Accounting for Solar Panels on Roof 1

[STrctPono]

I appreciate everyone's help so far with my Engineering related questions as it pertains to residential homes.

How do we account for solar panel weight? Is it merely additive to the dead load of the roof? Is there a standard uniform load for this? I remember reading somewhere that to correctly account for it, you should be treating it as concentrated loads... Does anybody actually do this?

Does the 20 psf Roof Live Load account for things such as item storage in uninhabitable roofs? Or is it just for things such as maintenance loads? If you have solar panels over a large area of your roof do you include both the weight of the solar panels and Roof Live Load simultaneously?

 

[jayrod12]

So in my experience dealing with these on wood trusses, they generally only fasten to every second truss, and the truss guys account for the lost material in the top chords due to the anchorage.

So perhaps an additional line load on each truss equivalent to 4 ft width of the panel weights. Unless you can specify that they have a track system that is mounted to each truss. Also have the truss guy design for the maximum expected point load to happen anywhere along the top chords but not in conjunction with the addition line loads. The intent there would be each truss can support the additional weight globally, and fastening of the post to the top chord is also covered locally.

 

[SteelPE]

I may be wrong, but how can you have solar panel load and roof load at the same time? seems like that's not possible as someone would be walking on a $300 piece of glass..... especially in HI where you are located.,

Now here in snow country, placement of the anchors to the trusses is an issue. We specify that the attachment shall load the roof system uniformly. If they attach to every other truss then they must use a diamond pattern as to not concentrate the load on any single roof member.

As Jayrod said, it may be a good idea to have the truss manufacturer design the TC for an additional concentrated load of X anywhere along the top chord of the truss...... this requirement will help ease the nerves with regards to local bending of the truss TC.

 

[STrctPono]

I may be wrong, but how can you have solar panel load and roof load at the same time? seems like that's not possible as someone would be walking on a $300 piece of glass

This is what I figured but wasn't sure if a portion of that roof live load was reserved for uninhabitable attic storage space load or if all 20 psf is intended for occupancy. From what I read, when checking global reactions, solar panels are only approx. 4psf anyway. If they are not intended to be accounted for simultaneously then I guess assuming that the roof live load is present while the panels are being installed would govern. In essence, when checking global effects, I would just ignore the solar panels and consider the roof live load.

The information about the concentrated load helps. I will speak with the truss manufacturer about the concentrated load requirement for local effects.

 

[SteelPE]

I work on mostly flat roofs... you bring up a good point about the attic space. As I live in snow country I don't know what a truss manufacturer would typically do to the truss TC when there isn't a solar panel on the roof. I would guess they should be applying the 20psf to the TC of the truss. If they are, then how can you have live load on the TC and solar panels at the same time? When you are talking to the truss mfr you might want to ask them about this.

 

[Aesur]

The problem with trying to account for solar is there are way too many possible installations to accurately guess at what will be there in the future; you may have a tilted and elevated system in which case live load could be on the roof, in this case you also would have higher wind loading. The different racking systems allow for attachments spaced at far spans or close spans, it's really impossible to tell unless the owner/architect gives you an idea of the future solar system. Most engineers typically just throw an additional 5 psf dead load on the roof to account for the dead load and assume it will be parallel to the roof and meet the gap and offset requirements of IBC and ASCE7 to not add any additional wind loading. Doing this technically meets many requirements to design a solar ready roof, however what can be installed will be very limited and in some cases so inefficient for thh solar system that it is worthless. As for live load on the roof review 1607.13.5 in IBC 2018 for the requirements.

 

[XR250]

I don't usually get too excited about solar panel loads. There is generally enough fat in truss designs to account for the additional 3 PSF. As other have mentioned, the panel area basically precludes human loads which are accounted for in the 20 PSF. To deal with the unknown rack attachment spacing and locations, I usually show site installed 4x4 strong-backs under the top chords that will distribute the load to the nearby trusses.

 

[STrctPono]

Thank you all for the help!