I’m attaching a photo of a rotting roof(sagging between the rafters)that is less than 20 years old. The sagging is happening only on the north side, and there are no visible roof leaks or damage. The home has cathedral ceilings with fiberglass batt insulation and no ceiling penetrations (ie. can lights) in the ceiling. There is not excessive humidity in the home, owners use bath fans regularly, no humidifier, steam shower etc. There is no roof venting. A roofer’s and general contractor’s opinion is that this amount of damage cannot be just from ordinary household humidity, that there must be some other problem. Our only idea is a possible breach in the integrity of the furnace flue that runs from the basement through the roof, which has yet to be checked. This house is in Boulder, Colorado. Any other ideas why this is happening?
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7/16" osb on rafters that are 24" center???
since this is the north side....
excessive snow loads on the roof???
doubt that you have an 80 or 120 roof... more like 50....
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WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
Whooooo!!!!!
I am no expert, far from it.
Piffin will comment on this soon.
But I would bet it has to do with the lack of venting.
What is the roof sheeted with ply or osb, size??
here's the roof info from the house plans:2x10 rafters at 24" OC
1/2 APA rating sheathing
roof pitch 12:12
R30 fiberglass batt insulationWe are the contractor who is going to repair this roof, but have yet to open it up to see actual conditions. It appears to be a problem bigger than roof venting.
What is the ceiling covered with, drywall, T&G board, ect...what does that look like?
those are the specs...
what is there IRL???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
" It appears to be a problem bigger than roof venting."In your opinion, what makes it appear tht way?
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Piffin nailed it; this is caused primarily by a lack of venting, and I'll bet with him that the sheathing is OSB, too.
With respects to those who like the stuff for other reasons, OSB simply does not resist sag as well as plywood, and once it gets wet it just gives up the ghost. Game over....
When you replace the sheathing, go for 5/8 ply. You might get away with 1/2" because it's 12/12 pitch, but those archy shingles will hold the snow even better than 3-tabs so I wouldn't bet on it. The extra cost to use 5/8" is cheap insurance.
Vent with 2x2 stringers and homosote (or 1" XPS) panels; don't even think about using Raft-R-Mates.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
What's wrong with Raft-R-Mate?
What's wrong with Raft-R-Mate?
Raft-R-Mate is made of ¼" polystyrene; it's not strong enough to resist being squashed somewhat by FG batts shoved up under it by enthusiastic installers (which describes most anybody who simply wants to be done with handling glass fibre insulation overhead). The ribs only provide for about an inch of airspace to start with; once it starts getting squished, the effective airspace below the sheathing winds up being somewhere in the neighbourhood of ½-¾". Not enough.
Run 2x2s up both sides of each rafter bay and nail Black Joe or 1" XPS to them and you get a true 1.5" of clear, uninterrupted ventway in each rafter bay. Yeh, it's a bit more trouble to do it that way, but you wind up with a much better result.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Hey, thanks Dinosaur. I'm trying to figure out what to do with my attic this fall and this info is really helpful. I'll start another thread soon with my own attic insulation issue.
With respects to those who like the stuff for other reasons, OSB simply does not resist sag as well as plywood, and once it gets wet it just gives up the ghost. Game over....
It sounds like OSB has one additional benefit over 5/8" ply. Since it's prone to "giving up the ghost" when moisture conditions are improper, it's probably a better choice. It's acting like the canary in the mines. If the original poster had 5/8" ply up there, the framing would be rotting and mold spreading before any there were any indicators that a problem existed.
The one I fixed was sagging less obviously than the pix the op put up, but it was plywood.
Realized just how bad it was when I put my boot through it.
No rafter damage on my example.
Bing
I see your point.But... if the roof assembly was constructed correctly to NOT let any moisture in from below or above then the OP wouldn't have the problem, right?Even with that "early warning" saggy sheathing, I bet there is gonna be mold and soggy insulation. Perhaps enough damage to some of the rafters needing replacement. I'd like to see the excavation pics of that roof.I still scratch my head about 1/2 inch of any kind of wood sheathing on 24" spacing for a roof. Especially where there is a lot of snow. Just my personal opinion.
Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?
I lived in a house for 20 years with 3/8" ply with trusses 24" oc. Ply clips were used. 4/12 slope. In Michigan.
No problems. The current owner and no one on the block is having problems.
You worries are worryless. It is an approved APA installation.
I don't think you ever saw the kind of snow loads that Boulder gets, Jim
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I hope the OP comes back and lets us know the source of the moisture. I think most here have decided it's due to a lack of ventilation. I'm not convinced it's not from a source of moisture that shouldn't be dumping into that space. Loose fart fan hose connection, damaged or loosely connected waste vent pipe, loose joint in the B vent. I've found them all, the 1st two items on nearly a weekly basis.copper p0rn
could be that - I don't see much gas vents, but OTOH, you boys down south never get to see enough of the kind of plain old winter condensation I am talking about that is so common in severe cold climates like his is, so you have a hard time imagining it. I lost count long time ago of how many times I was called to fix a "roof leak" that turned out to be melting ice in the roof space from frozen condensation
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I would like to see the OP post pictures as he goes. I would vote this my favorite thread then.
I don't think that there is any one problem causing sheathing failure. If it were just the venting issue, the whole roof would look like that. My guess is the fart fan hose is not connected or non-existant, combined with no vents. I think someone has said this already.
Good thread, we need more pix.
The reason only the north side of that roof looks like that is that the temperature diff between the north face and south face of a roof in that kind of climate can be as much as 20ºC (68ºF) on a sunny day. The south face is so warm that very little condensation occurs; the much colder north face condenses pretty much all the humidity available and freezes it into a constantly thickening layer of ice on the underside of the sheathing. Then spring comes...and it all starts to melt.
OTOH, a fart fan blowing hot, moist air into an attic will usually blow it into the entire space, so the whole roof would be affected. In this case, there's no attic as it's a cathedral ceiling job; so if the fan vent is the cause, it would be localised to the one rafter bay that the ductwork passes through.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I don't think you used the right conversion ratio for °C to °F. My memory is something like 2.58 which would make it 52°FCommon error because the zero point is diff for both
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It's 9/5C + 32 = F 20C*9/5 + 32 = 68F
OK 9/5 is the ratio. When using the degrees as a diference the way Dinosaur did, you do not deal with the 32 between zzero points. He was saying how many degrees diff there can be from south side to north side of a roof
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20ºC=68ºF is one of those 'waypoints' I have memorised. After living in metric for nigh on 30 years, I have actually lost my 'feel' for winter Farenheit temps and when someone says, "Brrr! It was 14º here today," I have no idea how cold that feels unless I do the arithmatic or look at a twin-scaled thermometer.
Still, I have never learned to use ºC in baking; I think in Farenheit for that. And while I think in centimeters for wrench sizes and snow depths, I still use inches and feet for carpentry.
Go figger....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
A 20 C temperature DIFFERENCE is multiplied by 1.8 (or 9/5) to get a 36 degree F temperature difference.
Sounds about right to me. Thanks for doing the math ;o)
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
That's what I was saying, just had the wrong ratio.
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I agree, good thread. Everyone loves a mystery."If it were just the venting issue, the whole roof would look like that."
i've seen several that were spotty this way, always worst over a bathroom or kitchen on the north slope, with nothing else amise.It's just because those area produce the most moisture and it rises.I'm saying too that the odds are good that this has a paneled ceiling with no - or poorly sealed- VB above it, so normal moisture is rising into t he rafter spaces. They probably depended on the kraft face only for a VB
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No, we rarely got more than 12" in MI. Sometimes 15 to 18. My comments included the state so the reader could qualify the statement.
I think your canary in the mine shaft is a good point. I have seen as much as 2-3 inches of ice on the inside of sheathing in situations like this where extreme cold and condensation occurs, with no real damage to plywood, but extreme damage to OSB.I think that ply tends to fail more from heat and OSB more from moisture., but no objective facts to back that one up, just an instinct.
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We are in agreement.
It's acting like the canary in the mines.
Jim, I hear what you're saying, but going on that theory, the 'best' houses would be constructed exclusively from materials known to fail earlier rather than later. Sorry, but I just can't bring myself to embrace such a philosophy.
As you know, I do a lot of re-roofs and remods, so I see a fair amount of rot. In my experience, it takes a much longer time for the framing to die than for the sheathing, no matter what the sheathing is made of. I have literally pulled disintegrating roof sheathing off with my bare hands, and still the rafters weren't so far gone they couldn't be saved with a bit of scraping, sistering, and poisoning.
For me, OSB is good for two things: filler, where you've gotta fatten up an existing ply or plank subfloor to match levels; and 'throw-away' sheathing for buildings not expected to last.
The difference in price between OSB and ply isn't sufficient to justify using it in serious construction. Say you might save a few hundred or even a thou by sheathing a new house with OSB instead of ply--looked at in the context of a $90,000 materials bill, the up front savings just aren't worth the cost in loss of longevity.
My two cents worth....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Say you might save a few hundred or even a thou by sheathing a new house with OSB instead of ply--looked at in the context of a $90,000 materials bill, the up front savings just aren't worth the cost in loss of longevity.
I wouldn't allow my house to be built with today's plywood even if it was cheaper. OSB is far superior.
Well, I know you and you're nobody's fool. So all I can say in response is that the plywood you're talking about must be far inferior to the stuff I'm getting. FWIW....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I do think the long term solution will be to build a cold roof onto this with 2x2s over the new sheathing, but since he is opening things all the way up, I'd be inspecting the insulation close too to be sure it is not sopping wet and not supporting mold growth. He could blow in chopped FG or cells to get better insualtion and resistance to dew point condensation. Better yet, he could have the space sprayed with polyurethene foam. That might be the only way to warrantee the fix.
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...build a cold roof onto this with 2x2s over the new sheathing, but since he is opening things all the way up, I'd be inspecting the insulation close too to be sure it is not sopping wet and not supporting mold growth.
The OP didn't mention the interior treatment of this cathedral ceiling, but I am assuming there must be some interior finish up there that would allow venting below the roof deck. That would be my first approach; to re-build it the way it should have been built in the first place.
In any event, as soon as he yanks that sheathing, he's gonna get an up-close-and-personal look at the insulation, and if it's all soaked, well, that'll be another piece of bad news for the HO. "Pardon me, sir? I need you to sign this change order, please...?
This looks like one of those contracts that could just go on, and on, and on....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
OSB simply does not resist sag as well as plywood
you know the funny thing is all the plywood I ever installed was already so bowed and warped it looked bad as soon as it was on.
must be something about osb and you peoples up north :)
Or Plywood and you people down south! :)
...all the plywood I ever installed was already so bowed and warped it looked bad as soon as it was on. must be something about osb and you peoples up north :)
Well, maybe y'all orta git some o' this here good Canukian plywood 'stead o' that southern stuff made outa cottonwood and magnolias....
;-)
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I understand that Canuck osb gets an abundance of hardwoods in the chips as opposed to southern mills with a smiggin here and there.
Seems that might make a case for quality comparison.
Now that makes one wonder if the glues used are regimented to certain guidelines to receive the stamp
and how much variance there might me within those guidelines.
Question all. The world is full of liars, charlatans and cheats
We ran into a similar problem a few months ago on a house that had a roof that ran into a cheek wall that had no venting. The room below was also catheidral ceilings. Apon further inspection the insulation (that had no proper vent) was soaked and the 1/2" plywood sheathing was so rotten you could stick your pencil through it. Every bay that ran into the cheek had the same problem. I belive what was happening was moiture from the air was getting traped between the roof membrane and the insulation moisture barrier. My remedy was to cut in soffit vents, add a proper vent air space, and a special trasition flashing that had a vent in it to vent the transition from the roof to the cheek wall. Had the roof been vented properly I think this costly problem would have been avoided
I think it's from lack of venting. Here's one I fixed. It was also the north side. When I went into the attic the underside was wet. Warm, moisture-laden air rises and condenses on the cool underside of the roof decking. It's worse on the north side because the north side is cooler and more condensation takes place there.
Edited 9/1/2009 1:17 pm ET by Mudslinger
That one looks like graham crackers were used for sheathing.
Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?
I agree. It's roof venting. I don't need to open it up anymore...it needs to be vented in every bay.
Continuous soffit and continuous ridge vent and new ply. Lets hope the rafters aren't compromised.
Its venting. Period. Oh...I already put the period on there :)
Now that is just plain funny!except that it's so sad
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That roof looks WELL ventilated now.
I would wager the excess moisture is from the waste vent right below the damage or as noted, a leaky gas vent. Both should be checked for integrity. Due to the location, I suspect the WV. Also, since there's a WV there, there must be some sort of moisture source below the damage.
copper p0rn
This stuff is most common over areas like kitchens and bathrooms on a north side of house.I also see what appears to be a wood stove metal chimney that is not high enough to draw properly, which gives rise to another thought.
I ahve seen a lot of people store three or four cords of green firewood in the basement and suddenly find the house a whole lot more humid and frosting on the winodws etc, but can't figure out why. It never occours to them that they just dumped the equivalent of a thousand cgallons of water on the floor
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as a roofer whoes opinion i respect what do you think about o.s.b.?
of course i have my own :)
Mike -
I don't think it's the best material in the world, but it has it's uses. I don't think it has a place as roof sheathing under a copper roof. But, Advantech is an OSB type product and I think it's great for roof sheathing, especially under copper.
A tract house that probably won't last more than 35-40 years before it's bulldozed should be sheathed with osb because it's cheap.
I've seen more problems with plywood delaminating than osb failing, but I don't think osb holds nails as well as plywood in general.copper p0rn
thank you
I've seen way more plywood that has failed than o.s.b
and have seen plywood do what the picture of the op's showed ,but not o.s.b.
I've never seen failure like that before.copper p0rn
Where do the bath fans vent to? Through the wall/roof or into the soffits?
bath fans vent through the wall, appear to be functioning fine.plumbing vent and H20 heater vent are our strongest suspicions for moisture leaks
style of that house does not appear to have any soffits.
NBut checking where the dryer exhausts is a good point to consider.
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That could be fire retardant plywood roof sheathing, or FRT plywood delamination.
Saw a alot of that on ply from the sevemnties, but I thought they had that fixed now
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I agree w/ above, could be fire retardant.
Have seen this exact same failure on a repair about 10 years ago. Cathedral ceiling, NO soffit vent, NO ridge vent. Top 3 feet of ply was nada. Fortunately for them, it was a nasty T1-11 ceiling that we were removing to be plastered. Pulled ceiling, removed insulation, installed proper vent, ridge vent, soffit vents, re-did top row of ply, re-roofed.....Good as new 10 years later.
Bing
There is no mystery there at all. He has an unvented roof and probably OSB sheathing.
The opinion of the roofer is wrong. In colorado I saw a lot of this sort of ccondensation damage and much more common on both north slopes and on cathedral cieling situations
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Years ago I saw some homes sheeted with 3/8" plywood on trusses 24" oc, but they ran the sheets vertically, looked similar to that. They could have sheeted it with 24" scraps, anything is possible.
Take some shingles off and show us what you see, the suspense is killing me.
Greg in Connecticut
Should be APA-rated 'Exterior' or 'Exposure I' - is it possible that plywood without exterior glue contributed to this?
It certainly *does* look a lot like the FRTW plywood failures too.
There's also no mention of roof felt - if omitted condensation under the shingles (alone) would do it.
Jeff
Edited 9/1/2009 9:24 pm ET by Jeff_Clarke
Note the attached structure which I assume is a garage, probably built with same materials. The same north facing slope shows none of the same sagging. Also the west facing slopes show little or none of the same problem.
The sad part is Disney would pay you big bucks to get that look
I petty much agree with Piffin and the oter "no vnts" crowd, but with a slight change in emphasis.
First thing is to find and stop the source of the moisture.
There is warm moist air getting into the attic in that area -- Since it is close to the vent stack, I would check that first for a gap (ad the gap may well be inside the walls.
Check that the bath fans do not vent below a soffit vent or gap
It there is a moist basement of crawl, the moisture could be finding it's way up, but I wouldn't expect it to be condensing in such a "concentrated area."
As noted, such condensation is always more pronounced on the north slope, but my gut tells me that the source has to be fairly close to the underside of the roof in that area.
And roof vents are a good idea, also.
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Mystery solved?
mystery rotting roof update:thanks for all the comments. I'm attaching photos of what we saw when the roof was opened. Sheathing was able to be peeled away by hand. Rafters rotten top 1-2". original vapor barrier (4 mil plastic)installed between fiberglass and drywall plane appears to be intact. tar paper correctly lapped. Fiberglass insulation currently dry but shows signs of moisture in top 1" only. the worst damage is closest to the ridge.Hot water heater flue and plumbing vents were scoped with a camera, all appear to be intact with no signs of moisture penetration.Many of you suggested venting, however this shape of this roof makes it almost impossible to vent correctly- there are no soffits, and many dormers. Ridge vent is possible, though only in a few places can you get an "eyebrow" vent down low. Since the vapor barrier appears to be intact, it is still mysterious how this much moisture could be getting into the roof plane.Solutions: replace roofing/sheathing, insulation on north side only? south side of roof (and all other aspects of dormers) seem fine. do we replace north side insulation with closed cell foam to create bomb proof vapor barrier, since venting is challenging? Should the entire north side have to be replaced, or just the top 4-8 feet where rotting is?
Thanks for checking back in. This job crossed my mind a few days ago.
Is the sheathing showing rot all over or just in localized areas?copper p0rn
I would guess that the problem is from the plastic vapor barrier.
The shingles and tar paper are not 100% vapor proof... but the plastic is.
Moist air is getting in but can't get out.
Looking at the 2nd pic in your first post of the thread I see no reason that the house can't have ridge vent. Also, there is a ventilated drip edge that could be used. Here is one example.
Edited 11/4/2009 9:44 pm ET by Matt
there is a ventilated drip edge that could be used.
Probably not in this instance. It's in CO (snow country) with gutters. copper p0rn