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

[retirede]

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.

Thank you.

 

[ggunn]

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.

FWIW, I think a structural engineer should look at every commercial rooftop PV system that uses ballasted racking, and most jurisdictions I have worked in require it. We ran into a couple of situations where even 5PSF could not have been tolerated.

 

[BackCountry]

FWIW, I think a structural engineer should look at every commercial rooftop PV system that uses ballasted racking, and most jurisdictions I have worked in require it. We ran into a couple of situations where even 5PSF could not have been tolerated.

It’s usually not an option, most jurisdictions require it. We submit without it, and wait to see what they’ll want, since that changes every single day.

 

[ggunn]

It’s usually not an option, most jurisdictions require it. We submit without it, and wait to see what they’ll want, since that changes every single day.

OK, but when you submit without a structural analysis and the AHJ lets it pass I believe that you are taking on some liability that I would not be comfortable with. As I said, I have encountered situations where the structural engineer we were dealing with would not allow a 5PSF dead load. He may have been being over conservative, but it's his stamp on the line, not mine.

It's likely a moot point, though; In the past five years or more I have not encountered any AHJ that did not require a structural engineer's letter or stamp on the plan set for a commercial PV project.

 

[mtnelect]

OK, but when you submit without a structural analysis and the AHJ lets it pass I believe that you are taking on some liability that I would not be comfortable with. As I said, I have encountered situations where the structural engineer we were dealing with would not allow a 5PSF dead load. He may have been being over conservative, but it's his stamp on the line, not mine.

It's likely a moot point, though; In the past five years or more I have not encountered any AHJ that did not require a structural engineer's letter or stamp on the plan set for a commercial PV project.

For HVAC units on any roof the AHJ always requires an engineering stamped set of plans usually only 1 or 2 sheets with load calculations. Some larger HVAC units and older structures require additional construction drawings.

 

[BackCountry]

OK, but when you submit without a structural analysis and the AHJ lets it pass I believe that you are taking on some liability that I would not be comfortable with. As I said, I have encountered situations where the structural engineer we were dealing with would not allow a 5PSF dead load. He may have been being over conservative, but it's his stamp on the line, not mine.

It's likely a moot point, though; In the past five years or more I have not encountered any AHJ that did not require a structural engineer's letter or stamp on the plan set for a commercial PV project.

Me either. We use a PE, they only stamp when required. Extra fee.

 

[jaggedben]

In California, pretty much any commercial permit requirements a Engineers stamp.

 

[BackCountry]

In California, pretty much any commercial permit requirements a Engineers stamp.

I’ve had some luck without a stamp on smaller commercial, we still have it all designed by the same PE, they just don’t stamp it.

Anything larger and for sure — structural, and EE.

 

[jaggedben]

Yeah, I think it depends on the AHJ.

 

[Zee]

My understanding has been ballasted systems almost never work or make sense for residential:
1. most residential roofs are too weak to support the weight of the ballasts
2. wind loads are much greater within a couple feet of roof edges....and that area is often needed real estate for a residential system
3. the perimeter of ballasted arrays is often still screwed down....and that is a large part of a resid. system but a small part of a comm. system (perimeter to surface area ratio)

 

[mtnelect]

My understanding has been ballasted systems almost never work or make sense for residential:
1. most residential roofs are too weak to support the weight of the ballasts
2. wind loads are much greater within a couple feet of roof edges....and that area is often needed real estate for a residential system
3. the perimeter of ballasted arrays is often still screwed down....and that is a large part of a resid. system but a small part of a comm. system (perimeter to surface area ratio)

I agree with that.

 

[BackCountry]

My understanding has been ballasted systems almost never work or make sense for residential:
1. most residential roofs are too weak to support the weight of the ballasts
2. wind loads are much greater within a couple feet of roof edges....and that area is often needed real estate for a residential system
3. the perimeter of ballasted arrays is often still screwed down....and that is a large part of a resid. system but a small part of a comm. system (perimeter to surface area ratio)

It really depends on the racking system that you’re using. I find that Unirac Ecofoot 2 works great with small scale residential and commercial.

Usually only one or two attachments are required max. We typically use a 4’ setback. And for the load, I haven’t found an issue — even with small roof spacing.

The neat thing about Ecofoot is it does portrait and landscape.

 

[retirede]

My understanding has been ballasted systems almost never work or make sense for residential:
1. most residential roofs are too weak to support the weight of the ballasts
2. wind loads are much greater within a couple feet of roof edges....and that area is often needed real estate for a residential system
3. the perimeter of ballasted arrays is often still screwed down....and that is a large part of a resid. system but a small part of a comm. system (perimeter to surface area ratio)

Around here, residential structures almost all have a roof pitch that would exclude a ballasted system anyway.

 

[ggunn]

My understanding has been ballasted systems almost never work or make sense for residential:
1. most residential roofs are too weak to support the weight of the ballasts
2. wind loads are much greater within a couple feet of roof edges....and that area is often needed real estate for a residential system
3. the perimeter of ballasted arrays is often still screwed down....and that is a large part of a resid. system but a small part of a comm. system (perimeter to surface area ratio)

I agree, and I would add that in ballasted systems the smaller a sub array is the more weight per module it requires, so as total area goes down the PSF goes up.

 

[BackCountry]

Around here, residential structures almost all have a roof pitch that would exclude a ballasted system anyway.

Lots of flat roofs over here, TPO with a parapet.

 

[Zee]

Lots of flat roofs over here, TPO with a parapet.

Bit of thread creep but:
Ironridge FRA (flat roof attachment) with matching and simple TPO membrane (flashing) saves roofing costs vs post mount and flashing.

Flat Roof Attachment

Engineering down to the deck. The IronRidge Flat Roof Attachment combines a high-strength cast aluminum base with a 16-point fastening pattern to enable the widest range of design options on low-sloped roofs.

www.ironridge.com

Also for PVC.

 

[BackCountry]

Bit of thread creep but:
Ironridge FRA (flat roof attachment) with matching and simple TPO membrane (flashing) saves roofing costs vs post mount and flashing.

Flat Roof Attachment

Engineering down to the deck. The IronRidge Flat Roof Attachment combines a high-strength cast aluminum base with a 16-point fastening pattern to enable the widest range of design options on low-sloped roofs.

www.ironridge.com

Also for PVC.

We use OMG PowerGrips if we’re doing attachments. I’ve used the IronRidge FRA’s and U Anchors also — both work fine, but they require a roofing contractor to weld the TPO piece. That slows us down and adds some complexity and cost. OMG PowerGrips are easy to install: use chemlink primer along with chemlink M1 sealant per OMG’s instructions and you’re golden.

 

[69gp]

I have inspected at least a hundred rooftop systems and have seen all types of attachments, and ballast systems. As long as the structural engineer/ racking system are stamped drawings then its adequate. On the ballast systems at least in the northeast the block has to be weather proof and designed for cold conditions. Block will absorb water and in the winter if it does it will freeze and then cause the block to spawl. Over time your block will turn to dust and your modules will not be secured.

1st visit to one site I see a crane flying pallets of block off a roof. Product is suppose to go up not down. Got on the roof and found out the contractor was not distributing the block according to the engineers drawing. The pallets were supposed to be placed on 20' centers. Contractor was placing them right next to each other. Wrote it up and their engineer had to do the calcs to see if the roof was overloaded. It was.