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Some Advice on Replacing a Mono Pitch Roof

Postby DHISRT » Wed Apr 01, 2020 11:23 am

Hi All

Just looking for a few pointers in replacing a mono pitch roof.

Some pictures attached to help set the scene

The roof has about a 5 degree pitch. Scope to increase this to just below 10, by tucking it up tighter to the main roof you can see (I've got about another 10 inches to play with). It's a single skin, block built design. About 3.5m by 8m. one 3.5m side is block built the other timber framed. Main facing wall block. It is butted up against a stone wall of a far larger building.

I understand that anything under 10 degree needs to be built like a flat roof. Trusses with a 20mm roof sheet for the roof deck. Was then going to put a membrane on, run some battens/ purlins left to right, fill in the gaps with rigid insulation then fixed some bituminuous corrugated sheets like 'Onduline'. Onduline sell an 'apron flashing' to finish the corrugated sheets against the wall. Do I need to flash over these or can i simply silicone the joint? I'm mindful of putting in a tray(?) as its a bit of a higgledy-piggledy stone wall with no natural single joint like you would get in a brick built wall (if that makes sense). It will be tucked up under the eaves of the main roof so very little, if any weather will hit the small bit of wall (if that makes sense). You will see from the pictures that this 'join' (roof to wall) has simply been covered with cement. In fairness there is no rain/ damp coming down that wall.

Ok and with apologies for the daft questions but this will be first project like this.

a) What size of wall plate do I need on the block wall and what sort on the far wall. How to fix both?

b) what sort of size trusses to go from wall plat to wall plate. What centres? How to fix at both ends?

c) Apart from the Trusses any other sort of supports I should be putting in?

Go gentle with me as this is all a bit new. No doubt I have got some of the terminology completely wrong.


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Postby stoneyboy » Thu Apr 02, 2020 4:45 pm

The answers to your questions will depend on whether the rooms under this roof will be habitable rooms. If you are converting from outbuildings used for storage to living accommodation you should be mindful of building regulations.
If you really must use Onduline you will need to provide some sort of sheet support under it or it will tend to sag between supports.
Regards, S
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