Common Mistakes When Installing a Shower (and How to Avoid Them)
Shower installations look simple from the outside. A tray, a screen, a mixer, a few tiles. Then the tray comes out of the box and the wall behind it runs out by 15mm, the waste needs to travel four metres to the stack, and the old pipework is 15mm copper feeding a pump that gave up years ago. Almost every shower failure traces back to a handful of repeated mistakes. None of them are exotic, and all of them are far cheaper to avoid than to put right. The trick is to think about water, movement and access before you think about tiles.
Getting the fall wrong on the waste pipe
Water has to leave the tray faster than the shower puts it there, and it has to keep moving all the way to the stack. Two things go wrong here, and they are opposite problems.
Too little fall and the pipe holds water. On a long run, an unsupported pipe sags into a shallow belly where soap scum and hair collect, and you end up with a shower that drains slowly and smells faintly of drains by the following morning. Too much fall is just as bad: the water races away and leaves the solids behind, and the trap can be siphoned dry, letting sewer gas back into the bathroom.
A steady fall in the region of 1 in 40 is a sensible target for a 40mm waste — check the tray manufacturer's guidance for the waste you are fitting. Support the pipe with clips along its whole run so it cannot settle, keep the route as short and straight as you can, and use slow-radius bends rather than tight elbows. Use 40mm waste rather than the 32mm pipe you would use on a basin, and make sure the trap has a proper deep water seal: a shower trap is not the same animal as a basin trap. If you cannot get the fall you need without lifting the floor, say so before you buy the tray — a low-profile trap or a pumped waste may suit the job better.
Skipping the tanking before you tile
Tiles and grout are not waterproof. Grout is porous, and the small movements a building makes over a few years open hairline cracks in it. Water gets behind the tiles, sits against the plasterboard or the adhesive, and appears months later as a damp patch on the far side of the wall, blown plaster, or mould in the next room.
Waterproofing — tanking the shower area with a coating or a membrane over the substrate — is the cheapest insurance in the whole job. Use a suitable moisture-resistant backer board, tape and seal the joints and internal corners, then tank over the top. Pay particular attention to the corners, the tray-to-wall junction and around pipe entries. Those are where leaks start.
Sealing in the wrong order
Silicone gets blamed for leaks that were really caused by movement underneath it. The order of work matters more than the brand of sealant:
- Fix the tray or former solidly first, so it cannot flex by a millimetre when someone stands on the edge of it.
- Tile and grout the walls, and let the grout cure fully.
- Then run a neat bead of sanitary silicone at the tray-to-wall junction and in the internal corners.
Grout the flat wall joints, but never the tray-to-wall joint or a vertical internal corner — use a flexible sanitary sealant there. Don't apply silicone to a dusty or damp surface, don't try to fill a deep gap in one thick bead (use a backer rod and seal over it), and don't smear a second layer over an old, failing bead. Strip it back and start again.
Ignoring what is holding the tray up
A shower tray is only as watertight as its base is rigid. A stone resin tray bedded onto a bouncy chipboard floor will flex every time it is used, and that flex will crack the seal at the wall and eventually the tray itself. Check the floor is level in both directions with a spirit level, add noggins or a sheet of suitable board if the joists are springy, and follow the manufacturer's bedding method precisely — mortar, legs or a former, depending on the tray. The same applies to the screen: if the wall it fixes to is not solid, the frame will move and the seals will follow.
Getting the water supply and pressure wrong
Mixer valves expect hot and cold supplies at broadly similar pressures. On a gravity system fed from a loft tank, or where someone has teed a shower off a long, undersized run, the two sides fight each other: the temperature jumps the moment a downstairs tap is opened. Before buying a valve, work out what system you have and check the minimum pressure it needs against what you can actually deliver.
Run 15mm pipe rather than 10mm over any distance, insulate hot runs to reduce condensation and heat loss, and fit isolation valves so the shower can be serviced without draining the house. Electric showers are a separate conversation entirely — they need their own circuit, a correctly sized cable and the right protection, and bathroom electrical work often needs notifying and testing. If you are unsure about any of that, use a registered electrician. It is not a job to guess at.
Leaving yourself no access and testing too late
It is easy to box in a waste and a pump behind tiles that will never come off again. Leave a removable panel, or plan access before you board and tile. And test before the tiles go on: run the shower for a good twenty minutes, watch underneath the tray, check every joint and the waste connection, then check again the following day. Water finds a route overnight that it did not find in five minutes.
Do it once, in the right order
Plan the waste route before you choose the tray. Tank and board before you tile. Fix everything rigidly before you seal anything. Test before you close it in. If the floor needs lifting, the stack is on the other side of the room, or the electrics need work, that is the point to bring in a qualified plumber or electrician — a second visit costs far less than a new ceiling. Get those basics right and the rest of the shower is just tiling.
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