Fitting a shower waste pipe in a concrete floor with wet UFH

[ruggers]

Not sure if this one is for the plumbing, bathrooms or the design section.

 

For my self build, I was planning on having a beam & block ground floor construction and having a small bathroom or shower room with toilet and sink. The architect has changed the bath to a shower and also now using a concrete slab floor. The make up from top down will now be. 50mm screed with wet UFH inside, membrane, 150mm insulation, 100mm concrete slab, membrane, sand blinding, stone.

 

With a P shaped bath I was planning to keep the waste pipe above the finished screed tight against the wall with some boxing in as it leaves through the wall to an outside drain. How if possible can I fit a shower trap into a screed floor along with the 40mm waste pipe and not clash with the UFH pipe which was planned to cross its path? Normally you'd bed the stone resin tray down onto a finished surface and ensure the trap fits correctly but the pipe would already be encased at this point with no play in it. 

 

I could possibly replan the ufh pipe route so it wasn't in the way but I'm not sure how to install a shower waste pipe and trap so that it's in a finished position before the floor screed. Once set if wrong it's a disaster.

 

Any advice from plumbers or designers would be appreciated.

[Temp]

This is what we did on a beam and block floor. Same would work on yours I think. Basically you run the waste pipe in the insulation. UFH in the screed. Tray can be recessed or above the screed.  

 

Use a "top access, top fitting" waste trap. These have a part that goes through the tray and screws into the part below the tray. Trap can be removed for cleaning from above through the drain hole. No need for access below. Example..

 

https://mcalpineplumbing.com/traps/shower-traps-accessories/st90cp10-hpc-2-90mm-hi-flow-shower-trap

 

I would use a 50mm/2" waste pipe not 40mm.

 

Procedure..

 

Lay concrete slab.

 

Construct timber support platform where tray will go at the right height. We used pressure treated frame with WBP on top. WBP has hole for trap in it. If the tray is being recessed into the screed you need shuttering as well. This can be temporarily screwd to the support frame.

 

Trap and pipe should be installed before adding the WPB top to the frame. Support the waste and pipe but don't fix it down. It's best if it can move a bit (including vertically so it can be pulled up to the hottom of the tray later). The frame can be partly filled with insulation if you like.

 

Temporarily position tray to check location of trap.

 

Test waste for leaks.

 

Lay insulation, ,membrane, UFH and screed. Remove any shuttering when set. 

 

To fit tray..

 

Mortar on WPB

Lower tray into place, check level

Fit sealer around waste, screw top of waste through tray into trap. 

Tile down to top of tray. Etc

 

If you recess the tray best leave the top about 5mm above the floor tiles. That way the door seal doesn't rub on the tiles.

 

If you need a drawing let me know. 

Edited March 24, 2022 by Temp

[Temp]

PS McAlpine make several traps of this type. They have various heights but the shallower ones have lower flow rates. I think one also has a shallower water seal so more prone to siphoning and evaporation. I think the one I linked to is the best one. 

[ruggers]

Thanks Temp, I've just used one of them traps for a mates bathroom, I installed a 40mm tray onto 6mm ply which was on top of 22 Weyrock. I cut out a circle into the weyrock to fit the trap but the only problem I found was the top of the nut securing the waste pipe to the trap was fouling the underside of the weyrock leaving a gap between the trap and the underside of the tray, so i needed to cut more out of the floor. It would be nice to find trap where the outlet sits lower down the trap body.

 

So with the method you describe above, you mention installing the pipe then the insulation, membranes and screed. Do you split the insulation either side of the pipe, lay some under and over or cut a channel out of the PIR that sits above the waste pipe?

 

I suppose I'll need to purchase the tray early on if installing pre screed, i was hoping to install stud walls after the screed for this room but might have to revise this.

[Temp]

 

13 hours ago, ruggers said:

Do you split the insulation either side of the pipe, lay some under and over or cut a channel out of the PIR that sits above the waste pipe?

Rather than cutting 150mm sheets.. it might be possible to fit layers of say 25 or 50mm above and below the pipe. Eg make up the 150mm depth  near the pipe from thinner sheets so they bridge over or under it. Lot depends on the heights and fall.

 

I normally use 12mm WBP which seems to clear the trap connector. The latest mcalpine is super shallow and has a solvent weld connector to reduce depth further (eg no nut). But only had 25mm water seal.

 

You don't want the outlet pipe too low as that frequently reduces the fall available for the waste pipe.

 

It might actually be easier to build the stud wall next to the tray rather than install tray afterwards. Some trays are v.heavy and easier if you can have one person each side.

Edited March 25, 2022 by Temp

[ruggers]

Thanks temp, all good advice and should be doable, a little more technical than id of liked at that stage in the build but workable.

 

Unlike a 110mm floor installed toilet soil pipe, you can generally adapt new toilets to always fit. Locking a shower waste into the floor screed, thats then fixed and any new shower tray replacement will require the exact same hole position in the tray. Im not sure they always match up between brands?

 

It shouldn't be a problem for a long time if looked after and using stone resin tray, i just dont like the idea of having to destruct a good floor at a later date if i had to change it.

 

Does a solvent weld hold up as many yrs as the screw fit?

 

Pipe run will be 2.2 to 2.5m.

[Onoff]

How about a Geberit wall drain?

[IanR]

2 hours ago, ruggers said:

Unlike a 110mm floor installed toilet soil pipe, you can generally adapt new toilets to always fit. Locking a shower waste into the floor screed, thats then fixed and any new shower tray replacement will require the exact same hole position in the tray. Im not sure they always match up between brands?

 

I went with 110mm pipe coming up under the slab, directly to where I wanted the shower drain, but then used a 110mm to 50mm offset connector that allowed me to rotate the connector to get a +/-50mm tolerance in all directions.

 

I didn't use a shower tray, just set a linear drain like this:

 

 

into the slab, sloped the slab itself and tanked it before tiling

 

 

I did all my showers the same.

Edited March 26, 2022 by IanR

[Temp]

10 hours ago, ruggers said:

Locking a shower waste into the floor screed, thats then fixed and any new shower tray replacement will require the exact same hole position in the tray. Im not sure they always match up between brands?

 

The drain can be moved a certain amount within the footprint of the tray. Perhaps just not right where the pipe enters.  I fitted a very shallow tray then had a change of mind and decided to fit a deeper one as we like high flow rate showers. The waste moved from the corner to the middle of one side. Just had to rip out the supporting frame and rebuild it to suit the new waste position. I can imagine it isn't always that easy.

 

 

[Nickfromwales]

I dig out a keyhole shaped trough in the concrete and terminate with a flexible hose if it's a real difficult install. If it's more straightforward I will set the tray into its final position with packers and work out the height from the base to the top of the trap. I'll then make all the pipework up ( in the same keyhole ) dry, eg not glued, and put everything into its final position, resting the tray back over to check all is 'good'. I then lift the tray off and mark 3 or 4 lines across each junction of pipe and fitting, numbering and marking which way it top / arrow to show direction etc and disassemble. I start gluing it all together using the marks for confirmation that it's all going back in exactly as it was mocked up at the dry fit.

Then I will chase a small channel out from the head of the keyhole to just outside the outer edge of the tray as a kind of funnel. If the pipework goes to an outside wall or disappears down through the slab, make sure you use a few hand fulls of dry tile adhesive powder to fill up all those gaps so the pipe is completely sealed. You'll realise why shortly.....

 

Then mix up a wedge of rapid set flexible tile adhesive, mixed so it doesn't slump. Use that to prep / clean the underside of the tray by putting some adhesive on a cotton dishcloth  so you can scour it and remove any contaminants from manufacture. Then you can set the tray down on big piers of adhesive, aiming for around 70-80% coverage is ample, staying away from the area immediately where the trap / waste will sit. Put some CT1 around the underside of the hole in the tray where the seal of the trap will make contact, not going mad with it.

Once the tray has been set in place, and whilst the tile adhesive is still wet, screw the waste into the trap sealing up as the underside of the waste so you have wet CT1 from under to over. The fittings will displace what it doesn't need, but you need to be sure the rubber seal under the tray doesn't skate off centre whilst lining the waste up with the trap and screwing it in. Once tight, clean all excess CT1 away with baby wipes ( lots of them ). Check the levels one last time, and leave the tray to set for a couple of hours. Then comes the magic bit  

 

Make a continuous line of sand about 6" deep 10mm higher than finished floor level like a dam in front of / around the tray, with the end of the funnel inside the dam eg so the sand will 'belly out' at that point. You then mix some self-levelling compound and pour it SLOWLY into the 'funnel' where it then flows towards the keyhole, thus filling it with SLC like filling a mould. Eventually the SLC will fill the trough / keyhole, will rise up to finished floor level, where you can then stop mixing and pouring. Job done. That process fully encapsulates the trap and pipework so you can stand on the trap whilst showering without any risk of it pushing down / through the thin part of the tray,plus this also fills in between all of the piers of tile adhesive that you used to set the tray on. You pour the SLC until it comes up and out of the funnel, and rises about 3-4mm above FFL. That way you know the underside of the tray is full of the SLC and zero voids. Tres bien.

[ruggers]

On 26/03/2022 at 11:54, IanR said:

 

I went with 110mm pipe coming up under the slab, directly to where I wanted the shower drain, but then used a 110mm to 50mm offset connector that allowed me to rotate the connector to get a +/-50mm tolerance in all directions.

Looks a really tidy job, did you have to add a U bend of some sort into this system or what was in place to prevent any smells coming back from the 110mm foul drain your connecting too since you opted not to use the shower trap onto the linear drain?

 

@Onoff thanks, I think the geberit looks another good idea but with it being a small bathroom i need to keep the drain within a tray rather than a wet room style.

[IanR]

42 minutes ago, ruggers said:

did you have to add a U bend of some sort into this system 

 

The linear drain has an integral bottle trap, so no U bend required.

 

[ruggers]

@NickfromwalesThanks for the detailed reply. Whats the flexible hose your referring to that's compatible with the drain components? I'm going to read over this a few more times to get my head around it, at least i know theres options for this, yours and @Temp are probably most suited to this room. Whatever I choose, I'll come back to update it at a later stage for anyone elses benefit.

 

My floor make up is now, 100mm slab, 150mm rigid sheet insulation, then 50mm liquid screed with 16mm pipes encased. I'm not overly fussed about 40mm shallow trays or the walk in look, but by placing a tray on top of the finished screed floor whatever depth, due to the traps I've seen, the crown of the pipe coming from the trap will be then sitting within the floor screed for the first 1m of fall If I position the tray closest to the exit wall to outside.

 

My options to avoid this seem to be,

1. turn the 1200x900mm tray 180 degrees so that the waste is a little further away from the exit wall and instal a timber support frame screeded up to and around the frame as Temp suggested, this will provide me 1m at 18mm/m fall so that when the pipe reaches the screeded floor area, the top of the pipe is then sitting under the screed within the insulation. downside to this would be, tray timber support having to rest on the concrete slab so no insulation underneath and cold bridging between screed and slab. 

 

2. Buy a deeper tray around 90mm and have it set down 50mm into the floor so only 40mm is showing above FFL, this would then lower the trap height resulting in the top of the waste pipe being below the underside of the finish screed and wouldn't interfere with my UFH layout. I'm just unsure how to have a solid insulated base underneath the shower tray because sitting the tray or timber frame direct onto sheet insulation wouldn't adhere or be stable. It might just have to be an area thats a weak point, infill a timber frame with some insulation.

 

 

[Temp]

This is a drawing I did for a similar previous question I think..

 

[Temp]

33 minutes ago, ruggers said:

My floor make up is now, 100mm slab, 150mm rigid sheet insulation, then 50mm liquid screed with 16mm pipes encased. I'm not overly fussed about 40mm shallow trays or the walk in look, but by placing a tray on top of the finished screed floor whatever depth, due to the traps I've seen, the crown of the pipe coming from the trap will be then sitting within the floor screed for the first 1m of fall If I position the tray closest to the exit wall to outside.

 

Sometimes life is easier if the waste has to cross the tray to get to the stack... The longer pipe will flex more easily to make minor corrections to the alignment with the tray and space for the pipe to dip below the screed. 

 

 

[ruggers]

@temp a picture speaks 1000 words as they say, exactly this thanks. The coloured image will be my floor make up, but the tray is in the back corner of the room not on the outside wall. So from the centre of the tray to the outside will be 2100mm. The nearest stack will be 2.2m further along the gable wall (Downstream) from where the pipe breaks out, so I'm thinking it might have to terminate into a gully which will tee into the main 110mm foul which runs down parallel to the gable wall. The ground floor toilet in the same room will exit via the floor slab with a sweeping bend into the mains too.

 

How do waste pipes normally connect to the mains if the stack is't near. My only concern now is referring to the coloured sketch you added which is how I'd have to do it to keep under the screed. With the top of the screed being level with DPC, the outside is 150mm 2 brick courses down to the ground path height. So deduct 50mm screed, then deduct 40mm for the 2.2m waste length at 18mm/m fall, then the diameter of the pipe 45mm = 15mm from pipe to path finished level. I'd have to make a hole in the DPC too. 

 

Can a 40/50mm waste tee into a 110mm vertical foul pipe without getting blocked with crap? I always thought basins/baths/showers had to join into SVP higher than where the toilet does.

[Nickfromwales]

44 minutes ago, ruggers said:

I'd have to make a hole in the DPC too. 

I've done this more times I can remember. As long as you seal up with a frame sealant, and do NOT create a damp bridge with cement etc, you'll be fine.

46 minutes ago, ruggers said:

Can a 40/50mm waste tee into a 110mm vertical foul pipe without getting blocked with crap? I always thought basins/baths/showers had to join into SVP higher than where the toilet does.

Yup. Take a look at a few of the houses around your area when driving about the place. They're commonplace. Horizontal needs a little thought, but vertical is pretty much safe as houses as long as it's not fitted opposite the fall, aka 'splodge' zone, of a discharging 110mm foul branch.

Where the 50mm waste pipe ( I never use 40mm on subfloor shower installs ) exits the building and changes direction, fit a 50mm T and put a cleaning eye / rodding access cap so you can clean back to the trap, and downstream to the foul pipe with ease.

51 minutes ago, ruggers said:

So from the centre of the tray to the outside will be 2100mm. The nearest stack will be 2.2m further along the gable wall (Downstream) from where the pipe breaks out,

Deffo 50mm for this instance

[Temp]

44 minutes ago, ruggers said:

So from the centre of the tray to the outside will be 2100mm. The nearest stack will be 2.2m further along the gable wal

 

Call it a run of about 4.3m. I think the pipe run can be entirely in the insulation..

 

If the 50mm pipe is just below the screed at the trap end then there is 100mm of fall available within the 150mm insulation. A 100mm fall over 4.3m works out at 23mm/m. That's within the 18-90mm recommended. 23mm/m is 1 in 43. What does @Nickfromwales think?

 

If that's OK your tray could sit on top of the screed or even be recessed into it by about 20mm depending on the trap used. 

 

If possible use large radius/swept bends where it goes around corners and support it regularly on to try and keep the fall uniform.

 

 

 

[Nickfromwales]

Yup. A 50mm pipe can run almost flat and still work perfectly due to the better air break internally, massively better than with a 40mm pipe. I recommend either swept bends ( vs elbows ) or 2x 45 degree bends to go around corners. 

[ruggers]

Thanks @Nickfromwales I'll definitely be using sweeping bends where i can, Is 50mm recommended so much just because it flows better than 40mm only? I've had no issues with 40mm on my current shower on a first floor but don't mind stepping it up. Do you use 50mm on the first floor also and 32  for the basin pipe work? 

 

I thought a 50mm pipe entering horizontally into a vertical soil stack at a lower point than the 110mm toilet enters could cause blockages on the 50mm waste entry point.

I might have to use a radon barrier as precaution so the cuts over pipes will need sleeved and taped if thats the case.

[Nickfromwales]

22 minutes ago, ruggers said:

Thanks @Nickfromwales I'll definitely be using sweeping bends where i can, Is 50mm recommended so much just because it flows better than 40mm only? I've had no issues with 40mm on my current shower on a first floor but don't mind stepping it up. Do you use 50mm on the first floor also and 32  for the basin pipe work? 

 

I thought a 50mm pipe entering horizontally into a vertical soil stack at a lower point than the 110mm toilet enters could cause blockages on the 50mm waste entry point.

I might have to use a radon barrier as precaution so the cuts over pipes will need sleeved and taped if thats the case.

I only use 50mm on showers, and run 40mm to basins as far ( close to the basin ) as is practicable. I only reduce down to 32mm pretty much at the basin. 
40mm works fine if you’ve got a decent fall, but the 50mm ( afaic ) is much better.

I like insurance btw because I post here about jobs I do for others ( as I run an M&E company ) so I tend to go belt and 3 pairs of braces, but I’ve always gone the extra mile tbh, the reason I’m always busy I suppose!!

[ruggers]

@Temp thanks. Can solvent weld pipe be used encased in the floor where you can't replace it? I was thinking if the pipe gets damaged where it comes out the wall or through UV damage over the years as they do, you can't add a new section to the part in the floor. Options would be to sleeve the pipe within another but the tray would still need lifted to join to the new trap.

 

I also found this image from a 2019 post that someone added on this forum which i thought was really good. Could have the tray FFL mounted and a smaller cut out of the screed under the tray to take the trap and connect to the 110/50 inlet. But not too large that it has no support under it. Maybe even infill part of the void with timber again once all connected up.

[ruggers]

It's always good to do things correctly, thats why often on here its good to hear other peoples opinions and how they go about it. Sometimes you can take all of the idea or even just parts of it. Theres often little tips or items you can buy that you didn't know existed if not in the trade. I've got a good idea now what to do so I'll take some photos when i get there and come back to these posts because someones always looking further down the line, so i appreciate the replies from all, cheers. 

[Nickfromwales]

11 hours ago, ruggers said:

Can solvent weld pipe be used encased in the floor where you can't replace it?

I wouldn't use anything else!!

11 hours ago, ruggers said:

I was thinking if the pipe gets damaged where it comes out the wall or through UV damage over the years as they do, you can't add a new section to the part in the floor. Options would be to sleeve the pipe within another but the tray would still need lifted to join to the new trap.

You would just excavate down enough to cut and joint the pipe. UV? That's only e real world concern if the pipes are outside, but then they are already produced to be outside in direct sunlight....al-a every house you drive past which has waste pipes / soil pipes on the external facades.

If there is a tray on top, zero damage or degradation can occur at the end of the day. You are, I'm afraid, worrying a little too much here. I still speak to people I have fitted bathrooms for, some over 17-18 years ago and some over 20 years ago ( before I had kids and my oldest is nearly 18 ) and they're still functioning perfectly well.

[Temp]

15 hours ago, ruggers said:

@Temp thanks. Can solvent weld pipe be used encased in the floor where you can't replace it? I was thinking if the pipe gets damaged where it comes out the wall or through UV damage over the years as they do, you can't add a new section to the part in the floor. Options would be to sleeve the pipe within another but the tray would still need lifted to join to the new trap.

 

I also found this image from a 2019 post that someone added on this forum which i thought was really good. Could have the tray FFL mounted and a smaller cut out of the screed under the tray to take the trap and connect to the 110/50 inlet. But not too large that it has no support under it. Maybe even infill part of the void with timber again once all connected up.

 

Yes to solvent weld. I find using plenty of adhesive gives you a few extra seconds to ensure everything is aligned. I also put depth and alignment marks on the pipe with a sharpie during the dry run.

 

You should switch to uv stable pipe where it's exposed or perhaps sleeve it between wall and stack. Our stacks are all internal so not an issue for us.

 

You can certainly run pipe in screed but a 50mm pipe in 75mm screed might be prone to causing the screed to crack. If the trap can't move you also need to be much more accurate with positioning it vertically and horizontally. If it can move slightly it can be pulled up to the underside of they tray if you are a few mm out.

 

I think putting the trap and pipe in screed is more appropriate for a tiled tray or wet room where you can screed to the level of the trap.

Edited April 2, 2022 by Temp