Constructing and Creating a Fall for a Shower Kerb

Why is the shower in so many flats a step higher than the rest of the bathroom? The waste needs room and a fall. The drain trap has a certain build-up height, and the pipe from it must run downhill all the way to the soil stack. If all of that does not fit within the floor thickness, the whole shower has to be raised.

So the height of the kerb is not a guess. You work it out from the bottom up:

• The height of the trap or linear drain, as stated by the manufacturer
• The fall of the waste pipe towards the stack, usually around 2 to 3% on short runs
• The thickness of the screed, waterproofing, adhesive and tiles
• The fall of the surface towards the drain, commonly about 1 to 2%

The further the drain is from the stack, the higher the platform ends up. It therefore pays to look for a low-profile trap and to place the drain as close to the stack as possible. In a block of flats, talk to the building management or owners' association before anyone touches the stack or shared pipework. If the bathroom refit includes larger building work, check with your local building authority what the local building regulations require.

The order matters. Each layer needs time to set or dry before the next one goes on.

1. Planning. First the drain position, the shape and size of the platform and the direction of the fall are decided. You also need to know where the glass screen will stand, because its profile is often fixed into the kerb.
2. Waste and drain. The trap or linear drain is set in place, connected to the pipe and tested for leaks while everything is still visible.
3. Building the perimeter. The outer edge of the platform is built from blocks or bricks. The inside is filled so that the surface is not hollow.
4. Screed to falls. Cement screed or a mortar for falls is laid on the base and ruled off with a straightedge along guide strips, so that everything leads to the drain.
5. Waterproofing. Once the screed is dry, a liquid waterproofing membrane goes on, normally in two coats. Sealing tape goes into the corners and along the kerb edge, and a sealing collar goes round the drain.
6. Tiling. Finally the floor tiles are laid, the front face of the kerb is tiled and the edge is finished with a trim or with mitred tiles.

One point that older guides often get wrong: the waterproofing belongs on top of the finished screed, not underneath it. The fall is formed by a separate layer, not by building up adhesive under the tiles. The adhesive should be roughly the same thickness everywhere.

• Aerated concrete blocks are light and easy to cut and shape. They soak up water readily, so they need a thorough primer and good waterproofing.
• Clay bricks are strong and easy to lay with mortar. They suit taller platforms in particular.
• Dense concrete blocks absorb less water and cope well with damp conditions. They are heavy, though, which may not suit older houses with timber floors.
• Pre-formed shower boards made of rigid foam come with the fall built in and often with an integrated drain. They save time, but they must match the height of the trap exactly.

The type of drain affects the choice of tiles. A centre point drain needs a fall from four sides, and the tiles have to be cut in an "envelope" pattern, so smaller tiles or mosaic work well. A linear drain along the wall needs a fall in one direction only, so it can take large tiles. On the entry and the shower floor, choose slip-resistant tiles made for wet areas.

How much work is involved depends mostly on the starting point. It is simpler with new waste pipework, a level base and a rectangular platform between two walls.

More work comes from:

• Moving the waste or making a new connection to the stack
• An uneven or damaged existing floor that has to be levelled first
• An unusual shape, such as a curve, a cut-off corner or a free-standing platform
• A point drain with falls from four sides and envelope cuts in the tiles
• Mosaic and lots of small cuts on the front face and edges
• Removing an old shower tray and the remains of old tiling

A handy DIYer can manage this with patience and a good spirit level. If the connection goes into the stack in a block of flats, it is wiser to call a plumber.

1. Mark the outline of the platform, the drain position and the direction of the fall on the floor.
2. Work out the height from the trap height, the pipe fall and the thickness of all layers.
3. Fit the drain, connect it to the waste and run water through it to check for leaks.
4. Clean the base of dust and apply primer.
5. Build the perimeter from blocks in adhesive or mortar and fill the inside.
6. Set guide strips to the fall and rule off the screed with a straightedge.
7. Let the screed dry as the manufacturer instructs, usually several days for cement screed.
8. Bed sealing tape into the corners and along the edges, and fit the collar round the drain.
9. Apply the waterproofing membrane in two coats, keeping to the drying time between them.
10. Pour water over the surface and check that all of it drains away.
11. Lay the tiles, tile the front face, grout, and seal the corners with sanitary silicone.

In practice it often happens that water sits in a corner of the shower or runs over the kerb into the bathroom. The cause is usually a fall set by eye or a missing back-fall at the entry. The kerb edge should sit slightly higher than the area at the drain, so water has no reason to run out.

• Waterproofing applied only to the floor, not taken up the walls and the front of the kerb
• No collar round the drain, so water gets under the tiles
• A fall made from a thick bed of adhesive instead of screed
• A trap built in so that it cannot be cleaned
• Tiles on the sloping area that are not slip-resistant
• Tiles laid before the screed and waterproofing had dried

• Water poured on different spots of the shower all drains away and nowhere stands
• No water runs out over the entry or the kerb edge
• The tiles do not sound hollow when tapped
• The grout joints are full and the corners have flexible silicone, not grout
• The edges and corners of the kerb are cleanly finished and not sharp
• The drain grate lifts out and the trap can be cleaned
• The tradesperson shows you or photographs the waterproofing before tiling