Overdeck insulation goes in when the roof is being built or re-covered. The board sits above the structural deck, under the waterproofing or the sheet, which keeps the slab out of the daily thermal cycle and takes a large part of the movement out of the membrane.
We work in XPS and PUF/PIR board, rockwool lamella where fire performance governs, and finish with bituminous membrane, tarfelt or metal sheeting. Falls, upstands, penetrations and edge details are part of the quote, not an extra.