Fireproof (resistant) cathedral roof
–Looking for modifications to make one of the “5 Cathedral Roofs that work” in Martin’s Dec. 11, 2020 article more FIRE-RESISTANT.
(https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-workcid=287281&discussion=comment#comment-287281)
We are building a new home with cathedral ceilings in climate zone 7 in the mountains and I would like to make the roof fire-resistant, durable (and ice dam resistant if possible). I started with the idea of using 8″ of exterior polyiso with the top layer being rockwool all above the roof deck but the roof became 22″ thick —it was too heavy and didn’t match the aesthetics. We have 11 7/8″ I-joists for the roof (part of the roof is on top of exposed timber frame trusses in the great room)–I now believe the only way to build this without a super thick roof assembly is with a flash and Batt approach similar to roof #5 above in the article. The problem is that none of the roofs in the article are fire-resistant. So I was hoping to combine this roof with some of the ideas in Joe Lstiberec’s 2013 article “ROCKS DON’T BURN” (https://buildingscience.com/documents/insights/bsi068_rocks_dont_burn) ——similar to his figure 2—hoping this community can help me create a fire-resistant R70 roof that is less than 16″ thick ——here is my current plan from the inside to the outside:
5/8″ drywall
11 7/8″ I-Joists with 6″ of Rockwool then 6″ CC spray foam against the plywood sheathing in the bays
plywood sheathing
vapor open breathable peel and stick membrane with around 20-30 perms
1 1/2″ or 2″ of Rockwool Comfortboard 70 or maybe Comfortboard 110 ??
2 x 4 strapping going vertical to create vent channel—-this would help prevent ice dams (could be talked out of the vent channel which is a fire risk but I believe I can reduce the risk with fire-proof vents and eaves.
Layer of Tyvek Commercial D over the Rockwool to protect it and a strip of peel and stick fire rated underlayment on top of 2 x 4’s
2nd layer of plywood
2nd layer of vapor open breathable peel and stick–fire rated like Polystick XFR or Titanium FR. although perm ratting of these products are only 0.5
Metal roof in some areas over the shed roofs and Bravo fire-rated class A rubber shingles in other areas over the gable roof areas.
Will this work?—looking for ideas to improve this—I do like the idea of having two sheathing layers and two membrane layers which I would think would reduce the chances of a roof leak—I also think both sheathing layers are breathable. Yes, a bit more expensive but maybe cheaper than re-roofing if a leak occurs in a single sheathed roof?. Am I really gaining anything here or is this overkill and a waste of money as I could easily just put a fire resistant membrane over the one layer of plywood and be done with it.—it would be sandwiched but maybe that’s ok—–Thanks in advance—-Brian
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Replies
I feel your system is too complex, and would be a pain to build, and doesn't gain you much.
For fire resistance, you want a good amount of rockwool on top to dissipate the heat. For example, 7½" of foam & ½" rockwool isn't robust.
Since you're using foam I can think of two simpler approaches:
Use 10" of cc spray foam in the joist bay,
¾" plywood sheathing
Bitumen based Wrb membrane
2" + 2" rockwool board (staggered seams)
2x4" furring
Standing seam steel roofing
If you don't want to use spray foam, another option is to increase the joist height from 11⅞ to 14", this gives you more room for rockwool batts, and then the same assembly as above. Yes there is a cost increase the I-joists, but that would probably be more cost effective than the spray foam.
I'll say a bit of a rant about materials selection. The ASTM E108 is a good test. I'm glad it exists. But note that it's not the be-all/end-all for if the material will catch on fire and propagate. In fact it actually actually says it's not a fire risk assessment of actual fire conditions, and it's a test to measure the response of materials to heat and flame in a controlled setting.
I know there are rubber, recycled plastic, or asphalt roof coverings that have a class A rating. Does that mean it's the most robust option? Personally, I don't think so.
If you're concerned about fire, using non-combustible materials is better than using combustible materials that take a little bit longer to combust.
Just my thoughts,
Jamie
Thanks Jamie---agree with your thoughts on fireproof materials---hence the metal roof----btw, for the Brava rubber roof tiles, I torched the tiles with MAPP gas (5,000 degrees) for 1 minute--they melted but wouldn't catch on fire!---I think your suggested roof is pretty much what I was proposing except I added a layer of plywood above the 2 x 4 furring and then the fireproof WRB over the plywood--because I believe most standing seam metal roof's can't be installed over furring---they require a firm surface under all of the metal ie plywood under them------but maybe I could eliminate the 2nd layer of plywood and fill in between the furring with another layer of rockwool comfortboard 110-----this way I could also eliminate the expense of the fire rated peel and stick membrane----brian
Brian,
I'll be interested in the views of more blue sky thinkers like Akos and Bill as to what might be simpler alternative assemblies. Looking at what you are proposing a few comments.
- The plywood over your I joists acts as a variable perm vapour-barrier, and if taped is a good air barrier. You don't need a membrane over it.
- Every type of metal roof panels I've used can be installed over horizontal strapping. The downside of doing that is it leaves voids to collect moisture, and no good drainage path. Adding rockwool board between the strapping would mitigate that. Then you could install your roof underlay directly under the metal panels, where it is more effective.
Not any blue sky ideas here. In heavy snow country a vented roof is the best to limit ice but that creates issues with ambers.
I would start from a rated assembly and work from there. You can't really go wrong with a metal roof and class A gypsum or fiberglass underlayment.
If you are bellow 60lb of snow, stick to simple unveted assembly (spray foam+batts).
If above 60lb, 2x4 on flat for vent channel followed by roof deck/class A underlayment/roof. Rated screens for all vents.
Adding the MW above doesn't make much sense to me as it won't buy you much additional resistance. By the time the fire gets through the metal/underlayment, your windows and doors are long gone.
If you do want MW above, I would do an over-roof 2x8 rafters with 2x6 mineral wool batts, the extra gap is your vent space. Roof deck/Class A underlayment/Metal over this. As long as the first deck over the I-joists is permeable (ie CDX/OSB with permeable underlayment), you push the condensation ratio a bit, so you can fill the I-joists with 2x10 batts to get you up the R49. This won't be that thick as you can stop the I-joist at the walls and only extend the over roof to form your soffit.
I know some there are other roofing materials that are fire rated but I really can't come to trust anything other than metal/slate/tile. When you look at post fire pictures, usually the houses left standing have metal or stone roof.
Since I'm more on the commercial side than the residential side, I start thinking "noncom construction" when I hear someone looking for fire resistance. This would mean steel trusses instead of wooden joists (and you could use pretty lightweight trusses here), and a steel roof deck. The conventional way to insulate a commercial roof is polyiso over the deck, then a membrane over that. The membrane would be the weak spot for fire resistance. Usually I see EPDM and TPO membranes, and while they are both fire resistant, I'm not sure how fire "proof" either of them actually are. TPO as I recall can fracture in really cold temperatures if you walk on it too (I might be mistaken on that -- I know of a roof membrane happening in my area, but it wasn't my project and I can't be sure what membrane was actually used).
Steel framing and a steel roof deck is pretty fire proof. Polyiso is resistant to fire spread, but needs something to protect it. Maybe you could put a standing seam metal roof over it? I have no experience trying that, but polyiso is a lot more rigid than "rigid" mineral wool is, and it's usually rigid mineral wool panels that are a problem for standing seam metal roofing if you don't have some kind of support framing for the metal roof over the insulation layer.
Your roof has way too many layers. Your contractors would be hating life building that, and I don't think all the extra layers are really helping you much here. If you want to stick with wood framing, and assuming your primary concern is protecting your home from EXTERNAL fire (i.e. wildfires setting your roof on fire), then I'd worry about protecting the exterior more than I'd worry about what you do UNDER the roof's structural sheathing. Basically spray foam under the sheathing between the TJIs would be fine. I'd use fiberglass to make up any extra space (cheaper than mineral wool, and I don't think mineral wool buys you much here in terms of fire resistance). I'd use noncom plywood for the roof sheathing, which is the plywood treated with fire retardant and dyed red(ish). I'd use 3/4" to get a thicker layer for more protection. I'd put standing seam metal roofing over that, as a sort of ignition barrier. If you want some additional fire protection between the metal roof and the plywood, to act as a thermal barrier (i.e. to help keep hot things on the roof from getting the plywood up to ignition temperature), I'd put a layer or two of 5/8" drywall there (something that is sometimes done on commercial roofs too, BTW).
This gives you a wood frame stackup, from outside to inside, of metal roof layer / 5/8" drywall (ideally two layers with staggered seams for max fire protection) / 3/4" noncom plywood structural sheathing / TJIs with closed cell spray foam sufficient for your climate zone and overall R value, then fiberglass to fill the remainder of the rafter space.
I always spec interiors to commercial standards, so 5/8" type X drywall throughout. That makes for solid and flat walls, and also 1 hour fire rated walls. Your weak spots inside then end up being your interior doors, which will usually be hollow core wood doors on residential projects.
Bill
Thanks all----snow load is hard to calculate up here--we get 200" of snow per year, but it is constantly melting so looking at existing roofs, I don't think we ever accumulate much above 60 lbs which is 3' of heavy wet snow or over 10 feet of light snow. With this in mind, maybe simplifying with an unvented roof per Akos's suggestion makes sense. With CC spray foam in the rafters, what are your thoughts of just using a fire-rated peel and stick like Polystick FXR over regular non-treated plywood with no drywall then the metal roof over that?----certainly more simple -- and I now think what is most important is making the roof "ignition-proof" as opposed to "fire-proof" and I would think the polystick does that----my only concern was that I thought I was getting more fire AND WATER protection by adding the extra layers ---but maybe I was just creating a headache--------I am a bit worried about the metal roof nail /screw penetrations through the polystick and if it is gooey enough---maybe do two layers??? obviously whether 1 or 2 layers, this is creating the dreaded non breathable sandwich but I think I am ok with that----thx, brian
There is no issues with impermeable cover over a spray foam roof. Just make sure the spray foam is installed onto a dry roof deck.
These modified bitumen underlayments all seal around screws. You also have a layer of spray foam under it so screws won't leak any air.
I know lot of these mod bit ones are fire rated, but I have trouble trusting something that is essentially fancy tar in fire. I would go for something that is non combustible like gypsum or fiberglass. Metal roofs are liquid tight, the peel and stick under it doesn't buy you much. If you want better than plain OSB/CDX, go for ZIP roofing with taped seams and fire rated underlayment.
To add to what Akos said, besides just having the surface dry prior to applying spray foam, make sure it's also at least about 50*F. I have found that trying to apply spray foam onto too-cold surfaces can cause problems with adhesion, and if it's really cold the curing process can get messed up too (below freezing is a big no-no).
Bill