Ground floor shower tray support over insulation

[Dunc]

I will be installing a shower tray on the ground floor. Build up is:

150mm slab

220mm PIR (don't ask - it was supposed to be 300mm EPS but various architectural and construction cockups and this is where we are)

Slip membrane

50mm screed.

I'll shutter off the entire shower tray area before the screeding is done (because SWMBO hasn't finalised a desicion on which shower tray so I don't know where the waste will go).

 

Is it reasonable to just lay the tray on a 50mm bed of mortar straight on the membrane & PIR?

Could I get away with a thinner mortar bed (if so how thin?) to drop the tray closer to tile level?

 

[Oz07]

Be interested to see the answer here. Id be worried about the shower tray rocking on the insulation. Like if you put all your weight on one corner surely it would dip?

Edited yesterday at 18:28 by Oz07

[Dunc]

That was my concern. I initially thought to construct a wooden frame topped with 22mm ply to support the tray from the slab and then stuff insulation underneath.

However, the rest of the ground floor is basically 50mm screed (i.e. a mortar-like substance) on the same insulation. So the question could equally be asked: What happens when you sit on a chair? Does the floor dip under the chair legs? Thus the lazy side of me got to thinking could I save a bit of woodwork...

It's rather an expensive experiment to give it a go so I thought it best to ask: I'm sure those with KNOWLEDGE (@Nickfromwales) will be along to put me right.

 

[Oz07]

No because the screed would spread the weight of the point load. The only way I could see something like this working is ommitting the insulation below the tray and say 300mm all round, then insulating to the same thickness less 25mm, then replacing this section of insulation with ply. It sounds messy and likely to cause problems, maybe it could work? I am sure others will be on with more tried and tested methods 

[Nickfromwales]

13 hours ago, Dunc said:

@Nickfromwales) will be along to put me right

Nothing to worry about here, other than you do NOT use mortar.

 

Flexible tile adhesive or builders screed (Mapei self lever with fibres), or for a full depth install a mixture of both.

 

On my current clients project we’re doing both trays exactly as you propose. One over 250mm of EPS and the other over 175mm of foil faced PIR ( depths differ here with old + new substructure levels, as it’s a conversion at kind of EnerPHit level). Got an airtightness score of high 0.1’s, so happy to declare a definite flat score of 0.20 ACH for bragging rights. 😎👌. 
 

Firstly, 220mm of PIR is very good, no need whatsoever to get that to 300mm ; PH is achieved with 300mm of EPS don’t forget!! Diminishing returns say not to bother much past 200mm, which you’ve gone over.

 

Anyhoo…

 

Trays don’t ‘rock’ unless you’ve got that section of insulation cut away and are infilling that also, but if there’s full sheet material locked in 4 sides around then it’s going nowhere.

 

My ‘trick’ is to use 150mm concrete screws to set into the main screed as catchers / rebar pins, so the infill of SLC / tile adhesive can bind to those. Zoom in and you’ll see these pushed half way into the EPS blocks I made as shuttering. You need to have these low enough not to interfere with the tray when set into the void.

 

The blue expansion skirting wasn’t necessary around this, but the 3rd party UFH company did it anyways….   
 

Down at the end is a buried 50mm waste pipe for me to jump on later, which all went in as 1st fix with hot / cold / waste plumbing all got installed / hidden below the insulation. I’ve set the waste pipe low enough to allow for the tray to be slightly recessed eg lower than the surrounding screed to reduce the step / lip left after tiling, however the clients have chosen very low profile pre-finished trays so probably only dropping those by 5mm or so to leave 10mm of shower tray showing above the tiled floor. 

All hot and cold feeds went in on a radial setup. 

 

Another 100mm of PIR got put down there after I sorted out DPM etc around the pita pipes / other obstacles in the plant cupboard. 
 

If you’ve still got an opportunity to get the 1st fix in the ground floor like this then it’s something to consider, as it makes 1st fix much easier imho, but more relevant here as it is a bungalow with vaulted ceilings / no attic void for services.

 

Shuttering worked perfectly for the screed pour (liquid cement)

 

 

Shuttering now removed and all walls gone up. Getting boarded atm ready for my very good plasterer mate to come and do his thing.

 

I can post some pics of the trays going in but prob a week or more away from that yet. @Dunc when are you doing all this?

[Dunc]

@Nickfromwales huge thanks for the detailed decription!

Screed in the next couple of weeks but shower tray not for months; just want to make sure I don't do something now that I regret later.

Rest of first fix is coming down from above so no problems there, ta.

 

Are the concrete screws those black ones horizontally through the blue expansion skirt?

 

In my case the shower will be going in against 2 walls and I'll need to build a third stud wall to separate the soil stack (green line). I was going to shutter off along the edge of the shower tray +50mm. (Only 1 of the 2 layers of PIR is down on the floor in this photo. Next set to go in with staggered joints).

So you suggest that this would be fine with just the PIR and self leveling compound topped with flexible tile adhesive - what relative depths? I have 50mm to make up.

 

Given we haven't decided on a tray yet, I am wondering whether leaving out the PIR in this area would be wiser to allow better access for installing the waste, rather than digging it out later. If so, are you suggesting that laying the tray on the added-back-in PIR plus tile adhesive is NOT appropriate in this case?

 

 

The other "exciting" challenge is that will I have to connnect the 50mm shower waste to the soil stack which is currently just that stub in the top right hand corner. A female/female straight coupler and a boss adapter (such as https://www.screwfix.com/p/floplast-push-fit-3-boss-single-socket-pipe-black-110mm/97309) puts the top of the 50mm boss at 190mm above the bottom of the coupler. With my 220mm PIR + 50mm screed the bottom of my shower tray is 270mm above the slab so I've got 80mm to play with. Even with a shallow waste, say the Macalpine 53mm (https://mcalpineplumbing.com/traps/shower-traps-accessories/90mm-shallow-shower-trap-53mm-high/) and assuming a drain in the long-side of the 1700mm tray (i.e. 1m from the stack) I'd need about 18mm fall so I'd be cutting the stub off with the coupler stopping only 20mm above slab level. Feels tight!  Solvent weld boss adaptor may be better here, assuming solvent weld is OK on the brown underground type pipe? But obviously comes with its own risks - one shot to get it right. Any other suggestions?

 

 

 

 

[Nickfromwales]

All sounding good atm, just not the take out / put back in of the insulation part. 
 

This should be continuous where it appears out from under the screed into the shuttered area.

 

When installing the layer of insulation where the trays going, use a load of exp foam and put some weight (sand / blocks) on top to keep it all bonded whilst it cures (30 mins or so).

 

I use the solvent weld boss on orange (underground) pipe quite often, just clean it and rough it up with some sandpaper.

 

I’ll try to remember to add some pics tomorrow.