What is a pergola?
A pergola is an open structure of posts carrying beams and cross rafters, with no solid roof covering. It is originally a planted walkway or seating area: the shade comes from climbing plants, not from the roof itself.
That is exactly what separates it from a veranda or canopy. As soon as you lay roof sheets, glass or fabric on top, it is no longer a pergola in building terms but a canopy — with different rules for permission, drainage and structure.
Build-up and sizes
A typical timber pergola consists of:
- Posts of 70×70 up to 120×120 mm, depending on height and span.
- Main beams of roughly 45×145 to 63×175 mm, connected over the posts.
- Cross rafters of 45×70 to 45×95 mm, at centres of usually 300 to 500 mm. The closer the centres, the more shade.
A clear headroom of 2.2 to 2.4 metres works well — higher and it reads as a carport, lower and you hit your head as soon as plants hang down. Spans over about 3 metres call for a heavier beam or an intermediate post; have this checked if a heavy climber such as wisteria is planned. A mature wisteria or grapevine weighs hundreds of kilograms and adds considerable wind load.
Foundations and timber choice
- Avoid post base rot. Do not set posts in concrete; use an adjustable post base that keeps the timber a few centimetres above the paving.
- Pads. A concrete pad of roughly 30×30 cm and 60 to 80 cm deep reaches below the frost line in most temperate climates.
- Timber species. Durable hardwood or pressure-treated softwood for the posts; larch or Douglas fir is a middle course. Untreated spruce lasts only a few years outdoors.
- End grain. Cut it at a fall and treat it; end grain draws up water like a straw.
Permission
Whether a pergola needs planning permission depends on your local rules — typically on its height, footprint and distance to the boundary, and on whether the property is listed or in a conservation area. Check with your local authority before building, especially for anything in a front garden.
Related terms
- Pergola beam
- Post base
- Treated timber
- Planning permission
- Boundary fence
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.