Bath Installation and Waste Connection

A badly fitted bath rarely causes trouble straight away. Water can seep slowly under the rim or through a leaking trap for years, into the floor, the wall or the flat below. Often the first sign is a stain on the ceiling downstairs.

The second issue is stability. A full bath with a person in it weighs several hundred kilograms. If the bath moves even slightly under that load, the silicone at the wall cracks and water finds its way into the gap. With acrylic baths, poor support under the base can even crack the material.

The third issue is drainage. If the waste pipe has the wrong fall or the trap is badly connected, water drains slowly, dirt builds up in the pipe and the bathroom can start to smell of drains.

Preparing the floor and the waste

The floor under the bath must be solid, level and clean. Uneven spots are corrected with levelling compound or screed. During a renovation it is worth checking the old waste pipe. Cast iron or cracked pipework is best replaced now, while everything is exposed. Waterproofing on the floor and on the wall behind the bath goes on before the bath is fitted, because you will not be able to reach those areas later.

Levelling and height

The bath stands on adjustable feet or a support frame and is levelled with a spirit level in both directions. Most baths already have the fall towards the plughole moulded into the base, so the rim itself is set level. The height is chosen so that the trap fits underneath and the rim lines up with the planned tiling.

Connecting the waste

The trap has to suit both the bath and the diameter of the waste pipe. The joint to the drain must be firm and watertight, and the horizontal run of the waste needs a gentle fall, usually around 2 per cent (roughly 2 cm per metre). Straight after connecting, the bath is filled and every joint is checked for drips while it empties.

Boxing in and tiling

A bath is often boxed in with aerated concrete blocks or a frame faced with cement board, then tiled. Some people prefer a ready-made bath panel instead. The boxing holds the bath in place sideways, but it must not lift the bath or press against the rim. Sealing tape goes between the rim and the wall and is bonded into the waterproofing. The wall tiles then overlap the rim, so water runs down the tiles into the bath, not behind it.

Access panel and silicone

An access panel is built into the boxing on the trap side. Without it, a blockage or leak means breaking tiles. Finally, the joint between bath and tiles is filled with sanitary silicone, ideally with the bath full of water. The bath then settles into the position it will have in normal use, and the silicone does not tear later.

• Acrylic bath: light and warm to the touch, but it needs good support under the base, so it is fitted on a frame, on feet with a base support or in a bed of mortar, as the manufacturer specifies
• Enamelled steel bath: stiffer and more scratch resistant, but it needs sound-deadening pads, otherwise it drums loudly while filling
• Cast iron bath: very heavy, so check that the floor can carry the load and plan for several people to carry it in
• Freestanding bath: not boxed in, which means the waste in the floor must be set out very precisely, because it is visible and hard to move later

With metal baths, electrical bonding may come up. Whether it is needed depends on the local wiring rules and the existing installation, and only a qualified electrician should assess or carry out this work.

The simplest case is a new bathroom with a level floor and the waste exactly where it should be. These things make the job longer:

• An uneven or damaged floor that has to be levelled first
• A waste pipe in the wrong place or in poor condition that has to be moved or replaced
• An unusual shape, such as a corner or asymmetric bath
• Elaborate boxing with shelves, a niche or curves
• A heavy bath that has to be carried up several floors without a lift

If you live in a block of flats and the work affects the shared soil stack, agree it in advance with the building management or owners' association. Replacing a bath usually does not need permission. If the renovation touches structural elements or shared services, check with your local building authority what the local building regulations require.

• The bath stands only on its feet without base support, even though the manufacturer requires it
• No waterproofing behind the bath or no sealing tape under the rim
• Tiles stop above the rim instead of overlapping it
• Silicone is used as the only protection against water
• Boxing without an access panel
• A waste pipe with no fall, or one that runs uphill
• No leak test before the bath is boxed in

If the waterproofing is done, the waste is ready and you are reasonably handy, you can fit a standard acrylic bath yourself. Call a professional if the drainage in a block of flats needs to be moved or if a metal bath needs bonding.

1. Check the floor is level and solid, and correct any unevenness.
2. Fit the feet or frame to the bath following the manufacturer's instructions.
3. Fit the trap with the waste and overflow; seat the washers without silicone unless the manufacturer says otherwise.
4. Set the bath in place and level the rim in both directions.
5. Connect the trap to the waste pipe and check the fall.
6. Fill the bath, empty it and check every joint.
7. Apply sealing tape between the rim and the wall and bond it into the waterproofing.
8. Box in the bath, leaving an opening for the access panel.
9. Tile the boxing and the walls so the tiles overlap the bath rim.
10. Fill the bath with water, run the silicone joint and drain the water only after the silicone has cured according to its manufacturer.

• The bath does not move or creak under load
• The rim is level and water on the base runs towards the plughole
• No joint drips after a full bath has been filled and emptied
• Water drains smoothly without gurgling
• The access panel opens and the trap can be reached through it
• The tiles overlap the bath rim and all joints are grouted
• The silicone is continuous, free of cracks and well bonded to both surfaces