How tall are double deck trailers?

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In the mid 2,000s, both double deck trailers and lifting deck trailers were built up to 4.88m (16'); however, that limit has been pushed in recent years and Don-Bur now regularly build up to 4.92m (16'2") and, on rare occasion, 4.95m (16'3").

A derogation exists in the UK which means that overall height is not considered when complying with construction and use regulations.  This means that UK manufacturers can build trailers to any height.  The practical restriction is bridge height.

Unmarked bridges in the UK should be 16'6" (5.0292m) or greater.  Any bridge that does not have this clearance should have the clearance clearly indicated.  All public highways (including dual carriageways and motorways) have a standard minimum clearance of 16'6" or greater.

This means that many double deck trailers have ~4" (101.6mm) clearance between the roof and underside of public highway bridges.  This is sufficient for suspension "bump & roll".