New Rooftop Solar Panels
New Rooftop Solar Panels
(OP)
I wanted to check my thought process on adding solar panels. The code allows a 5% change in gravity loads to avoid retrofitting the structural elements. The solar panels are fairly light and if you really think about it, wherever you place the solar panels, you're removing the area for roof live loads. So is it safe to assume that you're actually reducing the gravity load, because you are removing your roof live loads (assuming the solar panels are less than your live load)? Thanks in advance.






RE: New Rooftop Solar Panels
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
I have examined this for a conventional stick-framed gable roof in a 35 psf ground snow load. I would expect wind loads would also apply.
Panel installers (and the homeowner) want the fewest penetrations/attachments to the existing roof. If the rails that the panels attach to are connected to every 3rd roof rafter, what is the load path to where it no longer matters, and what is the tributary area for that load?
Both wind and snow loads on the panels are not permanent loads. But both loads can be applied to the panels and subsequently from the panels to the roof framing - the rafters supporting the panels must be checked. Could the panel installation actually increase the loading on select rafters? While the NDS might recognize load-sharing between multiple repetitive members, my observations of roof sheathing quality causes me to not use the permissible stress multiplier (for wood).
While I can agree that roof live load might never be realized after the panel installation, it will exist during panel installation or panel maintenance - should you really discount it?
Ralph
Structures Consulting
Northeast USA
RE: New Rooftop Solar Panels
I usually add strongbacks in the attic to distribute the load to all the rafters from the 4' O.C. point load.
RE: New Rooftop Solar Panels
XR250 - I have done the same. I have also required that the rail-to-roof connection be offset from one rail to another so that every rafter receives a portion of the load that the panels apply to the roof.
Ralph
Structures Consulting
Northeast USA
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
WARose - I agree that there cannot be significant live load AFTER the panels are in place. And, for the most part, live load may not be a controlling load. But there WILL be a live load on the roof during installation and any future maintenance of either the panels or the roof itself.
I keep getting telemarketers calling about installing "Free" solar panels on my roof. I usually tell them that if they'll put a new roof on my house, then they can install whatever solar panels they want. But until I have a roofing system with a 20 year+ life span, no go on the solar panels.
Ralph
Structures Consulting
Northeast USA
RE: New Rooftop Solar Panels
@WARose: I have the same thing, but it's tilt-up so adding solar panels didn't make a huge dent on the seismic load.
@RHTPE: San Francisco has already mandated PV panels. Just a heads up as everyone in California will probably follow.
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
This exchange is what I really enjoy about this forum. We all can't know everything about everything but each of us has a unique perspective on our craft, and the particular niche we evolved to (or fell into). The exchange of knowledge can't underestimated.
Ralph
Structures Consulting
Northeast USA
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
RE: New Rooftop Solar Panels
I would have thought the panels may increase the lateral wind load (possibly downward and uplift as well).