Joist spans explained
Span, centres and section depth — the three numbers that decide your subframe, and how to read a span table without getting it wrong.
A subframe fails at the design stage far more often than it fails in the garden, and nearly always for the same reason: someone read a span table as a single number instead of a relationship between three.
The three numbers
- Span is the clear distance a joist covers between its supports — from one bearer to the next. It is not the length of the joist and it is not the width of the deck.
- Centres is the spacing between joists, measured centre to centre. 400 mm centres means a joist every 400 mm.
- Section is the joist's size, quoted width × depth. Depth is what does the structural work: a 47 × 145 mm bearer spans far more than a 47 × 97 mm joist of the same timber.
The relationship is the part people miss. Tighter centres allow a longer span, because each joist carries less of the load. That is why a span figure without its centres value is meaningless.
The published figures
These are the guidance rows this site sizes subframes against, shown exactly as they are cited.
| Material | Section | Centres | Max span | Source |
|---|---|---|---|---|
| Cladco Plastic | 50 × 100 mm | 300 mm | 400 mm | Cladco Composite Decking and Joists Installation Guide (cladco.co.uk), Recycled Plastic Framework |
| Cladco Plastic | 50 × 100 mm | 400 mm | 400 mm | Cladco Composite Decking and Joists Installation Guide (cladco.co.uk), Recycled Plastic Framework |
| Plas Pro | 50 × 125 mm | 300 mm | 1800 mm | Millboard Plas-Pro installation guidance — supports at max 1800mm centres (domestic) |
| Plas Pro | 50 × 125 mm | 400 mm | 1500 mm | Millboard Plas-Pro installation guidance — supports at max 1800mm centres (domestic) |
| 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 |
Read a row as: this section, at these centres, may span up to this distance. If your span is longer, you have three levers — tighten the centres, go deeper, or add a bearer to split the span.
These are indicative figures for planning. They are not a structural determination for your site: confirm against the current published tables, your ground conditions and building regulations before you build.
Your boards set your centres
The other constraint comes from above. Every decking board has a maximum support spacing, and it is usually tighter than anything the joists would require:
- 32 mm pressure-treated softwood: typically 500 mm
- Most composite and Millboard boards: 400 mm
- Thin hardwood boards such as 19 mm balau: 350 mm or tighter
So the board almost always wins the argument. Pick a 19 mm hardwood board and you have chosen 350 mm centres, which means roughly 40% more joists than the same deck in 32 mm softwood — a subframe cost that arrives with the board choice rather than showing up on the board line.
Boards laid diagonally are a special case: the effective span across the joists is longer than the centres, so centres come in further again.
Why the deck gets a bearer in the middle
Once the joist span exceeds what the table allows, the frame needs an intermediate bearer, and the bearer needs its own supports. This is where deck size stops scaling linearly: a deck twice as wide is not twice the subframe, it is twice the subframe plus a line of bearers and posts.
The configurator does this sizing for you — it picks joist and bearer sections and places the supports so that no member exceeds its published span at the centres you have chosen.
An example to open
4.2 × 3.6 m on C24 joists at 400 mm centres
C24 rather than C16: the same 47 × 97 mm section spans 2000 mm instead of 1800 mm at these centres, which changes where the bearers land.
Open this deck in the configuratorChange the centres from 400 to 600 and watch the subframe reorganise: fewer joists, but shorter allowed spans, so more bearers and posts. Whether that saves money depends on your footprint, which is exactly why it is worth trying on your own deck rather than reasoning about it.