Repoint 30 Metres of York Stone Flags with a Marshalls Weber Slurry Primer

Prime the joint faces while the slurry is still live

Weber SPP is a styrene butadiene rubber slurry, sold in 10 kilogram tubs, and its job on York stone is bond. The primer goes on the cleaned vertical faces of the joint immediately before the pointing mortar is pressed into place. That wet contact is what ties the new mortar to dense sandstone edges, especially on reclaimed York flags with low porosity.

Mortar pushed against an unprimed dense flag edge can shrink away as it cures. After one or two freeze cycles the failure often shows as a fine break along the stone edge, then a loose strip of pointing. The primer does not fill the joint. It is a brushed film on the sides of the flags.

Mix the slurry to a brushable consistency, about the thickness of single cream. Use a 25mm brush and work it into both faces of the raked joint, including any small hollows left by the old pointing. The mortar should be packed while the primer still feels tacky.

On a warm dry day the useful window is usually around 20 to 40 minutes for each small section. That sets the pace: prime a metre or two, point that length, then carry on along the run. A full prime pass across all 30 metres would skin over before the last joints were filled.

Rake out the old joint to at least 20mm before brushing in the primer, giving the new mortar a proper shoulder to grip. With flags bedded at 40 to 50mm thick, there is plenty of depth to work into. Loose dust left in the joint weakens the whole operation, so rake, brush, clear the joint faces, prime the exposed stone, and fill while the slurry is live.

Check the fall before opening the primer tub

Standing water is the quickest route to failed pointing, so the fall across the patio is checked first. For a paved patio, the working fall is 1 in 80, equal to 12.5mm of drop for every metre of run. It should lead away from any building wall and toward a channel, gully or open bed.

Across 30 metres, that drop becomes too large to ignore. A line of flags that sits flat, or one that falls back toward the house, will hold water in the joints even when the mortar bond is sound.

Use a 2 metre spirit level with a tapered shim, or set a laser level at the high point and read the drop along the run. If the flags cannot be lifted and the existing fall is poor, give the lowest joints the most attention: fill them completely and leave them slightly proud so water sheds across the surface.

Porcelain areas on the same patio change the drainage behaviour. Vitrified slabs are effectively non porous, so water moves off the surface more readily and a fall closer to 1 in 100 can work. The joints still need a clear route to drainage, and on a mixed patio the York stone areas set the minimum fall.

Movement under the flag will crack the new joint

A repointing job only works if the flags beneath it are stable. Where a flag rocks underfoot, fresh pointing around its edges will crack as soon as the flag moves again. That is why the bedding and sub-base have to be checked before committing to the mortar work.

For a domestic patio carrying foot traffic, the usual sub-base is compacted MOT Type 1 granular fill. The minimum consolidated depth is 100mm, laid and compacted in layers no thicker than 75mm with a vibrating plate. Dropping 150mm of aggregate in one go and compacting it in a single pass leaves the lower material loose, which is where later settlement begins.

On soft or clay subgrades, the Type 1 depth increases to 150mm. A geotextile weed membrane is placed between the subgrade and the aggregate. It suppresses weed growth through the joints and separates the ground below from the Type 1, so fines do not pump upward into the sub-base on a wet site.

A non woven geotextile of around 100 grams per square metre is the normal specification. Lay it loose with 300mm overlaps at seams. Do not stretch it tight across the prepared ground.

The flags then need a full mortar bed. A 3 to 1 or 4 to 1 sharp sand and cement mix at roughly 40 to 50mm compacted thickness is the usual bedding. Spot bedding on five dabs is a familiar shortcut, and it is also a familiar cause of rocking flags, because the unsupported centre of the slab can flex under load. Once the slab flexes, the pointing takes the crack.

If only one flag has settled, lift that flag, rake out the old bed, check that the sub-base beneath is still firm, and rebed it on fresh mortar to the level of its neighbours before the joint is repointed.

Mortar quantity for a 30 metre run

Start with measured joints, since width changes the quantity quickly. A joint averaging 10mm wide and raked to 25mm deep has a cross-section of 0.00025 square metres. One metre of that joint uses 0.00025 cubic metres of mortar once filled.

Across 30 metres of primary joints, the base volume is 0.0075 cubic metres. Add the perpendicular joints between individual flags and the total commonly doubles or trebles, depending on the flag sizes. Working from a filled volume of about 0.02 to 0.025 cubic metres and a pointing mortar density near 2000 kilograms per cubic metre, the dry mix calculation lands around 40 to 50 kilograms before margin is added for waste and for mortar pressed below the visible surface.

A practical site allowance for a 30 metre York stone run can therefore sit nearer 90 to 110 kilograms once the actual joint width, joint depth, cross joints and waste are allowed for. Widen the average joint from 10mm to 15mm and the mortar volume rises by half, so measure the joints directly before ordering.

One 10 kilogram tub of Weber SPP covers the edge priming for a run of this size with material to spare, since the primer is only a thin brushed coating on the joint faces and not the material filling the joint.

Cutting and matching stone during the same repair

If a damaged flag has to be replaced during the repoint, York stone cuts readily on a wet cutting bench saw and can also be split along its natural bed with a bolster and lump hammer for a riven edge that matches reclaimed flags. Feed the cut slowly under water to keep the dust down and the blade cool.

Cleaning and curing the finished joint

After the mortar is struck, clean the flag faces before the residue hardens. Keep the joint profile full and neat, with no hollow line for water to sit in.

On heavily weathered reclaimed York stone, cleaning can expose friable edges that are already starting to delaminate, and a joint brushed tight against a crumbling face borrows its lifespan from stone that is quietly giving way underneath. That is the part no calculation for mortar or fall will tell you in advance: whether the edge you are bonding to still has the substance to hold what you press against it.