Flat roof array installation

[dave5358]

I'm planning a 32-panel DIY installation in SW Ohio. Most of the installation will be on a south-facing sloped roof. But a 12 x 24' flat roof is also available and relatively shade-free. The problem is that I do not want to penetrate the membrane on the flat roof. Leaking has been a problem in the past. Both the flat roof and the sloping roof were replaced two years ago and neither is leaking now.

Is there some clever way to attach an array or array framework to a flat roof without drilling through the roof? I thought about glue-attached sleepers (maybe several 4 x 4 x10' timbers) and attaching the array framework to the sleepers. Or, maybe wood sleepers with concrete blocks stacked on as ballast (don't laugh - some years ago we mounted a satellite dish in this way - worked like a charm without penetrating the roof). There must be other ways to do this.

Suggestions? Thanx.

[SunEagle]

Some of the solar pv systems installed on a flat roof is similar to what you state where the racking system is held down using heavy ballasts that do not penetrate the roof membrane.

I know you want to perform a DIY solar system but you should at least talk to a structural engineer and find out what codes and requirements must be met for that type of installation.

Wind load calculations along with roof support ability is very important or you could collapse your roof or a strong wind send the array flying away.

[Amy@altE]

You need a ballast mount, similar to what you described, but with a metal frame weighted with concrete blocks. DPW Solar makes some, as do a few other manufacturers.

[peakbagger]

Most large commercial installations that you see on warehouse and big box store roofs are ballast mount. The extra weight for the ballast can be significant. Unless the roof was designed for it there most likely will be need for structural bracing or modifications. Frequently on an older roof, the code requirements have changed to more stringent standard and adding additional load means having to upgrade the entire structure to the newer code. If someone is lucky they may be able to get away with installing a center column to cut the spans in half but its really something that a structural engineer has to review. Many building inspectors will requires a PE to sign off. Don't let the hassle scare you into doing an attached system as you regret the day you put in extra roof penetrations.

[dave5358]

Thank you to everyone who responded.

It seems the magic word is "ballast". Adding that into my Google search and all sorts of mounting systems appear. I also found a useful article on Solarprofessional.com which describes a variety of flat-roof mounting suppliers and systems. This is a descriptive article - no evaluations.

As for the flat roof in question, it has steel girders supporting a corrugated steel deck so it should handle just about any amount of weight. This is vaguely like the roof at our local Lowe's store. According to the Renusol folks, who make "plastic" flat-roof mounting systems, the ballast mostly needs to be in the exterior mounting pads - not so much on interior pads. With that in mind, the roof would probably support a Mack truck distributed around the perimeter.

[SunEagle]

Thank you to everyone who responded.

It seems the magic word is "ballast". Adding that into my Google search and all sorts of mounting systems appear. I also found a useful article on Solarprofessional.com which describes a variety of flat-roof mounting suppliers and systems. This is a descriptive article - no evaluations.

As for the flat roof in question, it has steel girders supporting a corrugated steel deck so it should handle just about any amount of weight. This is vaguely like the roof at our local Lowe's store. According to the Renusol folks, who make "plastic" flat-roof mounting systems, the ballast mostly needs to be in the exterior mounting pads - not so much on interior pads. With that in mind, the roof would probably support a Mack truck distributed around the perimeter.

Dave

Just because there are steel girders supporting a corrugated steel deck does NOT mean it should handle just about any weight.

You have to figure in the "live load" (people walking, performing maintenance, rain & snow accumulation,etc.) along with the "dead load" (pv system, air conditioning equipment, etc.)

Unless you are a structural engineer your estimate of what that roof can handle could be off by a factor of 5.

I use to work with someone that always said if the foundation is too small then just pore more concrete into the hole. He had at least two buildings sag on him due to lack of engineering design.

[peakbagger]

Flat roof collapses in New England happen frequently during winter especially when there is a real estate slump. Many marginal roofs are fine when the building is heated but take away the heat and the snow builds up as the roof drains are frozen due to lack of building heat.. Not many buildings built by contractors are overbuilt, they usually skirt on the edge of the code as otherwise the contractor would quickly be put out of business by the contractors that due skirt the code.

[dave5358]

Flat roof collapses in New England happen frequently during winter especially when there is a real estate slump. Many marginal roofs are fine when the building is heated but take away the heat and the snow builds up as the roof drains are frozen due to lack of building heat.. Not many buildings built by contractors are overbuilt, they usually skirt on the edge of the code as otherwise the contractor would quickly be put out of business by the contractors that due skirt the code.

This was an owner-built architect designed home. The roof of the small flat-roofed area (12x24') is up to commercial building standards - unlikely to collapse from snow or ballasted solar panels. My favorite highlight of this home is two 70' 10" I-beams supporting the first floor. Both of these beams are 1-piece - not welded. How did they even get them to the job site? Plus, there is a poured concrete 3-room bomb shelter on the property... just in case the 'big one' gets dropped.

Another suggestion - getting a structural engineer to review - is a good idea, but nigh unto impossible for a single aspect of a small job such as this. All risk - no gain. Even for commercial activities, my experience is that engineers don't like to get involved unless they are doing the whole job... or else working repeat jobs for, say, a solar installer... or possibly if they were involved in the original construction.

Two solar companies have made proposals on this project. Neither even bothered to look at the roof, support structure, or much of anything else. This didn't trouble me too much, at least from a structural standpoint, since I was familiar with the construction standards in the home. But I thought it was amusing. And, at least one proposed array would not fit in the space allotted. That should be very embarrassing to a solar company (or any contractor), but they just brushed it off when it was called to their attention. Amusing.

[foo1bar]

That should be very embarrassing to a solar company (or any contractor), but they just brushed it off when it was called to their attention. Amusing.

undoubtably was a salesperson - they get it "close" - and only if you go with them do they bring in someone that's actually qualified to do the design.
(Well - hopefully they will - I wouldn't guarantee it since there are crappy companies out there - just like there are crappy companies in *any* area of business.)

[MatthewThomas]

I want to know which solar panel will be good for home.