Natural Stone Paving: Sandstone vs Slate vs Granite, Bed Thickness, Pointing Mixes and Sealing

Quick Answer: Natural stone paving spans Indian sandstone (£35–£70/m² supply, soft, porous, riven texture), Welsh or Cornish slate (£60–£120/m² supply, dense, dark, fine cleft), and granite (£70–£140/m² supply, very dense, cut or flamed). Domestic patios go on a 30–40mm full mortar bed of 4:1 sharp sand:cement, jointed at 8–12mm with a 4:1 mortar or proprietary brush-in compound. Sealing is optional but recommended on light-coloured sandstone to resist iron staining.

Summary

Stone is the premium choice for visible domestic paving — patios, paths, terrace areas. The supply chain has changed dramatically over the past two decades. Indian sandstone, almost unknown in the UK before 2000, now dominates the value end of the market. Quarries in Rajasthan and Madhya Pradesh ship riven flagstones to UK importers (Marshalls, Brett, London Stone) in sufficient volume that prices have stabilised around £30–£50/m² ex-yard for standard buff/grey, with premium colours (Mint, Raj Green, Modak) commanding 30–50% more.

UK and Irish stone — Yorkshire stone, Pennine sandstone, Welsh slate, Cornish granite — costs 2–4× more but has a hard-to-fake authenticity. For a mid-budget domestic patio, Indian sandstone is now the default; UK stone is a deliberate choice on heritage or premium projects.

The construction technique differs from concrete block paving. Stone slabs are larger, irregular in thickness, and bedded on a full mortar bed rather than a sand laying course. Joints are wider (8–12mm vs 2–5mm for block), pointed with mortar or a proprietary slurry, and the surface is laid to a fall but with a slightly less precise tolerance than blockwork.

Key Facts

Quick Reference Table

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Stone Density (kg/m³) Water absorption Indicative supply £/m² (2026) Best use
Indian sandstone (Raj Green/Modak) 2,300–2,400 4–8% £30–£50 Budget patios
Yorkshire sandstone 2,400–2,600 2–4% £80–£140 Heritage patios, paths
Welsh slate 2,700–2,800 <1% £60–£100 Patios, paths, contemporary
Cornish granite 2,650–2,750 <0.5% £80–£140 Premium, vehicular, kerbs
Limestone (Portland, Bath) 2,200–2,400 4–10% £60–£120 Heritage, formal
Travertine (imported) 2,400 3–5% £40–£75 Patios (treated)
Porcelain "stone" 2,300 <0.5% £30–£60 Modern, low-maintenance

Detailed Guidance

Selecting stone — beyond colour

The selection drivers are surface finish, thickness consistency, and weathering performance. Riven (split natural face) is the most common patio finish — natural texture, good grip when dry, slippery when wet with algae. Sawn finishes (cut by saw to a flat face) are flatter and more contemporary. Flamed (granite, heat-treated) gives a rough textured surface with high grip.

Thickness consistency matters for laying speed. Calibrated stone (mechanically planed to a uniform thickness, typically 22mm or 30mm) lays much faster than uncalibrated. The premium for calibration is small (£3–£5/m²) and pays back in labour.

For sandstone, ask for the petrographic data: open-pore stones are more frost-prone. The Indian sandstone trade publishes water-absorption figures that vary 3–8% by colour — buy the lower-absorption stones for exposed sites.

Sub-base preparation

Same as for block or concrete: 100mm MOT Type 1 for patio, 150mm for vehicular, compacted in lifts. For natural stone over very heavy clay, a geotextile separation layer is recommended.

The mortar bed sits directly on the compacted sub-base — no DPM (which would cause the mortar to slip on a wet sub-base). The mortar bed itself acts as the moisture barrier between sub-base and stone.

Mortar bed — full bed only

BS 7533-12:2006 Code of practice for the structural design of pavements constructed with clay or natural stone pavers mandates a full mortar bed for stone paving. Spot-bedding (mortar dabs at corners) is unacceptable — it creates voids where water collects, freezes, and lifts the stone within 2–3 winters.

Mix: 4:1 sharp sand:cement, slumped to a workable but not wet consistency. Lay 30–40mm thick with a screed bar. As you bed each slab, tap level with a rubber mallet, checking falls with a long spirit level.

For sandstone, dampen the back of the slab before bedding — dry stone sucks water out of the mortar and weakens the bond. Slate and granite don't need this.

Joint pointing

Joints are 8–12mm — wider than block paving because of dimensional variation in stone. Two pointing options:

For Indian sandstone, brush-in with Probond Easy or Geofix is the trade default — fast and weed-resistant. For Yorkshire stone or premium granite, hand-pointed mortar gives a better finish.

Sealing — a soft topic

Sealers fall into two camps: penetrating (impregnating, no surface change) and topical (film-forming, gloss/matt finish).

For sandstone, a penetrating sealer reduces water absorption and largely prevents iron staining (the orange/rust marks that bleed through some pale stones). Apply to a clean dry surface in two coats; reapply every 3–5 years.

Topical sealers create a wet-look finish that some homeowners want and others find artificial. They wear away on traffic surfaces within 18 months and must be reapplied. They can also turn opaque if water gets trapped underneath.

For slate and granite, sealing is rarely needed — these stones are dense enough to resist absorption naturally. Some owners use a wet-look finish enhancer for aesthetic reasons.

Indian sandstone and the iron-staining issue

Pale Indian sandstones (Mint, Sagar Black, certain Modak batches) contain iron pyrite that oxidises in damp conditions and bleeds orange/rust streaks through the stone face. The staining is permanent and cannot be removed once it has migrated to the surface.

Mitigation: source from suppliers who hand-pick out iron-bearing slabs at the quarry; apply a penetrating sealer immediately after laying and curing; keep the patio dry during the first month if possible. Stains that have already developed can be reduced (not removed) with oxalic acid cleaning followed by sealing.

Heritage and conservation considerations

Yorkshire stone in conservation areas or on listed properties may need conservation officer approval before lifting and re-laying. The traditional bedding method was lime mortar on hardcore — modern cement mortar can damage soft stone over time. For listed-building work, specify NHL 3.5 or NHL 5 hydraulic lime mortar 3:1 sand:lime, no cement.

Frequently Asked Questions

Why is my new sandstone patio showing white marks?

Efflorescence — soluble salts (calcium carbonate, calcium sulphate) leaching from the cement mortar bed and depositing on the stone as water evaporates. It typically clears with weathering over 6–12 months. Hard cases respond to brick acid wash followed by a clean water rinse. Sealers prevent recurrence.

Can I lay sandstone on a concrete slab base?

Yes — a fresh concrete slab is an ideal base for stone paving. Wait at least 28 days for the slab to cure before laying. The mortar bed sits on the slab in the normal way, and saws cuts can extend through the slab to provide expansion joints.

My slabs are rocking — what's gone wrong?

Either spot-bedding was used (creating voids), the mortar bed wasn't fully bedded with the slab tapped down (air pockets), or the sub-base has settled. For a single rocking slab, lift, clean, replace mortar bed, re-bed. For multiple rocking slabs, the sub-base or mortar bed technique was wrong throughout — full lift and re-lay is the only durable fix.

Are porcelain pavers actually better than natural stone?

For uniformity and stain resistance, yes. Porcelain pavers are dimensionally consistent, frost-proof, very low water absorption, and easy to clean. They lack the natural variation of stone and the surface — when chipped — exposes the porcelain body which is a different colour from the surface print. For a low-maintenance contemporary patio, porcelain is the practical choice; for warmth and authenticity, natural stone wins.

How long should a stone patio last?

A correctly laid stone patio outlasts the homeowner. Yorkshire stone patios from the 1900s are still in service. Indian sandstone, being softer and more porous, has a shorter realistic life of 25–40 years before the surface laminates and becomes worn. The bed and sub-base last as long as the slab itself if installed correctly.

Regulations & Standards