Regrouting is the most under-used option in bathroom renovation. A great many bathrooms that get ripped out needed nothing more than new grout and new sealant.
What it costs
Almost all labour. At a day rate of £150 to £200, and with most bathrooms taking a day or less, the cost is modest. Grout is around £17 a bag covering roughly fifteen square metres, and silicone sealant about £20.
Compare that with a retile at £500 to £800 in labour before materials, plus removal at around £30 an hour, plus making good.
What it involves
The old grout is raked out to a consistent depth, either with a hand tool or a powered raker. This is the slow part and it is where the skill lies, because the blade must not chip the tile edges.
The joints are then cleaned and vacuumed, and new grout is worked in, cleaned off and polished once it has firmed up.
Silicone in the internal corners and around the bath, basin and shower tray is a separate material and should be replaced at the same time. Silicone is what accommodates movement; grout in those junctions cracks.
When regrouting is enough
If the tiles themselves are sound and only the grout is discoloured, stained or slightly crumbling at the surface, regrouting will transform the room.
Discoloured grout is what makes a bathroom look tired. People look at a grubby grout line, conclude the bathroom is past it, and spend thousands. New grout and new silicone frequently make the same room look renovated.
When it is not enough
If tiles sound hollow when tapped, the problem is the bond, not the grout. Regrouting a wall whose tiles are coming away buys nothing.
If grout is falling out repeatedly in the same place, something is moving, and new grout will crack in the same line. That is usually a substrate issue or a missing movement joint.
If there is any sign of water having got behind the tiles, staining on an adjacent wall, a soft patch, or a mark on the ceiling below, the tiles need to come off and the cause found. Regrouting seals the symptom and lets the damage continue.
And if the tiles are simply ones you dislike, no amount of grout will help.
Mould in grout
Black spotting in bathroom grout and silicone is mould growing in a damp, poorly ventilated room.
Regrouting removes it, but it will return unless the ventilation is dealt with. An extractor fan that actually runs long enough to clear the moisture, and gets used, does more for grout longevity than anything the tiler can do.
Mould in silicone in particular grows into the material and cannot be cleaned out. Replacing silicone is cheap and worth doing on its own.
Colour
Regrouting is a chance to change the grout colour. Moving from white to grey or a darker tone in a heavily used shower or behind a hob hides future staining substantially and is the single easiest way to make grout stay looking clean.
Test on a small area first, because grout colour looks different dry, and against your tiles rather than on the packet.