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Flat roof insulation and covering

Hi there,
I have a 1950s property with a Mansard roof with a flat felt covering on top which has been patched twice and reaching the end of its life. There's an internal access through a cupboard where I can see a very thin layer, approx 1cm of insulation on the rear of pine boards which appear to be the underside of the roof. Unsure if this thin, old insulation is only at this access point or spanning the entire underside of the roof as not possible to see the rest.
I've researched but had mixed messages from roofers. There has been discussion of risk of condensation if the roof were insulated from above. There has been mention of leaving the existing felt in place, then insulating/boarding over the top, with the new membrane over this.
I ready the difference between hot and cold roof constructions, but unsure if because the existing insulation is so old and thin, it can be disregarded and we could insulate on top. Also is it alright to reconstruct on top of the existing felt? Roofer said there's good benefit to this by putting a slight gradient on it. Personally I thought better to strip old off but uncertain.
Thanks in advance as very unsure. Job is in Stirling.

3 Answers from MyBuilder Roofers

Hi chris, i would strip off and re-insulate. The insulation MUST NOT touch the boards above as this WILL cause condensation, once stripped then you can change the fall or build up with timber. As a company i always strip off and start again. You can go on top and do what ever you like, have you thought of grp ( glass re-inforced polyester) fiberglass. the reason people are stopping doing built up with bitchumen is because if you have fire on your public liability then the price goes up by around £2500! hope this helps.


Answered 6th Aug 2017

Definitely strip off and create new fall with timber firing pieces.
Re-deck with 109mm insulation which is backed with Ply, then new membarane


Answered 28th Jul 2018

Strip existing coverings then recover using a vapour barrier before insulating .then 1 later 2mm underlay .followed by 4.5 mm capping sheet .


Answered 12th Dec 2018

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