Pavers on a flat roof

5 Jul 2007
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United Kingdom
Hi All,

I'm going to be laying paving slabs on a support system (ie Caro / Wallbarn) on top of a flat roof.

The roof is strong enough to support the weight (steel beams and concrete slabs). The existing flat roof is made up of 50mm hard foam insulation and 2 layers of EPDM.

We intend to lay ontop of the EPDM but want to ply it out first to spread the load. Given that the roof is already watertight with the EPDM, can we put a more cost effective membrane over the ply to protect it from wood rot etc? It's a fairly large area 121m2 so an EPDM skin is going to be quite costly when all we're trying to achieve is protecting the ply.

Any thoughts appreciated!
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Have you contacted the manufacturer of the insulation to see if the loadings would be OK without the plywood spreaders? The plywood should have gone down before the membrane, hindsight eh? I can't think of a half measure, which is what you're asking. You can't waterproof the ply without an additional membrane. I would just accept that you might be replacing some ply spreaders in a decade or two.
Thanks for the reply.

The existing flat roof has been down for 10 years.

It's concrete slabs, then insulation, then 2 EPDM skins.

There's currently decking ontop of the EPDM which is causing all sorts of problems (mainly the joists acting as a dam for drainage water).
Do you happen to recall what type of insulation it was? Was it polystyrene? Or was a yellow colour? Personally I don't think you'd have a problem without spreaders.
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So are you saying that you have bonded the rubber direct to the insulation.
No I haven't done anything yet.

The insulation / EPDM is already down, it's been down for 10 years.

The EPDM skins are loose by the looks of it and not bonded to each other or the insulation.

I'm trying to ascertain a cost effective way of protecting the ply I intend to spread the load of the paving supports, if ply is needed at all!
No, as I said in the OP, it's concrete slabs.
I think he's worried about squashing the insulation, I think it will be fine though. Paver supports aren't exactly tiny.

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