Niraj,
I had exactly the same problem on a previous house. The solution was an inverted or upside down roof. You use extruded polystyrene boards that will not pick up moisture laid directly on top of the existing roofing membrane Before people start shouting about the loading yes typically the insulation needs to be ballasted. That's fine if you're sure the roof can take that load. I wasn't and so went for a loose laid lightweight inverted roof system. My system was based on Styrofoam from Dow and was called Roofmate LG (stands for LiteGuard in the US) but available in the UK. The insulation boards have a specially designed T&G profile to all 4 sides so that they interlock, on top of each board is a 10mm lightweight modified cementitious screed that allows light trafficking and protects the XPS from UV. To prevent against wind uplift the requirement is for concrete paving slabs to be laid around the perimeter of the roof's open sides on top of the insulation boards and that's it. I laid a few in a hit and miss pattern. The inverted roof system made a tremendous difference to the comfort levels in the kitchen below. As an added bonus it shields the roof membrane from UV and possible degradation and prevents the contraction and expansion of the membrane due to thermal hot and cold cycles. Coolag (now owned by Celotex I believe) also made a similar product manufactured in Heysham?

I have always thought that inverted roofs are an excellent idea for the reasons that Saint gives. I have specified it commercially in conjunction with a single membrane roof. (Sarnafil) detail as their manual.
Be aware, though, in a downpour the water can take longer to percolate through loose lay gravel and could, in extreme circumstances, overwhelm weak points in the flashing, or lead to damp...worth checking state of existing. Sarna recommend the use of pads that lift the extruded poly above the exising mambrane.
Apart from the saving of energy, the styro will not be that green I suspect...! Would a planted roof be a better option?

Hi Peter,
The roof was on an single storey extension built into the L shape of the existing house so one complete side and an end butted up against the two storey house. There was a minor allowance for ventilation, I recall the fascia to the two open sides standing off about 10mm but I guess due to the configuration of the roof there was little or no through draft. The extension was also on the north side of the existing house and shielded pretty well from any direct sun so it was cold for most of the year, until of course i did the inverted roof. I was lucky enough to get hold of some 100mm product so the difference before and after was enormous.

Caliwag, yes its important to avoid ponding or slowing the runoff, there were small rebates around the underside edge of the foam that allowed drainage if the water percolated around the T&G joints but when raining hard most of the water ran off across the top surface.
One point about raising the XPS on pads is that by encouraging run off beneath the boards that will also "drain" off some of the heat and reduce the insulation factor. Dow quote a 20% reduction for USD rooofing. A planted green roof would be great but in retrofit there's a concern about the loading especially during wet weather when water retention levels in the soil could be high

helo saint,

Can you send me some links for example sites ? Unfortunately, i don't have any understanding of building work and i know you have explained it something very technical (from my point of view) but due to lack of my building work knowledge i am not able to visualise it.

Please help and send me few internet links.

Thanking You

Kind Regards

Niraj