Another day — another question on mounting solar collectors. Like I said in an earlier post, it’s definitely a penetrating issue. Here’s the question I received: How do I ensure the collectors are adequately attached to the roof so the wind won’t catch them like some big, expensive kite?
This contractor was wondering not only how to attach the collectors to the roof, but what kinds of structural supports were necessary underneath the roof. The structure in question has a metal roof. Here’s the contractor’s question:
“What type of backing under the actual metal roof will we need to adequately withstand windy circumstances? Do you recommend 4×4” wood supports or metal beams? And what overall length do you recommend? Is tying them to the metal deck itself enough?”
Now, the mounting on a standing-seam steel roof is easy. There are great products available for attaching both flat plate and vacuum tube collectors to steel roofs, such as the S-5 clip (for more information, Google S-5).
As for the wind questions, a good evasive answer is: It depends.
If the roof is a standing-seam metal roof like I just mentioned, and S-5 clips are used, the roofing attachment to the subsurface is adequate and no under blocking is needed.
If direct penetration through the roof is necessary, I like to see a 2”x6” board attached flush on the underside between the roof beams. Tack a couple of nails into the 2”x6” from each left and right roof beam (truss). This leaves you an “H” pattern. Now you have a bigger target to bolt to, and it’s strong. There are a couple of other different types of structural roof-fastening methods illustrated on page 21 of the HTP Flat Plate Collector Installation Manual (link here). If the collectors are being tilted up on rear legs, then fastening becomes more important when the wind comes up.
The above blocking considerations can be used with smaller residential applications. When it’s a commercial project, you should look to an engineer for advice and a stamp.