Slab thickness, pedestal spacing and overhang for a pedestal deck

Slab thickness, spacing and overhang are governed by whichever product standard applies to the material you are laying. Each standard sets its own dimensional and performance requirements, and the slab type decides which one applies.

• Concrete flags fall under BS EN 1339, which covers thickness tolerance and breaking load.
• Natural stone paving falls under BS EN 1341, which sets its own breaking load and flexural strength tests, different from the concrete flag standard because the material behaves differently under point load.
• Ceramic and porcelain tiles fall under BS EN 14411, which classifies by water absorption and breaking strength.

Why the standard matters more than the datasheet

A pedestal manufacturer's spacing table is written for a generic slab of a given thickness. It does not and cannot certify that your specific slab, from your specific supplier, meets the breaking load the relevant standard requires at that span. The standard governs the slab; the pedestal datasheet only describes the support.

What to check before you specify a span

Identify the slab material first, then confirm which of the three standards applies, then ask your slab supplier for the test data against that standard at the thickness you intend to use. Spacing and overhang follow from that data.

No published document gives a single span and overhang figure that applies to every slab, so the number you need has to come from the supplier, confirmed against the slab you are actually specifying.

Why a general figure will not cover your installation

Allowable span and overhang depend on the slab's thickness, its material and its bearing strength together with the pedestal head size and the spacing between pedestals. Change any one of those and the safe span changes with it. A figure printed for a different slab, a different thickness or a different spacing is not a figure for your slab, even if it comes from the same manufacturer's datasheet.

What to ask the supplier to confirm in writing

Put the request in terms the supplier can answer against a specific build-up, not a general one. Ask them to state:

• The slab product, material and thickness you are specifying
• The pedestal head type and the maximum centre-to-centre spacing it supports for that slab
• The maximum overhang permitted beyond the last pedestal on an edge or corner slab
• The standard or test the figures are based on, or a statement that none exists if the figure is the manufacturer's own engineering judgement

A reply that answers all four points against your named slab is a written commitment you can hold the supplier to. A reply that repeats a datasheet figure without tying it to your slab is not, whatever table it is copied from.

When two manufacturers give you different slab thickness, spacing or overhang figures, the usual next step is to look for a source that is not selling anything. LABC Warranty's technical guidance is often the first document specifiers reach for, because it sits with the warranty body. That does not mean the table in it is beyond checking before you quote it.

Does every figure in the table carry its own standard?

A table that lists slab thickness, pedestal spacing and overhang together is only as useful as the standard named beside each figure. Where a row gives a dimension with no standard and no test method stated next to it, that row cannot be checked against anything, and it cannot be set beside a manufacturer datasheet that does name its standard. Ask the warranty surveyor citing the table, or LABC Warranty directly, which standard each figure in the row was tested to before it goes into your specification.

Is the kerb-standard citation written for your slab?

A citation to a kerb standard appearing in a table about paving slabs is worth reading at source before it is repeated. A standard written for kerb units bedded continuously in the ground describes a different geometry from a slab resting on pedestals spaced a set distance apart, and a bedding depth or thickness figure derived for one does not transfer to the other without restating the basis for the figure. Check what product the cited standard actually covers before treating its figure as applicable to a pedestal-supported slab.

Where either check comes back unclear, put the question to whoever published the table in writing and keep the reply with your specification.