Greetings and salutations,
I have a general inquiry for which I’ve heard no definitive answer. I’m in the process of renovating an old cape and adding a timberframe ell and barn where a rotting ell and in-law apartment currently sit. My plan is to cover the timberframe with Structural Insulated Panels, with which I’ve never worked.
I’ve seen literature that suggests you should strap a SIPped roof and lay 1/2″ plywood over the SIP before applying roofing to allow ventilation. If SIPs are as efficient as I’m led to believe, it seems to me that there’s no real benefit to strapping and sheathing my roof. The only concern I’ve seen that makes sense is that when the frame dries, there could be some shingle damage at the seams of the SIPs where movement occurs. To combat the effects of that movement, which I’d assume would be minimal, I’ve heard people suggest the use of a heavier shingle — like a 310 pound fiberglass.
Do any of you have any thoughts on the perils of putting shingles directly on a SIP roof? I’d love to hear your thoughts.
Thanks
Replies
Venting aspect makes no sense. What are you venting in this case?
May be required for roofing material warranty to be valid, though.
Check with the shingle manufacturers. Several warrant their products for use on unvented roofs, such as SIPs.
Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
I hope someone with real experience with this will chime in. I'm guessing the venting with strapping and sheathing on SIPs is to protect the shingles themselves. I once saw an SIP roof with not-very-old shingles that were bedly cupped. I was thinking it was heat damage.
Al Mollitor, Sharon MA
the venting with strapping and sheathing on SIPs is
Or, because the local AHJ requires roofs to be vented.
We probably do not wish to re-enter the never ending vented v. non-vented roof brouhaha.
The question really becomes, if you strap the roof deck over the sips-where does the eave venting go in? The soffit detail where the wall sips meet the roof sips is the key, and that's what we don't know.Occupational hazard of my occupation not being around (sorry Bubba)
OK...more theory here since I haven't done it, but I'd look at vented aluminum drip edge with the usual ridge vent.
Al
more theory here since
LoL! Just when we need a SIP pro, Kevin pipes up with an authoritative shingle answer.
I always figured that you'd use the fascia/soffit "box" to "collect" air for roof venting on a sip. Which gets back to the vent v. non-vent argument (the change from soffit to sip being where the ice dam allegedly occurs).
But, now, we're both speculating . . . Occupational hazard of my occupation not being around (sorry Bubba)
We overhang the strapping 1/2" past the sip fascia trim, screen the overhang, nail on the sheathing, fasten a small piece of fascia over the strapping and sheathing and leave a 1/2" screened gap between the two layers of fascia. Pretty fast and looks good. Biggest drawback is the extra time and materials for the strapping and extra sheathing. That's hard to justify if there is little if any practical difference.
That's hard to justify if there is little if any practical difference
And that's where the going gets muddy.
How deep an overhang are you dealing with in that situation? In my lattitude, 24" ought to be a minimum, and 36" is not unreasonable (got to keep the sun off those walls in the summer). Just curious (and I know what that got the cat <g>).
Occupational hazard of my occupation not being around (sorry Bubba)
edit for spelling . . .
Edited 12/30/2004 11:55 am ET by CapnMac
Don't remember the overhangs on either house but guess 18", no more. I like 24" myself but didn't have nay say on these jobs.
meathat,
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It is my understanding (based on what I've read here at Breaktime, I have no experience with SIPs) that if the SIPs are properly installed then the only reason to vent is to preserve shingle warranty.
However, some shingles will be warranted without the venting when installed on SIPs. Check with the manufacturer.
Rich Beckman
Another day, another tool.
Thanks for the quick response. I didn't even know about the profile section -- so I've now followed up on your request. This is a really terrific forum.
We have been the gc on two timberframe houses sheathed with sips. We put a cold roof on one and put the shingles directly on the sips on the other. There appears to be no difference as far as the condition of the shingles is concerned but the unvented roof has some small ice dams. These have not resulted in any problems but ...
I probably would not spend the extra money for a cold roof if it were my own house.
Thanks for the feedback. I had heard that ice damming might be an issue, but wasn't sure if it would warrant the extra time, effort and cost of the cold roof.
I'm building a timberframe right now, and the plan calls for a cold roof system - strapping over the SIPs and an additional layer of sheathing. I would be hesitant to attach anything directly to the SIPs. Even metal roofing should be installed over strapping. The extra layer of sheathing helps to prevent ice dams.
It seems to me ice dams would be unlikely since they are caused by poor insulation and ventilation. If the roof SIPs are thick enough, there should be little heat loss and, therefore, no ice dams. I have to think the ventilation is to cool and dry the shingles to prolong their life.
Al Mollitor, Sharon MA
Ice dams can be caused by solar-driven snow melt in very cold weather, and have little to do with insulation or ventilation. Happens on my driveway all the time.
AndyAndy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
OK. I've got the concept of the solar driven ice dam.
So...If Mojo Man is correct in positing ice dams caused by poor insulation and ventilation are unlikely on a SIP roof, then adding a cold roof will have no benefit except to a hypothetical shingle manufacturer's warranty? Or am I missing something?
There is more to shingle warrantee than ice damn. The reason some manufacturers void warrantees over unvented roof suface is that such surface has a median temperature higher than vented. This higher temp has been shown to decrease life of shingles containing volatile oils because the heat cooks off the oils and shortens the time before onset of brittleness and cracking - loss of water resistance.But - IMO - the decrease in life is minimal.
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Appreciate the thought, sir. When we reroofed our (not SIP) home, old shingles on the South face of the roof were more brittle than those on the North. Also, old shingles on the plywood decked roof of the addition were more brittle than those on the skip decked roof of the original portion of the house. We plan to build a frame & SIP home, and will watch shingle warrantees carefully. Still, like a dog worrying a bone:
The question I was attempting to frame in my post to Andy still nags. I will rephrase:
In a properly constructed home with a properly constructed SIP roof deck is a cold roof going to do anything vis a vis non solar powered ice dams? Or, will the SIP deck itself be sufficiently cold to prevent ordinary damming?
Ice damns haven't formed on my roofs & I have about a 2 foot overhang. I'm going into my third year
I did end my use of SIP's at the wall and the overhang is just a single layer of 1 inch thick boards nailed over the beams I extended from the roof..
I did it that way for two reasons, first why pay for insulation you don't use? Second the wide facia that SIP's present, is awkward to deal with.
I don't have test figures, but IMO, the SIP surface would be sufficiently temperate to avoid ice damns
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No, I don't think you're missing anything. As long as your SIP roof is well enough insulated to prevent the roof deck from rising above freezing, I don't see much gain from a cold roof. No harm, either, execept to your checkbook.Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
You should look into Ceared Breather. Its a 1/2" thick nylon mat similar to the synthetic scrubies you use to clean dishes. Comes in 3' x 61' rolls. Specificly made to ' provide a continuous airflow between the solid roof deck and the shingle's '
Go to http://www.benjaminobdyke.com to learn more and find a local distributor.
Edited 12/29/2004 6:49 pm ET by reinvent
you can't use cedar breather under comp. shingles, only wood.
Spheramid Enterprises Architectural Woodworks
I'll just do it>
Why not?
Too soft for an underlayment, it would be wavy as all hell, and the nails would just rip through. I used 20 sq. of it this past summer, for cedar. read the package, that's what it's made for.suggested to allow a 1/4" for compression, a wood plank can do that, a comp. shingle can't.
Spheramid Enterprises Architectural Woodworks
I'll just do it>
it won't do a dang thing for composition shingles other than waste money. It is designed to let wood cedar shingles breathe from the backside when they get dampened to avoid rot and cupping. on this whole subject - I don't have the article, but there was a study done by a group of manufacturers on this whole thing - heat shortening the life span of asphalt shingles. They found out that it does probably do so, by about something like .008% or whatever. marginal in my opinion. When I read it, I calculated a pro-rata life and it worked out to something like a couple months on 20 yr shingles and a couple more on the heavies. I see more diff than that between north and south sides of a roof. My own conclusion is that it is so negligable that it is not worth worrying over, regardless of the warrantee, with the exception thatif you buy poor quality shingles such as JM was making for a few years, you are giving them an excuse to side-step the failure and disclaim your roof.
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And I forgot to add.......BECAUSE, I said so. {G}
Spheramid Enterprises Architectural Woodworks
I'll just do it>
Here is a pretty authoritative voice. I hope it helps with your concerns.
Thanks for the link. If it's good enough for the manufacturer to warranty, it's good enough for me. I really appreciate your help.
You're welcome. I originally planned a cold roof with metal panels for my SIP house but the trigger work and expense finally made me decide it wasn't worth it. My main objective wasn't the insulation value, plenty of that in the SIP, but the desire to get a little more meat for the screws to sink their teeth into. I ultimately just decided to add a few more fasteners for that insurance instead.
I did do strapping under the fiber cement siding - again, primarily for some extra meat for the fasteners more than the "rain screen wall" benefits. I did weigh the benefits of the rain screen concept pretty heavily and ultimately felt the benefits of the rain screen wall system warranted the extra expense and effort on the walls that I couldn't justify for the roof.
I'll tell you this - that SIP shell is one serious insulation package. We still don't have the main power connected to the house so I have yet to fire up the HVAC system but that house has been comfortable enough in a t-shirt on the hottest days of summer and the coldest days of winter here in Texas. Especially after I finally got the glass block openings filled in.
We built a SIP house three years ago, and put the shingles right on the panels. To the best of my knowlege it worked fine, but then it has not been very long ago. One problem we did encounter, which we never discovered the reason for was rain water dripping through the ridge about the center of the house. The ridge cap all looked good. We finally speculated that water was being driven into the end of the ridge, and running along the felt paper where it was depressed slightly where the sips joined in the center, and found a hole through which to drip near the center of the building. We caulked the exposed edges of all of the ridge cap shingles, and the ridge ends well, and have not heard anything more about the problem.
Sorry for the slowness of my answer,
I have a timberframe and use SIP's. The solution to the problem of shingles on top of SIP's isn't as simple as some would have you believe.
Much of it depends on your local climate and the type of shingle you intend to use.
If you plan on using wooden shingles, (I'm using cedar shingles) then you should use something that will allow air movement underneath the shingles in order that the underside of shingles dry out between rains.I use a product called cedar breather which is a plastic mesh that effectively holds the shingle off the OSB allowing the back side of the shingle to dry.. You can also achieve that by building skip sheathing (that's what strapping effectively does),. but to put plywood back over the skip sheathing effectively negates anything. The shingle will lie on plywood, remaining in full contact will the damp wood underneath and giving mold and decay a place to grow.
As for ashpalt shingles, who cares? They have a finite life, more due to the effects of sunlite drying them out than the effect of trapped moisture. There would be some advantage in placing them over a layer of plywood laid over strapping simply because should the underlayment under the shingle decay it would be simple to pull the layer off and nail on a new layer, something that would be really difficult to do if you nail them directly on the OSB..
Now here is where the local conditions come in play, If you are in a dry area where there is seldom long periods of time where the roof gets damp and stays damp then some of the concern involded won't affect you. if on the other hand you are in a coastal area or rainy area extra care will need to be taken..
* please understand my feelings regarding ashalt shingles. I'm using old growth cedar shingles on an extremely steep pitched roof (27/12& 17/12) combined with the cedar breather I have every expectation of very long life, measured in multiples of decades. I've never seen a ashalt roof go more than three decades.
I recently visited one house in Wisconsin using steep pitched roof and skip sheathing that remains dry close to 150 years after the roof was put on. To give you an idea of how old it is the shingles weren't evan nailed on, rather they were put on with little white oak pegs hand drilled and tapped in place..
'little white oak pegs hand drilled and tapped in place'
At this point that might be cheaper than stainless nails;-)
Can you imagine what it was like to drill the thousand of holes by hand in those precordless days? worse, spending all winter sitting around the fire with your carving knife creating those thousand of little pegs?
well of course , they cudda had the local blacksmith whip them up a keg of cut nails..
hmm whittle little oak pegs ... or bang out nails on the anvil....
what to do ... what to do ?Mike Smith Rhode Island : Design / Build / Repair / Restore
you people out east seem to forget what things were like here in the center of the country 150 years ago. Heck, Wisconsin was barely a state, and parts of it were pure forest. Finding a blacksmith probably took two days ride (if you could afford the cost of them when you got there.
Assume you were some immigrant fresh off the boat and had nothing more than a few tools and a burning ambition, now do the cost benefit calculation,....
"I've never seen a ashalt roof go more than three decades. "Are you ready for a big change in your life?if not, don't open these.
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paul... yikes !
my mom's house was built in '27.. i reroofed it in '77.. they were in pretty good shape , too.. 50 years !
it had some very thick single tab asphalt shingles.. if i remember right they were 12x12 .. 5" to weather.. sound familiar ?Mike Smith Rhode Island : Design / Build / Repair / Restore
I've seen some two tabs that were 18" by 36" and laid 5" to the weather. As far as i know, they are still on, from back in the late thirties, I heard. the exposures all checked and crumbly but lots of viable asphalt in under.I guess the ones in the pic - they liked the mossy look, but you see they had to replace a few in the second photo.I have torn off roofs like that laid over wood shingles on lathe. One I remember from '78 was the second roof ever on a house built in 1890. Figure 40-60 years for the first cedar roof still under it, and there was still at least forty years on the asphalt and mossy roof.
No leaks except right at the eave where the metal had rusted or worn out and shingles breaking down from tree limbs hitting and rubbing. It was a shady glenn so not much sun hit it.
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Looks like they're pretty well broken in.
Are you saying those roofs were more than three decades old? If so where were they? Hopefully you aren't suggesting that southern California should be the example the rest of us should follow. But I think that those exceptions prove the rule. The maximum practical life of any asphalt roof is about three decades.
I think you'd have to agree with me that your tile roofs are a far more durable answer than asphalt. (at least in that climate)... Here in the midwest I've seen very old cedar roofs. They were properly installed and flashed and almost always done prior to the use of plywood for roof decking.. We don't have the fire threat southern California does nor is the sun as much of a factor (what with cloud cover and differant angle of the sun),....
* I really do like the look of the moss covered roof!
Don't know much about S CalI'm not arguing that we should expect that kind of life expectancy, but it is not unknown when good product is well installed.Same idea with wood shingles. installation is half the battle, and you cannot get cedar shingles today that are asa good as the ones installed a century ago.
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I appreciate the answer, Frenchy. My initial posting has given me more feedback than I could have expected, and it's better feedback than I'd ever get from any of the manufacturers involved. We're building in Maine, so long-term exposure to moisture is definitely an issue. The pitch isn't anywhere near as dramatic as yours, either. We're looking at 10/12, maybe 12/12 if I'm feeling adventurous.
We'll definitely be going with asphalt, and I think you're right -- they're not supposed to last forever. It would be mighty hard, however, to replace a decayed layer of OSB if it's part of a SIP. I've got plenty to think about.
Thanks again.
If I could have afforded it I would have loved to put either slate or concrete tiles on the roof. To me the concept of durability had the highest priority followed by a style that was in harmony with the rest of the house..
Good news on the use of SIP's I have a great room with 28 foot high ceilings.. I also have one of those digitile thermometers. The greatest tempurature differance from the floor to the ceiling is 1/2 of one degree. Yet if I go outside the roof is covered with snow and except for the southern exposure filly covered with no sign of any heat escaping..
I'm living in the house while it's under construction and there is only one 4 inch heat vent going to the middle of the half with the SIP's. That portion is as warm if not warmer than the traditional stick built half with 14 heat vents.
As I've taken the old stick built half down I noted the great care they had spent on properly insulating and sealing the house and one of it's original selling features was how well insulated it was..
As one final nail in the coffin of fiberglas insulation everytime it drops below 5 degrees below at night my pipes to the masterbedroom shower freeze up. It has traditional stick building and that means about 5 inches from the outside wall the tempurature drops below freezing.
Now I'm using a 11 inch panel on the roof since 40 below isn't that uncommon but I figure that if I pay for some heat it darn well should stay inside and work for me rather than drifting off into outer space!
Fools that use tradional stick building and fiberglas insulation need to have their mistakes pointed out to them often!
Maine recieves far less solar radiation than most of this country, offsetting concerns about shortened lifespan over unvented surface
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That's for sure. In fact, we don't expect to see the sun again at all until sometime in mid-July.
I'm facing a similar decision for my TF house.
I've seen a couple reasons cited for strapping and sheathing a SIP roof.
1. Shortened shingle life due to elevated temperature. This is probably not a substantial problem.
2. Shingle warranty. Some manufacturers are better about this than others. Some provide the same warranty of unvented roofs as on vented. Some reduce their warranty, and some void the warranty entirely.
3. Moisture migration through SIP joints. Improperly sealed joints, or joints that are pulled apart as the frame dries, can allow moisture to reach the bottom of the underlayment. A vented roof will allow this moisture to evaporate. An unvented roof will trap this moisture and rot the OSB. There have been some high-profile failures of SIP roofs in Alaska due to this effect - though it should be noted that these roofs were installed quite poorly (beer cans and lunchtime garbage found in the SIP seams) and HRV's were not operated properly. A local designer told me of two houses in my area that suffered the same failure. I have no idea if they were installed as poorly as the ones in Alaska.
Also, in the FWIW department, I've found no SIP manufacturer/installer who will warranty their work (including the moisture migration problem) for more than one year. It makes me very uneasy that the people who know these panels best won't stand behind them for longer than that.
My opinion is that the only real issue is the SIP joints. If you're confident that they'll be sealed properly and stay that way, I don't think there's any justification for the expense of a cold roof. If you're concerned that the joints might not be sealed that well, you might compare the expense of strapping/sheathing to the cost of replacing roof SIP's and re-shingling.
It's a risk/reward balance that you'll have to evaluate for yourself.
Jon