The short answer
The decking subframe — the joists, bearers and posts beneath the boards — commonly costs around £20 to £50 per square metre in materials for a timber frame, with the figure rising for raised decks needing posts and bracing, and for steel or aluminium frames which cost more than timber but last longer. The subframe is a major part of a deck's total, often a similar order of cost to the boards once labour to build it is added. It carries the entire load, so its size and spacing decide whether the deck feels solid or bouncy. Most timber subframes use pressure-treated softwood even beneath a hardwood surface, since the frame is hidden and protected. Foundations such as concrete pads or ground screws add to the figure.
The subframe is the part of a deck you never see but rely on completely. Because it carries the load and takes real time to build square and level, it is one of the larger costs in any deck — and the worst place to economise.
Decking subframe cost at a glance
- Timber subframe materialsAround £20–£50 per m2
- Rises withRaised decks needing posts and bracing
- Steel / aluminium frameCosts more, lasts longer than timber
- Usual timberPressure-treated softwood
- Plus foundationsConcrete pads or ground screws
What the subframe is and what it costs
The subframe is the skeleton of the deck: the joists that the boards screw down to, the bearers or beams that carry the joists, and on a raised deck the posts and bracing that hold it all up. It does the structural work, so it must be built square, level and to the right spacing for the boards above. The main material costs are:
- Joists: the closely spaced timbers the boards sit on, the bulk of the frame's material.
- Bearers or beams: heavier timbers carrying the joists, more prominent on raised decks.
- Posts and bracing: on a raised deck, the uprights and the diagonal members that stop it swaying.
- Fixings and connectors: joist hangers, post anchors, brackets and screws.
- Foundations: concrete pads, post anchors or ground screws the frame stands on.
The figures below are indicative material ranges for guidance only; labour to build the frame is on top, and on a fitted deck the frame plus its labour is a large share of the total.
| Frame type | Indicative material cost per m2 | Notes |
|---|---|---|
| Timber, ground level | Around £20–£35 | Pressure-treated softwood, shallow frame |
| Timber, raised | Around £30–£50 | Adds posts, bracing, deeper foundations |
| Steel / aluminium | Higher than timber | More costly, very durable, resists rot |
Indicative UK figures for guidance only; excludes labour and foundations on raised decks. Sources: Checkatrade and MyJobQuote 2026 decking cost guides.
Why the frame is worth the cost
It is tempting to economise on a part of the deck nobody sees, but the subframe is exactly where cutting corners shows up worst. The frame decides how the finished deck behaves underfoot and how long it lasts, so its specification matters more than almost any other choice:
- Joist spacing: joists set too far apart let the boards flex and bounce. Composite boards in particular specify a maximum spacing, and ignoring it causes sagging.
- Timber size: undersized joists or bearers deflect under load. A frame built from substantial, correctly sized timber stays rigid.
- Treatment: the frame is closest to the damp ground, so it must be properly pressure-treated to resist rot. A cheap, poorly treated frame can fail before the boards do.
- Ventilation: a frame that lets air move beneath the deck dries out and lasts; a sealed, airless underside traps moisture.
Because of all this, a low subframe cost is often a warning sign rather than a saving. A frame with wide-spaced, undersized or poorly treated joists is the classic cause of a deck that bounces, sags or rots early, and replacing a failed frame means lifting the whole deck. Spending properly on the part you cannot see is what keeps the part you can see sound for years.
Timber, steel and the foundations question
Most domestic decks use a pressure-treated softwood frame, which is cost-effective and, when properly treated and ventilated, lasts well. Some decks, particularly larger or raised ones, use a steel or aluminium frame instead. These cost more than timber but are very durable, do not rot, and can allow longer spans with fewer supports. Whether the extra cost is worth it depends on the deck's size, height and how long you want it to last.
Separately, the frame has to stand on something, and the foundations add to the subframe figure:
- Concrete pads or footings: the traditional base, dug and poured, more involved but very stable, especially for raised decks.
- Ground screws: steel screws driven into the ground that the frame fixes to, often quicker to install with less digging and spoil.
- Post anchors: brackets set into concrete that hold posts off the ground to keep them dry.
The right foundation depends on the ground and the deck's height. A ground-level deck on firm soil needs less than a tall deck on a slope, which needs deeper, stronger support. When pricing a deck, treat the subframe and its foundations together, since the frame and what it stands on jointly decide how solid and long-lasting the deck will be — and jointly make up a large part of the cost beneath the boards.
Frequently asked questions
Can I use softwood for the frame under a hardwood deck?
Yes, and it is common practice. The subframe is hidden and protected from the weather by the boards above, so pressure-treated softwood joists are fine beneath a hardwood or composite surface. This keeps the frame cost down while giving the more expensive board on top, without compromising the structure.
Why does joist spacing matter for cost and quality?
Joist spacing decides how solid the deck feels and how much timber the frame uses. Closer spacing uses more joists, costing a little more, but gives firmer support. Boards spaced too far apart flex and bounce, and composite boards in particular specify a maximum spacing, so the frame must suit the board you choose.
Is a steel decking frame worth the extra cost?
A steel or aluminium frame costs more than timber but is very durable, does not rot, and can span further with fewer supports. Whether the premium is worthwhile depends on the deck's size and height and how long you want it to last. For many domestic decks a properly treated, well-ventilated timber frame is sufficient.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific garden. They are guidance, not a quotation.