Advanced options
Assumes 100x100mm posts (max 3m apart), two 150x50mm beams along the length, and 100x50mm rafters across the width. Posts want 600mm in the ground in postcrete, or bolt-down shoes on existing concrete/decking. Free-standing garden pergolas under 2.5m tall generally sit within permitted development, but attached-to-house and front-garden versions can differ: check.
What The Pergola Calculator Works Out
- Posts: one post every 3m of length plus one at the end, doubled for the two sides, so a 3m x 3m garden classic is four posts and anything over 3m long is six. Each post set 600mm in the ground takes two bags of postcrete.
- Main beams: two 150 x 50mm beams running the full length, one along each row of posts, bolted through the posts with two coach bolts per junction. The single-bolt pivot is the wobble every inspection finds.
- Rafters: 100 x 50mm timbers across the width at 600mm or 450mm centres (length / spacing plus one), each cut 600mm over-width so a 300mm overhang at each end can take the decorative end profile.
- Maximum post spacing and post length (advanced): tighten the spacing for heavier timber or a climber such as wisteria that will one day test every bolt, and choose 2.4m to 3.6m posts. The head-height card shows what each combination leaves under the beams.
- Post fixing (advanced): switching to bolt-down shoes removes the postcrete and the 600mm embedment, which gains headroom and, on an existing slab or deck, outlasts post-in-ground everywhere the base allows it.
- Rafter overhang and cross slats (advanced): set the overhang to suit your end profile, and add 50 x 25mm slats across the rafters at 300mm or 150mm centres for denser shade. Slats add a surprising amount of timber and screws.
- Total timber (advanced): linear metres for posts, beams, rafters, braces and slats combined, with knee braces of 0.8m at each post head included because they kill the parallelogram sway that plagues the unbraced frame.
- Fixings (advanced): M10 x 150mm coach bolts (two per post-to-beam junction) and an estimate of structural screws for rafters, braces and slats. Rafters sit in notches or on birdsmouth cuts that lock them against the beam.
- Head height (advanced): post length minus the embedment and the 150mm beam. Aim for at least 2.1m, ideally 2.2m, so the tall guest and the hanging basket coexist; a free-standing pergola over 2.5m overall may fall outside permitted development.
- Timber cost and cutting list (advanced): total metres x your price per metre (treated softwood averages GBP 4 to GBP 8 a metre across these sections, oak about three times that) plus postcrete at roughly GBP 7 a bag, with a cutting list you can hand to the timber yard. Re-treat every cut end: it is the forgotten step that rots first.