Issues with concrete flat roof

I have a concrete flat roof over the downstairs loo. Its been letting in water and there are several cracks in the concrete which I can see having removed the cladding from the loo ceiling. Water's obviously getting under the felt outside, the source of which I am still searching for. However, I'd already decided that I hate the look of the roof, so was planning to construct a pitched slated roof canopy to shroud it, hence killing two issues at once.

The loo has always been very chilly, and I guess the concrete is acting as a large cold bridge to the outside world. The issue I have is that the ceiling (the bare concrete) in the loo is quite low, so adding insulation within would be an issue. So, could I rip off the felt on the outside, insulate from above then build my canopy over the whole lot? If so, what's the best material to use? Do I need to rip off the felt (not that that's a great issue)?

Pics attached for reference but not adding much.

Yes technically your plan will work. What to use- solid insulation (Celotex/Kingspan stuff) since the void will be inaccessible once you've roofed it.
You may need planning permission for the new pitched roof though

The problem you will have with insulating above the concrete is that it will be a waste of time. Being a toilet, unheated or randomly, the concrete's thermal mass and thermal bridge will mean that it is always relatively cold and no heat will be getting into it let alone through it to make the room below warmer via an insulated top surface.

This is a situation where insulation on the underside would be beneficial, but that may not be practical. But still, even a 15 or 25mm layer of foil backed PIR would improve things if that will fit in.

Thanks guys. As I'm turning the room into a shower room while I'm at it, I'll see if I can insulate inside walls and ceiling, then. We've been getting condensation on the tiled inner walls already, so I'm guessing the walls are pretty cold, too.

^woody^ said:

The problem you will have with insulating above the concrete is that it will be a waste of time. Being a toilet, unheated or randomly, the concrete's thermal mass and thermal bridge will mean that it is always relatively cold and no heat will be getting into it let alone through it to make the room below warmer via an insulated top surface.

This is a situation where insulation on the underside would be beneficial, but that may not be practical. But still, even a 15 or 25mm layer of foil backed PIR would improve things if that will fit in.

Click to expand...

Should I foam the boards to the ceiling, or dot n dab? or is mechanical the only way? I'm not sure I want to be doing a lot of drilling into the 70 year old flat roof.

You should not be dot and dabing to ceilings.

If you've got the headroom you might be better putting a false ceiling in (battens fixed to the walls, struts across etc etc). You could then stuff the void with fibreglass & board underneath.