At this point in your solar thermal installation project, you should know what kind of collector you're installing, the model you've chosen, what kind of attachment brackets it uses, where the collector will be installed and that your roof is in good repair and can carry the weight. If you haven't checked those things, you should do so.

There are three main ways to securely mount solar collector on a roof:

1. Lag or regular bolts which penetrate the roof and are secured by spanners installed underneath the sheathing, between the rafters.
2. Lag bolts which penetrate the roof and rafters, and are secured by the rafters themselves.
3. Long bolts which penetratre the roof and rafters, and are secured by spanners on the underside of the rafters.

Of the three options, the third is the most secure, though the other two are still very strong. If your area suffers from extremely high winds, you should only consider the third, though in normal climatic regions, the first two choices are usually secure enough.

The diagram shows all three methods in a simplified schematic form:

1. This method works well if you're going to install a finished ceiling attached to the rafters. The roof bracket is bolted to a 2'x4' or 2'x6' spanner which fits tightly between the rafters, just under the roof sheathing. The spanner itself is pushed up against the sheathing and attached to the rafters by two nails or screws at each end. The bolt holes are drilled a bit bigger than the bolt size. Large washers are used on the bolts.
2. This method needs pre-drilling, as screwing bolts through rafters without making holes first is likely to crack the wood. Drill the hole for the lag bolt through the rafter, using a bit slightly smaller than the bolt itself, so that the bolt's thread bites into the wood. The hole should also end slightly prematurely, so that the bolt end screws into the rafter itself. If you need to use a spacer between the mounting bracket and the roof, drill right through it with a bit slightly larger than the bolt so it slips through easily.
3. The most secure method: a long bolt or threaded metal rod fastens the mounting bracket to the spanner (some people use Unistrut as the spanner). Blocking with the same dimensions as the rafters is installed to ensure the rod is supported and that the roof doesn't cave in when the nut is tightened.

Remember to use plenty of sealant whenever you make holes in your roof. Leaks can be prevented by slapping on butyl sealant or roofing cement inside any penetrations, under the mounting brackets and shims, and all around any fasteners.

Flat roof installations

If you're installing your collector array on a flat roof, you'll need to set up a mounting rack so that the collectors sit at the right angle in the right direction. These racks are much like ground-mounting racks. Unfortunately, installation can be complex, so it's best to contact a professional roofer and ask them to install a pitch-pot (or curb), which is