Refurbishing and insulating a flat roof

17 Aug 2009
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United Kingdom
I am looking at refurbishing my L shaped flat roof which has been patched over for years and now needs totally redone. To keep costs down I'm considering replacing the deck, along with a joiner friend, and then doing a Diy-Epdm job. The whole roof is about 150sqm. I'd also like to take the opportunity to insulate the roof but the more I read the more confused I become, so I've a few questions! First off, should I remove the old felt, remove any warped, broken board (I think it's chipboard), and use that as a base to build up my new warm deck, or should I remove it completely and lay the insulation boards onto the joists? Secondly, I was originally looking at using Celotex TD4000 and laying the epdm straight onto the top but their website appears to suggest that you then need to lay 12mm ply on top of that. It's making the whole project mega expensive! If that is the case what is the advantage of using TD4000? Would I be as well using the existing deck, laying down a vapour barrier, adding insulation boards and either laying the epdm direct onto that or adding ply then the epdm? As I say, I'm rather confused and any help would be appreciated. I'm sure I'll then have further questions!
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I think you need a deck to support the insulation boards- OSB is probably what is used most nowadays. But insulation boards are not usually designed to have mechanical strength.
OSB at about £15 a sheet isn;t all that expensive compared to the EPDM and insulation board
Whether the existing chipboard is OK is of course only knowable when - and if- you strip the felt.

How you want to construct your roof is up to you: there are plenty of options.
There is a good overall description of principles here:
Thanks for your reply. It's probably the cost of the combined components that's hiking the cost, ie osb, insulation, another board on top (if that's required) then the epdm membrane. Can you place the epdm straight onto the insulation boards?
I've had a look on the Celotex website and it appears that you can place TD4000 straight onto the joists (it has a 5.5mm WBP ply bonded to it)although it has to be overlayed with 12mm ply before the EDPM can be put down. I suppose my next question is should I completely rip up my existing roof and start again with TD4000 or would it be acceptable to lay the TD4000 straight onto the existing roof (with or without the felt removed)? Would that cause any issues with regards to the vapour barrier? I would rather not lift the whole roof due to time, and exposing the roof to the elements, but I want to do it properly. Any thoughts?
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Typically, in residential work, the whole flat roof is stripped down to bare joists.

Carefully removed sheets of ply or OSB can usually be re-used - thats if no rot or damage is present. Anything with fungal damage, then ditch it.

Opening up the roof gives you opportunity to do a variety of updates in the roof space without disturbing the finished ceiling.

An L - shaped footprint definitely needs stripping or you will possibly end up with no falls, wrong falls or ponding.

WBP ply is not required. You can make up your own inexpensive design and list of materials without using Celotex composites etc.

Remember that you have flashings and edgings and possibly Fascia and rainwater goods to tie into the total design.
And access.
And debris removal.

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