How do I build a pergola that will not move?

A pergola is four or six posts, two beams and a set of rafters. There is nothing complicated in it, which is why so many are built without the two things that keep them standing: foundations sized for the wind load rather than the weight, and bracing that stops the rectangle becoming a parallelogram. Add a mature climbing plant and both become urgent.

Safety

What you'll need

• Posts, 100 x 100mm minimum, treated
• Beams and rafters, sized for the span
• Galvanised post anchors or post bases
• Concrete for the pads
• Coach bolts or structural screws, exterior rated
• Post hole digger or a spade and a bar
• Long level, string line and a framing square
• Circular saw and a chisel for the notches
• End-grain preservative

Step by step

1. Check what the structure will have to resist.An empty pergola is light. A pergola carrying an established wisteria or a grapevine is heavy, catches wind like a sail and can gain a great deal of load in a storm. Design for the planted, wet, windy case, not the day you build it.
2. Check the planning position if it is large or near a boundary.A pergola is usually a small garden structure with no planning implications, but height, position relative to a boundary, and anything at the front of a property or attached to the house can change that, and listed buildings and conservation areas are different again. Ask the local authority if there is any doubt.
3. Set out the plan and check the diagonals.Pegs and string lines at the post centres, with both diagonals measured equal so the frame is square. This is much easier now than after the posts are set, and a pergola out of square is visible from every angle.
4. Size the posts and the spans honestly.100 x 100mm is the practical minimum for a domestic pergola and 150 x 150mm looks and behaves far better on anything over about 2.4m high. The beam section is set by its span between posts; a deep beam on edge spans much further than a wide one laid flat.
5. Dig pads to firm ground, not to a rule of thumb.Excavate through topsoil and any soft or made ground until you are into firm subsoil. In a normal UK garden that is commonly 450 to 600mm; on soft ground it is deeper. Cast a concrete pad sized to spread the load.
6. Use post anchors rather than burying the posts.A galvanised anchor cast into the pad, holding the post 50 to 75mm above the surface, means the timber never sits in wet ground. A buried post rots at ground level within ten to fifteen years and takes the pergola with it.
7. Get the posts plumb and hold them there.Plumb in both directions, temporarily braced with battens to stakes in the ground, and left until the concrete has real strength. Do not fit beams to posts that are still being held by wet concrete.
8. Notch or bolt the beams properly.The beam should sit in a notch cut into the post, or on a shoulder, so the timber carries the load in bearing, with bolts holding it in position. Beams screwed to the side of a post rely entirely on the fixings in shear, which is a much weaker arrangement.
9. Brace every post in both directions.Diagonal knee braces from post to beam, at 45 degrees, at each junction, bolted at both ends. This is the difference between a rigid frame and one that leans a little further every winter. It is the step most often left out.
10. Fit the rafters with a proper connection and an overhang.Notched over the beams or in hangers, at consistent centres, with a generous overhang beyond the beams. Shaped rafter ends are the detail that makes a pergola look designed rather than assembled.
11. Treat every cut end, and treat the whole frame before it is planted.End-grain preservative on every cut without exception. Then whatever finish you intend, applied while you can still reach everything — once a climber is established, re-treating the frame is effectively impossible.

Tips

• Build the two beam-and-post assemblies flat on the ground where you can, then stand them. It is far easier than working overhead, if you have the space and the help.
• Wisteria in particular will eventually crush and distort undersized timber. If that is the plan, go up a section size everywhere.
• A pergola attached to a house wall at one side uses a ledger, and everything in the deck ledger guide applies — fixings into structural masonry, a spacer behind, and no bridging of the damp proof course.
• Rafter spacing is a shading decision as much as a structural one. Closer rafters give more shade and a heavier appearance; work it out by holding a couple up before fixing them all.
• Buy the timber a few weeks early and stack it flat, off the ground and covered. Treated timber arrives wet and moves considerably as it dries.

Common mistakes

• Concreting posts directly into the ground — It is quicker and it puts the timber in the one place guaranteed to rot it, at the point of highest bending stress. An anchor on a pad costs a few pounds more per post.
• No bracing — A four-post frame with pinned connections is a mechanism, not a structure. It racks in wind, the joints loosen, and it leans progressively until something gives.
• Screwing the beams to the sides of the posts — All the load goes through the screws in shear rather than through timber in bearing. It works on the day and loosens over years of movement.
• Sizing the timber for how it looks empty — The design load is a mature plant, wet, in a gale. Timber that looks generous on an empty frame is often marginal by year five.

Questions people ask