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 — pergola cuts are unforgiving, and a wrong-length post is a real problem.
Main risk: Undersized posts or skipped bracing — the structure starts racking in wind within a couple of years and gets worse from there.
Tips & traps
- Undersized posts — 75×75mm looks fine on day one but bows under climbing plants within two years.
- Posts in concrete with no airflow at the base — rot starts at the soil line.
- No diagonal bracing — pergola rocks in wind.
- Cross-rafters spaced too widely — droops along the run.
- Forgetting that wisteria/grape weighs a LOT once mature.
Planning & timber detail
Why build this?
A pergola is one of the most transformative garden structures you can build. It defines an outdoor room without enclosing it, supports climbing plants that take a garden vertical, and gives a sense of permanence to an otherwise temporary outdoor space. Built properly it lasts 20+ years.
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 height or attached to a house may need planning permission — check before pouring posts in concrete. Mark out the footprint with stakes and string before committing — most people underestimate the size that 'feels right' from the deck and overestimate from a drawing.
Typical sizes
A 3×3m freestanding pergola at 2.4m height uses roughly: 4× 3.0m 100×100mm posts, 2× 3.6m 200×47mm beams, 5–7× 3.6m 150×47mm cross-rafters. Larger structures scale up post size faster than length.
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 or proper post-anchor brackets for all beam-to-post connections. Generic woodscrews are not enough for a structure carrying its own weight plus climbing plants in wet weather.
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 — or use galvanised post anchors keeping the timber 100mm above ground level. UC3 minimum for above-ground structure. Caps on post tops prevent water sitting 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
Get the Timber Buying Companion
An 8-page practical guide to choosing better boards, avoiding waste and spotting common timber problems before you buy.