On 20/12/2012 09:27, Terry Fields wrote:
On Thu, 20 Dec 2012 00:00:04 -0800, harry wrote:

On Dec 19, 9:56 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.



Strange how often people notice condensation during periods of heavy
rain Must be warm(er) air coming from somewhere if it's condensing on
the underside of the roof.
Maybe asbestos becomes porous over time?