Underlay for low pitch roof

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With the purchase of a house I have inherited a low pitch roof. According to plans, the pitch is 10.5 degrees. The construction is a warm roof (ply on top layer), then monarfoil underlay, then battoned and counter battoned, then concrete interlocking tiles.

The problem I have is that water gets behind the tiles during heavy rainfall. I recently had a leak on the eaves which I fixed but noticed that lots of the batons are rotting and the ply beneathe the monarfoil is wet - I assume through 'sweat'.

I don't really want to fork out for a new flat roof as it's quite big (36m2). I am planning on re-battoning the roof and using a breathable underlay. My question is, will the breathable underlay be water resistance enough considering the low pitch of the roof?

I was planning on battoning, then underlay, then counter battoning. Is this correct?


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i am no expert but 10.5 degrees sounds more suitable for sheet roofing material than tiles...

i don't have any input for you about the correctness of your plans. i just wanted to remind you to use only treated timber and it would probably be good to give the plywood a coat or two also.

low pitched roofs are often associated with lean-to type buildings. if this is so your water through the tiles might actually be finding ways past any flashing. it is also possible that moss has a good foothold and is beginning to lift the tiles a bit or is separating them...

i feel the underlay should either be bonded to the ply in order to completely exclude water or should have a good gap for air circulation between the layers. i have no experience with breathable stuff - but my gore-tex jacket was a huge disappointment. it would cope with generated moisture while at rest or strolling but any effort would have me drenched and took hours to dry (it is a quality police issue jacket) so i am very suspicious about things claiming to be breathable...

treating the tiles with a water repellant would probably help them to shed rain more quickly and efficiently thus reducing the chances of leaks and seepage and also reduce frost damage.

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