Silicone Finishing in London Renovations: Why It Matters and How It Is Done Correctly

Silicone finishing is the last operation in a bathroom renovation and among the first things to fail when applied incorrectly. A silicone joint at the junction of a bath and a tiled wall, around a shower tray, or at a worktop-to-wall junction is not decorative — it is the waterproof seal that prevents moisture ingress behind the tile substrate. When it fails, water finds its way behind the tiles and into the wall structure, causing adhesive failure, mould growth, and eventually tile movement. The remediation of a wet-area moisture penetration behind failed silicone is one of the most disruptive and expensive bathroom repairs.

This guide covers why silicone finishing is critical, what the correct specification and application procedure is, and how to identify quality work from poor work.

Why silicone rather than grout at junctions

At all internal corners and at junctions between a tiled surface and another material (bath, shower tray, basin, worktop), movement occurs. The tile substrate and the adjacent material expand and contract at different rates under thermal cycling and structural movement. A cementitious grout joint at these positions will crack — always, eventually, and typically within 1–3 years in a bathroom in regular use. Once cracked, it is permeable to water.

Silicone sealant is flexible and elastic. It accommodates the movement at these junctions without cracking. It bonds to both surfaces and maintains a continuous waterproof seal across the joint. This is why BS 5385 (the British Standard for tile installation) specifies silicone sealant at all movement joints — it is not optional, and specifying grout at these positions is a defect.

The positions requiring silicone (not grout): - All internal corners (floor/wall junction; wall/wall junction) - Bath/shower tray to tiled wall junction - Basin to tiled wall junction - Kitchen worktop to tiled wall splash back junction - Any junction between tile and a different material (wood, stone, sanitary ware) - Perimeter of large tile areas where movement joints are required (every 4–5m in a large floor)

Silicone product specification

Not all silicone sealants are equal. The relevant distinctions:

Sanitary vs standard: sanitary silicone contains an anti-mould biocide that inhibits the growth of black mould on the joint surface. In a bathroom, sanitary silicone is mandatory. Standard construction silicone without mould inhibitor will develop black mould within months in a humid bathroom environment.

Acetoxy vs neutral cure: acetoxy-cure silicone releases acetic acid during curing (the vinegar smell). It bonds excellently to glass and ceramic but can cause corrosion of some metals (particularly mirror edges and non-anodised aluminium) and staining on certain natural stones. Neutral-cure silicone is safer for use adjacent to sensitive materials — specify neutral cure for marble, limestone, and mirror-edge applications.

Colour matching: silicone must be specified to match the grout colour. All major grout manufacturers (Mapei, BAL, Ardex, Laticrete) produce matching silicone sealant colours for their grout ranges. Specifying white silicone with grey grout, or off-white silicone with a warm-toned grout, produces a visually inconsistent joint that reads as a defect even if the application is technically correct.

Quality tier: use a branded, professional-grade product. Evo-Stik, Soudal, Geocel Pro, and Dow Corning 786 (for neutral cure) are the standard trade products. Supermarket silicone in an unbranded tube should not be used in a renovation context.

Surface preparation: the step that determines outcome

The most common cause of premature silicone failure is inadequate surface preparation. The surfaces to be siliconed must be:

Clean: free from tile adhesive, grout residue, dust, and any contamination. Tile adhesive and grout that remain in the joint from the tiling operation must be fully removed before silicone is applied. Use a grout rake or oscillating tool to clean back to a clean substrate. Any residue in the joint prevents the silicone from achieving a full bond.

Dry: silicone does not bond to wet surfaces. In a new bathroom, allow at least 24 hours after grouting before siliconing, and ensure the room is ventilated to reduce humidity. In an older bathroom being re-siliconed, the old silicone must be fully removed (a silicone removal tool and chemical silicone remover) and the substrate allowed to dry completely — any moisture behind the old silicone that is now exposed will prevent the new silicone from bonding.

Primed where required: on some porous substrates (natural stone, certain tiles), a silicone primer improves adhesion. Check the silicone manufacturer's guidance for the specific tile type.

Application technique

Masking tape: apply low-tack masking tape to both surfaces on each side of the joint, leaving a gap of the intended joint width (typically 4–8mm in a bathroom). The tape gives a clean line and prevents the silicone from smearing onto the tile surface.

Cutting the nozzle: the nozzle should be cut at 45° to an aperture approximately 70% of the joint width. A nozzle cut too large produces excess silicone that is difficult to tool; too small produces insufficient fill.

Application: apply the silicone in a single continuous bead, maintaining consistent pressure and gun speed. Do not stop and start mid-joint — the join point between two applications is a weak point. Work from one end to the other in a single pass.

Tooling: immediately after application, tool the silicone with a wetted finger, a purpose-made silicone tool, or the back of a spoon to compress the silicone into the joint and create a slightly concave profile. The concave profile is not merely aesthetic — it means the centre of the joint is in compression against both tile surfaces, maintaining the bond. A convex profile means the joint is pulling away from the centre. Work quickly — most sanitary silicones begin skinning within 10–20 minutes.

Remove the tape: pull the masking tape immediately after tooling, before the silicone begins to cure. If the tape is removed after curing has started, the silicone tears rather than cutting cleanly.

Curing time: most sanitary silicones require 24 hours to skin and 72 hours to fully cure before the bathroom is used. Do not expose the joint to water during this period.

Quality assessment

A correctly applied silicone joint: - Has a consistent width throughout its length - Is slightly concave in profile (compressed against both tile surfaces) - Has clean, sharp edges where it meets the tile surface (no smearing or ragged edges) - Is a uniform colour with no voids or bubbles - Sounds solid when tapped (not hollow — hollowness indicates incomplete bonding or voids)

A poor silicone joint: - Has variable width (particularly at corners where the tiler has struggled with the geometry) - Is convex or uneven in profile - Has silicone smeared onto the tile surface outside the joint - Has grout or adhesive contamination visible through the joint - Shows mould within months (indicating non-sanitary product or poor surface preparation)

Re-siliconing: when and how

In a bathroom in regular use, silicone joints have a lifespan of 5–15 years depending on quality of original application and maintenance. Signs that re-siliconing is required: visible mould that cannot be cleaned, cracking or separation at the joint edges, discolouration that cannot be removed.

Re-siliconing requires complete removal of the old silicone — cutting back with a silicone remover tool, chemical treatment with a dedicated silicone dissolver, and thorough cleaning of the joint faces. Applying new silicone over old silicone without full removal produces an inadequate bond and doubles the time to the next failure. Done correctly, a re-silicone job restores the bathroom to a fresh appearance and restores the waterproof integrity of the junctions.

ASAAN has carried out silicone finishing works as part of both new bathroom installations and snagging/remediation works on completed renovations in prime London properties.