Sandwich flat roof

[Mark Hindley]

Hi,

I am in the process of recovering an extension flat roof, about 4m square. Waterproof covering is to be EPDM membrane, but I have fitted

100mm Kingspan TR27 over the existing felt as (I thought) the roof was completely uninsulated.

When fitting the insulation, I noticed that it was relatively easy to dent the surface with a point load, even by just kneeling on them. As there is work to be done next spring on a dormer in the adjoining pitched roof, I was wondering if it was sensible to sheath the insulation boards with a thin layer of ply on top, screwed through the insulation to the deck, to provide them with a bit of protection?

I realise that the additional fixings for this would increase the overall U value a bit, but as I discovered there was already a 20mm layer of PUR board underneath the felt when I began fixing the new insulation, that gives me a total of 120mm PUR insulation overall which isn't too bad.

Questions that I would like opinions on, please:

Is this worthwhile/a good idea?

Would 4mm exterior ply be sufficient?

Any problems people can see?

Thanks for your help

Mark

[Lobster]

IIRC Kingspan sell a product comprising sheets of their insulation bonded to sheets of 4mm ply, just for this purpose, so i would have thought it should be fine

David

[fred]

In article , Mark Hindley writes

Overall it sounds like a nice solution although I might have gone for something tougher for the waterproofing but I'm not exactly sure what.

On the 4mm question, I suppose it depends how flat the flat roof really is, I assume there is a small fall to shed water. If you use 4mm then you risk pulling dimples into the surface at the fixing points, when you tighten the screws, and water may end up pooling there. If you use 12mm then I think you would avoid that.

[meow2222]

could you use 4mm and glue it? never tried it

NT

[fred]

I thought 12mm would be more resilient over all, particularly with the prospect of ongoing works, steps bearing etc, just too much capacity for doh! factor with the thin stuff.