Ballasted Solar Panel mounting system

[EBF]

First time doing a ballasted solar panel system on a new building. I'm trying to figure out what information we need to provide on our drawings/specs. We call out the assumed average ballast weight for the design of the roof joists and we have designed the building accordingly. I thought the mounting system/number of ballast blocks would be a delegated design but the architect seems to expect we will specify that. Anyone dealt with this before? I looked at the spec for the panels section 263100 and it doesn't have any information on the mounting system, is that a separate spec? Any input would be appreciated.

 

[haynewp]

Does the architect have a product basis he is using to decide that you will design the ballast? The ones I recall designing for didn't require us to design the ballast and came as part of the system.

 

[DaveAtkins]

The system I designed around had "trays" that sit directly on the roof membrane, and the trays are filled with concrete blocks. I agree with you, it is a proprietary system and you need not get into the design of the solar panel system itself.

I designed for the added weight of the system and for the snow drifting that occurred between the panels.

DaveAtkins

 

[Once20036]

I`ve never designed these ballasted systems, and I`m consistently surprised at how light they are (6psf ish).
When you look at wind uplift pressures on a roof, they`re consistently much higher than this. The solar guys will say that they`ve done their own wind tunnel testing and don't need to apply the GCp factors defined by ASCE.

If you design anything, I suspect it will be much heavier than the architect is expecting and heavier than what the manufacturer would provide.

 

[Aesur]

I have designed a large amount of solar, both ground mounted and roof mounted, of which the roof mounted was either fixed or ballasted. I have see systems that were tray based filled with gravel or CMU blocks and I have seen others that had a precast block that the rails attached too. The trick here is it needs to go through a racking manufacturer as many have proprietary systems and wind tunnel studies and can optimize the location on the roof using their studies to provide the least amount of ballast possible. If you designed a ballast system for wind uplift per ASCE for a tilted solar system (7-16 has a whole section on this) you will get fairly large ballast requirements to counteract uplift and sliding (both happen at the same time and should have a load combination adjustment factor - ASD is 0.6D). I just can't see designing a roof to support such a system without a proprietary system as being cost effective.

Note that SEAoC has some PV guidelines which ASCE7-16 was based on. This was developed with wind tunnel studies by one of the big testing agencies. I recall there may be guidelines that allow for movement on the roof, ie sliding, but with the caveat that after they move they have to be placed back in their original location, but this may be for seismic only. I also recall some discussion on ballast not being allowed for seismic without some exceptions that SEAoC PV1 and PV2 documents gave.

 

[EBF]

Thanks all! The architect has decided they will specify the mounting system and we only need to check the structure for the additional ballast weight (which we calculated based on solar panel wind loads in ASCE 7-16 to be about 18psf on average). It will be interesting to see what actually get submitted.