What a pedestal deck does to the room below
Two defensible thermal calculations of the same deck can differ by half, because one of the figures they both need has never been published. This is for the architects and specifiers who have to put a number on it anyway.
The BBA Information Sheet No 4 method and the missing f value
Working out the U-value for a pedestal deck means deciding how much air moves through the open joints between the paving units and up into the void under the deck, because that air exchange changes the heat loss from the room below. BBA Information Sheet No 4 sets out a method for that calculation. It is a guidance document that builds the calculation around a correction factor usually written as f, applied to the ventilated air layer beneath the paving.
What the method gives you and what it does not
BBA Information Sheet No 4 tells you where the f value sits in the calculation and how it is applied once you have it. It does not state what the value should be for open-jointed pedestal paving. No published figure for it exists, so the method leaves that number for the person doing the calculation to select.
Why two correct calculations can disagree
Two specifiers working from the same build-up, the same slab and the same insulation thickness can both follow BBA Information Sheet No 4 correctly and still produce different U-values, because each has chosen an f value. The gap between a cautious choice and an optimistic one can run to around half the final result. Neither calculation is wrong under the method; they simply rest on different assumptions about a number the method does not supply.
What to ask for
Where a U-value calculation for a pedestal deck build-up reaches you, ask what f value was used and what it was based on. A calculation that states its f value and its reasoning can be checked and defended; one that states only the answer cannot. Where the calculation carries BBA's name, it is also worth reading what the suspension of BBA's UKAS accreditation means for certificates that rely on it.
Why Approved Document E has nothing to say about your terrace deck
A search for the acoustic rating of a pedestal deck build-up usually comes back empty, and that is because the document most readers expect to hold the answer was not written for this situation. Approved Document E is the guidance that accompanies Part E of the Building Regulations, covering resistance to the passage of sound. It sets out standard floor and wall constructions and references the Robust Details scheme, and every one of those constructions sits inside a single dwelling.
The test is room to room.
The sound test procedure behind Approved Document E compares a source room and a receiving room within the same building, usually one flat above another. A paved terrace over a habitable room below is not that arrangement. There is no source room, only an open deck, a membrane, a slab and the pedestals carrying the load between them. No standard construction in the document describes it, and no Robust Detail certificate covers it either.
What that means for the figure you are being asked to provide
Where a warranty surveyor or building control officer asks for an acoustic figure for a deck build-up, the request is being made against a document that has never defined a construction to test it against. Any figure in circulation will have come from a manufacturer's own testing or modelling.
Ask the manufacturer or acoustic designer for the test or calculation behind any figure they quote, and the exact construction it was measured on, before putting it in a specification.