Laying a new tiled floor is one of the most satisfying parts of a bathroom renovation. But before you reach for the adhesive and spacers, there's a crucial step that many DIYers skip: waterproofing. Also known as tanking, this hidden layer protects the room below from leaks, mould and costly repairs. Get it right and your tiles will sit on a stable, watertight base for years to come.

Why Tanking Your Bathroom Floor Is Non-Negotiable

Water is persistent. Even a tiny gap in your tiling can let moisture seep through the substrate, especially in areas with frequent splashes or direct spray. Over time, that moisture can rot joists, stain ceilings below, and encourage black mould. In a flat, a leak can affect neighbours and lead to disputes and repair bills. Tanking creates a continuous waterproof barrier beneath the tiles, stopping water in its tracks. It's particularly important in wet rooms, walk-in showers and anywhere a shower tray isn't fully contained. Building regulations don't always mandate tanking in domestic bathrooms, but it's best practice and often required by insurance policies. Think of it as cheap insurance for a room that sees a lot of water.

Getting the Subfloor Ready for Tanking

Your tanking system is only as good as the surface it sits on. Start by removing old tiles, adhesive and any loose screed. The subfloor must be clean, dry, and free from dust and grease. If you have timber floorboards, check for movement and screw down any squeaky boards. For concrete or screed, fill cracks and holes with a suitable repair mortar. Uneven floors should be levelled with a smoothing compound before you tank. Next, prime the surface. Most tanking manufacturers specify a primer to improve adhesion. Apply it with a roller or brush and let it dry completely. Don't skip this step, as it can make the difference between a lasting membrane and one that peels away.

Applying the Tanking Membrane Correctly

There are two main types of tanking: liquid waterproofing and sheet matting. Liquid systems are painted or rolled on, usually in two coats. Apply the first coat in one direction and the second at right angles once the first is touch dry. Use a brush for corners and a roller for larger areas. Don't thin the product unless the instructions say so. Sheet matting is embedded in a flexible adhesive or applied with a waterproof adhesive, then overlapped at joints by at least 50mm. Whichever system you choose, cover the entire floor and extend the membrane up the walls by at least 100mm. In shower areas, take it up to at least 150mm, or full height if it's a wet room. This creates a watertight tray that connects to your wall tanking.

Sealing Joints, Corners and Pipe Penetrations

The weakest points in any tanking job are the joints and penetrations. Use a flexible waterproof sealant or pre-formed corners specifically designed for tanking. Reinforce these areas with a scrim tape or jointing tape embedded into the first coat of liquid tanking or into the adhesive for matting. Around the waste pipe, apply extra coats of tanking and seal the gap between pipe and floor with a suitable silicone. Don't rely on grout and silicone alone; they will crack and let water through. Take your time here. A few extra minutes with tape and sealant can save you thousands in water damage.

• Floor-to-wall junctions
• Internal corners
• Waste pipe and shower drain
• Any joints in the substrate

Checking Falls and Drainage Around the Shower

A shower floor must direct water to the waste efficiently. If the floor is level or has a wrong slope, water will pool, leading to ponding and potential leaks. Check the fall with a spirit level. For a tiled shower area, a gradient of 1:60 to 1:80 (roughly 1.5% to 2%) is typical. That means for every metre, the floor drops 12 to 17mm. If your subfloor doesn't have this fall, create it with a screed before tanking. For linear drains, ensure the slope runs towards the channel. Once you've tanked, test by pouring a bucket of water onto the floor and watching how it flows. It should head straight for the waste without sitting in puddles. If it doesn't, adjust before tiling. Tiling over a bad fall is a recipe for standing water and future problems.

Final Steps Before You Lay Tiles

After tanking, let the membrane cure fully. This can take 24 hours or more, depending on the product and room conditions. Don't rush it. For wet rooms, a flood test is a good idea: block the waste, fill the floor with water to a depth of about 20mm, and leave it for 24 hours. Check the ceiling below for any signs of damp. Once you're satisfied, you can tile. Use a flexible adhesive and grout, as these will accommodate minor movement. Apply silicone sealant at all movement joints, such as where the floor meets the wall. Finally, keep an eye on the sealant over time and replace it if it becomes mouldy or loose. With proper waterproofing, your bathroom floor will stay beautiful and dry for years.