Lean to against fence

5 Aug 2013
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United Kingdom

I'm building a lean to shed in the garden, it's down the side of the house, so space is at a premium. Currently there is a 9ft high closeboarded fence (facing outwards so the posts and rails and visible from inside the garden). I'm going to build the lean to against that fence. I appreciate it would easier against the house, but it wouldn't be practical (you need access on that side).

The lean to is going to have to go right up to the fence. The structure is simple: I've put some posts parallel to the fence, put a timber across those at 6ft high, attached a timber to the existing fence posts at 8ft high, then cut some rafters across.. My plan is to use the fence as the back of the lean to, and I'll clad the front and sides with shiplap. For the roof I'm using OSB and felt.. I'll line the whole thing with breathable membrane to stop water ingress..

The only flaw in the whole plan is the roof.. as it's going to have to be cut around the existing posts (it can't go over them - way to high, and the fence is stepped anyway). So I'm stuck with trying to find a way of waterproofing around the posts, similarly I need stop ingress where the roof abuts the fence (which is complicated by a rail being in the way). My thinking is that as this is the top edge and I'm not too worried if a bit of water runs down the side of the fence, as the waterproof membrane will stop it getting into the building.. But whatever I come up with it seems like water will run down the side of the roof.

If it was against a house I could chase some lead flashing into the walls, but that's not an option. I did come up with a plan felting the back (folding back the bottom 8inches) first sliding the roof in place, then felting the front half then unfolding the back over it.. That will work, but it doesn't solve the issue of the water running down where the roof is cut around the posts.. so I'm wondering if anyone has any ideas? flashband? silicone?
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Actually, thinking about it I could take the boarding off the fence. Then I could relocate the rails below the roof line (6 inches down), cut the top 8 inches off the posts, then put roof over the posts, and put a drip trim from the roof into the gap between the posts and boards (essentially letting water run onto the rail) and reboard the fence - I could possibly get a small gutter in there to take the majority of the water - this is top edge so it's only water that lands there....

I could even take the top 4 inches off the boards and let the roof overhang the fence.. would look unsightly from the outside though (as it's a stepped fence at the side of a parking area).

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