3 Litres of Rompox Jointing Compound Sealing 15 Metres of Flagstone Gaps

One Tub Only Reaches 15 Metres on Narrow Joints

The coverage figure on a Rompox tub is based on joints around 5 mm to 8 mm wide and about 30 mm deep. Open those joints out to 15 mm and the same 12.5 kg tub, roughly 3 litres of usable slurry after mixing, is nearer an 8 or 9 metre product. The label number is easy to over-read on a first patio, especially when the paving is riven rather than sawn.

Marshalls Fairstone laid by hand rarely gives you one neat gap all the way across the work. One joint might be 6 mm, the next can be close to 20 mm where an uneven stone edge has been kept in the pattern. Any coverage number is therefore an average over the terrace, and the wide spots take a surprising amount of material.

Measure before buying the tubs. Take five or six joint widths across the patio, check the depth the compound can actually reach, then convert the total joint length into volume. Fifteen metres of joint at 10 mm wide and 30 mm deep needs about 4.5 litres of filled volume, already more than one tub provides. For most patios with 15 metres of joints, two tubs with a little left over is the safer figure. Running out mid-pour leaves a cold join that never quite blends.

Damp Stone, Drained Joints

Rompox Easy is a water-permeable resin and aggregate compound. It needs the stone face damp enough for the slurry to move cleanly over the surface and into the gaps, while the joint itself must be free of standing water. Hose the whole area, wait for the surface sheen to go, and begin while the slabs are still cool and slightly moist to the touch. Water pooled in the joint dilutes the binder, and the cured material can end up soft enough to thumb out weeks later.

Timing does most of the damage when jobs fail. In hot sun, the slabs can dry before the compound has settled properly, so resin and dust skin on the surface and leave a haze. A cold damp morning brings a different problem: water sits low in the joint and the cure stays weak. The workable spell is mild, dry and overcast, with the build below already drained.

A patio on MOT Type 1 with a proper fall usually clears water from the joints in ten or fifteen minutes. Compacted clay without a membrane below holds water far longer, and the sweeping becomes a fight against wet pockets under the compound.

A 5 Metre by 3 Metre Fairstone Pour

Take a 5 metre by 3 metre patio in Fairstone sandstone, laid in a random pattern with joints averaging 8 mm. Add every internal seam and the total joint length comes close to 34 metres, although the perimeter might make the job look like a 15 metre run at a glance.

Hose the slab down at about 9am while the stone is still cool. Wait until surface water has gone and the sandstone remains dark with damp. Tip half a tub onto the paving and pull it diagonally across the joints with a soft rubber squeegee, pushing the compound down and away from the stone face. Work in one direction first, then cross at ninety degrees to catch the joints missed by the first pass. Diagonal strokes keep the squeegee from dropping into the gap and dragging fresh compound back out.

When the joints are full and slightly proud, change to a soft broom and clear the slab face carefully. This part gets rushed because the joints already look finished. Leave Rompox sitting on Fairstone for twenty minutes in sun and a grey film can bond to the stone beyond ordinary brushing. A second dry broom should stay in use while the filling continues.

Mist the joints with a fine spray to settle the surface and begin the cure. A jet from a hose trigger can blast material out of the gap. A watering can with a fine rose gives a gentler wetting. The filled joint may drop about 2 mm as it settles, which is why it was left proud before misting.

On 34 metres of joint at that width and depth, expect to use close to two full tubs. One tub normally gets through the first patch and dies around the halfway mark. Having the second tub open and ready keeps the two halves curing together; batches that are hours apart can leave a faint boundary line.

Foot traffic should stay off for at least 24 hours, and longer if the weather turns cold or wet during the cure. Full hardness in a permeable resin compound takes several days. Heavy rain in the first hour can wash binder out of the top few millimetres before the surface has tightened.

Edges Fail First

The perimeter joint against a lawn, wall or resin bound driveway edge is usually the first place to crack out. The slab has more movement at a free edge and often lacks an adjacent stone to lock against, so a haunched mortar edge under the perimeter slabs holds the rim still and stops that outside joint flexing under foot traffic.

The Base Under the Slab Controls the Joint

A jointing compound can only stay as stable as the paving below it. Rompox Easy is permeable by design, so water passes through the joint and into the bedding layer. That bedding has to drain. Flagstones on a full mortar bed over 100 mm to 150 mm of compacted MOT Type 1, with a fall of around 1 in 80, allow water to move through the joints and away. The joint then has a much better chance of staying intact for years because the slab is not heaving or holding water against the underside of the compound.

Trouble starts with spot dabs of mortar and voids under the slabs, or with slabs laid straight onto sand over clay. Water drops through the permeable joint, reaches a layer it cannot drain through, sits there, and freezes. Frost lifts the slab by a fraction, the joint cracks, more water enters, and by the second winter the compound may be loose across a third of the patio because the base has moved beneath it.

A Buzon pedestal terrace takes the opposite approach. Porcelain or stone sits on adjustable feet over a waterproof deck, so the gaps stay open and water is meant to fall straight through. A slurry joint does not belong there. The build below the paving matters more than the brand name on the tub.

Kiln-Dried Sand Belongs Somewhere Else

People often reach for kiln-dried jointing sand out of habit and expect it to behave like Rompox. It does not. Kiln-dried sand suits block paving with tight 2 mm to 3 mm joints, where the blocks interlock and hold each other in place. The sand fills the void and stiffens the pavers, but it has no binder. In wide flagstone joints it washes out with the first heavy rain and grows weeds by the second month.

A resin compound bridges wider gaps because the binder locks the aggregate together and grips the slab edges. The cost is that failed resin cannot be topped up loosely like sand on a block drive. Loose material has to be raked out back to sound compound, then the section is re-poured wet and blended into the existing edge while it is still workable. A dab of fresh compound pressed onto a cured, dry edge will sit on top and lift off within a week.

Below 4 mm on interlocking blocks, sand does the job. Above 5 mm on independently laid flagstones, the resin compound earns its cost.

The awkward part is that the same open joint that lets water escape also lets a weak base show itself.