Plan the Pergola Before You Buy a Single Length of Timber

A pergola earns its keep in a British garden because it does three jobs at once: it frames a seating area, it casts dappled shade on a sunny afternoon, and it gives climbing plants something to scramble up. Get the proportions right and it will look as though it has always been there. Get them wrong and you will spend years explaining why a giant wooden box is loitering on your patio.

Start by marking out the footprint with pegs and string. A comfortable size for a garden dining set is roughly 3m wide by 3m deep, which suits four posts at the corners of a square. Anything wider than about 4m needs a third pair of posts in the middle, otherwise the beams will sag over time.

Think carefully about orientation. If the pergola sits against a south-facing wall or fence, the shade falls on the wrong side in the afternoon. Push it out from the boundary by a metre or so, or rotate it, and you will get usable shade when the sun is high. Also check where the posts land relative to any drains, manhole covers or buried cables. If you are unsure about services, keep the footings shallow and hand-dig rather than reaching for a breaker.

Finally, check your local planning rules. Most garden structures under 2.5m high, and not within 2m of a boundary if taller than that, fall under permitted development, but it is worth a quick look before you commit.

Timber, Fixings and Tools You Will Actually Need

For a pergola of this size, pressure-treated softwood is the sensible choice. It is affordable, widely available and, with a decent preservative top-up every couple of years, will comfortably outlast a decade. Avoid untreated timber unless you plan to paint it thoroughly.

• Posts: four lengths of 100mm x 100mm sawn timber, 2.4m long, giving roughly 2.1m of height above ground.
• Main beams: two lengths of 150mm x 47mm timber, cut to your overall width, fixed in pairs at the top of the posts.
• Rafters: 100mm x 47mm timber, spaced at 400mm centres, cut to the depth of the pergola plus a 150mm overhang at each end.
• Fixings: galvanised or stainless steel throughout. M10 coach bolts with washers for the post-to-beam joints, and 100mm exterior-grade screws or joist hangers for the rafters.
• Footings: four bags of post-mix concrete, or a ballast and cement mix at roughly 6:1.
• Other bits: post supports or a length of damp-proof membrane, clear wood preserver, a spirit level, a string line and a 1m post hole digger.

Buy slightly more timber than you think you need. Cupping, twisting and a rogue knot are facts of life with softwood, and one spare length saves a return trip mid-build.

Setting the Footings Square and Solid

Everything downstream depends on this stage, so take your time. Dig four holes 600mm deep and around 300mm square. In heavy clay, widen the base a little so the concrete forms a bell shape that resists frost heave; in sandy soil, keep the sides as vertical as you can.

Set the two front posts first. Drop each in, brace it with two battens screwed to the post and staked into the ground, then check it is plumb with a spirit level in both directions. Repeat at the back, then run a string line corner to corner to check the diagonals are equal — that is the only reliable way to confirm a rectangle is truly square.

Pour the concrete, tamp it down with a spare batten to remove air pockets, and slope the top of the mix slightly away from the post so water runs off. Leave it to cure for at least 24 hours, ideally 48, before you put any weight on the frame. Do not be tempted to rush this. A pergola that leans after two winters is nearly always a footing problem.

Building the Frame and Fitting the Rafters

With the posts set, cut them all to the same finished height — usually 2.1m from the concrete to the top — so the finished structure clears head height comfortably. Cap the tops with a small angled cut or a proprietary post cap so rain runs off rather than soaking into the end grain, which is where rot begins.

Fix the main beams to the posts in pairs, one either side of each post, clamped and bolted through with two M10 coach bolts per joint. This sandwich arrangement is far stronger than notching a single beam over the top and looks neater too. Check the beams are level across the width before you tighten everything down.

Now lay out the rafters on the ground first and mark them out at 400mm centres. Working on the ground saves a lot of ladder time. Lift them into place one at a time, checking each is square to the beam, and fix with two screws driven at opposing angles or with metal joist hangers for a cleaner finish. Space the outermost rafters in from the ends by about 100mm so the structure reads as deliberate rather than accidental.

Weatherproofing and Looking After the Timber

Once the frame is up, treat every cut end with preservative. Sawn timber is at its most vulnerable where you have sliced through the treated outer layer, and those are exactly the spots where water sits.

Give the whole structure two coats of a good quality wood preserver or UV-resistant stain. Clear treatments let the timber silver down naturally, which suits a rustic planting scheme; a coloured stain holds its finish longer and hides the inevitable weathering.

Then plant it. A grape vine, a jasmine or a climbing rose will soften the hard lines within a season, but keep climbers a hand's width away from the posts and train them up canes rather than letting them hug the timber. Air moving around the wood is what keeps it dry, and a pergola smothered in damp foliage will rot long before its time. Check the bolts each spring, tighten anything that has worked loose, and it will quietly do its job for years.