Up to 40% Less Efflorescence from a Lithofin Sealer on a Sandstone Path

Efflorescence is the white crust left behind when soluble salts move through sandstone and crystallise as water evaporates. Those salts can originate in the flag, the bedding mortar, and the MOT Type 1 sub-base beneath the paving. A Lithofin impregnating sealer such as Lithofin MN Stain-Stop slows the movement of liquid water through the stone, which is the route behind the up to 40% less efflorescence claim. The percentage describes a comparison between treated and untreated stone under similar exposure and salt loading.

An impregnating sealer works inside the pore structure of the slab. It leaves the surface film-free, reduces water ingress, and still allows moisture already present in the stone to dry out. Salt below the slab can keep feeding bloom for months, so applying sealer over saturated, salt-loaded sandstone gives poor results. The 40% figure assumes that water movement has already been controlled by fall, drainage and a compacted sub-base.

Sub-base work that changes the result

Start below the slab. Poorly compacted ground moves under load, holds water after rain, and feeds salt movement upward through the paving. MOT Type 1 is the granular material specified across most UK and Irish paving work. It needs to be laid and compacted in layers no thicker than 100mm to 150mm before consolidation. A single 250mm dump compacted in one pass leaves voids in the centre of the layer, and those voids can hold water for weeks.

For footpath widths, a wacker plate of about 100kg is the usual tool. Driveway-grade sub-bases use larger reversible plates in the 300kg to 400kg range. The finished layer should ring hard underfoot and remain visibly still as the plate crosses it. A light spray of water during compaction helps the fines lock into the aggregate. Too much water turns Type 1 into a slurry that shifts under the plate and later under traffic.

Water held in an uncompacted sub-base has a simple escape route: upward through the flags. Every litre that evaporates through sandstone leaves dissolved salts on the surface. This is why two identical paths can look different after their first winter. One laid on 150mm of well-consolidated Type 1 has fewer wet pockets under the flags. One laid on loose backfill keeps feeding moisture into the stone.

Drainage above the sub-base still matters. If the bedding and joints allow water in faster than the structure can shed it, the stone remains wet for longer after each storm. A sealer can reduce absorption at the slab face, while the build beneath decides how often the slab is forced to dry upward.

Clay subgrades add another weakness. A geotextile membrane between the subgrade and Type 1 stops aggregate migrating into soft ground under load. Terram-type separation fabric is cheap beside the labour involved in relaying a path that has dropped 40mm at the joints.

The visible failure often appears at the surface, although the cause sits lower. White bloom, moss, rocking slabs and sunken joint lines all point back to water, movement, or both. Sealer becomes useful once those routes have been narrowed.

Cross fall and the 20mm edge drop

Set a fall of roughly 1 in 60 to 1 in 80 across the path, leading water away from any building. A rotating laser level with a receiver on a staff reads the drop directly. On a 1.2m wide path at 1 in 60, one edge sits 20mm below the other. An error of half a degree can create low points, and pooled water is the main driver of both efflorescence and moss.

String lines and a spirit level with a graduated shim still work on short runs. Across a 6m path, a laser removes the cumulative error that creeps into string-and-eye setting. Check the fall at three positions along the run, including both ends and the middle, because a path can fall correctly overall while carrying a flat belly where the sub-base dipped during compaction.

Joints: resin, mortar and kiln-dried sand

Sandstone flags are usually pointed with mortar or a resin-based jointing compound. Products such as GftK vdw 850 and Marshalls Weatherpoint 365 flow into 5mm to 20mm joints and cure to a semi-flexible finish. That finish handles frost heave better than a rigid sand-cement mix. Resin jointing also closes the joint against side-entry water, which matters because the joint face remains exposed even after the slab surface has been treated.

Kiln-dried jointing sand belongs with concrete block pavers. It is brushed into tight 2mm to 5mm joints, where it creates interlock and transfers load between blocks. In wide sandstone joints, the same sand leaves an open water path and washes out in the first heavy rain. The two products serve separate jobs, and a merchant selling jointing sand for a riven sandstone patio has either misread the work or is clearing stock.

Resin compounds have their own working window. Most need 5C or above and a dry stone surface to cure properly. Application on a damp autumn morning wastes the material. The joint also needs to be at least as deep as it is wide so the resin can key properly. On 22mm sandstone, that usually means filling the full slab thickness down to the bedding.

The permeable block option

Marshalls Priora is a permeable block system. The blocks carry nibs that create a permanent 5mm to 6mm gap, filled with 2mm to 6mm angular grit so water can pass through the surface into a clean, open-graded sub-base.

This uses a different water strategy from a conventional sandstone path with cross fall. The surface accepts rain through the joints. On driveways where planning rules discourage water running onto the highway, that can change the drainage compliance of the installation.

MOT Type 1 is unsuitable as the sub-base for a permeable build because its fines block the voids. The correct layer is clean, single-sized crushed stone, typically 4/20mm, laid over a permeable geotextile so the water has somewhere to go. Build a permeable surface on standard Type 1 and the first storm leaves water held at the interface. The system works only when every layer supports permeability, from the laying course grit down to the subgrade.

Where the 40% claim has room to show

A 10m by 1.2m sandstone path gives 12 square metres of paving. Put that path on 150mm of compacted Type 1, set it to a 1 in 60 cross fall, and resin-joint it with a frost-resistant compound. On that build, the sealer is treating stone after the main water pathways have already been narrowed. The remaining salt movement through the flag face becomes the route most affected by the sealer.

Use the same flags on loose backfill with kiln-dried sand in the joints and the result changes. Water enters through the open joints, sits in the sub-base, and evaporates through the stone continuously. A sealer on that path addresses only one of several open pathways, so the observed reduction will fall well below the headline figure. The percentage is arithmetic on the treated surface, while the installation decides how much water reaches that surface from below and from the sides.

Stone type changes the number

Sandstone varies from dense to open-textured. A dense Indian sandstone such as Kandla Grey absorbs less water than a soft, open Yorkstone reclaim, so the same sealer on the same build can produce different visible results. The 40% figure is an estimate across typical stone.

Porcelain, clips and retrofit spending

Porcelain slabs sidestep most efflorescence because vitrified porcelain has near-zero water absorption and almost no salt reservoir to bring to the surface. Levelling clips, the plastic wedge-and-base systems used to hold adjacent 20mm porcelain slabs flush during bedding, solve lippage on large-format tiles that would otherwise rock on a slurry primer bed.

After two winters of bloom, repeated cleaner and sealer coats compete with lifting the flags, correcting the fall and sub-base, then resealing. That leaves the awkward choice between spending on the visible crust and paying to expose the hidden layers beneath it.