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

Summary

Natural stone paving — primarily imported Indian Sandstone but also limestone, granite, slate, and travertine — became dominant in UK gardens and patios from the early 2000s. It remains popular because of its genuine variation in colour and texture, its premium aesthetic, and the perception (often accurate) that it will outlast man-made alternatives if properly installed.

However, natural stone has specific installation requirements that differ from concrete block paving. The most important difference is the bedding method: natural stone must be laid on a full mortar bed, not a sand bed. Spot (five-spot) bedding — placing mortar only at the corners and centre of each stone — causes hollow spots under the stone which flex and crack under load, eventually causing the stone surface to break or the stone to rock. This is the most common installation failure in natural stone paving.

Pointing is the other critical area. Mortar pointing that is too strong (high cement content) is brittle and cracks as the slab moves under temperature cycles and ground movement. The correct mix is weaker than most people expect.

Understanding stone type, sourcing, bedding method, pointing mix, and sealing options is essential for anyone working regularly with natural stone paving.

Key Facts

• Full bed only — All natural stone must be laid on a full mortar bed; spot or five-spot bedding is not acceptable for stone over 40mm thick or any stone subject to vehicle loading
• Bed mix — Semi-dry 6:1 sharp sand:cement (or 5:1 for denser stones); mix should clump when squeezed but not be wet; consistency like damp crumble topping
• Bed depth — 25–30mm compacted semi-dry mortar bed for most stone; adjust to meet finished level requirements
• Sub-base — 100mm Type 1 MOT minimum; 150mm for vehicle access areas; same as block paving (see block paving installation)
• Stone thickness — Patio: 20–25mm minimum; driveway: 30–50mm minimum (thinner stone cannot take vehicle loading); granite setts typically 50–100mm deep
• Indian Sandstone — Most popular; buff, silver, autumn, and kandla grey colours; BS EN 1341:2012 natural stone flags product standard; hardness varies by quarry
• Granite — Very hard, frost-resistant; setts (cobbles) 50–100mm deep; long-lasting; heavier to cut
• Slate — Laminated sedimentary rock; smooth surface (slippery when wet — must be riven or brushed for external use); 20–30mm typical thickness for paving slabs
• Limestone — Porous; stains easily; requires sealing; not recommended for driveways with vehicle traffic (softer than granite or good sandstone)
• Pointing mix — 4:1 sharp sand:cement or 3:1 for exposed positions; never use 3:1 or stronger as a standard pointing mix — too rigid and will crack; alternatively use specialist dry-packed jointing mortar (Sandstone Fix, Rompox, EasyJoint)
• Colour bleeding — Some Indian sandstone (especially kandla grey, autumn mix) bleeds iron staining when wet and can stain adjacent walls or other paving; test seal first
• Sealing — Impregnating (penetrating) sealers preferred for external use; do not use film-forming sealers outdoors as moisture becomes trapped and causes spalling; apply after 4 weeks minimum
• Ethical sourcing — Marshalls and other reputable suppliers audit Indian stone quarries; insist on audited supply chain especially for projects where councils or housing developers are involved (see marshalls register contractors)

Quick Reference Table

Detailed Guidance

Stone Selection

Indian Sandstone — The workhorse of UK patio installation. Imported primarily from Rajasthan, it comes in consistent calibrated thicknesses (20mm, 22mm, 38mm, 44mm) for easy laying. Common colour groups:

• Buff — warm golden/cream; popular, ages to silver-grey
• Silver Grey — cool contemporary tone; suits modern properties
• Autumn Brown — rust, orange and chocolate tones; high iron content, bleeds readily; seal before and after laying
• Kandla Grey — blue-grey; high iron; also bleeds; popular but requires careful handling

For driveways, specify 38–44mm calibrated Indian Sandstone. 20–22mm is insufficient under vehicle tyres.

Yorkstone (British Sandstone) — Quarried in Yorkshire and Lancashire. Much harder and more consistent than Indian sandstone, and genuinely frost-resistant due to UK climate adaptation. The premium choice for durability but significantly more expensive than Indian sandstone. Natural riven or sawn faces available.

Granite — Imported or UK-quarried. Technically the most durable natural stone for external use. Granite setts (small rectangular blocks, typically 100×100×100 or 200×100×50) are the traditional UK street paving material. Resistant to frost, heavy loads, and chemical staining. Difficult to cut accurately without a wet tile saw. Much heavier than sandstone.

Slate — Riven (natural split) slate gives a textured non-slip surface suitable for paths. Smooth sawn slate is too slippery for external use when wet. Not suitable for vehicle areas — too thin and brittle under concentrated point loads. Welsh slate (Penrhyn, Delabole) is higher quality and longer-lasting than Chinese imports.

Sub-Base and Preparation

The sub-base specification is the same as for block paving (100mm Type 1 MOT), but natural stone has one additional step: a concrete or mortar blinding layer over the sub-base.

Blinding — A 50–75mm layer of C20 concrete (or lean mix concrete 8:1) laid over the sub-base provides a rigid working platform. Allow to cure 24–48 hours before applying mortar bed. Blinding prevents the semi-dry mortar bed from mixing with the granular sub-base, and provides a consistent base for setting out levels.

Some installers omit blinding and lay the semi-dry bed directly on compacted sub-base. This is acceptable for patio areas but not recommended for driveways.

Laying Method

Semi-dry mortar bed — Mix 6 parts sharp sand to 1 part Portland cement; add just enough water to achieve a damp crumble consistency (squeeze in hand, holds shape, does not drip). This is NOT the same as a wet screed. The semi-dry bed allows minor adjustment of individual stones while remaining firm enough to hold them.

Screeding the bed — Screed the mortar bed to the required depth across the working area using parallel screed rails. Bed depth allows for the stone thickness: finished level = bed depth + stone thickness.

Laying stones — Place each stone onto the semi-dry bed, press and tap firmly with a rubber mallet. Check for full contact: if the stone rocks or sounds hollow, lift and adjust the bed. For large slabs (600×600mm or larger), the semi-dry bed must be fully supporting all four corners and centre.

Bond pattern — Stretcher bond (staggered joints) is most common. Random rectangular (three or four sizes mixed) gives the most natural appearance and is typical for Indian sandstone. Keep joint width consistent: 8–12mm for natural stone is typical.

Cutting — Wet tile saw gives the cleanest cuts on sandstone and granite. Angle grinder with diamond disc also works. Safety: silica dust from stone cutting; same controls as block cutting — wet cutting preferred, FFP3 if dry cutting (see block paving installation for full COSHH guidance).

Pointing

Allow the laid stone to set for at least 24–48 hours before pointing, so stones don't move during the pointing process.

Semi-dry pointing — Mix 4:1 sharp sand:cement; dry enough to pack with a pointing iron or trowel. Press firmly into the joint with a pointing iron, ensuring no voids. Brush off excess with a stiff brush. This is the traditional method.

Dry-pack jointing mortars (EasyJoint, Rompox Traffic, Sandstone Fix) — Pre-mixed, brush-in products that require only water activation. Easier to use, generally consistent results. Some are suitable for driveways (check load rating). Product-specific application instructions must be followed.

Crucial rules:

• Never point in temperatures below 5°C or above 25°C
• Keep joints damp for 3 days after pointing to assist curing
• Protect from heavy rain for 24 hours after pointing
• Remove surplus mortar from stone face before it stains (use soapy water on freshly-laid mortar; chemical efflorescence remover for cured stains)

Sealing Natural Stone

Why seal? — Sealing reduces water absorption, prevents oil and vehicle fluid staining, reduces biological growth (algae, lichen), and can enhance colour. It does not change the texture significantly if an impregnating sealer is used.

Types of sealer:

• Impregnating (penetrating) sealers — Penetrate the stone pores; create a hydrophobic barrier; do not change appearance significantly; water beads on surface; re-apply every 3–5 years; recommended for external use
• Film-forming sealers — Sit on the surface; create a visible sheen; trap moisture in the stone → spalling in frost → not recommended for external use
• Colour-enhancing sealers — Impregnating but add a wet-look effect; popular for Indian sandstone; change appearance permanently (test a small area first)

Application — Stone must be dry (at least 4 weeks after laying, in dry weather). Clean stone thoroughly first. Apply sealer with a lambswool applicator or brush; two coats with 30-minute interval between coats; wipe off excess immediately before it dries as a residue.

Frequently Asked Questions

Why is my Indian Sandstone going orange-brown?

Iron oxide staining from within the stone. High-iron sandstones (Kandla Grey, Autumn Brown) release iron when the stone first gets wet — the iron oxidises and stains the surface and any adjacent surfaces. Treatment: apply a proprietary iron stain remover (muriatic acid-based products — follow safety instructions carefully). Prevention: apply a penetrating sealer to the dry stone before wetting it for the first time.

Can I lay natural stone on a bed of sharp sand like block paving?

No. Natural stone flags must be laid on a mortar bed. Sharp sand (sand-only) bedding does not support the full underside of the stone, and any hollow area will flex and crack the stone under foot or vehicle loading. The mortar bed sets rigid and provides full support.

Should I use lime mortar or cement mortar for pointing?

For most residential natural stone paving, a 4:1 sand:cement mix is appropriate. Lime mortar (NHL 3.5 or NHL 5 + sharp sand, 1:3) is used in heritage/listed building contexts where the softer, self-healing properties of lime are required, and where cement pointing would be refused by planners. Lime pointing requires longer cure protection from rain and cold.

How long before vehicle use after laying natural stone?

Allow mortar bed and pointing to cure for minimum 7 days before any vehicle loading. 14 days in cold weather (below 10°C). Natural stone on full mortar bed cures to sufficient strength within 7 days under normal conditions.

Regulations & Standards

• BS EN 1341:2012 — Slabs of natural stone for external paving; requirements and test methods

• BS EN 1342:2012 — Setts of natural stone for external paving; requirements and test methods

• BS 7533-4:2006 — Pavements constructed with clay, natural stone or concrete pavers; Part 4 code of practice for laying natural stone setts and cobbles

• BS 7533-12:2014 — Code of practice for manufacture, prefabrication and construction of flag pavements using natural stone

• COSHH Regulations 2002 — Silica dust from cutting; risk assessment required (see block paving installation)

• GPDO 2008 — Natural stone with open joints may be permeable (depends on jointing); dense/closed-jointed stone is impermeable and subject to 5m² planning rule on front driveways

• Natural Stone Institute guidance — international; relevant technical guidance on stone types and installation

• Stone Federation Great Britain — UK trade body for natural stone; technical guidance and ethical sourcing

• Interpave technical guidance — covers some natural stone paving guidance alongside concrete products

• suds regulations driveways — planning permission for natural stone driveways depending on joint type

• marshalls register contractors — ethical sourcing requirements for Indian and other imported stone

• block paving installation — COSHH silica dust controls for cutting; sub-base specification

• paving near dpc level — finished level requirements adjacent to buildings