Updating Roof Sheathing and Metal Roof
My house is located in the mountains of northern Colorado at 8000 ft, climate zone 5. It was built to the current county code in 2000; the plans mention the UBC 1997 as applicable. Since the house was built, the county has changed the design ground snow load from 40 psf to 100 psf for the location, and the design wind speed from 105 mph to 173 mph. I want to replace the metal roof in order to change color and to rebuild the roof as much as possible to meet current code. Several years ago I had the experience of winter winds destroying my ‘regular’ garage door, which was then replaced with a highly up-rated door. I am talking to local roofers about replacing the 7/16 OSB sheathing with 5/8 in, now minimum required, and getting it nailed to the new standard spacing of 6 in, and also increasing the metal gauge from 29 to 26. I have found that the 7/16 OSB holds the roofing screws poorly, even oversize replacement screws. This has met some resistance from the roofers as unnecessary; however, it seems to me that it would be prudent. Advice on whether this seems reasonable given the situation?
I also want to re-insulate the roof at the end of the house having vaulted ceilings, as the wind which commonly blows 40-60 mph during the winter readily wind washes through the inadequate 7 1/2 in fg batts in the 2×12 rafters. The best advice seems to be 8 in CCSF applied from above leaving a vent channel under the sheathing. There doesn’t seem to be another way to air-seal the roof as is. Advice?
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Replies
wmf,
I agree with you - and so do many roofers here who refuse to install metal panels on 7/16" OSB. I prefer 1/2" plywood as a substrate, but if it prices out too high, I'd go with the 5/8" OSB.
I agree about going with 26 gauge too. I've only used 29 gauge roof panels once. They felt like tin-foil.
I can't think of any useful alternatives to your insulation plan. Maybe others will have some good ideas.
One additional question that has come up is whether the old 7/16 in sheathing should be removed over the rest of the roof and garage before adding the 5/8 in layer. An engineer at the county building department did not think so. It would be cheaper to just put on the additional layer over, and either duplicate the layers on the vaulted section or use thicker sheathing to match the overall thickness.
wfm,
I was going to suggest that. If the existing sheathing is in good shape, and the roof can take the additional weight, it would be a lot easier to sheath over. Then the question becomes whether there is much benefit to that second layer being 5/8" when you get the strength and holding power of both layers?
I didn't bring it up as an option because I thought you needed access to the rafter cavities to re-insulate and air-seal.
Most of my 30-year career has been designing or building homes in coastal New England and I have always used 5/8" roof sheathing--CDX in the old days but usually Zip these days. Anything thinner feels flimsy to me.
Michael,
Decades ago I worked on a framing crew that used 7/16" OSB on roofs. You had to be really careful about the layout to prevent too narrow strips at the peak. Even with H-clips, and only weighing 160lbs, I remember going though a couple that were about 12" wide.
Hi,
Screws hold better in plywood. . Have the roofer show the calcs on the screw spacing for the wind speed & the substrate.
Tim
wmf,
Sounds like you must be on the front range in Boulder-ish region. If so, reflect upon the recent perempt0ry shut down of power due to 100+ mph winds and the similar extremes during the major fire a few years back. Depending on your overhang size and house siting, it may be a good time to at minimum peel some sheathing at the wall line to enhance the tie down details on your rafters. I would also suggest making the cathedral section unvented not vented.
You seem to be saying that someone proposed to apply spray foam into the rafter bays from the roof side without removing the fiberglass batts. I have grave doubts that anyone could successfully place closed cell foam down to your existing interior ceiling with any degree of uniformity let alone manage to leave a consistent air gap for venting under the sheathing plane. Foam simply does not spray that way. If you peeled the entire roof sheathing off the cathedral area, stripped out the batts, and the sprayed to a fill depth of 9" average above interior ceiling, you MIGHT manage to achieve a useful venting cavity between foam and new roof sheathing.
You didn't mention if your cathedral ceiling has lighting or electrical penetrations, so if you do plan on the foam path, be mindful of how they will affect your useful insulation depth and contact restrictions on lighting cans. However, this still means a vented roof plan with need for fire resistant screening and soffit materials. The 173 mph wind limit is way beyond my experience.
I am southwest of you at 8,000 CZ6 with lower wind regs. The ground snow cover value is only a bit less. It is worth noting that the roof snow load is a different value, which can be significantly lower. There are engineering calculations that need to be done to derive roof from ground loads. Local code prevails of course. For me the wind load is 90 mph and roof snow load is 60 lbs. For fire and envelope insulation reasons I went unvented. R-32 insulation above first sheathing combined with R19 batt between 2x12 joists under the sheathing has worked well for my low cathedral ceilings. It has also worked for my low pitch roof areas as well. My lighting cans and electric boxes all reside in the roughly 5" gap under my R-19 batts. The insulation balance keeps the first sheathing layer warm enough to avoid condensation risks despite the ceiling penetrations.
Despite our famed Colorado low humidity conditions you need to be aware that it is the interior humidity migrating via air leaks and vapor transfer that can bite you. With such light insulation in the cathedral area and your perceived wind washing from the current venting set up you may find the sheathing suffering from moisture damage. If you were able to bite the bullet and drop the ceiling in the cathedral area to enable spraying to the underside of the new sheathing you would likely derive a better result for a fairly pricey foam installation. You would still be fighting the thermal losses of the 2x rafters, but at least the foam would (should) be providing a tight barrier against interior moisture gathering on the sheathing bottom.
Last notes. Locally we are required to place high temperature water shield from eave to ridge under metal roofing. I don't support the idea of "venting" metal roofs by raising the material on strapping. Keep the clip anchors on the sheathing/water shield. Plywood definitely holds screws better. Self adhesive water shields are some what gummy by nature and help seal the screws. Narrower panel widths are a bit less prone to "oil canning" and don't pop so much in high winds. The width may be driven by material choices or roof dimensions working better with some particular size. Ridge details and wall terminations must be free of pockets that can trap embers. Not a fan of snow brakes. Snow curls are my main problem when the sun warms the top limits of my roof, but the fall where it is harmless.
onslow, I am located just outside Estes Park in a particularly windy location at the top of a chute coming up from the valley. Winter winds are often brutal and chaotic. One winter morning I found that my garage door had been folded in half vertically and ripped off the tracks. It sounds like you know this kind of environment. I have two major objectives: to keep my roof on and to air seal the rafter insulation bays. I have only a few ceiling penetrations now, most of the original penetrations have been removed and sealed with drywall. Wind used to blow through them strongly. But wind still blows through and under the FG batts. Winter interior relative humidity is mostly around 20%, and my interior drywall is well sealed in all corners and joints and painted with multiple coats of latex. When I sealed the ceiling penetrations I saw no sign of surface mold. The plan for CCSF was to remove the sheathing and FG batts and spray onto the top of the drywall 8 in, leaving ~3 in for a vent channel to the ridge vent. There doesn't seem to be any other reliable way to seal the structure against the high winds and extreme differential pressure of the chaotic gusts. Wind direction often seems to reverse rapidly, often several times a minute. Filling the rafter bays with batts, either HD FG or mineral wool is attractive, but no contractor is confident in sealing the structure other than CCSF. External insulation doesn't solve this problem and would introduce a step in the roof beteen the vaulted end of the house and the other end with trusses and 8 ft ceilings.
My other concern has been the change in code requirements for my location since the house was built. Now 5/8 in OSB would be minimum; 7/16 in would not be allowed. The snow load requirement has almost doubled. Two weeks ago I had 42 in of wet snow on the roof. Several people have expressed concern about the added weigh from just adding another layer of sheathing over the existing layer, but that would only add about 1 psf, which sounds negligible compared to the snow load. And the nailing spacing requirement for sheathing has doubled the fastenings. Checking for hurricane clips is on my list - they are specified in the plans. It all seems like a good idea to improve the strength of the roof if I am already replacing the metal roofing.
The construction industry here is very conservative. They think anything over code-minimum is overkill, and if it hasn't already failed it's good enough. But then, the house across the street lost 1/3 of it's roof 15 years ago in a windstorm. I had a very hard time years ago when I replaced my propane furnace with cold climate heat pumps - which have worked wonderfully well. Even now people tell me that they won't work here. I did get lucky in that it was seemingly oversized at the time, but that the contractors didn't know enough to downrate for altitude and the low minimum design temperature.
WMF,
Sounds like you have already determined a viable plan given the circumstances you have. CCSP is a relatively costly way to get R value though you don't really have any good options if you don't want interior moisture to potentially accumulate in the sheathing or alternate insulation materials. Key question now is foam to ceiling or foam to sheathing.
Placing the foam downward to the interior ceiling and leaving a vent gap similar to what exists could be the preferred choice. If the expansion of the foam will not bow the drywall or violate code restrictions for existing lighting or electrical then foam could be placed in a few lifts to the correct depth with a relatively flat final face. IF the installer is good and conscientious. If they spray too deep, too fast it could result in poor setting and bubbles. I would want to see a detailed plan of attack for how they plan to move about on open rafters without hazard to themselves or your ceiling. I know a skilled carpenter that went through a ceiling to a very bad injury. Just one misstep and a year of recovery. Check insurance closely.
I concede an unvented roof plan is not very viable economically. Going to an unvented roof plan means placing foam to over top of rafters then trimming flush. Also, you would be stuck with having to create a false ceiling pan at an 8" depth from top of rafter. Lots of work, not enough gain, even if can lights were at play. Thermally, the rafter bridging degrades whole R values in essentially the same way either case. If you do want to run numbers, treat the rafter values as a 2x8 not 2x12. The whole roof R value will not be the R-48 of center of foam value. Still way better than before.
The vented plan does have the advantage for summer heat load reduction if the roof pitch is enough to be useful. During winter, if 42" of wet snow is covering your ridge vent, I wouldn't hold out much hope of air exchange. Having 42" of wet snow is large load. Hopefully the engineer has confirmed that the span you have with 2x12's is within range. I would worry some about ice damming just below a ridge vent opening. Consider a collector cap going to a few cupolas. Not elegant, but more water backup resistant. Both venting approaches have ember issues to address. Down at the soffits, plan on continuous fine screen or perforated metal to get useful air flow capacity. The junction of rafter and wall is a thermal loss point where the available depth for insulation often gets reduced. Be sure to place foam out to vertical sheathing and as deeply as possible.
Best of luck with your pending adventure.