Hi folks, a relatively minor one here, but I haven't been able to satisfy myself with manuf. detailing or opinions on this and other forums:

Does it matter if a warm flat roof construction has a second layer of decking above (on the cold side) of the insulation?

Here's my proposed warm flat roof construction:

Outside
EPDM membrane
150mm tissue-faced PIR (glued with Instastik or similar)
VCL
18mm OSB deck
Joists
Inside

-

Question:

The roofer has said that his standard detail is actually to use foil faced PIR and mechanical fixings, with a second layer of OSB on top of the PIR because the tissue faced stuff inevitably gets damaged during install.

I don't really like this idea because:

(a) it's more OSB;
(b) mechanical fixings introduce mini thermal bridges; and
(c) I've read that that second deck is more susceptible to rot because it's on the cold side of the insulation - but have no idea whether there's anything in this...?

What do folk here think?

For clarity, here's his proposed construction:

Outside
EPDM membrane
11mm OSB deck
150mm foil-faced PIR, mechanically fixed through to Joists
VCL
18mm OSB deck
Joists
Inside


Thanks

Posted By: Nick ParsonsI am someone for whom glues rarely do what they say on the tin, but I certainly would not do a warm roof w/out mech fixings, and if using them effectively the top deck is the 'washers'.

Useful, thanks Nick - is 11mm enough on top to act as a washer layer then do you think? Any particular mech fixings that you would suggest.

Could use mechanical fixings without a top deck presumably? To double deck o not, that is the remaining question :)

We had some EDPM flat roof done a few years ago, the construction was the same as you proposed, with only one layer of osb, apparently to follow the EDPM manufacturer's instructions and so be eligible for their 30 year guarantee. There was some story about how if osb were to be fitted directly underneath the EDPM, it would warp on hot sunny days and tear the membrane, so the membrane was instead laid onto the insulation, which protected the osb from hot sunshine. It was tough enough to walk on, but you could feel it 'give' a little underfoot.

But since then I've seen many details online with two layers of osb, so I don't know who to believe. So long as it is guaranteed.

The insulation was screwed down with plastic tubular washer fittings to avoid bridging the insulation.

That extra layer of osb on top, in the cold zone, helps stiffen things up so you can walk on the finished result. It also helps flatten, as it’s tg4. But I hate the idea of wood there ! Is there anything better? Has anybody tried EWI basecoat and mesh there - that would give a flat result, but it’s waterproof, just in case ! ? Daft idea ?

The BBA for one system of EPDM says (https://www.rubber4roofs.co.uk/mwdownloads/download/link/id/26 ):

14.8 When used as a fully adhered system, the resistance to wind uplift will be limited by the cohesive strength of the
insulation and the method of attachment. These factors should be taken into account when selecting the insulation
material. Polyurethane insulation boards must be mechanically fixed, bonded in a polyurethane adhesive or bitumen
bonded to prevent bowing.

14.9 The fully adhered application may not be used directly onto insulation materials (eg polystyrene) which would be adversely affected by the solvent in the adhesive, or onto bituminous materials, without consulting the manufacturer.

14.10 A layer of bonding adhesive is applied to both the substrate and the membrane by means of a roller at an
application rate of 0.75 litres per m 2 . When the adhesive has reached its tacky point, the membrane is applied to the substrate and rolled to ensure a full bond and that no air has been trapped beneath the membrane.

It also mentions that if the insulation is rigid batts of mineral wool, a 13 mm fibreboard must be used between the EPDM and the insulation.