Low pitched roof

23 Feb 2016
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United Kingdom
I have a low pitched roof, about 20 degrees, clad with fibre cement slates on battens on untearable breather felt. I formed a 50mm air gap by fixing 50mm x 25mm battens to the inside of the rafters and cladding the underside with plastic sheeting and then hardboard. There is a considerable amount of water running down the underside of the rafters, enough to fill several buckets in just over two weeks. The water only appears to come in when it is raining. I have stripped the roof and the only other thing I have found is that some of the glass fibre insulations appears to have been close to the underside of felt. Could this just be condensation?
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IIRC, 20° is the minimum recommended pitch, as by their nature, these slates are very susceptible to capillary creep of water between the slates, and the lower the pitch the more likely this will be.

This would appear as water penetration every time it rains, but in lesser quantities than a leak.
Yeah 20 degrees is the minimum for 600 x 300s on general roofing @ 245mm gauge, but I'm sure I read somewhere you can go down to 15 for short rafter runs as the amount of water captured is less and runs off of the whole roof quicker.

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