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!