How do I seal the gap between the worktop and the wall?

The gap behind a worktop is a nuisance in daily use and a genuine problem for the worktop, because it is where water runs off the back and reaches the raw rear edge of the board. Closing it properly is worth the hour it takes, and choosing the wrong closure is worth doing twice.

What you'll need

• Neutral-cure sanitary silicone and a gun
• Sealant remover and a scraper (optional)
• Masking tape (optional)
• Tape measure
• Upstand material, if fitting one (optional)

Step by step

1. Push the worktop tight to the wall if it is not already.Base units sometimes stand off the wall on a skirting board that was never removed, or on a lump of plaster. A worktop pushed hard back closes a surprising proportion of the gap.
2. Measure the gap every 300mm along the whole run.This is the measurement that decides everything, and almost nobody takes it. Write the numbers down. A gap that runs 2, 3, 2, 8, 11, 4mm is not a silicone job however neat you are.
3. Use silicone only where the gap is even and small.Under about 6mm and consistent, a neutral-cure sanitary silicone bead is correct, quick and invisible. Above that, or where it varies, silicone slumps into the wide parts, cracks as it cures unevenly, and traps dirt.
4. Remove every trace of the old sealant first.Cut the bulk away with a blade, then use a silicone remover on the residue. New silicone will not bond to old silicone, and the invisible film left behind is what makes a new bead peel within weeks.
5. Clean and dry both surfaces properly.Methylated spirit on a lint-free cloth, then let it flash off. Grease on a worktop back edge is the norm and it stops sealant sticking.
6. Mask both sides for a straight line.Two lines of tape, the bead run between them, tooled once with a wetted finger or a tool, tape pulled immediately while the sealant is wet. This is what makes the difference between neat and obvious.
7. Fit an upstand where the gap varies.A strip of worktop material, usually around 100mm high, stood against the wall and bedded on silicone at the bottom and at the back. It is rigid, so it bridges a wavy wall, and it only has to be sealed along two straight lines.
8. Consider a splashback if the wall behind gets wet.Behind a hob or a sink, a full-height glass, acrylic or tiled splashback taken down onto the worktop closes the gap and protects the wall in one move. It is the only option here that deals with a splash zone properly.
9. Scribe the worktop if it has not been cut yet.Best-looking answer of all, and only available before cutting. The back edge of the top follows the wall's shape, so there is nothing to close but a hairline.
10. Let it cure before the worktop gets wet.Sanitary silicone needs time to skin and cure through. Keep the area dry for the period on the tube, which is usually a day or more for a full cure.

Tips

• Neutral-cure silicone rather than acetoxy on stone and quartz. Acetoxy silicone releases acetic acid as it cures and can mark natural stone and some metals.
• A slight bevel on the top of the bead, running away from the wall, sheds water rather than holding it in a dished line.
• If the gap is very large — 15mm and more — the worktop is sitting well off the wall for a reason. Find out why before filling it. A skirting board left in place is the usual answer, and taking it out is the better fix.
• Sanitary grade sealant contains a fungicide and resists mould far better than general-purpose silicone. It matters more at a sink than anywhere else in the kitchen.

Common mistakes

• Running silicone into a gap that varies — It slumps into the wide parts and skins over a hollow, which then cracks. Six months later there is a split line full of grime and it has to be cut out and done again.
• Applying new silicone over old — Silicone does not bond to cured silicone. The new bead lifts in strips within weeks, and the residue has to be removed chemically before anything will stick.
• Using decorator's caulk — It is not waterproof, it shrinks, and it cracks along a joint that moves as the worktop expands and contracts. This is a silicone joint.

Questions people ask