Pergola
A freestanding or lean-to pergola for shade, climbing plants, an outdoor dining area or a sheltered seating zone. The structural detailing makes the difference between an elegant garden feature and an alarming structure.
What you'll need
Materials
- Posts (100×100mm minimum, 150×150mm for larger structures), main beams, cross-rafters, post anchors or concrete-set bases, structural screws or coach bolts, optional shade slats or canopy.
Tools
- Impact driver, circular saw or mitre saw, spirit level (long), tape measure, post-hole digger or auger, clamps, ladder, string line.
Material complexity: High
Allow one spare on each component type — a wrong-length post is a real problem.
Main risk: Undersized posts or woodscrew joints — the frame racked by wind and the weight of wet climbers.
Cut list
3×3m freestanding pergola at about 2.4m head height. Member counts match the live page.
- Posts ×4100×100mm × 3000mm (about 600mm in the ground) — or 2400mm on galvanised shoes kept 100mm above ground
- Beams ×2200×47mm × 3600mm
- Rafters ×6150×47mm × 3600mm (use 5 on a lighter look, 7 if you want denser shade)
- Optional shade slats: 38×19mm × 3000mm, count to suit the gaps you want
- Post caps ×420×150mm squares, or bought chamfered finials
- Post anchors ×4, or post-mix if you bury the posts
- Coach bolts: M10 or M12 × 150–200mm, two per beam-to-post joint, with washers both sides
- Rafter fixings: structural screws or joist-style brackets, two per rafter end
Step-by-step
Mark a square 3×3m on the ground
Peg the four corners and check the diagonals are equal. This is the moment to live with the size — walk around it, put a table in the square, and only then dig or fix shoes.
Tip: String lines beat guessing. A 50mm error at the base becomes a twisted roof.
Set four posts plumb
Set 100×100mm posts — 3000mm if 600mm is going in the ground (UC4 timber, because that end sits in soil or stays wet), or 2400mm on galvanised shoes kept 100mm above ground (UC3 is then enough). Brace each post both ways until the mix is set or the shoes are tight.
Watch out: Do not skip the plumb check. A leaning post cannot be hidden later.
Seat the two beams
Offer the 200×47mm × 3600mm beams across the posts. Notch the post tops if you can, then through-bolt with M10 or M12 coach bolts, two per joint, washers both sides. Check the beams are level with each other.
Watch out: Woodscrews will tear out. This joint carries the whole roof.
Space the rafters and fix them
Lay five to seven 150×47mm × 3600mm rafters across the beams. Equal gaps look deliberate; start from the centre and work out. Two structural screws or a bracket at each end. Optional shade slats go on last, at right angles to the rafters.
Cap the posts and re-check every bolt
Fit a sloped cap or finial on each post so water cannot sit on end grain. Recheck plumb and bolt tightness. After the first winter, go round every bolt again — the timber settles and what felt tight on day one will have slack.
Tip: If you plant climbers, keep them off the post bases so you can still inspect for rot.
Tips & traps
- Posts in the ground with only UC3 treatment — they rot at the soil line.
- Woodscrews instead of coach bolts at the beam joints.
- No post caps, so end grain soaks.
- Building over 2.5m or onto the house without checking permission.
Planning & timber detail
Why build this?
A pergola defines an outdoor room without enclosing it, takes climbing plants vertical, and gives a garden a sense of permanence. Built properly it lasts decades; built with undersized posts and woodscrews it becomes an alarming structure.
Where it works best
On level or near-level ground where posts can be set plumb and square. Avoid extremely exposed sites without sizing posts and bracing up — wind loading on a pergola with climbing plants is significant.
Planning notes
Anything over 2.5m high or attached to a house may need planning permission — check before you pour concrete. Mark the footprint with stakes and string first. Most people underestimate the size that feels right from the deck and overestimate it from a drawing.
Typical sizes
Default: 3×3m footprint, 2.4m head height.
Four 100×100mm posts. If you set them 600mm in the ground you need 3.0m posts to finish at 2.4m. On bolt-down shoes, 2.4m posts are enough.
Two 200×47mm beams at 3.6m (overhang past the posts). Five to seven 150×47mm rafters at 3.6m.
Default dimensions
Footprint: 3000 × 3000mm.
Head height: 2400mm (2700mm if you want more air).
Posts: 100×100mm. Beams: 200×47mm. Rafters: 150×47mm, five to seven.
Design variants
Freestanding 3×3m (this page) — four posts, two beams, 5–7 rafters.
Lean-to — two posts and a wall plate. Check the wall can take the load and whether attachment needs permission.
Larger than 3m — step posts up to 150×150mm. Span grows faster than length in terms of post size.
Suitable timber options
Treated redwood for budget and longevity. Oak for a heirloom structure with the weight and cost to match. Douglas fir for the structural sweet spot — strong, straight, mid-priced. Cedar only for smaller spans.
Fixing and finishing
Use structural coach bolts (M10 or M12) or proper post-anchor brackets for every beam-to-post joint. Generic woodscrews are not enough for a frame that carries its own weight plus wet climbing plants. Notch beams into post tops if you can; face-fixing with coach bolts works but looks more utilitarian. Re-tighten every bolt after the first winter.
Notch the beams into the post tops for structural strength and a clean look — face-fixing with coach bolts works but reads as utility. Cross-rafters typically sit on top of the beams with proprietary brackets or notch joints. Cap the post tops with a chamfered finial or a flat cap to shed water.
Maintenance
Annual check of all bolted connections — re-tighten any that have loosened. Check post bases for rot. Re-oil if maintaining colour.
Re-tighten ALL structural bolts after the first winter. The timber settles, the load redistributes, and what felt tight on installation day will have a quarter-turn of slack.
Timber behaviour
Durability
UC4 for any post going in the ground — UC4 means timber that sits in soil or stays wet. Or use galvanised post anchors and keep the timber 100mm above ground, in which case UC3 (timber kept off the soil) is enough for the whole frame. Cap the post tops so water cannot sit on end grain.
Movement
Pergola beams are big enough that movement matters — long beams can twist as they dry. Use kiln-dried structural stock if available, and over-spec slightly so any twist is cosmetic not structural.
Go deeper
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