Fixing a pergola to an existing patio

How to tell whether your patio can take a pergola's bolts, and what to do if it cannot.

Most pergolas end up on a patio that was laid years before anyone thought about bolting a roof to it. Before you order, find out what is under the slabs.

Find out what the patio is

Slab type and thickness. Natural stone, concrete flags and porcelain behave differently when drilled. Porcelain can crack, and one retailer we read lists bolting down to porcelain as an extra charge.
Bedding. A full mortar bed on a compacted sub-base is a different fixing from slabs on dabs of mortar or on sand.
Concrete underneath. Some patios are laid on a concrete slab. That is the best case for bolting.
Edges and joints. Bolts near a slab edge or a joint have less to hold on to. The maker's instructions set minimum edge distances.

Lift a slab where a post will go if you can, or drill a test hole somewhere hidden.

Follow the maker's fixing and base instructions

The maker's instructions set the anchor type, the hole depth and the minimum slab or concrete thickness. Some kits include anchors and some do not. If your patio does not meet the instructions, the stated wind and snow ratings do not apply to your build.

If the patio is not good enough

Lift the slabs at each post and cast a pad to the maker's size, then re-lay or cut the slabs round the post. Plankley works out concrete quantities.
Move the pergola so the posts land on a sound part of the patio, if the furniture still fits. Check in the planner.
Ask the maker what they accept. Write down their answer.

Before you drill

Check for drains, cables and pipes under the patio.
Check the falls: the patio should shed water away from the house, and the pergola's downpipe should not drain onto it.
Mark out post positions from the maker's drawing, measured diagonally to keep the frame square.

Can I use resin anchors in a patio slab?

Only if the maker lists them for that surface. The anchor has to suit the base and the load.

What if my patio is on sand?

Slabs on sand give a bolt little to hold. Plan for pads at the posts.