In the mid-2000s, double deck trailers were typically built up to 4.88m (16'); however, that limit has since increased. Don-Bur now regularly build trailers up to 4.92m (16'2"), and occasionally up to 4.95m (16'3").
In the UK, a derogation means overall trailer height is not restricted under Construction & Use regulations, so trailers can be built to any height — the practical limitation is bridge clearance.
Unmarked UK bridges are required to provide a minimum clearance of 16'6" (5.03m). Any bridge below this must have a warning sign. Public highways, including motorways and dual carriageways, are designed with this minimum height in mind.
Across mainland Europe, however, the maximum legal height is restricted to 4.0 metres. This limitation presents a significant design challenge for double deck trailers and often requires compromises in deck aperture depending on the load type.
What About Minimum Heights?
There is no fixed minimum height for a double deck trailer — it depends entirely on the load and the necessary internal aperture height.
Where headroom isn’t critical (e.g., side-loading by forklift), the overall trailer height can be reduced. Fitting smaller wheels is one method of lowering the deck and maximising internal height.
For example, a step frame trailer at 4.0m overall height can achieve a main deck height of just 880mm, providing a total internal aperture of approximately 3.033m — or roughly 1.5m per deck. While not ideal for manual loading, this can be ideal for forklift or automated systems.
In highly specialised designs, using independent suspension and boxed-in wheel arches, the rear lower deck height can be reduced to just 250mm. This enables an internal aperture of 3.663m within the 4.0m EU height limit, offering approximately 1.831m clearance per deck.
