A roof flashing can include a bottom half and a top half. The two halves can each have a plate and a neck portion projecting away from the respective plate. The neck portions define portions of an aperture configured to encircle and seal against a post projecting away from a roof. At least part of the first neck portion can be slidably receivable in the second neck portion and the two portions can define a neck when the bottom half and the top half are engaged with one another.