Stone worktop thickness, overhang and support

At a glance

Factors multiply the value you had. Exact beam statics for a strip of one material, load and width, not safe limits. Stone strength and stiffness vary by stone and by slab and no stone value is used.

What an overhang asks of stone

An overhang is a cantilever: the part of the top beyond the last support carries its own weight and any load put on it, and passes it back to the support as a bending moment. Stone is stiff but brittle, and its strength varies from slab to slab and with the direction of the veining or bedding, so the question is never only ‘how far’ but ‘how far for this stone, this thickness and this load’.

Three dimensions and one material property decide the result: the thickness t, the overhang a and the depth D of the whole top, with the stone’s stiffness. For a strip of top of width b the section stiffness is E × I with I = b t³ / 12. For a load P on the tip, the sag is P a³ / (3 E I) and the bending moment at the support is P a; for a load spread along the overhang the sag is w a&sup4; / (8 E I) (Wood Handbook, equations 9-2 and 9-3 and Table 9-1, which give these as beam formulae for any material). Those few lines produce the table above. They are exact for a strip of a single material and constant section.

What the table cannot give is a safe size. That needs the load the edge must carry (a person leaning, sitting on a stool and pushing back, or climbing onto the top) and a strength that can be trusted. We found no published rule for either for stone worktops. The one source that could supply it, the Natural Stone Institute’s Dimension Stone Design Manual, is paywalled and we did not obtain it. The web pages that quote an unsupported overhang for 30 mm stone disagree, and none links to a primary document. So the limit is the partner fabricator’s to state for the stone it is cutting.

Two practical points follow from the arithmetic. First, thickness is the strongest lever: going from 20 to 30 mm cuts the sag to under a third and the bending stress to well under half, for half as much again in weight. Second, the overhang is the most sensitive dimension: a few extra hundred millimetres at a seating edge is a larger change than any change of thickness the design is likely to make.

How heavy is the top

Stone is heavy, and the weight sets how the top is delivered, lifted and carried by the cabinets. At an assumed density of 2,700 kg per cubic metre (our assumption; the stone’s own density test report governs) the weights are:

Weights are for a plain top with no cut-outs. A cut-out removes weight but also removes the part of the stone that carries the load beside it: see the cut-outs section. Ask the partner for the weight of each piece, because the heaviest pieces are the ones most often damaged in transit and on site.

Overhang, fixing and tipping

A loose top stays down under its own weight only while its centre of mass is over the support. For a top of depth D resting on a support of depth D − a, that holds while the overhang a is less than half the depth. Beyond that, the top tips about the front edge of the support unless something holds it down. A load on the edge tips it sooner.

A worked example of the principle, not a design figure. Take a 30 mm top, 900 mm deep, with a 300 mm overhang along a 1 m run. Its weight is 81 kg per square metre, so 48.6 kg sits over the support and 24.3 kg beyond it. The restoring moment about the front edge of the support is 48.6 × 0.30 ÷ 2 − 24.3 × 0.15 ÷ 2 = 10.9 kg·m, and a load on the edge of about 36 kg per metre of length of the top would tip it, if nothing at all fixes it and the support is rigid: about 36 kg for a 1 m top and about 109 kg for a 3 m top. A person can put that much on an edge. It is arithmetic for a loose top, not a claim about any installed top: stone tops are normally bonded or fixed to the cabinets or brackets, and the fixing, not the self-weight, is what holds them. The point is that overhang and fixing have to be designed together, and the fabricator should say how the top is held down.

Cut-outs, ligaments and seams

A cut-out is a hole in a plate under bending, and the stone beside it is what carries the load. We give no minimum figure because none was found in a primary document and it depends on the stone. The principles are general mechanics and trade practice, and the fabricator confirms the numbers.

• Ligament. The strip of stone left between a cut-out and the front edge, an end or another cut-out. If a ligament is half the width of the full top, the bending stress in it is twice that of the full width for the same moment. A sink or hob close to the front edge of an island, or a narrow strip between a hob and a sink, is the most common place for a crack to start.
• Corners. Stress concentrates at a sharp inside corner, and cracks start there. Cut-outs are made with a rounded corner; the radius is the fabricator’s to state.
• Position against the supports. A cut-out is best over, or between, cabinet walls, not at an overhang where there is no support. The sink or hob is also where the top is heavily loaded, wetted and heated.
• Seams. A joint is a line of least strength. It is best over a support and clear of every cut-out and of the overhang. Where a joint is unavoidable near a cut-out, the bonding and the reinforcement under it are the fabricator’s to specify.
• Edge build-up. A thickened or mitred edge adds weight and looks heavy, but only adds stiffness where it is bonded as one section with the top. See our sheet on edge profiles for the profiles themselves.

For the materials these questions apply to, see worktop materials compared, and for the same statics applied to dining and meeting tables see table top thickness, overhang and span.

What the partner fabricator confirms

Every limit on a stone worktop is a fabricator’s limit for a particular slab. What a designer can draw safely is therefore whatever the partner fabricator has agreed in writing for that stone, with the drawing below the line. Ask for these before the layout is fixed:

Stone worktop: [30] mm [named stone, finish] on [cabinet carcass / 18 mm ply carrier / steel frame] as drawing [ref], depth D [ ] mm, overhang a [ ] mm at [the seating edge / the island end], supports at [ ] mm and [ ] mm from the end. The partner fabricator is to confirm in writing, before production, the overhang it is willing to leave unsupported for this stone and thickness, the load case it is based on, and whether that is from test or calculation.

Cut-outs: [hob] and [sink] positioned as drawing [ref], with a minimum ligament of [ ] mm to the front edge and [ ] mm to any end or other cut-out, inside corners radiused to [ ] mm, made in the workshop. Seams: positioned over [a cabinet wall], not less than [ ] mm from any cut-out, joint width [ ] mm, bonded and levelled with [colour-matched adhesive]. The fabricator is to propose the figures; none is to be assumed from this drawing.

The top is to be fixed to the supports by [adhesive and clips / brackets / mechanical fixings] so that it cannot tip or lift when a load of [ ] kg is applied at the edge at its most unfavourable point. The partner is to state how the top is held down and the load at which it would tip if the fixing failed. No reliance is to be placed on the weight of the top alone.

Provide the flexural strength test report for the slab supplied [BS EN 12372:2022 or BS EN 13161:2008, edition stated], the density, and the weight of each piece. Pieces are to be [carried flat on edge on an A-frame] and the access route checked by [contractor] before delivery.

Standards and sources

• USDA Forest Products Laboratory, Wood Handbook, FPL-GTR-282 (2021): equations 9-2 and 9-3, Table 9-1: beam formulae used for the ratios. No timber value used.
• BS EN 12372:2022 and BS EN 13161:2008: flexural strength of natural stone; BSI Knowledge showed 12372:2022 Current (2006 edition withdrawn 11 Mar 2022) and 13161:2008 Current, under review, on 5 and 6 Nov 2026 (seen in our flooring notes). Texts not read.
• Natural Stone Institute, Dimension Stone Design Manual: the primary reference for stone design in the US; paywalled and not obtained. Not read.
• Our sheet: Table top thickness, overhang and span: same statics for tables.

Questions we are asked

How far can a 30 mm stone worktop overhang without support?

There is no figure we could stand behind. Web guides give numbers that disagree and none links to a primary source, and the primary design manual is paywalled. The answer depends on the stone, the slab, the depth and the load on the edge, so the partner fabricator states it in writing for the stone being cut. Statics tell you the direction: twice the overhang is eight times the sag under a load on the edge.

Is 30 mm stone always stronger than 20 mm?

For the same stone, load and support, a 30 mm top bends about a third as much and carries about 44 per cent of the bending stress, because stiffness rises with the cube of thickness and stress falls with its square. It also weighs half as much again, so supports and handling change. It does not make a long overhang safe: that is for the fabricator to confirm.

How close can a hob or sink be to the front edge?

We give no minimum, because none was found in a primary document and it depends on the stone. The strip left beside a cut-out, the ligament, carries the load round the hole: halve its width and the bending stress in it doubles for the same moment. Ask the fabricator for the ligament it needs for this stone, and for the corner radius.

Where should a seam go?

Best over a cabinet wall or other support, well away from every cut-out and from the overhang, since a joint is a line of least strength. Where the layout forces a seam near a cut-out, the fabricator specifies the bonding and the reinforcement. This is general practice, not a published rule; ask the partner.

Does an overhang need a bracket or a leg?

That depends on the overhang, the thickness, the stone and what people will do on the edge (lean, sit, climb). Brackets and legs add a support so the stone is no longer a cantilever, and they are normally specified once the fabricator has said what the stone can carry alone. A seating overhang is the case to settle first.

Can a loose stone top tip?

Yes, in principle. A top held down only by its own weight stays put only while the overhang is less than half its depth, and a load on the edge tips it sooner. In practice tops are bonded or fixed to the cabinets, and the fixing is what holds them. State how the top is held down.