Flat roof repair / insulation - Update and more questions!

17 Feb 2008
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United Kingdom
Over the winter the flat roof on our extension (40 sqm approx) has developed a leak. it appears that it has been patched on numerous occasions with a number of different materials including GRP.....the original layer appears to be felt or some derivative.

I have come to the conclusion that further patching is unlikely to be worth the time and hassle and I'm looking to fit a new EPDM membrane.

At the moment there appears to be no insulation at all with the current waterproofing fixed directly to the wooden roof deck and just an air gap between deck and the plasterboard ceiling.

Due to the unpredicatable nature of the weather (We live in western Scotland) I would much prefer to add an additional layer to the existing roof rather than strip everything back to the deck.

Clearly the simplest method is to bond the EPDM directly to the existing roof but I know that "Cold Roofs" are now not permitted in Scotland...In addition, additional insulation would seem a sensible idea.

What I am proposing is to utilise the existing covering as the "vapour control layer" and add a layer of 'kingspan type' insulation topped with a Ply / OSB deck to give a nice new surface to bond the rubber to.

EDIT - I have a 500mm parapet around the roof so there is plenty of depth available to play with.

Will this work? Are there any Gotchas? If it is not possible, then am I allowed to merely repair the current roof using an EPDM sheet or will that put me foul of building regs?

Thanks in advance for your help

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You've had leaks over a number of years, the latest leak is showing now but could have been going on for some time. This might mean that you have some kind of rot inside the roof envelope.

Strip the roof back to the joists.
Examine the state of the exposed roof area and joists.
Seal off or use sealed devices for any items that pierce the plaster board ceiling eg. pipes or lights.
Build a warm roof.
Be aware that parapets can give endless moisture penetration difficulties.

By and large, my view is that its not worth patching domestic flat roofs.
Thanks for the reply....I've got access to the underside of the deck and it and joists look OK. You say parapet walls can cause problems......is it worth building the roof up to the same level as the parapet with extra insulation and having a completely flat roof?
Without seeing your roof and parapets its difficult to say but i would say no, dont go building up the roof to parapet height.
One difficulty with parapets is if they need parapet drainage outlets: hidden leaks and blockages. Another problem could be getting sound flashing details along with parapet capping arrangements.

I'd suggest that you forget about any unconventional ideas and stick to straightforward roofing.
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Well it seems i've been guilty of making too many assumptions! The extension roof covers 3 and a half rooms and i've previously had eyes in the ceiling void of 2 of them (a bathroom and entrance vestibule). Neither of these had any insulation at all and I incorrectly assumed that this was the case throughout.......however after pulling down a section of the ceiling in the bedroom it would appear that this room only has been insulated with a thin (approx 20mm) layer of Kingspan straight above the plasterboard with the rest of the void stuffed with rockwool. I can see no evidence of any cross ventilation at all in this roof.

As far as I can tell this roof doesn't conform to the requirements of either a warm or a cold roof but notwithstanding the leak, this part of the house is warm and dry with no evidence of condensation issues (Unlike the rest of the house which has an approx 150 sqm concrete cold flat roof which causes no end of troubles but is at least watertight).

My priority is obviously to stop the water coming in but I would like to increase the insulation levels and I would prefer to incorporate a greater slope to assist drainage if practicable while minimising the time that the roof is exposed to the local Scottish weather. While undoubtably the best solution is to completely strip the roof and rebuild from scratch, that is outside my abilities as a DIY'er and I would struggle to afford a pro job in the timescales required.

My original plan was to fit a layer of kingspan thermataper insulation ontop of the existing roof with a new EPDM membrane ontop....this would provide improved drainage as well as improving the Uvalues of the roof.

I have looked at the advice regarding a hybrid roof and the general consensus appears to be dont do it! Accepting this, and assuming that the existing deck is sound (I still believe it is but will do further inspections using an endoscope to confirm) I am struggling to see how the extra insulation could hurt......the roof deck temperature will be somewhat higher and thus less susceptible to generation of condensation........the existing ventilation (or lack thereof) has not resulted in condensation issues and I cant see how this would change....In addition I would really rather not rip out the entire ceiling to remove the existing insulation.

Can anyone point out the flaws in my thinking? The roof is basically flat with no 'ponding' so a simple recover with EPDM may be my best option in the short term but I would prefer an improved runoff and any increase in insulation values wouldnt hurt. The kingspan is expensive but is much quicker and easier than trying to build a sloping deck from scratch and would provide a nice smooth surface for the EDPM.

I can post photos once I'm at home if that would help anyone visualise what i'm talking about as its definitely into what I would call "non-standard" territory. Sooner or later the whole roof is going to need doing but, until then, I need a watertight half decent job that is better than the current bodge job that is within the capability of a DIYer in a week or so......I would welcome advice even if its along the lines of "you're being a bloody idiot"

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