On our flat roof, it looks like I need to join two sections of wall plate together. Thanks to everyone on this thread
I'll make the join so that at least three joists are resting on the plate (NHBC Standards 2018, 7.2.6 Wall Plates). (Thanks Gary @nod )
The same guidance suggests I should use a coloured timber preservative (7.1.4)
And I bet they don't mean the stuff you get from B+Q. An Internet search gets a flurry of probably irrelevant results.
Do they mean the blue color stuff you get on roofing battens? Ideas anyone?