by Mike G » 23 Sep 2021, 16:15
Right, thanks, that helps a lot. I actually like what you are trying to achieve, but we've some details to sort out beforehand.
For a start, a 100mm thick wall isn't going to work well with rawl bolts in this circumstance. If you want to bolt it then I think through bolts are the only sensible answer. They'll obviously show up on the brickwork on the other side, and they'll need a big washer/ spreader plate. Three out of the five posts should be fine bolted like that. It's the two free-standing ones that you are going to have an issue with.
The photo shows a distinct difference from your 3D image, in that there is a low brick wall in the area of the 2 free standing posts. This wall isn't enough to hold the posts up on its own, but you could retain the bottom end of the post by bolting through the wall in the same way as with the other posts. You will need a second fixing higher up, though, and this is where you will need some steel (stainless). I'm afraid this is going to be something quite substantial, possibly an "L" shaped piece set into the concrete base and extending up inside the post.
The principle of Mark's suggestion is fine, and sort-of solves the structural issues. However, there are 2 reasons it won't work well. For a start, green oak will shrink substantially, and leave the post flapping about in an oversize hole. Secondly, the post will start rotting-off at ground level.
Personally, I would be looking to add visible structure which both does the job of holding the posts up, and LOOKS like it is doing the job of holding the posts up. Some tensioned wires in a cross formation bracing all of the posts laterally, for starters. I would similarly tension the small overhang of the rafter back to the post, to counteract the sagging you'll get as the timber in the joints shrinks. Then whatever design of stainless steel you came up with for the free-standing posts I would look to apply to the other three, for the sake of continuity, or, if it suits your plans for the area behind the wall, tensioned wires holding each post up could look quite sweet.
Finally, the bottom ends of all the posts should be raised above the ground. Twenty or 25mm would be a reasonable compromise between looks and durability. If you want, this could be covered with timber "skirtings", which would need replacing every few years. Don't forget that the tannins washing from the oak will seriously stain the ground around the posts. If you put fancy stone, tile or slabs down on the surface they will be destroyed very quickly. It might be best to leave them off for a year.