01
Map the affected roof area
Have the designer show where new valleys, sidewalls, and roof planes meet the existing slate. Identify retained areas and ask the slater how the new work will tie into them. A dormer quotation limited to its visible walls may omit roofing work necessary to make those junctions complete.
02
Coordinate material and sequence
Review matching material, access, temporary protection, and the point at which existing slate is removed. Keep the agreed limits of work on a marked drawing. If investigation changes those limits, document the reason before proceeding. This helps the owner understand which work belongs to the dormer addition and which concerns were already present on the retained roof.