What load does a pedestal need to carry?
This page gives the governing imposed load for a pedestal deck and sets the manufacturer figures in circulation beside each other, for anyone specifying or signing off a balcony or roof-terrace build-up.
See how a published load figure gets correctedImposed load, wind action and restraint strap: what each document actually governs
A pedestal system is sized against more than one requirement, and each one comes from a different document with a different status. Treating them as one figure is where specifications go wrong.
| Document | Status | What it governs |
|---|---|---|
| National Annex to BS EN 1991-1-1 | British Standard (National Annex), paywalled through BSI | The imposed load for this use, given in Table NA.2 category A |
| BS EN 1991-1-4 | British Standard (Eurocode), paywalled through BSI | Wind action, including the uplift a roof terrace or balcony deck has to resist |
| NHBC Standards | Warranty body requirement, not a regulation | The restraint-strap provision applied where a warranty surveyor is signing off the build-up |
The imposed load
The imposed load a pedestal has to carry for this use is given in Table NA.2 of the National Annex to BS EN 1991-1-1, under category A. This is a structural Eurocode document, held and sold by BSI. It states a loading in kilonewtons per square metre (kN/m²), which is the figure a manufacturer's own load rating should be checked against.
The wind action
BS EN 1991-1-4 covers wind action on a structure, including the suction that can lift loose-laid paving off its pedestals. It is a separate Eurocode from the one governing imposed load, and it is read alongside the building's height, exposure and the detail of the deck edge. Where a deck is exposed, this is the document a wind-uplift calculation has to be traced back to.
The NHBC restraint-strap requirement
NHBC Standards sit apart from both of the above: they are a warranty body's own requirement, and they apply where NHBC cover is the route being taken. The restraint-strap provision addresses how paving is held down against wind uplift, and it is worth checking against whatever fixing method a pedestal manufacturer ships as standard, because a surveyor working to NHBC Standards is assessing the strap detail against that document specifically.
What to check before comparing load figures
A load figure printed on its own says little. Check its unit, its test house and the standard it was tested to before you set it against another supplier's number.
| What to check | What it shows | Why you need it |
|---|---|---|
| Unit | kN, kN/m², kg or kg/m² | kN and kN/m² are not interchangeable |
| Test house | The laboratory that ran the test | A figure with no test house behind it cannot be checked |
| Standard tested to | The British or European standard used | Different standards mean different load cases |
| None of the three stated | No unit, test house or standard given | Ask the supplier to confirm it in writing |
This sets out what to look for on any datasheet. It does not state what any particular manufacturer currently publishes.
Is there an uplift calculation for loose-laid pedestal paving?
Paving on a pedestal sits loose on its support, held down by its own weight. Wind moving across the deck, particularly near a parapet or a roof edge, can lift a slab if the suction building up underneath it exceeds that weight. No British Standard, Approved Document or warranty body guidance sets out a method for calculating that uplift for a loose-laid pedestal deck.
What the published documents do cover
BS EN 1991-1-4 gives wind actions on buildings and structures generally, and a wind engineer can use it to establish the pressures and suctions acting on a terrace or balcony. What it does not give is a worked method for applying that wind action to a paving slab resting loose on a height-adjustable pedestal, where the only thing resisting the upward force is the slab's own weight.
A manufacturer's load figure for vertical bearing says nothing about resistance to a force acting the other way, upward through the gaps between slabs. Where a deck sits at height, near an edge, or in an exposed location, closing that gap falls to the specifier, because no document does it for them.
The question to put to the designer
Where wind uplift is a live concern on a given site, ask the structural engineer or wind consultant to confirm, in writing, the wind suction calculated for the paving zone and the minimum slab weight per unit area needed to resist it at the pedestal spacing proposed. Ask separately whether that figure assumes loose-laid paving or has already priced in a mechanical fixing, because those are two different answers to the same question. The slab weight the calculation relies on should also match the slab thickness, spacing and overhang actually specified.