Structural values for roof spray foam insulation?

[anythingyoucanimagine]

Doing some snow load and wind sheer numbers for solar panels. I think I've boxed myself into a corner. If I can't use structural values for spray foam insulation I'm going to have to go with shorter rafter/truss spacing and that's going to be a pain in the balls because it means custom panel brackets.

Have building zoning height variance too so using bigger dimensional means ceilings go down, not roof goes up.

I realize money can solve any (most) problems. I've got history with this AHJ and I'm planning on hiring out the insulation. If I owned a spray foam setup I'd specify the product to be used, provide engineer's stamps or manufacturer's numbers, etc. I'm not looking to push it that far but at same time I'm assuming I can't call around to a few different manufacturers and submit plans based on least common denominator.

Looking for 3rd party, recognized numbers for structural values of spray foams. Thanks.

 

[dcg9381]

I don't see any real information on the structural values (calculable) of closed cell foam. Most engineers are plugging stuff into a calculator (or software) - not sure I've ever seen it considered. Not sure it should be considered as a structural roof component. Might ping Icynene - they had pretty good support at one time.

Is there a chance that you're not using the entire roof for solar? Could spreading those panels a bit solve your issue without adding rafter/truss density?

 

[anythingyoucanimagine]

I don't see any real information on the structural values (calculable) of closed cell foam. Most engineers are plugging stuff into a calculator (or software) - not sure I've ever seen it considered. Not sure it should be considered as a structural roof component. Might ping Icynene - they had pretty good support at one time.

Is there a chance that you're not using the entire roof for solar? Could spreading those panels a bit solve your issue without adding rafter/truss density?

Heh... I was thinking opposite: decrease rafter/truss spacing.

I could swear that I saw a site from somewhere legit (maybe Green Building Advisor) that mentioned how rigid spray foam on underside of roof deck could be applied to span and structural tables within IRC or IBC. I must have been dreaming (or drinking).

I'm probably going to do pre-built trusses with a metal roof. It's not so much spreading out the panels that's an issue, it's fitting them all on the plane of the south facing roof.

I have calls into a couple places but thank you for Icynene, I'll call them. I could swear that I read somewhere that spraying foam onto the underside of a wood/stick-frame roof-deck added XYZ structure to the roof (from a legit source). Since this will be new I have material options. Just really need everything to have a margin of error so it goes through easy.

People insulate over roof sheathing. Can you spray on top of a roof vs. under? What I thought I saw was numbers for structural improvements when spraying directly on wood sheathing to the underside of a roof deck (between rafters). My case is a little different but that's what I wanted to use as a starting platform. I want to do a metal roof with zip then an air gap between the underside of the roof deck and the spray insulation. Use furring strips and Tyvek or similar to create air gap between ridge and soffit vents.

I was trying to put numbers behind adhesion strength of the foam to wood (sheathing, rafters) vs. the actual structural values of a hunk/sheet of spray foam, etc. Dow Chemicals publishes numbers on some of their rigid foam panels but since they are pre-cut panels, there's nothing about adhesion or attachment to the wood. Great Stuff publishes specs too so technically I could use Tough-R panels and seal them in with Great Stuff but that would be a ****-show. And not as strong/uniform as just spraying direct.

Seems crazy but if I can find a few inches I can stay within height zoning variance and do everything we (she) wants. If I can't find the few inches then either things start getting really expensive or we have to compromise.

Why shouldn't it be considered as a roof component? (curious) Sheathing is considered right?

Other issue I'm having is that even though it'll have a metal roof, AHJ wants materials to be able to support two layers of asphalt roofing shingles plus the panels. With metal roof only (not two layers of asphalt shingles) and solar panels I far exceed target numbers. He's just being a **** about wind sheer. It's turned into a ******* contest because I made my own panel brackets and got them stamped by an engineer (his brother sells solar and I'm DIY'ing everything). I'm basically at my numbers but very close. I really need 3-5% more for an FU factor.

Thanks.

 

[dcg9381]

Heh... I was thinking opposite: decrease rafter/truss spacing.

You have it right.. Sorry about that.

All I can find (and I looked) is indications on thermal bridging - nothing structural. Like you, I was taught that closed cell has a structural component, but I've never seen it counted in terms of engineering. As an engineer, I'd be wary of putting a stamp on something that required structural application of foam as there is so much darn variation between installers.

With metal roof only (not two layers of asphalt shingles) and solar panels I far exceed target numbers. He's just being a **** about wind sheer.

A bundle of shingles covers like 33sqft, weigh 50-80, lbs, and he wants you to design for two layers? I'm kinda surprised that you're coming up short with a metal roof and solar panels.

Can you use any of the engineering data provided by the manufacturer of the solar mount system? I use Unirac, I could swear that they had a shitload of data. Course, if they're considering sheer design at the rafters/truss - the engineering of the panels and mounting doesn't help.

Only other thoughts - wider distribution of the panels, changes in roof pitch to hit your require height limit (re-pitch the panels with mounts), add truss(es), move to a shed-roof. Some additional mounting systems for trusses - if he's worried about the roof coming up...

 

[anythingyoucanimagine]

As an engineer, I'd be wary of putting a stamp on something that required structural application of foam as there is so much darn variation between installers.

Interesting you look at it that way. Aren't there differences in Concrete or asphalt? Or even dimensional lumber with drought and wet years so same two pieces get stamps but one has thicker sap rings than the other?

With foam I was thinking about it being brittle or breaking down. Don't know the words for it. Almost like how copper work hardens and breaks. Open spray foam is great because it fully off-gasses but what is in the bubbles inside closed? What are the thermal expansion properties of the gasses (and foam). Is it going to go ******** and turn to dust in 100 years.

Or... What happens when I miss a rafter with a panel bracket lag screw? Does the water get into the foam then break it apart during thermal cycles forming a big void? Sorry that is many hypothetical situations.

I'd tend to think that if there was an industry standard, they would dumb it down to least common denominator and account for the installer huffing fumes while spraying.

A bundle of shingles covers like 33sqft, weigh 50-80, lbs, and he wants you to design for two layers? I'm kinda surprised that you're coming up short with a metal roof and solar panels.

I want my cake and eat it too. Hurricane zone in 4/5 marine and because we have limited specific zoning height variance (42 ft. above grade) I can either make it work or fight the town and probably also get sued by the back neighbors when their one square inch of "waterview" (during the winter when all the leaves have fallen) goes away. This isn't 10# in a 5# bag, I'm trying to shove 50# into a 2# bag.

A bundle of shingles

He's jerking me around because of a personal beef unrelated to this property. Answer I got was: What if next homeowner wants asphalt roof with solar... or what if next owner wants slate roof AND solar. Property is in historical district, keep busting my balls and I'll take it to the historical committee that since you are approved for slate or asphalt and you have variance for metal roof, I'll make it so that if you want solar your roof needs to support 2x asphalt or slate.

Can you use any of the engineering data provided by the manufacturer of the solar mount system? I use Unirac, I could swear that they had a shitload of data. Course, if they're considering sheer design at the rafters/truss - the engineering of the panels and mounting doesn't help.

Yeah that's another beef too. I made my own solar mount system and already fought to get them certified as same or better than (insert other brand name fastening system). They have refused to consider that my mounting system is lighter than comparable widely available brands stating "what if new homeowner needs new roof and replaces with XYZ system not your lighter system". (honestly a valid point)


They are dicking me around about both. Mostly at the rafter/truss but he isn't happy about the panel mounts either.

Only other thoughts - wider distribution of the panels, changes in roof pitch to hit your require height limit (re-pitch the panels with mounts), add truss(es), move to a shed-roof. Some additional mounting systems for trusses - if he's worried about the roof coming up...

I think he's worried about proving a point and not letting me get what I want. I don't want to change things but I can easily change some stuff to make it work.

 

[strutaeng]

Doing some snow load and wind sheer numbers for solar panels. I think I've boxed myself into a corner. If I can't use structural values for spray foam insulation I'm going to have to go with shorter rafter/truss spacing and that's going to be a pain in the balls because it means custom panel brackets.

Have building zoning height variance too so using bigger dimensional means ceilings go down, not roof goes up.

I realize money can solve any (most) problems. I've got history with this AHJ and I'm planning on hiring out the insulation. If I owned a spray foam setup I'd specify the product to be used, provide engineer's stamps or manufacturer's numbers, etc. I'm not looking to push it that far but at same time I'm assuming I can't call around to a few different manufacturers and submit plans based on least common denominator.

Looking for 3rd party, recognized numbers for structural values of spray foams. Thanks.

Sorry, just briefly skimmed though your question, but NO, foam has no structural capacity as far as spanning in such a way you are describing (sprayed or rigid sheets.)

You can, however, use foam for full bearing conditions, such as fill for concrete stairs and built-up concrete floors. Hard to describe using words, but this is definitely different as you are describing. Similar to geofoam. Search "geofoam" for a visual.

 

[finn]

Sorry, just briefly skimmed though your question, but NO, foam has no structural capacity as far as spanning in such a way you are describing (sprayed or rigid sheets.)

You can, however, use foam for full bearing conditions, such as fill for concrete stairs and built-up concrete floors. Hard to describe using words, but this is definitely different as you are describing. Similar to geofoam. Search "geofoam" for a visual.

It’s got compressive load / stress capability but no tensile capability.

 

[3onthetree]

You might be thinking of structural insulated panels (SIP). The insulation is part of a factory mechanically adhered assembly. Spray foam in itself applied in the field to the underside of a roof deck has no structural value.

As others say, for instance "foam block" like used to raise grade can be calculated for compressive loads, but does not have to resist lateral or tensile loads.

 

[jbwilkins]

There are a 'few' manufacturers that claim closed cell foam can improve structural values (BASF comes to mind)....From what I remember it's for wall cavity with 16" pr 24" on center studs and it's strictly shear value.......

 

[WNYflyer]

Maybe I am missing something, but why can't the roof sheathing (OSB, Plywood, etc.) be sized for the weight of two layers of asphalt shingles plus snow load, wind load, etc like normal?
Then the rafter/trusses get designed for those loads as well as point loads from the solar panel support brackets assuming the brackets are designed for some spacing of typical rafter/truss spacing 16", 24", etc.? I would assume any solar panel support brackets are designed to span the distance between rafters/trusses and mount to the rafters/trusses rather than the sheathing?

 

[dcg9381]

My question - is "future proof" normal in your jurisdiction? It sounds like you have a roof that totally meets code and requirements for your local authority, but you're being asked "what if" and being asked to over build for some future roof design that may never happen.

I don't know how it works where you are, but around here, an inspector can refuse to sign off on anything for any reason (or no reason) - that's the reality of it. But the inspector reports to someone - a city manager (or eventually) a city counsel. If he/she can't site code and can't point to a local ordinance being violated, they're opening the city up to a legal issue.

I'm all for "playing nice" and do agree that local inspectors - especially in small jurisdictions - have a ton of power, but if I was asked to design a truss system for some "other" roof than what I was putting on AND such design was not required, I'd probably have a very frank discussion - ask for the denial / red-tag in writing, then I'd take it up with a city manager.

I know this isn't what you want to hear.

Around here, metal roofs are much more likely to stand up long term than shingle roofs - we're not a coastal city, so that may be the difference - but I agree, sounds like you're getting screwed unless that inspector can provide a reference to what you're violating...