Leaking Shower? How to Tell If It's the Grout, Sealant, or Tanking

If your shower is leaking, the source is almost always one of three things: the silicone at the joints, the grout, or the tanking behind the tiles. The good news is that silicone and grout faults are surface problems you can often fix. The bad news is that a leak with sound grout and sound silicone usually means there was never any tanking, and that is a tiles-off repair. Here is how to work out which one you are dealing with before you start pulling things apart.

Key takeaways

• Grout is water resistant, not waterproof. It is meant to let some water through.
• Tanking is the waterproof layer behind the tiles, and it is what actually keeps a wet area dry.
• Failed silicone and cracked grout are usually fixable in place.
• A leak with sound grout and silicone almost always means missing tanking, which needs a redo.

A stone shower with built-in niches in West Wickham. Niches and tray junctions are exactly where leaks start, so the tanking behind them has to be continuous and lapped correctly. Wet room tiling service

Start by understanding what each layer actually does

People assume tiles and grout keep water out of a shower. They do not, and they were never designed to. Water gets behind tiles in every shower, every time it is used. What stops that water reaching the structure is the waterproofing underneath. Get the three layers straight and the diagnosis becomes simple.

• Tiles shed most of the water but are not a sealed surface. Water sits in the joints.
• Grout fills the joints and is water resistant, not waterproof. Cement grout is porous and lets water through slowly. That is normal and expected.
• Silicone seals the moving joints where rigid grout would crack: tray to wall, floor to wall, around the screen.
• Tanking is the hidden waterproof membrane behind the tiles. Mapei Mapelastic, BAL Tank-it, or a Schluter Kerdi membrane are the usual specifications. This is the layer that catches everything grout lets through and sends it back to the drain.

Miss the tanking and you are relying on grout to be waterproof, which it is not. See what is tanking for the full explanation of why this layer is non-negotiable in a wet area.

How do I tell if it’s the silicone?

Silicone is the first suspect because it is the layer that moves and ages fastest. Look at every internal corner, the joint where the floor or tray meets the wall, and the seal around the screen or glass.

You are looking for splits, gaps, lifting edges, or black mould growth that has got under the seal. Press it. If it has gone hard and brittle, or you can lift an edge with a fingernail, it has failed. Failed silicone lets water straight into the joint behind it, and a tray-to-wall joint is the most common single point of a shower leak.

The fix here is genuine: rake the old silicone out completely, clean and dry the joint, and apply a quality mould-resistant sanitary silicone. Painting fresh silicone over old is a bodge that traps water and fails within months.

How do I tell if it’s the grout?

Run your eye and a fingernail along the grout lines. Cracked grout, missing sections, powdery or crumbling grout, and dark damp lines that stay wet long after the shower is off all point to grout failure.

Cracked grout in the field of tiles often means movement underneath, which is a related issue, but localised cracking and gaps can be raked out and regrouted. Just be clear about what regrouting does and does not do. New grout makes the surface tidy and slows water down again. It does not make the shower waterproof, because grout is not the waterproof layer. If water was getting into the wall through old grout, it will eventually get through new grout too, unless there is tanking behind to catch it.

How do I tell if it’s the tanking?

This is the one nobody wants to hear, and it is the most common real cause of a shower that leaks into the ceiling or the room below. The tell is simple: the silicone is sound, the grout is sound, and water is still appearing where it should not be.

If the surface is intact and water is still getting through, it is getting through the way it always does, slowly through the grout and joints, and there is nothing behind the tiles to stop it. That means no tanking, or tanking that was applied badly and is not continuous. You cannot see this from the bathroom side. You usually find it as a damp patch on a ceiling, a soft skirting in the next room, or staining on a wall below.

There is no surface fix for missing tanking. Regrouting and re-siliconing a shower with no waterproof membrane is treating the symptom and ignoring the cause. The honest answer is tiles off, a proper tanking membrane installed across the whole wet zone with correct laps at the corners and the tray junction, then retiled. It is a bigger job, but it is the only one that actually stops the leak. If you are weighing up the format of the rebuild while you are at it, wet room versus shower tray is worth a read.

Why missing tanking is so common

A lot of leaking showers across Bromley and the surrounding area were tiled by someone who treated grout as the waterproofing. The tiles went straight onto plasterboard or a bare wall, grouted neatly, and it looked perfect on handover. It stays dry for a while because it takes time for water to work through the grout and saturate the wall. By the time it shows, the tiler is long gone and the damage is done. Neat tiling is no proof of a waterproof shower. The waterproofing is the part you cannot see, which is exactly why it gets skipped on cheap jobs.

Getting a leaking shower diagnosed

A photo rarely tells the full story with a leak, because the cause is usually hidden behind the tiles. I assess and repair leaking showers across Bromley, Beckenham, Orpington, Chislehurst, and West Wickham, including full strip-outs where the original was never tanked. I will tell you honestly whether you are looking at a silicone job, a regrout, or a proper rebuild with wet room or bathroom tiling done correctly from the substrate up.

If your shower is leaking and you want a straight diagnosis, get in touch and I will come and find the real source rather than just papering over the joints.

See: what is tanking | wet room vs shower tray | why black mould keeps coming back