Putting a pergola on decking

A deck looks like a ready-made base. It is flat, it is already there and the pergola would frame it nicely. The catch is that decking boards are not a foundation. What holds a pergola down on a deck is the frame under the boards and what that frame stands on.

What a deck is made of

Most garden decks have three layers:

• Boards that you walk on, screwed to
• Joists running underneath, which sit on
• Bearers or posts, which stand on pads, blocks or the ground.

A pergola post screwed only into a deck board is held by a few screws in a thin piece of timber. In wind, a roofed pergola pulls up as well as pushes sideways. Boards and their screws are not designed for that.

Find out what is under the boards

Before you decide anything, lift a board or look underneath where the posts would go.

• Where are the joists? A post that lands between joists has nothing solid below it.
• What do the joists sit on? Concrete pads, blocks on soil, or posts set in the ground all behave differently.
• What condition is it in? Soft or dark timber, rusty fixings or a deck that bounces underfoot all need sorting before it carries anything else.
• How old is it? A deck that has been down for years may have rot that does not show from above. The repairing a rotten post guide describes what to look for.

The usual approaches

These are the broad options. Which one suits your deck depends on its structure and the pergola maker's instructions.

1. Fix through to the structure. The post base is fixed into a joist or into added timber between joists, not just a board. Some makers allow this and some do not.
2. Take the posts through the deck. Cut the boards round each post and give each post its own footing below the deck. This keeps the pergola independent of the deck structure.
3. Put the pergola beside the deck. Stand the posts on proper footings just off the deck edge, with the roof over the deck.

The maker's instructions come first. If they only cover concrete, ask the maker in writing whether a deck is acceptable for that model and how they want it fixed. Keep the reply. We do not give bolt sizes or fixing counts here, because they depend on the kit, the deck and the exposure of the site.

When to get a competent installer

Get someone who understands timber structures to look at it if:

• the pergola has a solid or louvred roof, which adds weight and wind load
• the deck is raised well above the ground, or on a slope
• you cannot tell what the joists sit on
• the maker's instructions do not mention decking
• the deck already moves, bounces or has soft areas.

An installer may tell you that the deck needs extra framing, or that independent footings are the honest answer. That costs more than screwing into the boards, but it is cheaper than a pergola lifting in a storm.

Wind, height and planning

A roofed pergola on a deck catches the same wind as one on a patio, but the deck has to resist it. The wind and snow guide covers what makers state about loads. A raised deck also lifts the whole structure, which adds to its overall height. That can matter for planning, and the planning permission page explains how raised platforms and height are treated. Rules differ across the UK, so check your council if in doubt.

For other bases, see pergola base and fixings and fixing a pergola to an existing patio. If the deck is against the house, the wall-mounted or freestanding guide covers the wall side.

Can I screw a pergola straight into decking boards?

Not as the only fixing. Boards are not designed to hold a pergola down in wind. Fix into the structure underneath, or give the posts their own footings, as the maker's instructions allow.

Is a freestanding pergola on a deck safer than a wall-mounted one?

Not automatically. Each depends on what it is fixed to. A freestanding pergola relies on the deck or footings at all four posts. A wall-mounted one relies on the wall and two outer posts.

Will a pergola damage my deck?

It can if the posts trap water against the boards or the fixings loosen. Keep post bases clear of standing water and check the fixings each year.