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TOPIC: Underlayment

Condensation on underside of metal roof

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(I initially added this to a previous post put thought I better start a new post.) My contractor installed metal roof on my house in Central Virginia. It is a ribbed material. It was installed without underlayment. He said he checked code and was told we didn't need it. I had no clue if this was a proper installation. We just had a 3 day cold rain period. Saw water damage to drywall in bathroom. Went into attic and found heavy condensation on underside of roofing. Don't know what to do now. Should it have been installed with underlayment and/or vapor barrier? There is plenty of air ventilation in the attic. Help!!! Thanks
Guest User

10/7/2006
Underlayment should be installed and, in all cases, manufacturer instructions should overrule code unless the code is more strict. I would make sure that you ventilation is well balanced. The idea should be to continually "bathe" the underside of the roof deck with fresh air. Also, are you positive that the problem is not actually a roof leak? Todd Miller
Todd Miller
Isaiah Industries, Inc.

10/9/2006
Yes, not a roof leak as it was across the entire underside surface of one side (east facing) of the roof which I'm curious about. The other side (west facing) has a very steep pitch - east side much more shallow pitch. The condition manifested during a very hard 3 day rain storm from the east that we had in Virgina this past Thursday-Saturday. Some 10 inches reported. Freak storm. So very humid and cold (in the 50's during the day) the entire three days. This would certainly explain the condensation I guess. The building is only 28 feet long x 32 feet. Large gargage with 2nd story 900 sq ft apartment on 2nd floor. There are gable vents on both ends so there should be plenty of air flow I would think. I've been thinking about installing a switch controlled fan at one end for times like this and for hot summer. I talked to the local roofing company that did some final enclosure work on the roof (I fired the original contractor). He said this roof system, which he is very familiar with, should not need a vapor barrier as long as there is good ventilation. He is going to re-check it in the next couple of weeks. County code does not call for it either. He said his may be a very infrequent condition. Certainly, knowing what I have experienced, if I had it to do over, I would have put down underlayment and vapor barrier. But at the time I really didn't think it would be an issue. Besides, I would think condensation trapped between the vapor barrier on top of an underlayment would not evaporate fast thus causing rust over time. I checked the attic again yesterday after the sun finally came out and all the condensation was gone. Any additional thoughts based on what I have just told you? Thanks for your help Todd.
Guest User

10/9/2006
First of all, I would not rule out the possibility of a roof leak. I understand it may seem unlikely but there could be a problem at the ridge taking on water, for an example. Opposing gable vents are very ineffective for ventilation. They create a direct air exchange between the two vents but they do not create a total attic air exchange. If possible, I would close them off and add eave soffit vents and a ridge vent. Todd Miller
Todd Miller
Isaiah Industries, Inc.

10/9/2006
Thanks Todd I do have both soffett and roof vents but it occurs to me that foam plugging was installed to keep out criters. So as a result that venting may no longer be effective. I will go over all this with the roofing contractor. John Rindge
Guest User

10/11/2006
All best. Todd Miller
Todd Miller
Isaiah Industries, Inc.

10/11/2006