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So I'm finally starting on the ground floor buildup. It's 50mm poured screed on 150mm insulation (100mm PIR/50mm graphite EPS). I'm unsure of how the tray works in this situation...

 

The tray is (IIRC) about 45mm thick, so I'll need to set it on ply/OSB/cementboard to get the levels right. That bit's OK. But how does said underlayer sit on the PIR, is it just bonded down or should it extend under the screed edge too, to "key" it in? It's in a corner so two walls and two floor edges.

 

ta

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Are you looking to get the tray dead level with the finished floor ..? What tiles / floor finish are you using and how big is the tray ..?? With stone cast trays you normally bed them on a mortar bed and this would be the same. 3:1 sand cement with SBR added I think is what Uncle @Nickfromwales recommended previously ..? That would bond rock solid to everything around it and the bottom of the tray. 

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@PeterW  1400x900 trays. No I don't want flush with FFL. I'm aiming for 10mm or so above, but final finish isn't decided yet. Guessing 15mm will cover the final finish, which is likely to be tiles.

So (say) 18mm plus a thick mortar bed or 25mm and a thin bed of tile adhesive will get me to the height I want

 

I was thinking/ wondering more about how much movement there might be in the PIR rather than how the tray would be supported as such.

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I left a rectangular hole in my screed, used 3*2 to form the opening, to allow my finished floor to go down. Once I had my tiles picked out I could work out my final height and used a strong dry mix of sand and cement to fill the hole up to the height it needed to be. My tray was also the same size and it's heavy so it's not the sort of thing you would move when you stub your toe on it. The weight of the surrounding slab keeps the insulation from moving plus with the gap round the outside of the tray filled in once you where happy with how it set it can't move so was solid.

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+1

How did you do the waste? On the few I've done its easier if the waste can move around a bit so it can be pulled to the underside of the tray when you screw the top part into it. So I left a channel for the pipe and trap to move around in rather than encasing it in the sand and cement. I just put a small block of wood under the trap so it wouldn't drop down so far the threaded top part couldn't reach.

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There was a 100 waste pipe already set in the sub floor so I used the correct cap and bend to get it roughly where it needed to be. I had a channel cut out of the floor insulation which allowed me some wiggle room for moving the pipe and waste trap. Dry fitted the tray to make sure the trap was where it needed to be and then covered the waste pipe in sand then the dry sand cement mix. 

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The most recent one I did was the following;

Cut out DPM, cut out insulation, foam insulation to block and beam, weigh down until cured.

Then apply foam to top of upper insulation and spread DPM back on top until all in contact and keep doing so whilst foam cures so the DPM is bonded to the insulation. You could make a plywood baffle and weigh down if easier. 
 

( Note, obvious waste connections done in parallel to these processes ). 

Then reinstate DPM and tape it up to death and back. 
 

Tray then simply laid onto plywood pattresses which were foam bonded to the DPM and then flexible tile adhesive onto those pattresses and tray onto that ( cleaning and preparation of the underside of the tray carried out as required ). 
 

Tray set into place and tile adhesive left for an hour or two to cure. 
 

Cardboard funnel then made and self levelling compound flooded into the remaining void under the tray ( nearly 2 bags in this instance ) to a) fully fill the void and leave the tray ‘locked in’ at all 4 corners and all 4 sides, and b) to mechanically support the trap ( which is best practice imho ). 
 

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Other option could be to use Wedi insulated panels, which you can build up in layers of differing thicknesses, to achieve the same result. DPM will remain intact as the Wedi system is watertight. Just foam around edges then, instead of pouring SLC as the SLC would bridge damp. 

Edited by Nickfromwales

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Thanks all. Damp isn't ever going to be a problem anyway as the whole subfloor is poured on a Radon barrier and they're internal walls.

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