Pergola· 3 min read
Pergola Rafter Spacing: Centre to Centre or Clear Gap? How to Work It Out
A 600 mm rafter spacing can mean two different things. On a 4 m pergola we show how to work out the number of rafters for a centre spacing or a clear gap.
“Put the rafters at 600.” Sounds clear, but on a 4 m (about 13 ft) pergola it can mean 7 or 8 rafters. It depends on whether 600 mm is measured centre to centre or as the clear gap between rafters — and where the first rafter starts.
Centre to centre vs clear gap
- Centre to centre (on centre) is the distance between the centres of two neighbouring rafters.
- Clear gap is the open space between them — centre to centre minus the rafter thickness.
- With 60 mm rafters, 600 mm centre to centre is the same as a 540 mm clear gap.
Example: a 4 m wide pergola, 60 mm rafters
| Setting | Rafters | Clear gap | Centre to centre |
|---|---|---|---|
| Gap at most 600 mm, outer rafters at the edges | 7 | 596.7 mm | 656.7 mm |
| Centres at most 600 mm, outer rafters at the edges | 8 | 502.9 mm | 562.9 mm |
| Gap at most 600 mm, a gap at the edges too | 6 | 520 mm | 580 mm |
| Gap at most 600 mm, half a gap at the edges | 7 | 511.4 mm | 571.4 mm |
How the first row works: 7 rafters take up 7 × 60 = 420 mm, leaving 3,580 mm for 6 gaps — 596.7 mm each. With a 600 mm centre-to-centre limit, the same 7 rafters would be 656.7 mm apart — too much, so you need 8. One word in the brief means one more rafter.
Outer rafters: flush or with a gap?
- Flush with the edge — the outer rafters frame the pergola. With slats or a cover on top it is the logical choice, because the edge is supported.
- A gap at the edges — the first rafter starts one gap in from the beam end. You need fewer rafters, but the edge is left unsupported.
- Half a gap at the edges — useful when you build several bays side by side: where two bays meet you get a full gap and the rhythm carries on.
Slats on top of the rafters
Lay 40 mm slats with an 80 mm gap across the 7 rafters and over the 3 m depth you get 24 slats of 4,000 mm, with 100 mm left at both edges. Every slat crosses every rafter — that is 168 crossings to fix.
The slats rest on the rafters, so the rafter spacing (about 657 mm centre to centre here) is also the span each slat has to bridge. Check with your supplier whether a slat of that thickness can do it without sagging. The same goes for polycarbonate or any other cover — its manufacturer sets the support spacing.
Where exactly the rafters go
With 7 rafters and a 596.7 mm clear gap, the rafter centres are at 30, 686.7, 1,343.3, 2,000, 2,656.7, 3,313.3 and 3,970 mm from the left end of the beam. The middle rafter is exactly in the centre, above the middle post. Always mark from one end, not from the previous rafter — errors would add up.
PergolaTry rafter spacings in the pergola calculatorHow to choose the spacing
- First find out what the supplier or instructions mean: centre to centre or clear gap. If in doubt, ask.
- Rafter spacing is set by what sits on top and by the structural design — not by looks.
- Equal gaps look better than a round number with an odd remainder at the end. The calculator splits the gaps exactly; mark the centres from one end of the beam.
The calculator handles geometry and materials, not structural design. Rafter size, span and spacing, connections and snow and wind loads must follow the supplier’s system or an engineer’s design.