mystery rotting roof- boulder, CO

[bw-construction]

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. Any other ideas why this is happening?

[Dancing Dan]

Have you opened it up from the inside?

[caddis]

How about rot from condensation because of the lack of ventilation with the fiberglass insulation?

[Dick Seibert]

Wow, that is really bad, I had one a few years ago where shakes were replaced by OSB and asphalt shingles 5 years earlier, we decided the moisture from within the home going through the knots and joints in the hemlock 2x6 decking causing the OSB to swell, I removed everything and replaced with plywood and a Cor-A-Vent system. I'm looking at a similar one now after 5 years with OSB and asphalt shingles replacing breathable shakes. Neither of these is as nearly as bad as yours, asphalt shingles seal roofs up stopping air movement, most shingle manufacturers won't warrant their shingles if the roof is sealed like yours, put OSB in there and it turns to mush which looks like your situation. This is not OSB in roofs but walls, but it looks like what you've probably got under there.

Take Dan's advice and open it up now, before the rains, it's got to come off anyway.

[DickRussell]

Some good replies to this set of pics also over on Breaktime.

[NW Architect]

There is a gabled roof section perpendicular to, and immediately below, the affected area. My money is on some moisture source originating there and moving up into the enclosed rafter spaces of the affected plane.

North side also hints at condensation turning to ice, then to liquid water if the plane is in shadow all winter

[MarkMc]

There is a gabled roof section perpendicular to, and immediately below, the affected area. My money is on some moisture source originating there and moving up into the enclosed rafter spaces of the affected plane.

North side also hints at condensation turning to ice, then to liquid water if the plane is in shadow all winter

That sounds and looks much more probable than leaky flue pipe gases traversing that distance w/o having any clues on the int. surfaces or CO asphyxiations.

[Mark G]

This is what happens with an unvented roof assembly. Basic building science, I believe.

[Pete Engle]

This is what happens with an unvented roof assembly. Basic building science, I believe.

Yep. Boulder is more than cold enough for the normal moisture of living to cause that kind of damage in an unvented & unmanaged roof assembly. "Normal" building practices don't provide nearly enough vapor management to keep an unvented cathedral dry. That's why attic ventilation is required in the first place.

[Martin Holladay]

I tend to agree with previous posters who identify the problem as condensed moisture originating from the home's interior. However, it isn't as simple as "this is what happens with an unvented roof assembly." Unless I'm mistaken, there are three factors present that made this failure possible:
1. No venting.
2. Fiberglass insulation (instead of dense-packed cellulose or spray foam).
3. An imperfect ceiling air barrier.

The problem could have been avoided (or at least lessened) if some of these factors had been different. But if you have an unvented cathedral ceiling with the worst possible insulation and an imperfect air barrier, you have a disaster.

[Overbuilders]

3. An imperfect ceiling air barrier.

Martin,

What type of ceiling air barrier do you recommend and where should it be applied?

[Martin Holladay]

In most houses, the ceiling air barrier consits of gypsum drywall. Possible flaws include:
-- Ceiling electrical boxes or can lights
-- Vent pipe penetrations
-- Wiring penetrations of partition top plates
-- Long cracks between the wall drywall and the partition top plates.

[newleaf]

I will add I've seen a similar sagging pattern like a starving dog if you will. It was about 20+ years old.

This home had an interior poly "vapor barrier" and the deck was aspenite. I think the barrier helped to hold the moisture venting from the home into the roof cavity.

The house was relatively tight, but the chimney was not and once the roof was stripped you could see the plume of decay above it

[Hausdok]

Agreed,

Lack of ventilation, poor insulation, lousy air sealing. It's north side and it's a steep pitch. That means that most of the time it's shaded from the sun, cool and interior moisture is condensing on the interior surface and is feeding whatever is eating away at that roof deck. You've got some kind of organic nightmare going on with that roof deck. Better get it opened stat.

ONE TEAM - ONE FIGHT!!!

Mike