Bathroom ventilation and waterproofing done properly

Nearly every ugly bathroom problem is a water problem in disguise. Peeling paint, black grout, a soft patch of floor outside the shower, a ceiling stain in the room below. Tiles are not waterproof and neither is grout, so this article covers what actually keeps water out, how to size and site extraction that works, and the sequence in which these things have to be built.

Why bathrooms fail

Two mechanisms account for most damage.

Liquid water finds any unsealed junction: the gap behind a bath rim, the joint where a shower tray meets a wall, a poorly fixed screen, a penetration for a mixer or a shower rail plug. Water travels sideways along a boarding joint and appears somewhere unrelated.

Water vapour condenses on any surface below the dew point. A hot shower puts a surprising quantity of moisture into the air in a few minutes. If it cannot leave the room quickly, it condenses on the coldest surfaces available: window reveals, external wall corners, the ceiling near an uninsulated void. Mould follows within weeks.

You have to solve both. Perfect tanking with no extraction gives you a mouldy ceiling. Perfect extraction over unsealed boards gives you a leak.

One point underpins everything below. Tiles are ceramic and impervious, but the grout lines and the substrate behind them are not. Grout absorbs water, and the boards behind it will swell and rot if they are the wrong material. The waterproof layer must sit behind the tiles rather than being the tiles.

Getting the ventilation right

Mechanical extraction is not optional

An openable window is not a reliable substitute for a fan, because nobody opens it in winter and vapour moves into the rest of the house before it moves outside. Fit a mechanical extract fan in every bathroom and shower room.

What matters:

• Extract rate. Fans are rated by volume moved, typically in litres per second or cubic metres per hour. Building regulations in most jurisdictions set a minimum intermittent extract rate for bathrooms. Check the figure that applies where you live and specify a fan that achieves it through the actual duct run, not in free air.
• Duct length and bends. This is where most installations fail. Every metre of flexible duct and every ninety-degree bend cuts real-world performance substantially. Rigid duct, the shortest possible route and gentle bends will outperform a nominally more powerful fan on a long concertina of flexible hose.
• Duct to outside. Terminate through an external wall or the roof, with a proper external grille and a backdraught shutter. Never terminate into a loft or a void. That just relocates the condensation onto your roof timbers.
• Run-on timer or humidity sensing. A fan that runs only while the light is on stops before the room has cleared. A humidity-sensing fan or a run-on overrun of several minutes deals with the vapour that is still in the air after you leave.
• Make-up air. A fan can only extract what it can replace. Leave a gap of roughly 10mm (0.4") under the door, or fit a vent, or the fan will simply strain against a sealed room.

Placement

Site the extract as close to the moisture source as you can, generally in the ceiling above or just outside the shower, and as far as possible from the make-up air path so air is drawn across the room rather than short-circuiting. Fittings in bathrooms must be rated for the zone they sit in, and anything within reach of water needs to be appropriate for that location.

:::warning Electrical work in a bathroom is subject to specific zoning and protection rules in most jurisdictions, and is commonly notifiable work. Have it designed and certified by a qualified electrician. This is not a category for improvisation. :::

Heating and insulation help

A cold surface will condense water no matter how good the fan is. A heated towel rail sized to actually warm the room, insulation in an external wall, and thermally broken window frames all raise surface temperatures and reduce condensation. In a bathroom on an external wall, insulating that wall properly during works is one of the highest-value things you can do, and it belongs in the first-fix stage described in renovation order of work.

Waterproofing, layer by layer

The right substrate

Standard plasterboard behind a shower is a mistake even where it is labelled moisture-resistant, because moisture-resistant is not waterproof. Options, roughly in ascending order of robustness:

Substrate	Suitability	Notes
Standard plasterboard	Dry areas only	Not for any splash zone
Moisture-resistant board	Splash zones, tanked	Handles humidity, not standing water
Cement board	Wet areas	Stable, heavy, unaffected by water
Foam-core tile backer board	Wet areas	Light, insulating, easy to cut, needs correct washers and tape

Whichever board you use, the joints and fixing heads are the weak points, and they need the manufacturer's tape and sealant.

Tanking, properly

Tanking is a continuous waterproof membrane applied over the substrate before tiling. It comes as a liquid applied in two coats or as a sheet membrane bonded down.

The parts people get wrong:

1. Coverage area. In a shower enclosure, take the membrane full height, at least 1.8m (6') and preferably to the ceiling, and continue at least 300mm (12") beyond the enclosure on every side.
2. Corners and junctions. Every internal corner and wall-to-floor junction needs a reinforcing band or preformed corner bedded into the membrane. Corners are where movement happens and where liquid coatings thin out.
3. Penetrations. Every pipe, mixer body, screen fixing and shower rail plug is a hole through your waterproof layer. Use the correct sealing collars, and bed screen fixings in sealant rather than drilling and hoping.
4. Floors. In a wet room, the membrane goes over the whole floor and turns up the walls, lapped over by the wall membrane so gravity works in your favour.
5. Curing. Let each coat cure fully. Tiling onto a green membrane traps solvent and breaks the bond.

Falls and drainage

A wet room floor needs a fall to the drain. Around 1:60 to 1:80 is a conventional working range for a tiled shower floor, which means roughly 12–17mm of drop per metre. Too flat and water sits; too steep and large tiles will not lie flat and the floor feels odd underfoot.

Small-format tiles or mosaics accommodate falls better than large-format ones, because each tile only has to follow a small part of the curve. Linear drains at one edge allow a single-direction fall, which lets you use larger tiles than a centre gully with four-way falls.

Sealants and movement joints

Silicone, not grout, at every change of plane: wall to wall, wall to floor, wall to bath, wall to tray. These junctions move, and grout cracks when it does. Use a sanitary-grade silicone with a fungicide, cut the nozzle small, tool it in one pass and do not go back over it.

For a bath, fill it with water before sealing the rim and leave it full until the silicone has skinned. The bath drops under load, and a seal made on an empty bath will tear the first time somebody gets in.

:::tip Where a bath or tray meets a tiled wall, a purpose-made sealing tape or upstand under the tiles is far more reliable than a bead of silicone on top of them. Silicone on top is the last line of defence, not the only one. :::

Layout decisions that reduce risk

• Keep the WC and basin off the wet zone where the layout allows, so the tanked area stays compact and the fixings are simpler.
• Prefer a wall-hung basin or WC with a proper supporting frame. It makes the floor easy to clean and dry, and standing water is the enemy.
• Use a screen rather than a curtain where you can, and fit it so any drips run back inside the tray, not onto the floor.
• Site the shower away from external corners if the wall is uninsulated, because that corner will be the coldest surface in the room.
• Leave access. A removable panel to the trap and valves turns a future leak from a demolition job into an afternoon.

Working clearances in a bathroom follow the same discipline as elsewhere in the house. Allow roughly 600mm (24") of clear space in front of a WC and 700–750mm (28–30") in front of a basin for comfortable use, and keep the door swing clear of everything. The general principle of drawing circulation before fixtures is covered in living room layout dimensions, and it applies just as strictly here.

:::budget Tanking materials are a small fraction of a bathroom's total spend and the cheapest insurance in the project. The tiles, screen and sanitaryware you can economise on. The membrane, the substrate and the fan are not the place to save, because failure means removing everything above them. :::

Maintenance that actually prevents mould

• Run the fan during the shower and let the overrun finish. Do not switch it off at the isolator.
• Squeegee the screen and tiles after showering. It takes twenty seconds and removes most of the water that would otherwise evaporate into the room.
• Leave the door closed and the fan running for a few minutes after, then open up. Closing the door stops vapour reaching cold bedrooms.
• Clean the fan grille and check the external terminal annually. Lint and insects block both.
• Inspect silicone twice a year. Cut out and replace anything that has lifted or blackened, rather than applying new silicone over old.

The short version

• You must solve liquid water and water vapour separately. One without the other still fails.
• Fit a fan ducted to outside with rigid duct, the shortest route, humidity sensing or a run-on timer, and an air gap under the door.
• The waterproof layer is the tanking membrane behind the tiles. Tiles and grout are not waterproof.
• Tank full height in the shower plus 300mm (12") beyond it, and reinforce every corner and penetration.
• Fall of about 1:60 to 1:80 to a wet room drain, with small-format tiles or a linear drain.
• Silicone every change of plane, fill the bath before sealing its rim, and leave access to the trap.