Reinforcing garage roof for solar, any structural engineers?

[Marco]

My sister is considering joining the fray and fleecing non solar users while the subsidized net metering is still going (going to end in a couple of years) and putting some solar on her garage (6 panels, unfixed on the roof so with a couple 100 kg of tiles to hold them down). Roof beams seem a bit iffy though at 160x60 mm with 800mm in between for a 3 meter span, calculators show they are undersized as is.

Would it be sufficient to add a steel strip on one side of each beam with lag bolts to reinforce these? Say 80x4 or 100x5?

[Nusa]

"unfixed", which I presume means unattached, sounds like a very bad decision when talking about large flat surfaces laying on a roof. One high wind event could send them flying despite whatever unattached weights you had in mind. Plus I suspect it wouldn't meet whatever building codes are applicable to your area.

As for everything else, we don't have enough information. Whats the type and pitch of the roof and how is it covered currently? What are the snow loads in your area, and will the panel installation interfere with how the roof sheds snow? The weight of the panels themselves (which will add to the dead load of the roof) is probably not as significant as the potential side effects.

But if you do end up decide the roof needs reinforcing, perhaps you also need to look at the rest of the structure that supports the roof as well. And unless there's a space issue, reinforcing wood with more wood might be a better choice.

[Marco]

Sorry, thought it was implied that it's a flat roof. Just weighing the feet down is a standard way of doing it for flat roofs here, especially low altitude ones. There are conservative guidelines for keeping the panels away from the edges to avoid eddys.

The garage survived snowloads for decades, so it just needs to support the extra ~300 kg of the ballast+solar panels. Which will be centered on the roof and is around 5x2 meter (assuming this mounting system). So it will spread its load across around 6-7 beams.

The beams are simply set into brick work, I'm not worried about that.

[IanB]

Sorry, thought it was implied that it's a flat roof. Just weighing the feet down is a standard way of doing it for flat roofs here, especially low altitude ones. There are conservative guidelines for keeping the panels away from the edges to avoid eddys.

There have been wind events in southern England that have lifted concrete roof tiles and sent fences flying like kites. I would not be very confident about large flat objects on a roof that are not bolted down. Standard way of doing it or not, it seems unsafe.

[bdunham7]

My sister is considering joining the fray and fleecing non solar users while the subsidized net metering is still going (going to end in a couple of years) and putting some solar on her garage (6 panels, unfixed on the roof so with a couple 100 kg of tiles to hold them down). Roof beams seem a bit iffy though at 160x60 mm with 800mm in between for a 3 meter span, calculators show they are undersized as is.

Would it be sufficient to add a steel strip on one side of each beam with lag bolts to reinforce these? Say 80x4 or 100x5?

It sounds like your beams are big enough for the span, but further apart than ideal.  I'm guessing 500mm would be better, (obviously you can't change tha) but I don't know your building code requirements.  I think the steel strips will stiffen, but not really reinforce--I don't think the weight rating increases that way.  I could be wrong about all of that too, so take it FWIW.

Is there a parapet--extension of the brickwork above the roof surface?  What material is the roof deck and surface?  And is the reason for using ballasted systems to avoid roof penetrations?

[Marco]

I don't know your building code requirements.

We don't generally do the minutia rule stuff to that extent, not like the land of the free.

I think the steel strips will stiffen, but not really reinforce--I don't think the weight rating increases that way.

It does, but the calculations are a bit complex ... and that just gives the max absorbed moment, which I assume is some function of the length of the span and the load ... which I don't know the formula for.

Is there a parapet--extension of the brickwork above the roof surface?

Nah.

What material is the roof deck and surface?

Bitumen on tongue and groove slats.

And is the reason for using ballasted systems to avoid roof penetrations?

Yep, keeps it cheap and easy to DIY.

[Red Squirrel]

I would definitely find a way to attach them, but this is very tricky on a flat roof.  Maybe check with HVAC techs that work on rooftop AC units, they may be familiar with proper procedures for this.

Worse case you could build a shroud on the back side so the wind gusts can't catch the panels.  it's not just regular wind you have to worry about, but wind gusts.   

 

[bdunham7]

We don't generally do the minutia rule stuff to that extent, not like the land of the free.

States have freedom.  The people are under the yoke of the local governments, but we can always move. Things are differ vastly if you go from NYC to rural wherever. Where I'm at is somewhere in between and I only feel mildly oppressed at times.

Bitumen on tongue and groove slats.

That actually sounds promising--I've built stuff with tongue and groove and it adds a lot of strength as long as you disperse the joints.  You can engineer a wood structure all you want, but unless you use special engineered laminates, you don't know how strong that wood actually is closer than a factor of 2 or 3 at the best.  If your deck slats are interlocked and at least 20mm thick or so, you are probably just fine tossing a 300kg unit up there.  Just angle the panels at least 5 degrees. 

[nctnico]

Another option is to add panels (>18mm thick) on the underside of the beams. This adds a large amount of strength but with 80cm between the beams it may warp under load.
Another factor is the state of the bitumen roof. If this is more than 10 years old I'd replace it with EPDM rubber and add raised mounting points under the EPDM rubber in the process so you can bolt the solar panels to the roof. Still the roof beams may need better anchoring in the wall to deal with the wind pulling the panels up.

[woodchips]

Why not just put some parallel I section RSJs under the roof to strengthen it?

With global warming apparently coming then extreme weather could become more common, snow fall but more likely wind. We have had concrete tiles, with their fixing nails, lifted out of our roof, nearby the wind was measured at 99mph, not bad for Lincolnshire!

[paulca]

Mount them on a frame.  Strap the frame to the side walls with steel strips / roofing straps.

[Marco]

I'm really not worried about wind, it's in between two rows of houses, and multiple steal I beams are overdimensioned by two orders of magnitude. A somewhat strange investment for 1500 Euros of PV.