Ply for a dormer window

Would use 3/4" WBP ply

 

Are you that wedded to the past? 3/4" ply means 18mm in modern parlance.

 

2440 x 1220 means 8' x 4' plywood is made in imperial sizes.

 

I did know that but only an idiot living in the past would call 18mm ply 3/4"

 

It is not me who is the idiot.
Sheet Thickness

Sheet goods vary widely in their actual thicknesses, based on the type of plywood, manufacturer, origin, and batch. When the precise thickness of the sheet is important, it is always wise to measure.
A general though imprecise conversion of metric and imperial thicknesses would be as follows:

 

If the OP was given a drawing for his dormer would it be in imperial or metric?

 

Use 18mm class 3 exterior grade ply, wrap the plywood with Kingspan Nilvent or equal, batten out with treated timber to suit the tiles