Frost On Underside Of Roof Sheathing - Remodeling - Contractor Talk

Frost On Underside Of Roof Sheathing

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Old 01-14-2009, 06:58 PM   #1
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Frost On Underside Of Roof Sheathing

Hi there. After looking for a similar scenario already in the forums I came up with no results so I decided to start a new thread. Read only if you've got some time on your hands, I'm afraid I wrote a book.

First a little background; I'm a general contractor in southwest Virginia. We do some residential new construction but most of our work consists of remodels, additions, etc. All that to say that I've seen a good amount of nasty renovation situations.

I was called by my former landlord to check out some gutters that were leaking at the single story apartment I used to rent. It was built 50 years ago and has old asbestos siding without any housewrap. The apartment is attached to another apartment but at a 90 degree angle to the other one. So if you were to picture an L shape except that both portions of the L were the same size. The roof pitch is a 3/12 with asphalt shingles and felt. Some of the sheathing is CDX and some is the old 1x horizontal bracing. The attic space has neither soffit, ridge, or gable vents. The single bathroom is unvented.

When I got there the current tenant asked if the landlord had asked me to check the water and mold in the apartment, I said no but that I could check it out for her. All along the western facing side of the apartment they had moisture at the baseboard and in some places up 2' on the walls. On a portion of the south facing gable end there was moisture along the baseboard and it actually was moist all the way up to the ceiling. Mold was forming in all these locations. I'm pretty sure that some of the moisture on the western facing side of the house is due to the siding "failing" due to the tremendous amount of rain and stormy weather we have received in the last 1 1/2 months. The water coming from the ceiling was another matter that I wanted to explore in the attic.

The next morning I went into the attic which resulted in finding that frost had formed on the underside of the roof sheathing and on the roofing nails. There was mold on pretty much all the rafters, rot at the sheathing by the eaves (as a result of the gutters overflowing and wicking up). There was a lot of black and white mold on the southfacing gable end (the same end that the moisture was from floor to ceiling) and on the roof deck around where the chimney is.

I went back later to inspect both sides of the roof after things thawed out and this is what I found. What I found was that on the outside the chimney which is attached to the exterior of the apartment and runs through the soffit area of the roof was not flashed with aluminum, but merely had tar applied to the intersection where it projected from the roof. There were 1/4" gaps with nothing to keep the water from entering the attic. There were also several issues with the intersection of the roofs where the 2 apartments meet. On going back inside the attic, the frost had melted and the underside of the sheathing was wet, not so much that it was dripping, but enough that it was damp. The sheathing at the peak of the roof was saturated with moisture. Now, there is no ridge vent on this apartment, infact the roofer merely cut down 3-tab shingles to use for the ridge cap. Then of course there was the water getting in at the chimney.

It's my theory that water is getting in through the gaps around the chimney (obviously), the ridge cap, and also through the shingles in general. Here in SW Virginia we have a lot of freeze and thaw due to fluctuations in temps so as things thawed out I think the water could have been wicking in under the tabs, as well as the other areas mentioned.

The obvious starting point would be to seal up the roof in the areas identified (or better yet, tear the whole thing off, replace wood rot, ice and water shield and flash it, then install new shingles). The other thing is that since the building envelope is the exterior walls and the ceiling joists (due to the insulation in the ceiling), you have essentially exterior space in the attic which is unvented. So it seems like a good idea to add soffit vents and a ridge vent.

My main question at this point is why has this not surfaced sooner, especially without the ventilation? I suppose it's feasible that as the leaks in the roof grew more prominent that the problem has only surfaced at this point? The other question; is it possible that frost was forming without the introduction of water from exterior sources? My concern would be that we fix the outside leaks and still have frost forming and thawing into the house.

I moved into this apartment myself 3 years ago, and lived there 2 years. We moved out last May. During that time we saw no signs of moisture or mold in the apartment. The only difference is that the landlord added 6 in. of cellelose (sp?) insulation in the attic about 2 years ago (1 year into our time there). Before then there was R-11 batt insulation that had fallen apart. I'm not sure if this could have changed the dynamics that drastically, we didn't notice any difference.

Well, any thoughts are appreciated. I'm sorry to be so long winded.

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Old 01-14-2009, 07:15 PM   #2
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Re: Frost On Underside Of Roof Sheathing

It's more likely a vapor problem than a liquid water problem. The problems you pointed out are certainly real and need repair but getting the underside of the decking frosted requires vapor.

Look for what changed, like a new tenant. Some tenants are so stingy with heating costs that they won't ventilate the bath after a shower, or worse, vent the clothes dryer into the living space. It could also be a humidifier putting to much moisture into the building.

Obviously the roof needs proper eave and ridge ventilation. The moisture on the inside walls won't be fixed by fixing the attic. That almost has to come from interior vapor. that vapor will travel into the unheated attic resulting in the problems you described.


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Old 01-15-2009, 10:27 AM   #3
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Re: Frost On Underside Of Roof Sheathing

I work with Tim and am just as bewildered as he is about the moisture. Water does funny things no matter what state it is in (frozen, vaper, liquid). I have a question about what to do after we seal/ fix the moisture problems with the roof, chimney, etc. I have been told that household bleaches, even the "stronger" formulas are mainly composed of water. So the bleach kills the spores on the surface, but feeds airborn spores immediately. Is there any truth to this?

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Old 01-15-2009, 12:13 PM   #4
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Re: Frost On Underside Of Roof Sheathing

You've got to dry the place out first or your wasting your time and money. Not just the surface but the inner body of the plaster/sheetrock, the wall cavities, the insulation, everything. This will take time.
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Old 01-15-2009, 12:42 PM   #5
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Re: Frost On Underside Of Roof Sheathing

I have had this problem in the first house that I bought. It was 2 things-not enough insulation in the attic & not enough Ventilation. The underside of my roof was pure white with frost on a January day when I checked it out after I heard banging up there. Turned out to be the roofing nails popping up from the frost contracting the sheathing.
I put another layer of insulation in the attic & knocked out a bunch of bug blocks under the eaves & installed ventilated vinyl soffets............... Problem solved
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Old 01-15-2009, 01:55 PM   #6
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Re: Frost On Underside Of Roof Sheathing

I was reading this post and was thinking exactly what Mr. Mike above was saying, sounds like a ventilation issue, but if you lived in the above for a period of time with out any troubles, my mind starts wondering if something different is happening, but I would check proper ventilation and if you have some sort of proper vent or air chute and intake and exhaust.

Good luck!!


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