a sloping site or soft ground, this is the part worth taking advice on or leaving to a professional, since it is structural and hard to correct after the boards are down. A low deck on firm ground forgives a simpler base; a raised deck does not.
How ground type changes the support you need
The ground a deck sits on matters as much as the deck's height when deciding the support. The same deck can need a very different base depending on the soil:
- Firm, well-draining ground (gravelly or compacted): the most forgiving. A low deck may rest on slabs or pads over a compacted base, and footings, where needed, are straightforward.
- Clay soil: holds water and can shrink and swell with the seasons, which can move footings that are too shallow. Footings generally need to go deeper to firmer, more stable ground.
- Soft, made-up or recently disturbed ground: weak and prone to settling. Support points must reach down to firm bearing below the soft layer, or the deck will sink unevenly.
- Boggy or poorly draining ground: keeps timber wet and undermines footings. It needs drainage improvement and footings that shed water and reach stable ground.
The principle is consistent: the support must reach firm, stable bearing and stay well-drained. Weak or wet ground does not mean you cannot deck it — it means the foundations have to work harder, which is exactly the situation where proper footings, and often professional input, earn their place.
Getting the foundations right the first time
Because foundations are buried and load-bearing, they are the hardest part of a deck to put right after the boards are down — so they reward care at the planning stage. A few practical points pull it together. Decide the support from the deck and the ground together: height, size and load on one side; how firm and well-draining the ground is on the other. Do not under-build a raised deck — posts in proper concrete footings are not the place to economise, because the whole structure depends on them. Keep everything draining and ventilated so neither the timber frame nor the footings sit in water.
For a small, low deck on sound ground, a simple base of well-bedded slabs over compacted, free-draining ground is often entirely adequate, and this is part of why such decks are realistic DIY projects. For a raised deck, a sloping site, a large deck or any soft or clay ground, the foundations become genuinely structural, and that is the point at which advice or a professional build is worthwhile. Get the support right and the rest of the deck has a sound base to last on; get it wrong, and no quality of boards on top will save it.
Frequently asked questions
Can decking just sit on slabs?
A low, ground-level deck on firm, level, well-draining ground can rest on well-bedded paving slabs or pads, which spread the load and keep the frame off the soil. This is not enough for raised decks, sloping sites or soft ground, where posts set into concrete footings are needed to anchor the structure.
Do you need concrete footings for decking?
Not always. A low deck on firm ground may only need slabs or a compacted base. Concrete footings with posts are needed for raised decks, sloping or uneven ground, large decks, and soft or clay soils, where the supports carry real load and must be firmly anchored so the deck does not sink or lean.
How deep should decking foundations be?
There is no single figure, because it depends on the deck's height and load and the ground it sits in. Footings should reach firm, stable bearing below soft topsoil, and go deeper on weak or clay soils. A low deck on firm ground may need no footings at all, while a raised deck needs properly dug concrete footings.
Sources & further reading
- Checkatrade — decking foundations and cost guide
- RHS — paths, patios and decking
- Planning Portal (gov.uk) — decking
Figures on this page are typical UK ranges drawn from published sources and depend on your specific garden. They are guidance, not a quotation.