Underlay and Overlay for a Roof?

Hello, I'm thinking about replacing a small roof at the back of my house which covers a small portion of the kitchen and a toilet. Its about 160 slates which are the thin fibre cement slates.

I might as well replace the underlay and timber, but the guy in the building supply shop says I need underlay and overlay. I was of the understanding that underlay was all that's needed to protect the room from rain while you put up the slates.

can anyone clear this up for me , thanks

you require a vapour check membrane at ceiling level above the plasterboard, this generally doubles as the air-tightness layer. then, you have the insulation below and between the rafters as per building regs (and air-tight backers for spots if required), then probably breathable membrane and battened + cross battens followed by slates (if you opt for 'old school' felt you must be prepared to ventilate the roof space at the eaves and possibly at the wall junction location depending on your situation.

hmmmm- not as straight forward as I thought :/

Thanks for the reply

I was just going to replace whatever was up there with new stuff-

It appears to be "old school" black heavy felt underneath the battens ( as seen from a large hole in the kitchen ceiling from an old extraction fan) no membrane above the plasterboard nor any insulation between rafters.

The house is 35yrs old approx. and end of terrace. Would I get away with just replacing the slates, felt,battens and underlay? We will be replacing the ceiling eventually as it has wooden panels attached which are dated but not for a year or so, at that point we can insulate the rafters.

We had the wall cavity insulated a year or so a go and now we feel a bad draft coming from this roof area, a good few slates broken too and felt worn in places.

very confused, by your logic here

leukoshoopr said:

We had the wall cavity insulated a year or so a go and now we feel a bad draft coming from this roof area

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this has nothing to do with cavity being pumped
the 'draft' is related to the lack of a sealed and taped vapour control check as i explained early

a vapour check membrane at ceiling level above the plasterboard, this generally doubles as the air-tightness layer.

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(also known as vapour barrier)

leukoshoopr said:

Would I get away with just replacing the slates, felt,battens and underlay?

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i'm at a complete loss as to why you would fix the roof and not just install the insulation, membrane and new ceiling in the process?

thanks again for your expertise on my query, going to rethink the whole project based on your posts

much appreciated

best of luck, see here for eaves detail http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,18754,en.pdf
(for some reason the cavity wall details wont open http://www.environ.ie/en/TGD/ but you'll get the gist)

I would presume (*without having seen your situation*) that a warm roof construction would be better to avoid the need for ventilation in the roof space