I'm Alfie Jaques, a City & Guilds-qualified carpenter based in Bournemouth (BH2), and I build decks and pergolas across Poole, Christchurch and about 10 miles around. Every quote I give is written and itemised after a free visit to the garden.
Footings are the part of a pergola nobody sees once it's finished, and they're the part that decides whether it's still standing straight in five winters. This guide covers what holds a pergola down, where posts rot first, whether you can fix one to an existing patio or deck, and what changes on Bournemouth's mix of sand, clay and coastal wind.
Why does a pergola need footings at all?
Because wind tries to lift it, not only push it sideways.
A pergola is an open frame with a lot of surface up top. When wind gets under the rafters it pulls upward on the whole structure, and a gust from the side tries to rack it out of square. The footing is what stops both. That matters more here than in most towns, because gardens on the cliff tops at Southbourne, Highcliffe and Sandbanks take the full south-westerly wind off the sea.
The joinery up top only works if the bottom of every post is tied to something that can't move. See my guide to pergola wind load design for how the frame itself is built to cope, or my pergola installation page for what a build involves.
Should the posts be concreted in, or sit on metal bases?
Either can work. What matters is that the timber that touches wet ground is made for it.
The Wood Protection Association sorts treated timber into use classes. Its Use Class 4 covers timber "in contact with ground or fresh water" and lists "fence and deck post" as a typical example. Use Class 3 timber is meant to stay out of the ground. If you're not sure which applies, the Association's own advice is to move up a class, from UC3 to UC4.
So there are two sound routes:
- A post set into a concrete footing, cut from UC4 treated timber. Any end cut on site gets end-grain preservative, and it goes at the top, not in the ground: the Association's buyer's guide says "NEVER put cut ends in the ground, even if end grain coated".
- A metal post base bolted down to a concrete footing, which keeps the bottom of the post up off the ground and out of standing water.
- What doesn't hold up is ordinary above-ground timber pushed straight into soil.
Manufactured pergolas follow the same idea. The fitting guide for one UK aluminium pergola range (Hex Living) recommends a 400mm concrete footing under each foot, with every foot bolted down using the stainless expansion bolts supplied with the kit.
Which route suits your garden depends on the ground and what's already there, and I write it into the quote so you can see exactly what's going under each post.
Can a pergola go on an existing patio or deck?
Sometimes, but only once the ground underneath has been checked.
The same manufacturer's guide is blunt about it: if a paved patio or timber decking already exists, "the ground below will need to be inspected to ensure suitability", and if it isn't suitable, concrete footings are laid and the patio or deck put back on top.
That's because a patio was laid to be walked on, and a deck's joists were sized for people standing on it. Neither was designed for a frame that's trying to lift off in a gale. A post bolted to a single paving slab or to the top of a deck board is only as strong as that slab or board.
If you've already got a deck and want a pergola over it, the usual answer is to take each post down through the deck to its own footing. It's a little more work and it means the pergola and the deck don't pull on each other.
Are sandbags or paving slabs enough to hold a pergola down?
Not for a permanent timber pergola in an exposed garden.
Weights hold a structure down by sitting on top of something. A footing holds it down by being part of the ground. None of the fitting guidance referenced on this page uses loose weights for a fixed pergola: it fixes each foot to concrete. A pop-up gazebo that comes down every autumn is a different thing, and weights are what it's designed for.
Does the ground in Bournemouth and Poole change the footing?
Yes. Sand and clay sit side by side across the area, so the ground gets checked at your address.
The British Geological Survey describes the cliffs here as "generally very weak sandstones and mudstones" from the Boscombe Sand and Branksome Sand formations, and notes "the intimate juxtaposition of sand and clay" in them. Further inland the same mix turns up in back gardens: free-draining sand around Canford Heath and Talbot Heath, and pockets of clay that swell when wet and shrink when dry.
Sand needs a footing that won't wash out or settle. Clay needs one deep enough to sit below the layer that moves with the seasons. My guide to decking on clay soil in Poole goes into that in more detail.
Trees matter too. BCP Council says "you need our permission before you carry out any work if a tree has a Tree Preservation Order (TPO) on it", and the government's tree guidance treats cutting a protected tree's roots as work that needs consent. If there's a protected tree near where the pergola's going, see building pergolas near protected trees first.