We have been busy with pergolas recently. The other one we have recently finished is a very traditional rectangular pergola about 7m by 3.5m which covers a paved area in the back garden of my client just next to their large garage wall.
Braces are made just a little thinner than the beams and joists they fit into as this looks neatest. Joints are mortice and tenon fixed with a 5/8 inch (15mm) oak peg. The whole structure as massively strong and feels very rigid. I hate it when you get solid looking pergolas that aren’t braced properly and almost wave around in the wind.
We gave the whole structure a coat of oil – not necessary for preserving it but good to stop it seasoning too quickly in what remains of our summer.
For more information see Oxford Oak