I plan on roof mountin a 3 KW array with formed "C" 14 ga. steel (the C has a 3/8" lip so it's profile has all four corners and is similar to a box) I noticed most other rack systems use only 4 hold down clips bolted to the rails. My question: Is this really strong enough? I live near Phoenix and we get quite a bit of gusts here (70mph yesterday!) The C is roughly 1-1/2 X 3-1/2" and I would like to lag bolt it perpendicular to the rafters (trusses) and mount the panels to the 2 rails but I'm concerned that I might just be building really expensive kites. Any sugestions or is the 4 clip method going to do it?

Thanks!

Mikey

I thought I remembered my off-the-shelf rack system was rated at 125 pounds per square foot (tested by the company by simply loading sand bags to the appropriate weight).

I looked up the BP 4175 panels I have for my home and see that they are rated:

BP-4175-PDF

Qualification Test Parameters
Temperature cycling range -40°C to +85°C (-40°F to 185°F)
Humidity freeze, damp heat 85% RH
Static load front and back (e.g. wind) 50psf (2400 pascals)
Front loading (e.g. snow) 113psf (5400 pascals)
Hailstone impact 25mm (1 inch) at 23 m/s (52mph)

You can check with your local building department or possibly a structural engineer to get the load ratings for your area... But, it would appear that if your structure meets the 125 psf loading, it will be at least as strong as your panels (if they are rated similar to the BP-4175 -- 175 watt panels)...

I have seen other roof load ratings of 30 psf to 80 psf for snow loads and such just googling around. You need to look at the building codes for your area. I don't live in an area known for snow or very high winds (SF Bay Area)--so my requirements may not match yours.

If you multiply out the size of the 175 watt panels, you find that 125 psf works out to almost 1,700 lbs of load per panel--not an insignificant amount of weight.

And, my BP 4175 panels (with aluminium frames) are only held down with 4 clips each too (actually, the clips are shared between ajoining panels).

-Bill

Thanks Bill!

Here in AZ I'm primarily concerned with the shear strength of the clips rather than the snow load

All the racks I've seen use the 4 clip method, so I figure if I purchase off the shelf clips and bolt them to my rack, it should work out OK.

Mikey