Cracked Pointing Repaired across 18 Metres of Old Riven Slabs with a GftK VDW 840 Resin

GftK VDW 840 is sold as a two-component epoxy jointing resin for joints from 5 mm wide and 30 mm deep, and it was used across the 18 metre riven run described here. The previous joints were sand-and-cement. They had cracked along most of their length, with moss holding water against the slab edges. Before resin was poured, the failed mortar was raked out to the full 30 mm and the joints were jetted clean, because VDW 840 bonds to the joint walls and can lift within weeks if it is spread over loose debris.

Why the old pointing let go

Sand-and-cement pointing on riven slabs usually fails through a plain sequence. The mix shrinks as it cures, fine cracks appear, then frost and foot traffic widen those cracks season by season. On this 18 metre run, laid perhaps fifteen years earlier, the pointing had debonded from both slab edges along most joints. Whole sections could be lifted out by hand with a plugging chisel.

The surface made the failure easier. Riven sandstone and slate have an irregular split-face texture, so the joint width can wander between 8 mm and 25 mm along one line. On this patio the width changed every 300 mm in places. Cement pointing tooled into that shape has patchy depth and an uneven key, with thin edges first to spall.

VDW 840 suits that kind of joint because it flows into the irregular profile and cures as one flexible-but-hard band. It does not need a uniform joint width to stay in place.

Movement in the slab is a separate fault. If a flag rocks under load, the joint around it will crack again whatever material fills the gap. Several rocking flags on this patio were lifted and rebedded before pointing began, since missed bedding repairs are a common reason resin later takes blame for a failure that started below the slab.

Reading the fall before filling the gaps

A patio should shed water at roughly 1:80 to 1:60 away from the building, about 12 to 16 mm of drop per metre. Across an 18 metre run measured over its width, that fall needs to stay consistent enough to stop water sitting in the joints. Ponding damages both cement and resin pointing, because standing water finds micro-cracks and then freezes.

On this job, a 2 metre spirit level and tape showed a fall of about 1:70 across most of the area. Near a step, an earlier repair had left two slabs slightly proud, creating one flat spot. After rain, water sat there and left a dark tide-mark on the sandstone.

Those two flags were relifted and rebedded on a semi dry screed bedding mortar before the resin went near the joints. VDW 840 is effectively waterproof once cured, so a sound joint sends surface water sideways along the drainage falls. With a flat spot, the same closed joint can form the edge of a shallow puddle.

Mixing and washing VDW 840 on riven stone

VDW 840 is applied as a slurry. The two components are mixed in a forced-action mixer or in a bucket with a paddle for around 3 minutes, until the colour is uniform. The batch is then poured onto wetted paving and worked into the joints with a rubber squeegee.

The slabs need to be damp, with no standing water. The product is designed to move across a wet surface so excess material washes away from the stone face instead of grabbing it.

Riven sandstone is the awkward surface. Its open texture holds a resin film in small hollows, and slurry left to cure on the face can leave a permanent hazy sheen that cleaning will not recover. The practical defence is speed, plenty of clean water, and no waiting once the joints are full.

After filling, the surface was flushed with clean water. The squeegee was pulled diagonally across the joints to lift residue without emptying the fresh fill, then a soft broom finished the face. On warm days the pot life shortens, so the 18 metre area was split into three sections of roughly 6 metres each.

Temperature controlled the pace. GftK specifies application between about 3 and 25 degrees C, and the resin cures faster and stiffer toward the top of that range. Direct afternoon sun on dark riven stone can lift the surface temperature well above the air temperature, so the sections here were poured in the morning when the slabs were cooler.

VDW 840 forms a rigid, effectively closed joint suited to pedestrian and light vehicular traffic. Drainage-permeable resins elsewhere in the GftK range are built to let water pass through the joint rather than seal it, so they behave differently in wet joints and demand a bedding build that can carry that water away. The resin choice has to match the construction below it, and here the sealed joint sat over a solid screed rather than a draining base.

Bedding the flags that moved

Riven slabs normally sit on a semi dry screed bedding mortar, typically a 10:1 sand-to-cement mix laid around 40 to 50 mm and struck off to the fall. The backs of individual slabs are primed before laying.

On this repair, the relifted flags were rebedded that way. A cement-based priming slurry was brushed onto the underside of each slab so the flag bonded to the screed. A dry-laid flag on a semi dry bed is the classic rocker, and a rocker cracks the joint around it whether the joint is cement, block paving jointing compound, or resin.

Block paving jointing compound belongs to another kind of joint. The polymeric sand-type compounds sold for block paving are brushed dry into narrow, uniform block joints and set with a light misting. In the wandering 8 to 25 mm joints of riven stone, they can slump and wash out. VDW 840 was chosen because it fills an irregular joint as a poured slurry and holds without a consistent width.

A resin joint, a primed slab, a correct fall and a clean 30 mm joint depth still sit on whatever screed and sub-base were laid years earlier. On this patio the base was sound apart from two flags, so the 18 metre repair stayed within repointing and local rebedding. Where the base has moved across a whole area, the question shifts to whether the sub-base was compacted properly in the first place, which cannot be answered from the surface. The two rebedded flags had a fresher bond than their neighbours, and the surrounding fifteen-year-old screed remained the older layer in the system.

Ordering by joint volume

For the 18 metre run, the average joint width was about 15 mm and the full depth was 30 mm. With joints roughly every 600 mm both ways across a patio around 1.2 metres deep, the total joint volume came to a little under 20 litres. A wandering joint holds far more material than the neat average on a coverage table suggests, so a floor-area calculation on old split-face paving can leave a resin job short midway through the pour.

VDW 840 is sold by bag weight, and the coverage tables connect bag count to joint width, depth and slab size. Ordering was therefore based on the measured joint profile, including the wider riven sections, instead of the floor area alone.

What the finished run still hides

Once the sections were flushed, broomed and left to cure, the surface read as one continuous run of stone with dark, even joints and water tracking cleanly along the fall. The only visible sign of intervention was the two rebedded flags, which cannot be picked out from the neighbours they now sit level with.

What that clean surface will not tell anyone is how the older screed behaves under the flags that never moved. Those joints held, so nothing forced them open, and the bedding beneath them stayed sealed away under fifteen years of undisturbed stone. Whether that layer is as sound as the two fresh beds is a question the repair had no reason to ask, and no way to see.