Around 96 ish I did a couple of months for a mate pulling the old style chipliners out of Cowie and South Molton.
No easy sheet or sliding roof then, it was like a long letter box type hole section jobby, in the top of the trailer from back to the front.
You had to walk (or in my case crawl, as I aint too good with heights 
) either side of it about 18’’ either side, to roll the sheet over.
No probs on a nice dry day, but in the wet, the strong wind, or worse when iced, it was ■■■■ lethal up there.
Falling of a 14’ or so high trailer would have been a wheelchair job.
Those type of trailers may even be illegal now are they?