Table top thickness, overhang and span

At a glance

Factors multiply the value you had. Exact beam arithmetic for the same material, load and width (Wood Handbook equation 9-2, Table 9-1): direction and size of a change, not a safe limit.

What decides how a top behaves

Three dimensions and one material property decide how a top behaves under load: the thickness t, the span L between supports, the overhang a beyond a support, and the material's stiffness (its modulus of elasticity, E). For a strip of top the stiffness of the section is E x I, with I = b t³ / 12 for a rectangle of width b and thickness t (Wood Handbook, equation 9-3). Sag is the load times a length cubed divided by that stiffness, with a constant that depends on how the load is applied and how the strip is held (equation 9-2, Table 9-1). Those few lines give the whole table above, and they are exact for a strip of a given material: they say how much more or less sag and stress you get from a change, without needing the load.

What the table cannot give is a safe size. That needs the load a top must carry (a person leaning on the edge, a person sitting on it, a buffet of heavy dishes, someone climbing on it) and a strength that can be trusted. We found no published rule for either for table tops in the sources we could read, and the two European test standards for tables, BS EN 15372:2023 (non-domestic) and BS EN 12521:2023 (domestic), test a finished table, not a top on its own; we did not read their test loads. BSI's scope for BS EN 12521:2023 says one of its informative annexes is a table top deflection test: ask whether the partner has applied it. So the table of limits is the partner workshop's to supply, from a calculation or a test on the actual build-up.

Each material fails differently, which is why the same ratios do not give the same answer:

• Solid timber sags slowly. The Wood Handbook says that at typical design levels, after several years the extra deformation from creep may approximately equal the first, elastic deformation, so a top that looks fine at delivery can sag about twice as far in time. It also moves across the grain with humidity: see timber movement in panelling and veneer. The tabulated values are averages for clear specimens, with variation of 16 per cent in strength and 22 per cent in stiffness, so they are not design values.
• Natural stone barely sags and breaks without warning, at a flaw, a vein, a joint or a cut-out. How much load it takes is a property of that stone and that slab, shown by the supplier's test report, so we give no figure. Stone is heavy (see the weights below) and is usually carried on a frame or carrier.
• Glass is stiff and brittle, and its strength is statistical. The National Glass Association's paper says glass cannot be engineered like materials with a predictable strength, and quotes design values for a chosen chance of breakage: for annealed float glass 19 MPa, for fully tempered 77 MPa, at a breakage chance of 8 in 1,000, for a short load on undamaged glass supported on four sides. Edge flaws and point contacts are where it fails: see glass shelves and table tops.
• Metal sheet on a frame dents, ripples ('oil-canning') and yields before it breaks, so the frame spacing and the folds matter more than the thickness. Zinc, pewter, copper and stainless bar tops covers metal tops further.
• Boards and composites (veneered MDF, plywood, solid surface, engineered quartz, sintered stone) have their own stiffness and creep: ask the supplier for the data sheet. Board stiffness and spans are in shelf spans and deflection and the sheet materials selection chart.

How heavy is the top

Weight is where a top's thickness shows first: on the delivery route, the base, the lift and the floor. Weight per square metre is the thickness in millimetres times the density in kg per m³ divided by 1,000. The densities used are: glass 2,500 kg/m³ (National Glass Association paper); stainless steel 7,900 kg/m³ (Euro Inox, grade 1.4301); white oak 762 kg/m³ at 12 per cent moisture (specific gravity 0.68 in the Wood Handbook, which is on oven-dry mass, times 1.12); and for natural stone 2,700 kg/m³, which is our assumption, not a source figure: the supplier's density test report governs.

Whole tops, with no base, for two sizes:

The 40 mm stone top at 2,400 x 1,000 comes to about 260 kg on these figures, which agrees with the 250 kg we quote for a 240 cm marble dining top on our dining tables page. A top like that is lifted by several people or a hoist, is often delivered separately from its base, and needs the route and the floor checked: see delivery access survey. We give no floor-loading limit: tell the structural engineer where a heavy top and base stand on a raised or suspended floor.

Which thickness matches which

Because sag depends on E x t³, a stiffer material can be thinner. The drawing shows the thickness that gives the same stiffness as a 40 mm white oak top, using the published E of each: black walnut 11,600 MPa, glass 71,700 MPa, stainless steel 200,000 MPa. Glass needs about 22 mm and stainless steel about 16 mm, but at those thicknesses they weigh about 56 and 125 kg per m² against 30 for the oak: the thin tops are the heavy ones. For stone the figure is not given, because stiffness does not govern it.

Treat the bars as a way to compare materials, not as a thickness to specify. Equal stiffness gives equal sag for the same span, load and width, and says nothing about strength, edges, movement or how the top is fixed.

Where the supports go, and when a table tips

Where the legs go. For a load spread evenly along the top, the largest bending moment in the top is smallest when the supports are set in from each end by about 0.207 of the overall length (0.354 of the span between them). At that position the moment over each support equals the moment at mid-span. For a 2,400 mm top that is about 497 mm in from each end, leaving a span of about 1,406 mm, and the largest bending moment is about 5.8 times smaller than with the legs at the very ends. That is statics, not a recommendation: the legs would sit in the knee space of the diners at the end seats, and a top with large overhangs can tip, so the position is a trade between the top's stress, the seating plan and stability.

Tipping. A table tips when a load P at distance x beyond its tipping line (the edge of the base or the line through the outer legs) exceeds the mass M times the distance d from that line back to the centre of mass: it just tips when P = M x d / x. The drawing compares two pedestal tables of the same size, top 1,200 mm across on a base 500 mm across: a 12 mm glass top on a 20 kg base tips under about 39 kg on the rim; a 25 mm stone top on a 40 kg base takes about 83 kg. The masses are examples. BSI's listing of the 2015 edition of BS EN 12521 includes stability tests under vertical load among its tests, and its 2023 scope still refers to stability tests, so the finished table is tested for this: ask for the result and the edition.

Joints and cut-outs. Put a centre joint, an extension slide or a leaf joint over a support, not in the middle of a span; keep a sink or cable cut-out away from a support and from an overhang; and fix a rigid top (stone, glass, a wide solid timber top) so that it can move: slotted fixings or buttons for timber, soft pads and a stop for glass. These are practice, set out in the partner's drawing, and are listed in our unverified items.

What to ask the partner workshop for

The table of limits belongs to the partner workshop and its engineer. Ask for it in this form, and for the working behind it:

Table top: [40] mm [solid oak] on [60 x 40 mm steel rails] with supports at [ ] mm and [ ] mm from the ends, span [ ] mm, overhang [ ] mm each end and [ ] mm each side, as drawing [ref]. The partner is to submit the calculation or test report for this build-up, stating the load case, the material data used and the deflection limit, before production.

The table is to be stable against a vertical load of [ ] kg applied at the edge at its most unfavourable point, with the base footprint and mass shown on drawing [ref]. The partner is to state the load at which it tips, and the edition of [BS EN 15372 / BS EN 12521] and severity level to which a finished table has been tested, with the test report.

Weight of top [ ] kg and of the complete table [ ] kg, centre of mass shown on the drawing. The top is fixed [with slotted fixings so that it can move across the grain / on soft pads with a stop]; no rigid fixing of a stone, glass or wide timber top.

Standards and sources

• USDA Forest Products Laboratory, Wood Handbook, FPL-GTR-282 (2021): Tables 5-3a, 5-6; equation 9-2, Table 9-1.
• National Glass Association, Glass Technical Paper FM05-12 (2023): glass density, E and strength.
• Euro Inox, Tables of Technical Properties (2007): grade 1.4301 density and E.
• BS EN 15372:2023 and BS EN 12521:2023 (finished tables): current on BSI Knowledge; not read.

Questions we are asked

How thick should a table top be for a 3 m span?

No published rule gives it, and a top is rarely left to span 3 m on its own: a long table has rails, a stretcher or a steel frame that carries the span, and the top is chosen for looks, edges and movement. For a top that must span alone, the thickness comes from the partner workshop's calculation or a test on that build-up, which also states the load and the deflection allowed. The arithmetic on this sheet shows that twice the span needs sixteen times the stiffness for the same sag.

How far can a table top overhang?

There is no published figure we could read. Two checks govern it. The top must not sag or break at the tip: the tip sag grows with the cube of the overhang for a load on the edge, and the bending stress with the overhang itself. The table must not tip when someone leans on the edge: the load that tips it is the mass times the distance from the tipping line to the centre of mass, divided by the overhang. Both belong in the partner's drawing.

Does a stone table top need a support frame?

Stone is heavy and breaks without warning, so tops are normally carried on a frame or carrier, with the supports and any joints or cut-outs placed by the stone supplier and the partner. A 40 mm marble top at 2,400 x 1,000 mm weighs about 260 kg on an assumed density of 2,700 kg per cubic metre. We give no span or overhang figure for stone because it depends on the stone and the slab: ask for the supplier's test report.

Is a thick solid timber top always better?

It is stiffer (four times for 25 to 40 mm) and 60 per cent heavier, but it moves across the grain with humidity and creeps under load: the Wood Handbook says the extra deformation from creep may approximately equal the first deformation after several years. A thicker top also moves more in absolute terms across its width. The fixings must let it move.

Can a glass top be thinner than a timber one?

For the same stiffness a glass top is about 22 mm against 40 mm of oak, but it weighs about 56 kg per square metre against 30, and its strength depends on edge flaws and point contacts rather than on stiffness. Thickness and support come from the glass supplier's calculation: see our glass shelves and table tops sheet.

What do BS EN 15372 and BS EN 12521 cover?

They set strength, durability and safety requirements for finished tables, non-domestic and domestic respectively, including tables with glass, and test them with static and durability loads, impact and stability. We did not read their test loads. BS EN 12521:2023 also has an informative table top deflection test. Ask the partner which edition and severity level a table was tested to, and for the report.