On 19/12/2012 20:46, JakeD wrote:

I have a double garage with a sloping 'big 6' corrugated asbestos roof
which is about 30+ years old.

snip

Yesterday (which was a cold but rain-free day), I went
into the garage and inspected the roof, just after nightfall, and I
discovered that almost the whole underside of the roof had a lot of
condensation on it.

I have this problem with a garage, also roofed in Big 6. The problem is
far worse in areas where I'd slapped a coat of white paint on - could
have been a cheap masonry paint, or household emulsion, but definitely
water-based. The bare asbestos-cement sheets become damp, but I believe
they can absorb sufficient condensation that they don't drip. The
condensation is worst after a clear, frosty night, when presumably the
roof sheet temperature drops well below dewpoint of the air in the
garage. It's unheated, and draughty, and the floor has a good dpm.

Obviously I much regret painting it - almost as much as I regret using
the roofing sheets in the first place, but it was built for animal
housing with open-fronted hay and machinery storage bays. I've
considered fitting a ceiling and some insulation, but it would need a
vapour barrier to avoid condensation in places you can't see, and as
they were secondhand sheets 25 years ago they do have the odd crack and
nail hole that leak occasionally.

If I could be bothered, and thought I'd be here for a few more years,
I'd either re-roof it in insulated box-section steel or try
anti-condensation paint, which I understand works by providing an
absorbent layer.

--
Kevin