Slab thickness: 16, 18, 20 and 30 mm explained

Thickness is nominal, not exact

A slab sold as 18 mm is not 18.0 mm everywhere. Stone is sawn, not rolled, and the blade wanders a little. A millimetre or so of variation across a slab and between slabs of a lot is normal. A slab that is 15 mm at one end and 19 mm at the other is not, and will show as a step at every joint. Measure, and agree a minimum thickness rather than a nominal one (see the guide on buying granite by the square foot).

16 mm

The thinnest common slab. It is fine for vertical work: wall cladding, dado, the sides of a counter, cupboard faces. It is also fine for flooring in light-traffic rooms, because a floor slab is bedded on mortar across its whole back and never carries load across a gap.

It is not for overhangs, breakfast bars, or window sills that stick out. As a kitchen counter it works only on a solid bed (masonry, concrete or a well-supported ply deck) with no overhang, and even then the narrow bridge of stone beside a sink cut-out is the weak point.

18 mm

A common counter thickness in the Indian trade. It takes a standard sink and hob cut-out with reasonable margins, sits on a brick or ply base, and gives a clean edge. Plenty of kitchens are done in 18 mm and last for decades.

If the edge looks thin, a laminated edge, where a strip of the same stone is glued under the front and the two are polished together, gives the look of 36 mm without the weight.

20 mm

The safer choice for a working kitchen counter with cut-outs and for anywhere the slab has to carry itself a little. The extra 2 mm adds stiffness, which is what a bridge beside a sink cut-out needs. 20 mm also suits steps, window sills with a small projection, heavy-traffic flooring, and table tops on a frame. Small overhangs of around 100 to 150 mm are usually fine in 20 mm on a dense stone; beyond that, support it or go thicker.

30 mm

Steps and treads in heavy use, outdoor paving that takes vehicles, counters with a real overhang such as a bar, and anywhere the edge should look substantial without lamination. Not every stone or lot is available at 30 mm; ask what is in the yard.

Weight, so you can plan the lift

Granite weighs roughly 2.7 tonnes per cubic metre. Per square foot that is about 4 kg at 16 mm, 4.5 kg at 18 mm, 5 kg at 20 mm and 7.5 kg at 30 mm. A 20 mm slab 8 ft by 3 ft is therefore around 120 kg; a full gangsaw slab can be several times that. This matters for the stairs, for the base under the counter, for wall fixings, and for how many people you need on site.

Span and support

Stone is strong when you press on it and weak when you bend it. A slab supported along its whole back will carry a lot. A slab bridging a gap cracks at a surprisingly low load, usually from a cut-out corner.

Rules for counters: keep at least 75 mm of stone between any cut-out and the slab edge; radius the corners of cut-outs instead of cutting them square; put a support (brick pier, ply rib, steel angle) under the slab on both sides of a sink and under every joint; never let a joint fall across a cut-out.

Edge profiles and what thickness allows

On 16 and 18 mm the usual edges are a small bevel, a pencil round or a half-round. A full bullnose needs more stone and looks right from 30 mm, or on a laminated edge. Mitred edges, where the front face is cut at 45 degrees and joined to a drop strip, make a thin slab look like a thick block.

On a leather or flamed finish, any edge cut on site is a sawn edge and has to be re-brushed or re-flamed to match. Decide edge profiles before the slabs leave the yard.

Why thinner is not always cheaper

Per square foot, 16 mm costs less than 20 mm. Add the ply deck or extra masonry it needs, the steel angles under the overhang, the higher breakage in transport and cutting, and the wider margin a thin slab needs around cut-outs, and the saving shrinks or disappears. For counters, price the whole job, not the slab.

Gangsaw slabs and cutter slabs

Two kinds of machine make slabs, and we run one of each.

A gangsaw is a frame of many parallel steel blades that moves back and forth over a whole block, with abrasive slurry or diamond segments doing the cutting. It slices the entire block into slabs in one long pass. The result is large, full-size slabs the width and length of the block, which is what you want for a long counter run without joints or a feature wall. Thickness varies a little more across a gangsaw slab because the blades flex.

A cutter uses a circular diamond blade and cuts strips off a block one at a time. The slabs are narrower, typically two to three feet wide, and generally more even in thickness. They suit stairs, sills, skirting, dado and smaller kitchens, where the width is going to be cut down anyway and a narrower slab means less wastage.

They suit different jobs. Tell us the job and ask what is in the yard.

How to measure on site

Carry a vernier. Measure each slab at several points along both long edges and at the corners; write the thinnest reading on the slab with a wax crayon. Stand the slab on edge and sight along it for bow. Look at the back for deep saw marks, which are where thickness is lost. Do this at the yard, before loading, because a fault is harder to dispute once the slab is on the lorry.