SECURING SOLAR ARRAYS - Attaching to the structure or using cinder blocks

[mtnelect]

On my inspections, I am starting to see more solar companies using cinder blocks, instead of direct attachment to the roof structure. To me it doesn't seem right. Maybe I am wrong.

 

[winnie]

It isn't necessarily wrong.

The question is if a structural engineer actually analyzed things like the bearing capacity of the roof (to support the weight of the blocks) and analyzed things like wind uplift forces to determine if the blocks are heavy enough.

If they're just installed without analysis then there is a good chance they are wrong.

I am only a solar DIYer, but when noodling my (aspirational) system, the wind uplift forces were substantial.

Jon

 

[mtnelect]

It isn't necessarily wrong.

The question is if a structural engineer actually analyzed things like the bearing capacity of the roof (to support the weight of the blocks) and analyzed things like wind uplift forces to determine if the blocks are heavy enough.

If they're just installed without analysis then there is a good chance they are wrong.

I am only a solar DIYer, but when noodling my (aspirational) system, the wind uplift forces were substantial.

Jon

I agree, my gut feeling is it's on the cheap. Take a windstorm, and it is really solar !

 

[jaggedben]

Ballasted systems have been a thing for many years. I think most AHJs requiring engineering for resi (and definitely for commercial). Some people think it's easier or cheaper for small flat roofs but it's really not. For large roofs that are strong enough it can be.

Here's a partly ballasted system that survived a hurricane in Puerto Rico.

Solar survives the storms in Puerto Rico

In this op-ed Elie Rothschild of Sollega takes a look at the issue of mounting systems in extreme wind situations, such as Hurricane Maria in Puerto Rico

pv-magazine-usa.com

 

[retirede]

Almost all flat-roof installations around here are ballasted. A local school where I’m on the board is evaluating it now. The structural analysis is going to cost about $7K.

 

[electrofelon]

Don't all of the major racking mfgs do the analysis and design for you? (I know they do, can't remember if it's PE stamped though).

 

[electrofelon]

On my inspections, I am starting to see more solar companies using cinder blocks, instead of direct attachment to the roof structure. To me it doesn't seem right. Maybe I am wrong.

I think you are wrong ,. These are designed with software that takes all the variables into account. Are you saying you have seen lots of physical attachment until recently? Are you talking flat roofs? I don't think I've ever seen anything other than ballasted on flat roofs (although most ballasted systems still have several mechanical attachments).

 

[retirede]

Don't all of the major racking mfgs do the analysis and design for you? (I know they do, can't remember if it's PE stamped though).

I think they do the analysis with regard to what it’s going to take to hold the system in place per local codes and wind conditions. I don’t think they normally include structural analysis of the roof. At least that’s what I’m seeing in the project I’m involved with.

 

[electrofelon]

I think they do the analysis with regard to what it’s going to take to hold the system in place per local codes and wind conditions. I don’t think they normally include structural analysis of the roof. At least that’s what I’m seeing in the project I’m involved with.

Right correct, racking people don't do roof bearing analysis. But they do provide the ballast weight and distribution that an engineer can then use.

 

[ggunn]

On my inspections, I am starting to see more solar companies using cinder blocks, instead of direct attachment to the roof structure. To me it doesn't seem right. Maybe I am wrong.

I have been designing commercial rooftop PV systems for more than 12 years, and virtually all of them were installed with ballasted rather than attached racking. Attached racking of necessity involves a large number of membrane penetrations, any one of which could leak water into the building and touch off an expensive lawsuit and/or a voiding of the roof warranty.

Post #2 notwithstanding, it is more expensive, not cheaper than attached racking and it is engineered to ASCE standards, which take into account the maximum wind speeds anticipated for the site. A structural engineer verifies both the dead load and the wind loading for every project.

 

[mtnelect]

I have been designing commercial rooftop PV systems for more than 12 years, and virtually all of them were installed with ballasted rather than attached racking. Attached racking of necessity involves a large number of membrane penetrations, any one of which could leak water into the building and touch off an expensive lawsuit and/or a voiding of the roof warranty.

Post #2 notwithstanding, it is more expensive, not cheaper than attached racking and it is engineered to ASCE standards, which take into account the maximum wind speeds anticipated for the site. A structural engineer verifies both the dead load and the wind loading for every project.

I didn't expect these responses ...

 

[electrofelon]

I didn't expect these responses ...

How so? So you are an inspector and just starting to see ballasted systems? How long have you been inspecting and what were you seeing before? Are you talking about flat TPO/PVC/EPDM roofs?

 

[mtnelect]

How so? So you are an inspector and just starting to see ballasted systems? How long have you been inspecting and what were you seeing before? Are you talking about flat TPO/PVC/EPDM roofs?

You are correct ... All the solar roof installation that I have done where 4-12 roofs. This is the first flat roof.

 

[electrofelon]

You are correct ... All the solar roof installation that I have done where 4-12 roofs. This is the first flat roof.

Ok gotcha, that makes sense.

 

[hillbilly1]

Depends on where you’re at too, on the east coast, in hurricane zones they don’t allow ballasted equipment on roofs for good reason, unless they are tethered.

 

[ggunn]

You are correct ... All the solar roof installation that I have done where 4-12 roofs. This is the first flat roof.

That explains why you are seeing them for the first time; the maximum roof pitch for ballasted systems is usually about 5 degrees.

 

[mtnelect]

That explains why you are seeing them for the first time; the maximum roof pitch for ballasted systems is usually about 5 degrees.

Good information ... I am always dealing with complainants who have lost communication with the respondents.

 

[BackCountry]

Right correct, racking people don't do roof bearing analysis. But they do provide the ballast weight and distribution that an engineer can then use.

That is exactly correct, unirac u-builder for instance gives us the ballast layout, number of blocks, and provides a pounds per square foot (PSF) that an engineer then analyzes. The big fear is if your ballast is too heavy you can cause a dip in the roof deck and get some pooling. No bueno.

The other factor no one has discussed that I saw is the seismic hold down requirement. Most ballast jobs we do require a hold down or two in earth quake country so nothing hops off the roof.

For small resi with a lot of penetrations and obstructions, ballast mounts can be a nice tool.

For large commercial, ballast is the only way to go on a TPO flat roof. I prefer Unirac Ecofoot 2, it’s super easy to work with.

 

[retirede]

That is exactly correct, unirac u-builder for instance gives us the ballast layout, number of blocks, and provides a pounds per square foot (PSF) that an engineer then analyzes. The big fear is if your ballast is too heavy you can cause a dip in the roof deck and get some pooling. No bueno.

The other factor no one has discussed that I saw is the seismic hold down requirement. Most ballast jobs we do require a hold down or two in earth quake country so nothing hops off the roof.

For small resi with a lot of penetrations and obstructions, ballast mounts can be a nice tool.

For large commercial, ballast is the only way to go on a TPO flat roof. I prefer Unirac Ecofoot 2, it’s super easy to work with.

Just curious, what are typical PSF loads you see in practice?

 

[BackCountry]

Just curious, what are typical PSF loads you see in practice?

I pulled open a job in progress, this is a small one (21kw) but that doesn’t matter — since we’re looking at an average. This particular one is 4.26psf, which isn’t bad at all. We usually shoot for 5PSF or below before engineering is required.

This particular design doesn’t have any ballasts over two blocks, which are 35lb each.