Silicone Sealing of Corners and Joints

Why not grout the corner between two walls like every other joint? Because the two surfaces on either side of a corner move separately. A bath flexes slightly under the weight of water and a person. Floors and walls expand and shrink with temperature and humidity. Hard grout in a corner cracks sooner or later, and water gets in through the crack. Flexible silicone takes up this movement, so the joint stays watertight.

Silicone is not waterproofing, though. In a shower and around a bath, the wall needs a waterproofing layer under the tiles, often called tanking, with sealing tape bedded into the corners. Silicone is only the first, visible line of defence. In practice, when someone relies on silicone alone, water reaches the wall through the first small crack.

• Sanitary silicone is the standard choice for bathrooms and kitchens. It contains additives against mould and copes well with water. Use it around baths, showers, basins, sinks and in tiled corners.
• Neutral cure silicone is for places where ordinary acetoxy silicone could cause damage, for example on sensitive metals or some plastics. You can recognise acetoxy silicone by its strong vinegar smell while it cures.
• Natural stone silicone is made for marble, travertine and similar stone. Standard silicone can leave dark, greasy stains along the edges of stone, and these cannot be removed.
• Acrylic sealant belongs only in dry areas, such as the joint between a wall and plasterboard or around door frames. You can paint over it, but it does not belong in a shower or next to a bath because it does not stand up to water for long.

It is worth matching the silicone colour to the grout. Silicone comes in many shades, so a corner does not have to show a white line between grey grout joints. Silicone cannot be painted, so choose the colour at the start.

The simplest case is a new bathroom where the corners are clean, dry and free of old sealant. These things make the job take longer and cost more:

• Removing old silicone, especially when it is full of mould or has had a second layer applied over it
• Wide, deep joints, which first need a foam backer rod. The rod keeps the silicone from getting too thick and makes sure it bonds only to the two opposite sides
• Awkward spots behind a bath, under a tap, around a shower screen or in narrow recesses
• Delicate surfaces such as natural stone, acrylic baths or high-gloss tiles, which must not be scratched
• A damp surface, which has to dry out first because silicone will not stick to it

If you live in a flat and water has already leaked through to the neighbours below, new silicone alone will not fix it. Tell your building management or owners' association, and have someone find out where the water is really coming from.

A handy homeowner can replace the silicone around a bath, a basin or in a shower enclosure. You will need:

• A sealant gun
• A knife or scraper
• Silicone remover
• Isopropyl alcohol
• Masking tape
• A silicone smoothing tool

Plan to leave the shower unused for at least a day afterwards.

1. Cut along both edges of the old silicone and pull it out in one length. Soften any remains with silicone remover and scrape them off with a plastic scraper.
2. Wash the joint and degrease it thoroughly with isopropyl alcohol or the cleaner the sealant manufacturer recommends. Let it dry completely.
3. Fill the bath with water so it flexes as it does in use. Leave the water in until the silicone has cured.
4. Apply masking tape to both sides of the corner, leaving a strip of the width you want for the bead.
5. Cut the cartridge nozzle at an angle, with an opening that matches the joint width, and load the cartridge into the gun.
6. Apply the silicone in one smooth pass, with steady pressure and without stopping.
7. Smooth the joint straight away with a tool dipped in soapy water or smoothing solution. Wipe off the excess onto a cloth as you go.
8. Remove the tape right after smoothing, while the silicone is still soft. Pull it away from the joint at an angle.
9. Ventilate the bathroom and let the joint cure as the manufacturer specifies. Keep water away for at least 24 hours in most cases.

If the silicone starts to skin over and the joint has not turned out well, do not try to patch it. It is easier to pull the whole bead out and start again.

• New silicone over old. New silicone does not bond to old silicone, and the mould underneath keeps growing. Always remove all of the old sealant.
• A greasy or damp surface. Traces of detergent, soap or limescale, and even fingerprints, weaken the bond. The silicone then soon peels away at the edge.
• Too thick a bead. Thick silicone takes a long time to cure and can stay soft inside. Fill deep joints with backer rod first.
• The wrong sealant. Acrylic in a shower soaks up water and goes mouldy. Standard silicone on stone leaves stains.
• Removing the tape too late. If the tape stays on until the silicone has skinned over, it tears pieces out when you pull it off and leaves a ragged edge.
• Sealing an empty bath. The first time you fill the bath, the joint stretches and can tear away.

Simple care makes silicone last longer. Wipe the joints after showering, ventilate the bathroom, and clean with mild products and a soft cloth instead of stiff brushes. Replace the silicone when it darkens beneath the surface, loses its flexibility or comes away.

• The joint runs continuously along its whole length, with no gaps, bubbles or breaks
• The silicone bonds to both surfaces and you cannot easily lift the edge with a fingernail
• The surface is smooth and slightly concave, and the edges are straight
• There are no dried smears of silicone on the tiles, bath or taps
• No old sealant or mould shows through the new silicone
• The colour matches the grout and is the same in every corner
• There are no dark stains along the edges on stone or other delicate surfaces
• The tradesperson has told you when you can use the shower or bath again