the concrete floor slab in the planned extension is yet to be done. My question is, is it possible to do wet room not using any tray (only apropriate tanking) by fitting waste and drainage before they lay concrete?

Yes.. as long as they get the correct falls to the drain.. but that needs setting at the correct height before hand..

I understand the need for correct falls. i imagine i would have to do it myself to get it right, they won't be bothered while laying concrete. but my question is: is it ok to embed the drainage into concrete and whether it does have any structural impact on the concrete base?

No probs,, are you DPM;g before or after installation.. as in water proofing ..

You get them to concrete up to a blocked off section at the tray area then screed in the tray to create falls..

Similar to this video..

Thanks. Nearly covered my concerns in full legth. As i understand the usual way is to put rigid insulation on hardcore, then DPM and then lay concrete slab. So are you saying it is ok to sort of seperate this concrete slab into two sections? And after they did theirs I can do mine. Because i don't think there's gona be additional layer of screed on it. I don't want to compromise structural integrity of floor slab. On this youtube video the falls are created in the screed layer on top of the slab so the video doesn't cover my problem.
And not sure about DPM before or after instalation :P but assuming you mean the membrane. I want to use schluter ditra/kerdi combination with joint strips with special 2 part adhesive. And i wanted to put it on top of a concrete profiled by myself instead of using aquadeck for example. The purpose of this wet room is for disabled person to use the shower hence the need for no steps.

The video was just to show the forming of the falls... you can do this without having a retaining curb..

Personally i think it is better to tank on top as well as DPM to stop mould in the screed etc.. The issue of a break between screeds at the tray isn't a prob.. cover this with the Ditra etc..

Could you not discuss this with your builders

What's dpm??

Originally Posted by dilz000

What's dpm??

damp proof membrane

as Dave says or just recess floor were shower is to go (during concreting) then render falls in when ready defo put waterproofing on top

Originally Posted by whitebeam

Could you not discuss this with your builders

Thanks guys. The whole work is put on hold due to snow and low temp. So i have time to investigate. The builder will use DPM on the whole area and I will put Ditra on the whole floor as well. Creating falls shouldn't give me many problems. My main concern was whether I can seperate the slab. I know it may seem funny but I got construction book and there's concrete slab and concrete screen shown as seperate layers. As far as I know the builder will only lay slab on top of the DPM layed on polystyrene on top of ground(hardcore). I know this slab should work as one piece and this is my only concern.

Originally Posted by Dave

The video was just to show the forming of the falls... you can do this without having a retaining curb..

Personally i think it is better to tank on top as well as DPM to stop mould in the screed etc.. The issue of a break between screeds at the tray isn't a prob.. cover this with the Ditra etc..

I understand I can do this without a retaining curb. But as you can see on that video the drainage was chiseled into a concrete floor (slab) and falls were created with a screed layer. So really on this video structural integrity of a slab was not compromised. The floor will work as one surface so to speak. If I seperate needed square then I sort of create two surfaces next to each other - two blocks of concrete. I might be paranoid but i don't want any movement to occur between those blocks. I know that on continent this slab would have metal reinforcement all over and be one piece. I'll speak to the builder but I'm just afraid that he won't be bothered.

One option you've got is to use a pre formed former, talk to WetDecs.

Another option is to screed your own former as a separate operation.

Whichever option you go for you have several choices as to how to proceed.

A generic approach that I would suggest would be:

1) Decide where your drain has to go and shutter off a channel to accomodate it.

2) Pour your concrete slab in the normal way with insulation and DPM underneath, leaving adequate depth for screed. I would leave a good 3 inches bearing in mind you also need to accomodate tiles, adhesive, ditra matting if you need it etc. I usually put the insulation down first onto compacted hardcore blinded with sand or similar, then lay the DPM on top.

3) Your 'screed' will fall into 2 areas. Shower area and rest of floor.

The approach from here on in will differ slightly depending on whether you want to install a pre formed former or screed your own falls in the shower area.

If you are screeding it all yourself then the video will be a good indicator of what you should do in the shower area. However, you will also need to do the rest of the floor. The wetroom I'm doing at the moment has a fall of around 15mm in 1500mm before you get to the shower area. Obviously the datum you'll need to work from is the floor height near the door. Decide your falls and this will give you the drainage height.

In my case I used a pre formed former. The channel left in the concrete slab for the drain worked well in my case as I didn't have to commit myself to the correct position of the drain until the last minute. As I was using SPF for the tray I simply backfilled the channel with SPF after I had fitted the waste and drain etc, then applied more SPF to the area for the former and laid the former down as per the instructions.

If you are doing a screed, now would be the time to work out the position of the drain etc. Install it, back fill the channel with screed and then screed away. Start with the shower area then work back to the rest of the room, making sure you're achieving the required falls (or close as).

Then tank and tile.

This is a process you want to go through several times before you embark on the job. Your builder has only to give you a concrete slab that you specify.

As you will have a channel through part of the slab, assuming the slab will be say 6 inches thick I would reinforce around the slab where the channel is as a minimum.

I suspect your builder may be thinking of A142 mesh for the whole slab, unless it is fairly thick. A142 mesh is about 8 inch squares made out of welded 1/4 inch rod IIRC.

He should know what he's doing here, it's not difficult.

Hope this helps

Thanks. That's the answer i was seeking. I really aprieciate your effort.

No worries mate

It may not be 100% correct but you'll need to use it to think through the job, which you're trying to do anyway.

Do engage the builder on this. He may have some good ideas. Don't just hand him a specification and tell him to get on with it

Also do remember 7 weeks (normally) before you tile onto new concrete and agueably any screed layed on top of new concrete. That caught me out with this job I'm on at the moment to the tune of an extra £100 on materials and another day's labour.

You can shorten this but it costs.