f**k off. There's no such thing as a 20mm joist. In fact, thinking about it, are trusses 50mm? It's been a very long time since I've put a truss up.
Anyway, it's 2 inches, not 20mm. You've been looking at the wrong side of the tape again.
We once had a joiner (said he was) come on site when we were doing timber frame erecting....shout down to him Giz a bit of threebe at one one six oh.....
He had no idea what we were talking about and was trying to make some sort of sense of one one six oh in inches. Expecting a four foot bit of timber and he's handing up things eleven and a random bit inches.