Can you get back to the membrane once the deck is down?

Three documents get quoted when someone asks how often the membrane under a pedestal deck should be checked: BS 6229:2025, SPRA S01 and the LABC Warranty technical requirements. Each sets an inspection interval, but none of the three is answering the question most specifiers mean by it, which is how to get at the membrane itself once the deck is down.

BS 6229:2025

BS 6229:2025 is a British Standard covering flat roofs with continuously supported coverings, issued by BSI and available from BSI at a cost. Its maintenance provisions sit alongside its guidance on falls and outlets, and the inspection regime it sets out is written with the roof as a whole in view. Readers checking which edition is current, and where the withdrawn edition still turns up in other material, can see that set out in BS 6229:2018 Withdrawn — What It Means for Pedestal Deck Specification.

SPRA S01

SPRA S01 is a guidance note from the Single Ply Roofing Association, written for single-ply membrane roofs. Where a deck sits over a single-ply membrane this is the document most likely to be put in front of a warranty surveyor, and its inspection interval is set for the membrane as a whole.

LABC Warranty

LABC Warranty sets its own technical requirement for inspection of the waterproofing on a roof terrace or balcony, as part of the cover it writes for the building. That requirement is addressed to whoever is taking on the warranty, and it does not describe how an inspector is meant to reach the membrane once the pedestals and deck boards are in place.

The access detail is about outlets

Where these documents do call for an access point in the finished build-up, it is there so an outlet or gully can be found and cleared. A removable paver over a drain is an outlet access detail. It is not a membrane inspection detail, and treating the two as the same thing is where a specification goes wrong.

Ask the supplier, in writing, how the membrane itself is meant to be checked once the deck is laid, apart from at the outlets, and what the answer is if it cannot be, short of lifting the deck.

A pedestal does not spread its load across the slab the way a continuous screed does. It carries the deck down to a single bearing point, so a crack that runs under that point sits under the same concentrated load every time someone walks across the paver above it. A crack in open ground might be left to move. A crack directly under a pedestal foot is a different question, because the load keeps finding the same line.

What the governing documents say about a cracked slab

No published source sets out a crack width or a crack pattern that triggers slab repair or replacement under a pedestal deck. The standards and warranty documents that cover falls, inspection intervals and spreader plates do not extend to this decision. A contractor lifting a deck and finding a crack is working from site judgement, and a specifier relying on a supplier's assurance that a crack is acceptable is relying on that supplier's own criteria.

The exposed void during the works

Once a pedestal is lifted for inspection or repair, the cavity beneath it stops being sealed under the deck and becomes open ground for as long as the works take. Rain, debris and foot traffic can reach the membrane directly during that window, which is a risk the deck itself was built to prevent. Nothing in the documents that cover pedestal decks says who is responsible for protecting that opening while it is exposed, or for how long it may reasonably stay that way.

What to put in the maintenance plan at handover

Because the documents are silent on both points, the maintenance plan is where the responsibility has to be written down. Ask the installer or contractor to confirm in writing, before handover:

• What crack width or condition in the slab is treated as a trigger for further investigation
• Who is responsible for protecting the open void and the membrane beneath it while a pedestal is lifted for repair
• How a cracked area, once found, is recorded and reported back to the building owner, and in what form

None of this is a figure you can look up. It is a commitment you have to ask for, and get in writing, because nobody else is going to set the standard for you.

A pedestal carries the full load of the deck down to a single point. Below that point there is usually not a solid slab waiting to take it. On insulated roof terraces and podium decks, the pedestal often lands on rigid insulation, and that insulation has to resist the point load without crushing, creeping or punching through the membrane beneath it.

Three places where this risk is named have been identified in the course of building out this site's reference pages: a British Standard, a warranty body document and a manufacturer technical note among them. Each one says, in its own words, that the insulation under a pedestal needs enough compressive strength to take the load without failing over time. None of the three gives you a bearing area to design to, and none gives you a minimum compressive strength to specify against.

What that leaves you to establish yourself

Without a stated bearing area, you cannot work back from a pedestal base size to a contact pressure. Without a minimum compressive strength, you cannot check an insulation product's datasheet figure against anything the governing documents actually require. The risk is named; the numbers that would let you design against it are not.

That is a gap you can close with a supplier, in writing, before the build-up is signed off. The question is whether the insulation is suitable under this pedestal, at this spacing, carrying this load.

• Ask the insulation manufacturer for the compressive strength figure at the long-term load duration relevant to a roof terrace or podium deck.
• Ask for the bearing area the pedestal manufacturer assumes at its base, and check it against the insulation manufacturer's stated maximum point load for that product.
• Ask whether either manufacturer has tested the combination of their product with the other's.

Where a supplier cannot answer one of these in writing, that is itself worth recording against the project file. A figure given verbally, or implied by a product being marketed for roof terrace use, is not the same as a stated bearing area and a stated minimum compressive strength tied to your load case.