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:
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?
