Stopping condensation on underside of asbestos outhouse roof...

Hi all,
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. At first I thought it must be due to rainwater getting in through one or two of the holes through with the 6" galvanised roofing nails were driven. However, after carefully sealing any possible such leaks, the wetness continues. 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. Since the roof is gently sloping, that condensation runs along the enderide of each corrugation ridge, until it meets a joist. That is where the water gets into the wood. One weird thing I noticed is that some sheets of the asbestos seem more prone to condensation than others.
Are there any tricks I can employ to stop the condensation?
If not, does anyone know of any way to divert the water away from the joists? I thought of gluing a short length of string to the underside of each ridge, just upward of each joist, so that the water drips down the string and into the garage... but it's not a very satisfactory solution, because I have a lot of perishable stuff stored in the garage.
Can anyone offer any suggestions?
Thank you...
JakeD
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On Wednesday, December 19, 2012 8:46:29 PM UTC, JakeD wrote:

You need to reduce interior RH. Look for the sources of damp.
NT
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On Dec 19, 9:56 pm, snipped-for-privacy@care2.com wrote:

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

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

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On 20/12/2012 09:27, Terry Fields wrote:

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?
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On Thursday, December 20, 2012 9:47:33 AM UTC, stuart noble wrote:

A spike in RH and a sudden drop in temperature. There's also a bit of cooling due to evaporation after the rain.

no
no
NT
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On 19/12/2012 20:46, JakeD wrote:

<snip>
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


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Thanks to everyone for the input. Anti-condensation paint sounds interesting. I must look for that.
Yes, mine was built about 20 years ago, using second-hand asbestos too.
I'm mainly concerned about the roof joists rotting due to the moisture getting on them. Apart from that, the condensation is not really a problem.
Yes, it's interesting how and when the condensation appears. I took a look in there today, just after nightfall, after a rainy day, (lots of damp air) and there was no condensation at all.
The garage is already very well ventilated and there is no significant damp inside coming up through the floor or walls, as it has a proper DPC, etc.
Perhaps I should make it even more well-ventilated, somehow. I don't know though; you get dew forming on grass, outside, and "outside" is VERY well- ventilated! :)
JakeD
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On 20/12/2012 22:43, JakeD wrote: <snip>

Indeed - and look at the condensation /dew / hoar frost you get on a car parked outside, but not (so often) on one parked under a carport. Point your infrared thermometer at a clear night sky, and you'll see why your garage roof, or your car windscreen, is desperate to radiate to it, causing its temperature to drop below that of the air above and below it.
--
Kevin


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------------------------------------------------------------------------------- we bought a steel garden shed and had the same problem. manufacturers said to lay thin sheets of polyurathene foam right up against the roof and it completely solved the condensation problem.
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Kevin wrote:

I'd not heard of anti-condensation paint, so thanks for the mention. A friend's garage (sectional concrete with corrugated metal roof) is plagued with this condensation problem: perhaps a specialist paint would help. I'll investigate.
Bert
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Painting the inderside of the roofing sheets with polyurethane foam might be even more effective?
JakeD
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JakeD wrote:

Do you mean the expanding foam filler stuff? Can you paint with that? Perhaps I've just been unlucky (or cack-handed) but my experience with it has not been happy: it goes (and sticks) all over the place and the can becomes unusuable after only a few squirts.
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It can be sprayed on (given the right equipment). There are companies that specialise in spraying it onto the underside of roofs.
Al
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"AL_n" wrote:

I've come across that for house roofs but not for the sort of corrugated metal garage roof that I was referring to here: I was thinking more of a DIY solution. But I'll investigate, thanks.
Bert
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replying to Kevin, Tony wrote: Hi I have the same problem with my corrugated garage roof which has been cold tar sealed on the outside and painted with Matt emulsion on the inside which now explains all the condensation from the water based emulsion. I notice the roofing sheets from the door to midway roughly dry out (no dripping) when I have the garage door up for about 3-4 hours suggesting air circulation. Any advice gratefully received Tony
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On Tuesday, 30 January 2018 21:44:05 UTC, Tony wrote:

You need to reduce RH. Your condensation has nothing to do with the roof's coatings. There are 3 ways to reduce RH: 1. Ventilation 2. Heat 3. Dehumidifier
Relying on ventilation alone is likely to result in dampness when outdoor conditions are damp. Heat costs. Dehumidifiers are cheap to run, but cost to buy of course. If it's insulated, computer controlled heat just to limit RH to 80% is also an option. Ask Andrew Gabriel.
NT
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snipped-for-privacy@gmail.com has brought this to us :

It can have a lot to do with lack of any insulation. My garage and adjacent workshop I fitted with a modern corrugated metal roof. In colder weather lots of condensation would drip from the underside. It was much worse in icy weather, when ice would build up, then thaw as the weather warmed. I fixed it entirely by the addition of fire retardant expanded polystyrene insulation sheets, pushed up tight against the underside.
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posted on December 19, 2012, 8:46 pm Ahem.
Brian
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On Wednesday, December 19, 2012 8:46:29 PM UTC, JakeD wrote:

Almost anything, singly or in combination, is useful.
* Reduce the moisture indoors. Seal leaks, paint floors, fit a dehumidifier.
* Reduce the chill effect of the roof by improving its insulation. Hard work, but worth it for workshops.
* Reduce the moisture getting near the roof sheets. Just polythene sheet (or bubblewrap) stapled across the joists and with an inch or two clear of the roof is worth doing. If all else fails, it will at least tend to trap the condensation drips and route them to the edges!
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