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Roofing/wet plywood in middle of roof

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QUESTION: We have built an addition to our house and noticed that the one side of the roof is wet, the plywood but it is only in the middle top and bottom(which is the outside) is dry.  We had ventilation bats and insulation up but before we put the drywall up noticed water so we removed that and it was only wet where the drywall and ventilation bats are.  We did not finish the soffit on this side yet.  What do you think the problem could be?

ANSWER: Jamie,
Generally when we see water forming on the interior underside of the plywood, this points to a condensation issue. Condensation forms when inadequate or improper ventilation prevents warm moist air from evacuating the assembly, allowing the air to reach the dew point and form water. Is the area that you are adding on a cathedral ceiling where the insulation is installed to the underside of the joists, or are there trusses where the insulation is installed on the lower portion of the joists, and the plywood is on an upper portion of the truss?

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QUESTION: Yes it is a semi cathedral ceiling using collar ties that come down to make it 7 1/2 ft ceiling. This seems to only be on one side of the roof. Do you think that this may be due to not having soffit finished on that side?  Do you think that it will dry?

Answer
I believe that this is related to ventilation. With a cathedral ceiling there must be intake and exhaust (intake at the eave, exhaust at the ridge) in equal proportions. The area of ventilation must be 1:150 of the space that it is ventilating (in this case the area between the rafters, from top to bottom, divided by 150) equally distributed at the eave and ridge. There must be a 1" air space between the top of the insulation and the bottom of the plywood to allow air to flow and move the moisture laden air out of the assembly, thus preventing condensation. Provided that the ventilation is correct, the system should dry without further issues.
The fact that it occurs on only one side could be due to a number of factors. Amount of sun on a side of the roof, the area being above a kitchen or bathroom (which generally produce warm moist air), direction of the wind, etc.
The use of vented eave (through screened holes or vents between the joists) and a ridge vent will likely work for the condition you have described. Please remember that each space must be vented based on the 1:150 rule.
Thanks,
Aaron

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Aaron Nelson

Expertise

All types of questions with regard to roofing, waterproofing and sheet metal. This includes commercial and residential buildings, along with the various aspects of the construction means and methods, ventilation, steep-slope or low-slope, code requirements, installation errors and other such problems

Experience

I started in roofing fabricating and installing sheet metal flashings out of high school for a large commercial contractor. I worked for the same contractor in the office estimating and project managing commercial project for 10 years. I left the construction side of roofing and now work as a roof consultant, which I have been doing for 8-years.

Organizations
RCI, Inc.

Education/Credentials
I am a Registered Roof Consultant (RRC) and a Registered Roof Observer (RRO) through RCI, Inc. RCI is a international group of building envelope professionals. I am an approved instructor for RCI educational programs.

Past/Present Clients
We have worked with many local school districts, insurance companies, HOA's, building management companies, and we have a national client with buildings located throughout the US.

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