Screed and shower trays

[Benpointer]

Question for the BH Brains Trust:

 

For our 170m2 single storey new build we will be putting in a floor of B&B + 200m PIR + 75mm liquid screed over UFH.

 

We plan to have a couple of shower trays, shown dimensioned in the diagram below.  Obviously we could put trays on top of the screed, and maybe we will, but if we wanted to inset the traays at floor level, do we just box off the floor in that area and not screed it, then set the tray on the VCL over the PIR?  Presumably on marine ply or some water resistant strutural boards rather than straight on the PIR?  (Obviously we will need to arrange for a drain to emerge through the B&B and the PIR in that area, and no UFH where the tray will be obvs.)

 

 

Any suggestions gratefully received.

[JamesP]

Not a plumber, however I boxed off the areas where the trays would sit before the screed was laid.

Fitted the waste in position before screed, I recommend 50mm waste pipe for runs over 2 metres.

Then added a concrete pad of correct depth above PIR to allow for flush fitting of tray to final floor level.

Does depend on depth of tray, mine was 40mm but will need to sit on a pad to adhere to.

 

Edited March 20 by JamesP

[nod]

1 hour ago, JamesP said:

Not a plumber, however I boxed off the areas where the trays would sit before the screed was laid.

Fitted the waste in position before screed, I recommend 50mm waste pipe for runs over 2 metres.

Then added a concrete pad of correct depth above PIR to allow for flush fitting of tray to final floor level.

Does depend on depth of tray, mine was 40mm but will need to sit on a pad to adhere to.

 

Plus 1

Put your tray in afterwards and make up with a semi dry mix

[Bancroft]

2 hours ago, Benpointer said:

Question for the BH Brains Trust

Thanks for asking the question - I've been thinking about this exact issue for a few days now!

 

What a great site this is for building on people's experience.

[Russell griffiths]

I boxed off the areas, but didn’t use a tray, screeded the floor to the drain with the correct falls then tiled. 
you can screed lower then use one of the formers that you tile on top off also. 

[Alan Ambrose]

Did anyone have trouble getting the waste pipe to the foul pipe with the right fall and staying within the screed?

[Russell griffiths]

5 hours ago, Alan Ambrose said:

Did anyone have trouble getting the waste pipe to the foul pipe with the right fall and staying within the screed?

Pipes travel through insulation layer as well, not just screed. 

[Alan Ambrose]

Yeah, but if you have the insulation under the slab?

[Russell griffiths]

On 24/03/2025 at 08:07, Alan Ambrose said:

Yeah, but if you have the insulation under the slab?

Then your pipes should already be in, apart from the last little bit. 
You’re not going to have the depth to run a 40mm pipe in a 50mm screed or even an 80mm screed, with the fall.

[Benpointer]

On 24/03/2025 at 08:18, Russell griffiths said:

Then your pipes should already be in, apart from the last little bit. 
yor not going to have the depth to run a 40mm pipe in a 50mm screed or even an 80mm screed, with the fall. 

Now you've got me thinking.  How am I going to get the 3 WC soil pipes, 1 shower wast and 1 bath waste to the drain outside... on a single storey floor: B&B, 200mm PIR, 75mm screed?

 

Here's a diagram of the drain positions with some red lines lines showing how they might connect to the drain outside but each of those soil pipes and waste water drains have to go straight down through the insulation and the B&B (through a block presumably) and all the pipework is in the ventilated airspace under the B&B - where it is inaccessible forever, so I'd better hope there's never an issue with it. 😱

 

Have I got this right?  Is there a better way to do it?

 

 

 

[Russell griffiths]

All your 110 pipe will go in before you fit the block n beam, all bedded in a continuous bed of weak concrete mix, rest bends at the rends of the pipes bedded securely in concrete, these go up wards and pick up your smaller pipes. 
look at the regs for maximum distance a 40-50mm pipe can travel, upsize to 50mm if you can it flows so much better. 
the 110 will live under the floor very happily for the rest of your life. 

[Nickfromwales]

Showers will go to each of the 2 110’s for those back to back WCs, and the other WC will take the bath.

 

Basins T out of the 50mm runs to showers / bath or into 110mm boss’s, but should always rise out of the slab in 40mm, not 32mm, and I only reduce from 40 to 32mm at the elbow immediately prior to the basin trap. 

[Nickfromwales]

[Alan Ambrose]

@Nickfromwales - can I ask - what if the shower trays are level with the floor finish and you're on screed +  concrete slab with insulation under? If you can't get enough fall in the screed, do you chase out a bit from the the top of the slab?

[Nickfromwales]

On 25/03/2025 at 15:00, Alan Ambrose said:

@Nickfromwales - can I ask - what if the shower trays are level with the floor finish and you're on screed +  concrete slab with insulation under? If you can't get enough fall in the screed, do you chase out a bit from the the top of the slab?

Yup. 🥵. You work out the minimum amount you need to remove and no more. 
 

As it’s just soapy water you need very little fall. 

[Nickfromwales]

Hiring a diamond wall chaser (that an electrician would use) may be ok, but I’m not sure of the depth of cut they offer. 
 

That would defo remove a lot of the hard work and be worth the hire cost, but you may then have to chip out a bit at the deepest point. 
 

All could go to the nearest WC but it is what it is. If I plan these at the pre construction stage for clients projects I have a 110mm at each shower footprint. 20/20 hindsight is required obvs. 

[Alan Ambrose]

>>> have a 110mm at each shower footprint

 

Ooooo - underneath the shower tray?

[Nickfromwales]

1 hour ago, Alan Ambrose said:

>>> have a 110mm at each shower footprint

 

Ooooo - underneath the shower tray?

Yes, but offset to allow the trap and connectors / flexi, so about 400mm away from the ‘hole’ in the tray.

Impey wastes can drop into a 110mm fitting, very nice, but needs meticulous planning.

 

 

 

 

 

 

 

 

You opt for the “vertical waste outlet” and that will come ready to go into the 110mm fitting.
 

The white goop is CT1 but is not for the seal to 110mm, just the over goop from installing the waste / trap outlet.

 

Under the horizontal 110mm pipe I put some rockwool and left a gap of around 150mm. I then squirted a load of foam to encapsulate and support the 110mm pipe so it couldn’t ever drop down / away from the waste connection under the tray. 👌

[Alan Ambrose]

Wowee, nice