Many thanks for both of your replies. I apologise for the incorrect terminology being used.
Perhaps the dimension I'm described as 'the height of the bit of a narrowboat on which a sign is usually painted'. For example, in this arbitrarily-selected photo, the distance between the gunnel (which appears to be painted red) up to the roof edge (where the mans hand is resting).
I understand there's no definitive standard for this, but it must be within a range. I assume there's a maximum height/draft limit for the network, and so this dimension is fairly consistent. Accepting that there's no standard, though...
Could someone please measure this dimension on their own narrowboat, and tell me what it is?
Thanks again.
Sam.