Ground frames vs posts: which foundation does your deck need?
The height at which a deck stops being a ground frame and starts being a structure on posts, and what that changes in the bill of materials.
There are two ways a garden deck meets the ground, and the choice is mostly made for you by one number: how far the finished surface sits above the ground it is built on.
The two options
A ground frame lays the joists and bearers directly on a prepared, free-draining base — compacted MOT and slabs, or concrete pads under the bearers. There are no posts. The frame stack is the height of the deck, so a 47 × 97 mm joist sitting on a 47 × 145 mm bearer puts your boards a little over 270 mm up before you have done anything else.
Posts stand the frame off the ground on timber uprights bedded in concrete. You choose the height; the posts are cut to suit.
The rule of thumb
Below roughly 300 mm of finished height, a ground frame is usually the right answer and posts are impossible anyway — there is no room for them. Between 300 and 600 mm, either can work and it comes down to ground conditions and ventilation. Above 600 mm you are on posts, and you should also be thinking about a balustrade: most UK guidance treats 600 mm as the point where a fall becomes a real risk. The configurator flags that advisory for you as soon as you type a height over 600.
What changes in the bill of materials
Choosing a ground frame removes the posts and their concrete from the quote entirely. That is the obvious saving. Two less obvious effects:
- Ventilation costs you elsewhere. A deck sitting close to the ground needs the frame to stay dry, which in practice means a properly drained, free-draining base — cost that does not show up in a timber bill.
- Ground contact changes the sensible material. A frame lying on the ground is the classic argument for a plastic joist such as Plas-Pro rather than treated softwood. It is dearer per metre and spans less, so the subframe cost rises — see the Millboard range guide for the span figures.
Bearer and joist spans still apply
A ground frame is not exempt from span guidance. The bearers still span between their pads, and the joists still span between bearers. Widen the pad spacing and the frame needs deeper sections just as it would on posts.
| Material | Section | Centres | Max span | Source |
|---|---|---|---|---|
| Pressure Treated C16 | 47 × 145 mm | 300 mm | 3200 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — bearer span table, pressure-treated softwood C16, 47x145mm section |
| Pressure Treated C16 | 47 × 145 mm | 400 mm | 2600 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — bearer span table, pressure-treated softwood C16, 47x145mm section |
| Pressure Treated C16 | 47 × 145 mm | 600 mm | 1900 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — bearer span table, pressure-treated softwood C16, 47x145mm section |
| Pressure Treated C16 | 47 × 97 mm | 300 mm | 2400 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — joist span table, pressure-treated softwood C16, 47x97mm section |
| Pressure Treated C16 | 47 × 97 mm | 400 mm | 1800 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — joist span table, pressure-treated softwood C16, 47x97mm section |
| Pressure Treated C16 | 47 × 97 mm | 600 mm | 1200 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — joist span table, pressure-treated softwood C16, 47x97mm section |
| Pressure Treated C24 | 47 × 145 mm | 300 mm | 3400 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — bearer span table, pressure-treated softwood C24, 47x145mm section |
| Pressure Treated C24 | 47 × 145 mm | 400 mm | 2800 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — bearer span table, pressure-treated softwood C24, 47x145mm section |
| Pressure Treated C24 | 47 × 145 mm | 600 mm | 2100 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — bearer span table, pressure-treated softwood C24, 47x145mm section |
| Pressure Treated C24 | 47 × 97 mm | 300 mm | 2600 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — joist span table, pressure-treated softwood C24, 47x97mm section |
| Pressure Treated C24 | 47 × 97 mm | 400 mm | 2000 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — joist span table, pressure-treated softwood C24, 47x97mm section |
| Pressure Treated C24 | 47 × 97 mm | 600 mm | 1400 mm | TDCA (Timber Decking Association) Domestic Decking Handbook — joist span table, pressure-treated softwood C24, 47x97mm section |
A low ground-frame example
3.6 × 3.0 m ground frame at 150 mm finished height
No posts: the frame sits on the prepared base and the surface height is the frame stack. Treated softwood boards on C16 joists at 400 mm centres.
Open this deck in the configuratorRaise the height above 300 mm in the configurator and switch the foundation to posts — the posts appear in the bill of materials and the total moves. That is the comparison worth making with your own dimensions.