Rooftop solar options

A portion of our house has a flat roof - not ideal from an insurability perspective or from a heat avoidance perspective either, especially in north Florida. We have thought about adding solar to the house a few times, but I've always struggled to make the economic math work. Lately I've wondered if I could improve the math a bit by placing the solar on the flat roof section instead. Doing so would provide the obvious benefit of power generation, but would also mean that section of the house would be much more shaded, thereby lessening the amount of A/C needed to cool the house.

My main concern would be possible penetration issues. Panels are affixed to sloped roofs all the time but lots of commercial buildings with entire flat roofs have panels too. Is there anything special that would need to be done if we were to install racks and panels on the flat roof section? Would we need racks in order to get the correct angle? Picture below to help. The flat roof portion is the lighter gray in the middle. And the NW portion of that section is actually sloped up (meaning more southerly facing) as that room has a vaulted ceiling.

Interesting question. I was wondering if you could do a simple estimate of heat gain by measuring ceiling temp and room temp and plugging in some heat transfer numbers from painted drywall to air, but the calculators I found online were way above my knowledge level. Do we still have the ability to get paid back for the excess at higher than billed rates here?

Many of the flat roof options do not require penetration of the roof membrane. They are like big aluminum sleds that are ballasted down.

You'd need review of your roof structure to see how much weight it can comfortably hold, and if that's enough ballast for hurricane season.

Membrane roof? I'd reach out to some established local installers and ask about the details of the mounting. Mine's installed on a steel roof and I believe the fasteners have rubber washers. There's been no sign of any leakage, and it would be fairly obvious because there's nothing under that steel roof but some insulation.

I know that in my install, we didn't use angled racks. My roof has a slight slope that points half the array east and half south, but it's not by much. I think we're at the point where it's cheaper just to chuck another couple of panels up there than to do all the special racking.

I'm hoping to gain some benefit from the panels shading the roof as well, but I won't know until later in the summer if that's having an effect as I'm just coming into my first summer with it.

For commercial flat roof installations (typically membrane) the standard is to use a rack (5-10 deg tilt) with ballast to secure them. These (usually) don't require any penetrations through the membrane. The only requirement on your roof would be a membrane that is in good shape and adequate structural capacity to handle the ballast weight (typ. 3-6 PSF). I don't have any residential flat roof experience so I don't know if typical construction can handle the increased load. For reference, flush mounted systems on pitched roofs (that would probably include your metal standing seam Kieth) are typically 2.5-3 PSF.  If your roof couldn't handle the increased load, these racks can be installed with attachments in lieue of (or supplementing) ballast. Those would usually lag through to a structural member and you'd have a roofer flash it just like any other membrane penetration. With a competent installer, no increased risk of leaking compared a vent pipe.

Check out this page for reference: https://unirac.com/roofmount/rm5/

As far as lower roof/membrane temperatures under the panel, I'm confident you would see a reduction, but I don't know if there are standard rules of thumb to calculate the impact. 

Flat mount systems are typically more efficient than flush mount as they get better airflow behind the modules (module efficiency is inversely proportional to temperature) but you give up some density as there's spacing between titled rows. You can also do east-west facing systems to get better density. 

All of this should be well in the wheelhouse of your local PV installer. Make sure you find one with both residential & commerical experience as your system would have elements of both installation types.

Oh, and the weight I quoted above is for mid atlantic sites, if you're in a hurricane zone, those could easily double (or triple). I'm an electrical, not structural, engineer, so I deal mostly in order of magnitudes for reference when it comes to structural impacts.

Great feedback everyone. Thank you!

@MrJoshua - No is the short answer. I think we can sell back excess but only at market rate.