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How to lay new shower concrete floor with linear channel drain?

Hi all, I'm hoping you might have some handy advice.

I'm renovating a small bathroom. Walls are concrete block to 400mm and I'm raising the floor 200mm with 100mm polystyrene with 100mm concrete. The floor will all be tiled with a walk in shower.

How do I lay the concrete to get the linear channel drain at the right height? The plumber has already done the waste drain, but the channel drain will need to sit slightly lower than the final concrete floor level.

Any tips or advice would be very welcome.

Thanks.

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MitchellMc

Welcome to the Bunnings Workshop community @SimonH. It's sensational to have you join us, and many thanks for your question about installing a linear drain.

Creating a form or leave-out is the typical approach when you're pouring a new concrete slab and need to recess a linear channel drain. Since the channel drain needs to sit slightly below the finished concrete level to allow for tiling and correct water flow.

Before pouring the concrete, you'd build a formwork or void former in the exact spot where the channel drain will eventually go. This might be as simple as boxing out the area using timber or foam, sized to match the channel's dimensions plus any space you need for bedding mortar. The form should be securely fixed in place so it doesn’t shift when you pour the slab. It essentially acts as a placeholder, keeping the concrete out of that zone.

Once the rest of the floor is poured and cured, you remove the form, install the channel drain (bedding it if needed in mortar to fine-tune the height), and then waterproof and tile around it. Be sure to factor in the fall toward the drain — you’ll want to screed the shower floor separately after the slab is done, using a bonding agent if necessary, so you can create a consistent slope of around 1:80 toward the channel.

If the waste pipe is already roughed in, make sure you also have the channel's outlet location marked so the two line up.

Please let me know if you have any questions.

Mitchell

SimonH

Thanks Michell that's super helpful. Is it possible to get the right fall just with the concrete, or is screening really necessary? The shower will be the same floor level as the rest of the bathroom, so if I have to screed, do I need the shower part at a lower level. Or would I screed the whole bathroom floor to get the right fall and levels?

I've attached a photo of the area. The shower will be at the back (higher block wall level end), with the channel drain on the right side. The other photo (with window) looks from the shower to the end that will have the vanity.

Thanks again

MitchellMc

Hi @SimonH,

It is technically possible to get the right fall using just the concrete, but it’s usually much more practical to achieve a precise and consistent fall using a screed layer. Screeding gives you better control over the gradients and levels, especially for a linear drain setup where water needs to fall evenly towards one edge.

Since your shower floor will be the same level as the rest of the bathroom, you don’t necessarily need to recess the slab in that area, but you will need to factor in the thickness of your screed so that the finished surface still aligns across the room. That typically means applying screed across the whole floor, not just the shower zone. You can maintain a slight fall across the whole bathroom floor towards the drain (often done subtly), or you can do a more defined fall just in the shower area.

If you're keeping the main floor level consistent, waterproofing becomes especially important. The membrane will need to be continuous and carefully applied with upturns at walls and around the drain. Definitely a good idea to chat with a waterproofer or tiler to make sure your levels and fall plan work with the products you're using.

Mitchell

SimonH

Thanks Mitchell. Really helpful once again. It's a very narrow room, so precessing the shower concrete area shouldn't be too hard, it thatvwould make things easier.

Or, if I screed the whole floor, what's the minimum screed layer depth to ensure a robust final floor surface?

Thanks again 👍

JacobZ

Hi @SimonH,

It depends on the specific product you use, however, a layer of around 15-20mm is fairly typical when using screeds. Just make sure you read and follow the label on the specific product, or if you are mixing your own, use a layer around 15-20mm thick.

It should, however, be noted that screed cannot be used as a finished floor surface. It will need to be tiled over to finish the floor. Once tiled, you will have a robust finished floor with the correct falls towards your drain.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

SimonH

Thanks Jacob. That's really helpful.

One other issue occurs to me. If the floor is sloping towards the drain, and I want to put a fixed glass panel covering half the shower area on the drain / showerhead side, how do I get the glass panel to sit square against the floor and wall?

Thanks.

JacobZ

Hi @SimonH,

I'd always advise having a glazier install your glass for you due to the inherent risks working with glass poses. Asking your glazier what their preferred method of addressing this issue is before tiles go on would be ideal.

I suspect your glazier would do this by installing aluminium channels along the floor and wall that are level. This could be done by cutting a recess into the tile or screed, so the channel sits level. You could also pack the channel out with some extra screed so that the aluminium sits level. As long as the aluminium sits level, the glass will be plumb.

After the glass is installed, the channel and the glass will be sealed with silicone so that water can't get beneath it and will stay within the shower area where it will fall towards the drain.

Let me know what you think, and if you have further questions, please don't hesitate to ask.

Jacob

SimonH

Thanks for this Jacob.

There is one further complication, I overlooked.

I want to install underfloor heating for the floor area outside of the shower. That will add a bit of hight to that area of floor.

Should that go in the concrete; ontop and under the screed; or some other way?

Many thanks,

Simon

MitchellMc

That sounds like a question best directed to the manufacturer of the underfloor heating, @SimonH. Does the installation guide mention the positioning? I suspect they would have very specific depth instructions in the technical details.

Mitchell

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