Choosing the right sandstone slab thickness for your application

Slab thickness is one of the first decisions in a sandstone specification, and it is often made on cost grounds alone. That is not always wrong, but it can lead to problems if the structural requirements of the application are not properly considered. This note sets out the practical differences between the thicknesses we stock and the applications each is suited to.

20 mm slabs ¶

Twenty-millimetre slabs are the lightest and least expensive option. They are suitable for interior flooring on a solid, level substrate, and for wall cladding where the stone is fixed mechanically or with a full-bed adhesive to a stable backing. They are not suitable for external paving where they will be trafficked by vehicles, and they require a very flat, well-prepared substrate to avoid point loading that can cause cracking.

The weight saving is significant: a 20 mm slab weighs roughly 50 kg per m2 depending on the stone type, compared to 125 kg per m2 for a 50 mm slab. For upper-floor interior applications where structural loading is a consideration, 20 mm is often the only practical option.

30 mm and 40 mm slabs ¶

Thirty and forty millimetre thicknesses cover the majority of external paving applications for pedestrian use. BS 7533 guidance for natural stone paving recommends a minimum of 35 mm for pedestrian areas on a mortar bed, which means 40 mm is the more conservative and commonly specified choice for external terraces, paths, and public realm work.

Forty millimetres also gives the stonemason more tolerance when cutting to size or making adjustments on site. Thin slabs are more vulnerable to edge chipping during handling and cutting, and a 40 mm slab will survive the installation process with fewer losses than a 20 mm or 30 mm equivalent.

50 mm slabs ¶

Fifty-millimetre material is specified for applications with occasional light vehicle access, such as a driveway with infrequent car traffic, or for heavy pedestrian use in commercial or public settings. It is also used where the substrate preparation is less controlled, since the additional thickness provides more tolerance for minor irregularities in the bedding.

The weight of 50 mm slabs means that handling on site requires more care and, for larger format pieces, mechanical assistance. We can cut 50 mm material to smaller formats to make site handling more manageable, though this adds to the cutting cost and increases the number of joints in the finished surface.

A note on cladding applications ¶

For wall cladding, the structural requirement is different from paving. The slab needs to resist wind load and the stresses of the fixing system rather than vertical traffic loads. Twenty and thirty millimetre thicknesses are commonly used for cladding, but the fixing method, the backing structure, and the size of individual panels all affect whether a given thickness is adequate. If you are specifying sandstone cladding for a building facade, the fixing design should be reviewed by a structural engineer before the stone thickness is finalised.

If you are unsure which thickness is right for your project, send us the application details and we will give you a straightforward recommendation based on what we have seen work and what we have seen fail.