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I have a garage that has a metal frame holding the slate shingle roof. As it's way past its best days, I'd like to have a new roof installed. Almost all is clear, but I have no idea how to fix the wooden parts (sheathing?) to the metal parts (rafters?) - with breathable film between the two for full water insulation.

The frame consists of rectangular hollow iron pieces welded together, at least 5cm x 15cm in outer dimensions, with the longer sides vertical, unknown wall thickness. The roof has a slope of 15 degrees.

The goal would be to have a structure for a lightweight outer layer, either metal sheet roof, or asphalt based large panels (no idea what it is called).

Here is a diagram of what I'd like to be achieved for the structure:

Metal rafters, wood sheathing, vapor permeable film

I came up with these ideas, but all seem to fall short in a way or two:

  • drilling holes into the metal rafters and cutting threads in there, using metal screws for fixing the wood that will hold the battens - this way the water stays above the film but I think the wall of the rafters is too thin for this, the screws won't hold strong enough, and I'm not sure on the effect of drilling a lot of holes in the rafters doing any good.
  • drilling holes, but using some kind of anchor to hold the screws: slightly better, but I don't trust the anchors to much.
  • using U brackets to hold the wood: this is better on stability, but it requires the film to be cut twice with each fastening point, and nothing would cover these holes
  • use U brackets to hold another piece of wood on top of the metal rafters, and just do the rest of the roof as if it was an ordinary wooden structure, screwing the sheathing to the wood covered rafters - this would add significantly to the costs, but this seems technically sound.

What would be the proper way to do this, with lowest cost?

ppeterka
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    Do you know how the existing sheathing is affixed to the metal rafters? – Huesmann Sep 01 '23 at 13:11
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    @Huesmann What I know is that currently, there are 1-1.5 cm thick pieces of wood on top of the rafters fastened in a way it is not visible. I'd expect it to be some kind of ridiculous kludge (based on what "creative solutions" we found already). But you gave me an idea: I could drill a hole in the underside of one of the rafters and check with a small endoscope camera – ppeterka Sep 01 '23 at 13:20
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    Are your rafters some kind of rectangular metal tubing? – Huesmann Sep 01 '23 at 13:20
  • Instead of drilling more holes in the rafters, why not start removing the roofing from above, since it's going into the bin anyway? I'm not sure what kind of shingles you've got but both wood & asphalt shingles will come up quite easily with a pry bar. Just keep removing layers from the top until you get down to wood, then move alone a rafter until you find a fastener. – FreeMan Sep 01 '23 at 14:33
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    Additionally, all roofs (in the US, at least) are installed with some sort of exterior sheathing material (wood/asphalt/metal shingles, metal panels, slate, whatever) fastened _through_ the moisture barrier into the wooden substrate below. Nobody worries about the fact that I've got 10000 nails through my roofing felt. The overlap of roofing material (along with the appropriate sealer for the given material) is what makes and keeps the roof waterproof - not the barrier layer _under_ the outer layer. – FreeMan Sep 01 '23 at 14:35
  • @Huesmann yes, rectangular tubing. Actually the real thing has a bit more elaborate structure with to make it more sturdy. – ppeterka Sep 01 '23 at 15:18
  • @FreeMan it needs to go to the bin, but the question is whether now, or next Spring - decided by whether the funds for this are available with the required safety margin or not. So I hesitated to start digging in, but I'll try to find a way to check what's going on – ppeterka Sep 01 '23 at 15:36
  • @isherwood added some details - I'll be able to provide more accurate info a bit later – ppeterka Sep 01 '23 at 15:37
  • Do you have an idea how old is the slate shingles? Usually those last up to, if not over 100 years. – Jack Sep 02 '23 at 01:47
  • @FreeMan I think the OP wants to be able to have a plan before tearing into the thing. – Huesmann Sep 02 '23 at 14:57
  • I understand that, @Huesmann, but sometimes you've got to have a basic plan, then dig in to figure out what's there, then adjust the plan on the fly. I'd be so bold as to suggest that the first option (drill into the metal then screw down, but without tapping - that's what self-tapping screws are for, depending on how thick the metal of the frame is), is probably the best of the options. But... that plan may have to change on the fly depending on what he discovers there. We have a sketch of the desired outcome, but don't even get pics of what the current state is to make some guesses. – FreeMan Sep 02 '23 at 22:12

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