Had to replace the shingles on my roof.

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7,116
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MIchigan
Thought I had a leak because the roof was spongy in places. But upon removing the shingles the plywood was dry...to dry. Roofer said it dry rotted from improper ventilation. There wasn't any roofing vents on the house. Now the plywood on the garage was ok because it has three vents on it but I had to buy 40 sheets of plywood for the house which cost me an additional $500 on top of the roof job?
 
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4,837
Location
Central Texas
That's why your old ply rotted ie "no roof vents". Where you live it gets really cold. Any warm, moist air that leaks into a sealed attic like yours during Winter will immediately condense. At a minimum, you need some gable vents or (better) a ridge vent.
 
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17,947
Location
NH
40 sheets of plywood for $500? Did their original estimate assume a bunch already, and it was another 20? With the plywood off you can drop the (plenum, channel, whatever it is called, the styrofoam stuff for airflow) down, the plywood, then a ridge vent. Just soffit vents to finish.
 

Kestas

Staff member
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13,959
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The Motor City
During winter, the warm moist air rises into the attic where it condenses on the cold plywood. Attic ventilation is just as important in the winter as it is in the summer. When I reroofed my house (had to remove three layers), the 46 year old decking, which was thinner than what is allowed by today's standards, was also somewhat spongy. Even with proper ventilation the decking can absorb moisture and get soft. I too had to redeck my roof. I didn't want to put premium 40-year shingles on flimsy decking.
 

JHZR2

Staff member
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46,138
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New Jersey
Interesting. At our mountain house this happened despite attic vents. My home, and my parents' and grandparents' home, all late terms to 1920's structures, still have the original tongue and groove. Wonder what they treated it with back then...
 

Warstud

Thread starter
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7,116
Location
MIchigan
Originally Posted By: supton
40 sheets of plywood for $500? Did their original estimate assume a bunch already, and it was another 20? With the plywood off you can drop the (plenum, channel, whatever it is called, the styrofoam stuff for airflow) down, the plywood, then a ridge vent. Just soffit vents to finish.
I assumed about 5 sheets of plywood but once the shingles were off.... all of it looked bad. Glad I looked at the plywood before my roofer started nailing down the shingles because he didn't plan on re-decking the roof. We added a few more soffit vents and put on a ridge vent.
 
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8,022
Location
MI
I find it amazing how many people neglect proper attic ventilation. There's tons of info on the interweb. There is always some disagreement between the engineers and there are differences between practices between regions of the U.S.. From what I have read about our northern needs, it's important to have the total vent area sized correctly. The eave and ridge vent area should be somewhat balanced in size. If the ridge vent area is much larger than the eave vent area, it can result in a negative attic pressure that draws even more air out of the living space (summer air conditioned or winter warm/moist). Gable vents on a ridge/eave type system can mess up proper air flow as can gable fans. Make sure all openings into the attic are sealed: ceiling light fixtures, receptacles, etc.. Next winter, inspect the attic for condensation frost on the plywood. Proper ventilation also reduces ice dam problems.
 
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522
Location
SW Fla
Originally Posted By: doitmyself
I find it amazing how many people neglect proper attic ventilation. There's tons of info on the interweb. There is always some disagreement between the engineers and there are differences between practices between regions of the U.S.. From what I have read about our northern needs, it's important to have the total vent area sized correctly. The eave and ridge vent area should be somewhat balanced in size. If the ridge vent area is much larger than the eave vent area, it can result in a negative attic pressure that draws even more air out of the living space (summer air conditioned or winter warm/moist). Gable vents on a ridge/eave type system can mess up proper air flow as can gable fans. Make sure all openings into the attic are sealed: ceiling light fixtures, receptacles, etc.. Next winter, inspect the attic for condensation frost on the plywood. Proper ventilation also reduces ice dam problems.
You are bang on. Too many vents is as bad as no vents at all.
 
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4,837
Location
Central Texas
Other things you might check are bath fans that vent into the attic rather than outdoors. Same for the vent over the stove and even the dryer. All sources of warm, humid air. In order to rot that many sheets of plywood, you've got a major warm, humid air leak or leaks into your attic. Further, you're losing valuable and expensive heat you're paying for and then paying for it again by buying new ply.
 
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17,947
Location
NH
Originally Posted By: Warstud
Originally Posted By: supton
40 sheets of plywood for $500? Did their original estimate assume a bunch already, and it was another 20? With the plywood off you can drop the (plenum, channel, whatever it is called, the styrofoam stuff for airflow) down, the plywood, then a ridge vent. Just soffit vents to finish.
I assumed about 5 sheets of plywood but once the shingles were off.... all of it looked bad. Glad I looked at the plywood before my roofer started nailing down the shingles because he didn't plan on re-decking the roof. We added a few more soffit vents and put on a ridge vent.
I thought plywood was $30 a sheet? What with stuff like this makes me wary of very long life metal roofs. Get a step wrong underneath and it has to come off anyhow. Of course ice daming shouldn't occur on a steel roof, and one doesn't walk much on one, but still.
 
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9,783
Location
Saskatoon canada
The idea that too much venting is bad is truly absurd. An attic space in a perfect world should be the same temp as the ambient air temp outside. When an attic space is warmer than the outside air condensation occurs and in the winter ice dams can destroy the entire eaves. Common home building techniques here in Canada use aluminum soffit around the entire overhang which allows air to enter then vents out the common square can vents cut out close to the ridge. You can't have too much ventilation in a roof. Period.
 
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4,837
Location
Central Texas
Yep...you guys need a cold roof up in the Great White North. Ice daming can really wreck your day, not to mention your roof, rafter tails, sheetrock, inside walls....
 
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