Solar Panel Installation on Tar and Gravel Roof

[Jaxx]

Hello fellow solar fans

I am planning on installing a 2.2 KW micro-inverter system with 10 panels on a low slope tar and gravel roof.

The roof is built up as shown on attached picture. There is no attic and the rafters are exposed on the inside of the house (vaulted ceilings).

I'm currently looking for advice on how to mount the solar panel racking to the roof. Has anyone here done such an installation or has knowledge about how to do this? What hardware would I need?

Could I:

-Drill pilot holes through the particle board into the 4x6 rafters
-Open up hole size on the top portion down to the rafter (reason -> see 3 items further down)
-Put polyurethane into the hole
-Install standoff bolts
-Fill top of holes with roof patch/caulk
-Install flashing?
-Mount racking to standoff bolts with roughly 3-4 inches of clearance between panels and roof surface

Is this a good plan? How could it be improved?

Thanks a lot in advance for your help!

[Mike90250]

I{ think you need to get an engineer on this, as to how many & what size, holes you can punch into the beams. Generally, roof/ceilings are separate, and have sort of a "stick built truss" system. But you are adding weight, putting holes in the main support members, and till the span, loads, and perforations are calculated, I've got no idea if it's going to be safe or not.

[Jaxx]

What kind of engineer would this be?

[Mike90250]

What kind of engineer would this be?

Structural engineer, for housing.

[Naptown]

That is how it is done on 99% of installations. You could use a flashing and set that in roof cement but a good polyurethane sealant in the hole and the L foot bedded into the sealant will last a very long time (20+ years on average)
Just make sure the L on the mounting bracket is pointed downhill This keeps water running down the roof from flowing up and over the bolt which will be the weak point as the sealant will be exposed. Some good quality paint over the sealant will help with UV protection also.
Depending on the spacing on your rafters try to spread the attachment points out. For example on a horizontal row of modules with the racking running perpendicular to the rafters and the rafters on 24" centers on the bottom rail attach at the ends and every other rafter between. On the next rail up attach at the ends and every other rafter but attach to the rafters that the first row was NOT bolted to. This spreads the weight out.
For the most part a solar array flush to the roof doesn't need much more engineering than what the racking manufacturer reccomends. The array doesn't add much more than 3 -3 1/2 pounds per square foot to the dead loading or downward wind loading. It will however add to uplift so following the racking instructions is important.
You will need to check with your local AHJ however to see what their requirements are. Some require engineering and some do not.

[Jaxx]

Thank you for your detailed response.

Would L-brackets sit on the surface of the roof? I'm asking this because the 2" board that sits on the rafters is made of compressed wood fibers (similar to sound board that is used on walls for sound proofing). I don't expect this to be very structurally sound. I thought of an anchoring method where a long bolt is driven into the rafter which has coarse thread on the bottom half to go into the wood and then a regular fine thread at the top half to accept nuts. I would then mount the racking with standoff from the roof and would hold it in place with nuts (racking sandwiched between nuts). Would this work?

[Naptown]

Thank you for your detailed response.

Would L-brackets sit on the surface of the roof? I'm asking this because the 2" board that sits on the rafters is made of compressed wood fibers (similar to sound board that is used on walls for sound proofing). I don't expect this to be very structurally sound. I thought of an anchoring method where a long bolt is driven into the rafter which has coarse thread on the bottom half to go into the wood and then a regular fine thread at the top half to accept nuts. I would then mount the racking with standoff from the roof and would hold it in place with nuts (racking sandwiched between nuts). Would this work?

There are some flashing systems that are similar to that. I can't remember the name of them right now.
You may want to take a look at Eco fasten flashing systems they may have something that works.

[russ]

Thank you for your detailed response.

Would L-brackets sit on the surface of the roof? I'm asking this because the 2" board that sits on the rafters is made of compressed wood fibers (similar to sound board that is used on walls for sound proofing). I don't expect this to be very structurally sound. I thought of an anchoring method where a long bolt is driven into the rafter which has coarse thread on the bottom half to go into the wood and then a regular fine thread at the top half to accept nuts. I would then mount the racking with standoff from the roof and would hold it in place with nuts (racking sandwiched between nuts). Would this work?

Just needs the loving examination of a structual engineer - you don't want to weaken the trusses too much.

[Jaxx]

Thank you!

What would you expect this service to run at, cost-wise?

Thanks

[Mike90250]

Thank you!

What would you expect this service to run at, cost-wise?

Thanks

I just paid $1200 for an entire house set of plans. but everything was blueprints. Something already built will have to have a person on the engineers staff, verify the wood, size & spacing, and it might be $300- 800

[Naptown]

I just paid $1200 for an entire house set of plans. but everything was blueprints. Something already built will have to have a person on the engineers staff, verify the wood, size & spacing, and it might be $300- 800

And many jurisdictions have engineers on staff as plan reviewers. In which case an engineer may not be required. Many of the racking manufacturers have already done the engineering on the various systems and are willing to provide it at no cost.
If the house was built and inspected the roof engineering has already been done as prescribed by code at the time. Live and dead loads have already been established.
Average engineering cost for this in Maryland is about $500.00 Generally it is unnecessary and only required in a few locations. Most jurisdictions will accept the engineering done by the manufacturer.
Check with your local building department before spending your money on an engineer. Tens of thousands of these have been installed without an engineers stamp. I have not heard of a single structural failure on a roof built to code and the loads required to be supported.

[amira12]

Solar Panel Installation on Tar and Gravel Roof

Yes this is possible . I have seen Many pics where solar panels are installed on Gravel Roof , as well they are working well .

[Jaxx]

Yes this is possible . I have seen Many pics where solar panels are installed on Gravel Roof , as well they are working well .

Would you have some pictures that show installations on tar and gravel? Maybe even some closeups?

Thanks!

[Itsme]

Would you have some pictures that show installations on tar and gravel? Maybe even some closeups?

Thanks!

Jaxx, how did your project turn out? My wife's school has a similar roof, and are considering Solar. So your experience would be a great case study for them

Many thanks