Flat roof - EPDM over Celotex

20 Feb 2014
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United Kingdom
I intend to remove the covering from my extension flat roof (about 9 square metres) and install Celotex 126mm TD4000 (vapour control/insulation/5.5mm ply all in one) directly onto the joists and then put EPDM on top.

On the Celotex website it says that single ply waterproof layers laid on top of TD4000 usually require an additional 12mm of ply onto the Celotex board before applying the waterproof layer (EPDM).

Is this necessary? If so why?
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Or is there maybe a better/simpler/cheaper solution to creating a small insulated warm deck flat roof?
Could I put 11mm OSB3 board on top of the celotex instead of the more expensive ply?? Anyone is welcome to reply :D
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You can get away with EPDm over the existing 5.5 ply, it will just look like a patchwork quilt.

If you could use OSB3 and screw it down crossways to the existing lines with extra long screws getting the board joins perfect and then rubber over the top.
What will cause it to look like a patchwork quilt? People attach EPDM directly onto 18mm ply (for example) all the time. What exactly is the problem with attaching the EPDM to 126mm rigid TD4000? It is true that the integrated surface is 5.5mm ply but you have 126mm of rigid board in total.
Using those boards, you will see all the joints, 6mm or 11mm will pull down to much when screwed.
Use 18mm osb3 T&G 2.4x 600mm, for a nice flat even finish for the rubber.
OK, thanks for that. In that case is there a cheaper way to achieve the same level of insulation rather than TD4000?
Ive found the ply on the back of the TD4000 is to flexible for EDPM, you will see all the joins/fixing holes etc. Is there a reason your opting for a warm roof? Does the roof need stripping off? Would insulation between and below the joists be better?
If you opt for the warm roof with the Celotex/Kingspan or any other insulation you will be raising the complete roof by 120mm or more, so all the edges will need to be raised.
Look at all you options.
If iam building a new roof then I opt for some insulation in between the joists with insulation above them 18mm ply on top. Its ready for almost any finish then.

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