On Thu, 20 Dec 2012 00:00:04 -0800, harry wrote:

On Dec 19, 9:56Â*pm, wrote:

On Wednesday, December 19, 2012 8:46:29 PM UTC, JakeD wrote:

I have a double garage with a sloping 'big 6' corrugated asbestos
roof which is about 30+ years old. The roof joists are getting very
wet in places. I discovered that almost the whole
underside of the roof had a lot of condensation on it.

You need to reduce interior RH. Look for the sources of damp.

The above is right. You may find the floor has no damp proof membrane
under it and is very damp. Water evaporates from it and condenses on the
roof. Check if the floor is damp first.
Floor paint might fix the problem, also improve ventilation.

The problem might be less ventilation than heat radiation from the roof.

If the roof temperature drops below the local dew point due to radiating heat into a clear sky, water will condense
on it anyway, irrespective of the ventilation. Perhaps sheets of bubble-wrap tacked to the joists might help;
bubble-wrap is frequently used in greenhouses partly for this purpose.



--
Terry Fields