When it's time to renew bathroom silicone (and why the black bits are a problem)

Silicone is the least glamorous material in your bathroom. And also the most important, because it's the only thing stopping water running down the back of your bath and into the floorboards.

A good silicone bead should last 5 to 7 years. A great one, longer. The ones that are going black and pulling away from the tile after 18 months were either the wrong product or applied onto a damp surface. Either way, they need redoing before water gets behind.

The signs it's time

Any one of these is enough to justify a reseal:

• Black mould that doesn't come off with a bathroom cleaner or a bleach spray. If it's in the silicone itself, not on top, the mould has gone through the whole bead. It'll keep coming back.
• The silicone has lifted from the tile or the bath. Put a fingernail against the edge. If it peels away easily, it's not sealing anything.
• You can see a crack running through the bead. Sometimes a thin line, sometimes a gap wide enough to slip a piece of paper through.
• Water is pooling on the floor behind the bath panel after a shower. That's the late warning. By this point water may have been getting under for a while.
• A damp patch on the ceiling of the room below. This is the emergency.

Why old silicone goes black

Silicone itself doesn't rot, but the surface grows a biofilm of mould that eventually penetrates. Two reasons:

1. Non-bathroom silicone was used. Standard construction silicone doesn't contain the fungicides bathroom silicone does. Cheap to spot. Lasts half as long.
2. It was applied onto a damp joint. Water trapped behind a new bead becomes a breeding ground. Hence the "48 hours dry" rule before fresh silicone.

Removing the old silicone properly

This is where most DIY attempts come unstuck. A knife alone won't do it. You need:

• A sharp utility knife with a new blade
• A plastic silicone scraper (metal will scratch the bath or the tile)
• A bottle of silicone remover (De-Solv-It, or similar, £8 a bottle)
• Lots of clean cloth

Score along both sides of the old bead with the knife. Run the scraper down the middle to lift the main body. Pick off the last thin slivers with your fingernail. Apply silicone remover to what's left. Leave for 20 to 30 minutes. Wipe and scrape again.

The joint has to be absolutely clean and absolutely dry before new silicone goes on. Wipe with methylated spirits on a clean cloth for the final prep. Let it flash off (two minutes).

Applying the new bead

Masking tape is the difference between an OK job and a great one. Run a strip of tape along the tile about 4mm above the joint and another along the bath about 4mm below. Pierce the silicone nozzle at about 5 to 6mm, 45 degrees off the tip.

Apply steady pressure in one smooth pull. Don't stop mid-bath. If you run out of silicone halfway, the joint between the two beads is a weak point.

Smooth with a wetted finger or a proper silicone tool. Peel the tape off straight away before the silicone starts to skin, or you'll pull the bead with the tape.

Fill the bath before you seal

Old plumber's trick. Fill the bath with water before you apply the new silicone. The weight of water pulls the bath down to its lowest natural position. You seal it in that position. Then drain. When the bath is next used, it can't sink any further than where you sealed it, so the bead stays intact.

Without this, a new bead can tear when someone fills the bath the first time and the empty bath rises slightly when they step out.

Costs

• Strip and reseal a single bath or shower joint: 90 minutes. £65 to £85 including materials.
• Full bathroom (bath, shower tray, basin, maybe a back splashback): 2.5 to 3 hours. £120 to £160.

A small job. A very worthwhile one. Silicone keeps water where it's meant to be, which is the whole point of a bathroom.