Pergola Calculator — Timber Quantities for Pergola Build

Bury approximately one-third of the total post length, or a minimum of 600mm, whichever is greater. For a typical hole 300mm in diameter and 600mm deep around a 100x100mm post you will need about 4 x 20kg bags of Postcrete per post: the hole holds roughly 0.036 cubic metres of fill once the post is in place, and each 20kg bag sets around 0.009 cubic metres (about 9 litres). Round up and allow 10% spare.

Pressure-treated softwood (UC4 rated for ground contact) is the most common and cost-effective choice in the UK. Oak is a premium option that weathers beautifully. Cedar is naturally rot-resistant but expensive. All posts in ground contact must be UC4 treated.

A standard pergola height is 2.4m (8 feet) to the underside of the beams, which gives comfortable headroom. Under permitted development in England the height limit is 2.5m if any part is within 2m of a boundary; further than 2m from a boundary you can go up to 3m for a flat or mono-pitch roof, or up to 4m for a dual-pitched roof. Always allow for the beam and rafter depth on top of your headroom figure.

In most cases, a garden pergola falls under permitted development in the UK provided it is no higher than 2.5m if within 2m of a boundary, does not cover more than 50% of the garden, and is not in front of the house. Always check with your local planning authority.