Today we finished our latest green oak pergola. This is to a very traditional design with corner braces and beams across the top to provide some shade.
If you look carefully at the photo you’ll notice that the uprights seem to be floating. Actually they sit on galvanised steel stand-offs that isolate the oak posts from the ground. This has the advantage greatly extending the lifetime of the oak. Oak is pretty durable outside but it will eventually rot where it is both damp and the air can get to it – typically this is just at ground level. It takes a long time though – for posts of this size I would reckon on them lasting for 25 to 50 years if they are buried directly into soil. Isolated from the wet ground they are going to last much longer, perhaps more like 100 years.
This photo shows how the braces are fitted into each corner of the pergola. We use a traditional pegged mortice and tenon joint. The braces are essential to give the pergola rigidity. The curve cut on them is purely aesthetic but does look rather nice.