Published 2026-10-29 · 2 min read
How to design a wooden pergola: sizes and spans
A pergola looks simple: four posts and some beams. But if the beams are too thin for the span they sag within two years, and if the posts are not anchored properly the wind moves the whole thing. Here are the numbers to start from. Always check local rules and, for large spans, an engineer.
Basic dimensions
- Clear height under the beams: 2.30–2.50 m. Below 2.20 m it feels low; above 2.70 m it shades less.
- Timber posts: 12 × 12 cm for an ordinary pergola, 14 × 14 cm for bigger spans or a roof.
- Post spacing along the main beams: up to 3–3.5 m with 8 × 20 cm beams.
- For a terrace with a table for six: at least 3 × 4 m.
The beams on top
The main beams sit on the posts, and the secondary beams (rafters) sit across them. Rafter spacing decides how much shade you get.
- 5 × 15 cm rafters 50–60 cm apart: pleasant shade, especially with a vine or wisteria.
- 80–100 cm apart: mostly decorative; shade comes only from plants or a sail.
- Let the beams overhang the posts by 20–40 cm — it looks better and keeps rain off the ends.
Anchoring and roofing
- Do not bury posts directly in soil: they rot. Fix them with metal post bases on concrete footings at least 40 × 40 cm, below your local frost depth.
- 45° knee braces of 50–70 cm between post and beam stop it swaying in the wind.
- If you roof it with polycarbonate, give it a slope of at least 5° so water runs off, and space the supports as the maker specifies.
- Check whether your pergola needs a permit — rules differ by place.
Draw it before you cut the wood
GrafitGo's Pergola object takes length, width, height, number of rafters and post size as parameters. Put it next to the house, walk under it at eye level and move the time of day to see where the shade falls. The bill of materials and cut list then tell you how many pieces of what length to buy.
Which timber for a pergola?
Pressure-treated softwood (spruce, pine) is the most common and cheapest; larch resists rot better naturally. Any outdoor timber needs a stain every 2–3 years.
Can I fix the pergola to the house?
Yes, with a ledger beam fixed to the wall with chemical anchors and only two posts on the free side. Check first what is in the wall — thick insulation holds nothing.
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