Repoint a York Stone Path with a Sika Slurry Mix Over a 15-Metre Run

Slurry pointing fills the joint by spreading a wet, self-levelling mortar over the paving and clearing the surplus from the stone face before it hardens. Sika sells products under the FastFix name and in comparable brush-in ranges, and the wording on tubs can make two different materials look closer than they are. True slurry products are pourable cement mixes. Resin-bound jointing compounds, often stocked nearby, set as they dry and need bone-dry joints; cement slurry cures by hydration, so the stone is dampened at the surface during the work. Treating a resin tub as though it were a cement slurry is the common first-metre failure on a 15-metre run.

For a 15-metre path with joints around 20 mm wide and 25 mm deep, budget for two to three 20 kg units. That size of joint is typical on reclaimed York stone laid on a mortar bed. Irregular flags with wider gaps push the quantity higher.

Check the joint depth before buying the mix

Slurry needs enough depth to grip. Most manufacturers give 20 to 25 mm as the minimum for a cement-based pour. If the joint is shallower, the new mortar is left sitting proud, then it cracks under foot traffic and lifts within a season.

Work along the whole 15 metres with a screwdriver and test each joint. Mark the places where old mortar has failed to more than 30 mm and the places where the remaining pointing is only shallow.

Old lime or cement pointing that survives at 8 or 10 mm has to be raked out until the material below is sound, or until the joint reaches the depth the slurry needs. A grout rake works, and an angle grinder with a 3 mm diamond blade is faster than a bolster and lump hammer. The grinder throws silica dust, so the job calls for an FFP3 mask and a path wetted down first. York stone paths are often bedded on sharp sand with a spot of mortar, and hard raking near the arris can loosen a flag. Keep the tool in the centre of the joint and stop when bedding sand appears under the point.

The wash-off window decides how clean the flags stay

Once the mixed slurry is poured and pulled across the flags, a cement film starts forming on the face of the stone. On a warm dry day, the workable period can shrink to 20 or 30 minutes. In cool damp weather it can stretch to an hour or more. Across 15 metres, a slow wash-off at the far end leaves hazed York stone; once that film has hardened, brushing will not clear it and acid is left as the way to shift it.

Set up the work in short sections. Wet the flags with a fine spray so the stone does not steal water from the mortar. Tip the slurry onto the surface, push it into the joints with a rubber squeegee held at 45 degrees to the lines, then work from another direction to fill the places that have slumped.

The brush comes out only after the surface film has changed from wet-shiny to matt. Use a soft coco or nylon brush and sweep diagonally across the joints, which strikes them off while lifting residue from the flag face. If the brush arrives while the mortar is still loose it pulls material out of the joint; if the film has already keyed to the stone, the bristles skate over the problem and leave haze behind.

That timing is why professionals split a 15-metre path into two or three sections. They also keep each batch small enough to wash off inside the working window. One large mix spread across the whole run almost guarantees a dirty finish at the far end.

A traditional dry-mix or trowel-pointed joint takes longer and is harder on the knees. It avoids the haze risk because the mortar is placed into the joint without being flooded across the face of every flag. Slurry gains its speed from that flooding, so the saved time depends heavily on judging how quickly the particular stone in front of you is drying.

The mix should pour, but it should still have body, roughly like single cream. With excess water, the cement separates and the finished joint turns weak, pale and dusty. A stiff mix leaves voids at the bottom of a 25 mm joint, where trapped water can freeze and blow the pointing in the first hard frost.

The far end of the path is where discipline usually slips. By metre 12, the tempting move is to overfill the bucket to save a trip. That batch is the one most likely to haze.

Keep a second bucket of clean water and a sponge float beside the work for edges along walls and lawns. Slurry that spills onto grass will scorch it. A film at the foot of a brick wall is much harder to remove than residue on flat stone.

Weather sets the calendar

Cement slurry will not cure below about 5 degrees Celsius, and frost in the first 24 hours destroys a fresh joint. Beating sun causes another failure, with the surface skinning while the core stays soft. The practical slot through much of the year is a still, overcast day above 8 degrees with no rain forecast for 12 hours after finishing.

Where sealed joints send the water

A repointed path sheds more water than an open, weedy path that used to soak some of it away. On a 15-metre run beside a house, that extra runoff needs a route. If the paving falls toward the wall, sealed joints can drive water against brickwork and up under the damp course.

Check the fall with a spirit level on a straight edge before pointing. A path that drains toward the building rather than away from it has a bedding problem, and slurry can make that problem worse.

A rainwater diverter on the nearest downpipe, feeding a water butt, reduces the load on a gully that now receives water faster from the newly sealed path. It is a small downpipe alteration costing a few pounds and it cuts the volume hitting the path edge during a storm. Where the paving meets a border, a shallow gravel channel along the edge gives sheeting water somewhere to drop before it pools against the stone and freezes into the fresh joint.

Clean reclaimed York before the slurry is mixed

Reclaimed York stone often carries old mortar, moss, algae and a century of ingrained grime. Slurry bonds badly to a dirty or greasy joint arris, so the cleaning has to happen before pointing. A stiff brush with plain water removes loose moss. Black algal film usually lifts with sodium hypochlorite diluted at about one part bleach to four parts water, left for 10 minutes and rinsed hard, although it can lighten the stone unevenly and a corner test shows how the flag will react.

Pressure-washing immediately before slurry pointing causes a different failure. At 150 bar, the jet drives water deep into the bedding sand and can leave the joints saturated for days after the flag surface appears dry. Cement slurry poured onto a waterlogged joint base may sit as a plug instead of bonding, then pop out later. After washing, the path needs two or three dry days before pointing.

Reclaimed flags also vary in thickness by 20 or 30 mm across a single load. The depth measured beside one flag says little about its neighbour, so each joint has to be gauged on its own.

The surface matters as much as the depth. Smooth sawn flags usually wash off cleanly. Heavily riven York stone is full of pits where slurry residue lodges, and the first brush pass can miss the film sitting in those hollows. A second, lighter pass with a damp sponge after the joints have taken their initial set may be needed, working across the pits. The awkward leftover is the same riven face that gives reclaimed York stone its character also keeps hold of the last thin film.