help: pv rack mount to plank and beam roof

[tac92]

am looking to install pv panels on my south-facing roof, approx 7 x 12 pitch in Norcal. Location / exposure look great, and now I'm looking to see how the panels will attach to the roof. Problem is that the roof is a plank / exposed beam construction, with a rigid 2" foam piece sandwiched on top by 1/2" sheathing, some 3/4" spacers and from the bottom by 2" wood planking (the planking is also the ceiling of the home interior). Everything I read online says to simply "attach the rack to the rafters", but the rafters START some 4.5" below the sheathing surface. That'd be what, 5.5" screws?

Is there a normal way to do the rack installation for my setup? Here are my ideas so far: I could potentially use 4" lags to reach the 2" wood planks, but not sure if I'd need spacers of some kind to avoid crushing the rigid foam. Another possibility is that the 1/2" sheathing + 3/4" spacer boards are sufficient to hold the rack, depending on the rack mount spacing, but I don't know that for sure.

[andrewc]

Maybe I missed this part in your description. What sort of roofing material are we talking about?

[tac92]

update: roof materials

roof is asphalt dimensional shingles, probably felt paper underneath, then the half inch sheathing.

[tac92]

update: response from Unirac customer support

I posed the same question from this thread to UniRac, basically a "how do I install your system (SolarMount-I) to this roof" where I described the roof construction. Their response:

[...]our recommendation is to work with experienced professional roofing consultants and structural engineers to plan and address the guidelines as to what would be code compliant and structurally sound in regards to what would be required for attaching our engineered components to specific roof types and structures. This also helps ensure that the application will be compliant to building and inspection code guidelines specific to your planned project area.

well that was very corporate. here's the tldnr: no comment, go hire an engineer. I had expected some level of tech support in the response, but I suppose passing the buck is more cost effective. Anyone aware of a competitor that can do better?

[andrewc]

I know for something like a Quick Mount, you can switch out the hanger bolt (which is 6") with a longer bolt. I've seen and used 8" hanger bolts, and I know they also make 10-12" hanger bolts, but in larger diameters (which means you'd need to switch out more hardware, but I believe the whole in the middle of the mount still accepts larger diameters to some extent). Now, finding the rafters through all that foam and stuff... I don't envy the person who would be tasked with that.

There is also Sunpower's SmartMount which attaches directly to the sheathing with 8 screws per mount, and does not require you to hit rafters, but I have never used it outside of training, because of the hurricane wind loading in my area.

So the short of it: use longer hanger bolts with a conventional product, or use a product that does not require you to hit rafters.

[russ]

well that was very corporate. here's the tldnr: no comment, go hire an engineer. I had expected some level of tech support in the response, but I suppose passing the buck is more cost effective. Anyone aware of a competitor that can do better?

With them having no idea of your roof except your own input it is difficult for them to be precise - some lawyer could easily take their statement and run with it looking for a settlement.

[tac92]

thanks

I know for something like a Quick Mount, you can switch out the hanger bolt (which is 6") with a longer bolt. I've seen and used 8" hanger bolts, and I know they also make 10-12" hanger bolts, but in larger diameters (which means you'd need to switch out more hardware, but I believe the whole in the middle of the mount still accepts larger diameters to some extent). Now, finding the rafters through all that foam and stuff... I don't envy the person who would be tasked with that.

There is also Sunpower's SmartMount which attaches directly to the sheathing with 8 screws per mount, and does not require you to hit rafters, but I have never used it outside of training, because of the hurricane wind loading in my area.

So the short of it: use longer hanger bolts with a conventional product, or use a product that does not require you to hit rafters.

Thanks Andrew, that's very helpful.

[tac92]

With them having no idea of your roof except your own input it is difficult for them to be precise - some lawyer could easily take their statement and run with it looking for a settlement.

well sure, but that's much the nature of technical support; one provides guidance based on input from the customer. I think we've all seen helpful responses with the obligatory warnings and ymmv disclaimers included for lawyers. Anyway, post and beam roof construction is a standard technique and common for 70s period homes; there ought to be a "normal" way to get a rack system mounted to it safely. I'm looking for that normal way of mounting more so than a custom engineered solution.

[Naptown]

I would just use a bolt long enough to get into the beam by 2-2 1/2" depending on the species of wood. (some have higher pull out strength than others)

[russ]

well sure, but that's much the nature of technical support; one provides guidance based on input from the customer. I think we've all seen helpful responses with the obligatory warnings and ymmv disclaimers included for lawyers. Anyway, post and beam roof construction is a standard technique and common for 70s period homes; there ought to be a "normal" way to get a rack system mounted to it safely. I'm looking for that normal way of mounting more so than a custom engineered solution.

Wrong in a case like this - customer input only means they have zero reliable input. I have been on that end and you soon realize most customers are totally inept at describing what they are talking about.

[Mike90250]

I would just use a bolt long enough to get into the beam by 2-2 1/2" depending on the species of wood. (some have higher pull out strength than others)

The tough part is getting centered in the beam, from the outside. I've heard of drilling tiny pilot holes from the inside, through the shingles, to get the beams located, snap a chalk line on the shingles. Predrill bolt holes with proper size pilot hole, and have fun.

Patch the inside with wood dough, or a finish nail, or plant hanger !

Get a bolt off center, and you split the beam, and that's no fun.

[russ]

Often 2 x 4 or 2 x 6 rafters on older homes are of questionable quality timber - split etc over the years - does that ever cause a problem?

[Naptown]

The tough part is getting centered in the beam, from the outside. I've heard of drilling tiny pilot holes from the inside, through the shingles, to get the beams located, snap a chalk line on the shingles. Predrill bolt holes with proper size pilot hole, and have fun.

Patch the inside with wood dough, or a finish nail, or plant hanger !

Get a bolt off center, and you split the beam, and that's no fun.

Generally in this type of construction the Rafters or beams are 4x6 or 4x8 on three of 4 foot centers. A Deckhouse comes to mind (brand name) yes you can miss but once you find the first one the rest are easy.

Often 2 x 4 or 2 x 6 rafters on older homes are of questionable quality timber - split etc over the years - does that ever cause a problem?

Most splits don't occur in that axis (Vertically) as that is not the axis that supports the stresses and the amount that would have to split is a longer section.

This house was probably built in the 70's or 80's. Because of the nature of the construction the rafters beams whatever you want to call them are generally very high quality clear douglas fir as they are usually exposed and stained. Sometimes even redwood. (redwood is a group one structural lumber, very strong but pull out resistance is lower than say doug fir or SYP. ) IMHO pull out is more of an issue than dead load.

[tac92]

thanks all!

thanks all for the advice; just what I was looking for.