Mounting panels to the roof

1 Posts
Nov 10, 2015 10:10 pm
Mounting panels to the roof

I have purchased an IronRidge solar panel mounting system to mount 33 280 watt panels to a metal roof. The plan is to use Iron Ridge slotted L foot to go between the rails and the roof and use  lag screws to fasten the L foot to the roof. With all of this in mind I have three questions:

1. What is the optimal spacing between the panel and the roof. Currently with the height of the L-Foot and the height of the rail the bottom of the panel frame will be approximately 4 inches off of the surface.

2. How long and what size should the Lag screws be? They will be driven into 2" X 6" purlins.

3. Should I use stainless steel lag screws?

Thanks in advance for any feedback you can give.

47 Posts
Nov 19, 2015 10:46 am
Re: Mounting panels to the roof

Sorry for the delay in answering.
1. Depending on where you are, and how hot the roof gets, the standard space of the L-foot plus the width of the rail is generally enough air space between the roof and the panels. If you are in extreme heat, additional standoffs or a longer L-foot may be helpful to increase air flow and keep the panels cooler. For metal roofs, I like the EcoFasten L-foot with the EPDM bushing to help seal the hole. They are available in 3" or 6" lengths, for a higher mount, .
2. It depends. There are calculations to determine the length of the lag bolt needed that include wind loading for your area and type of wood for the rafters. Let's assume 10.6 sqft per panel and 4' spacing between L-feet (3 rows of 11), that would require 60 roof connections. Assuming the array needs to withstand 50psf wind uplift force the equation is 50 psi x 10.6 sqft = 530 lbs per panel x 33 panels = 17,490 lbs for the array / 60 L-feet = 291.5 lbs per L-foot. There is a chart in the Wood Engineering and Construction Handbook for what length Lag bolt is needed based on if the rafters are Southern Yellow Pine, White Spruce, or Douglas Fir. For example, a 5/16" bolt in White Spruce can handle 227 lbs per inch. 291.5 lbs / 227 lbs = 1.3" of the lag bolt must be in the rafter. Add an inch for going through the L-foot, roof, underlayment, etc, and you need at least a 2.5" stainless steel lag bolt. I'd use 3" for good luck ; )
3. Yes, use stainless steel lag bolts.

Amy Beaudet
Solar Queen
altE Store

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