A watertight chimney flashing must direct water onto the roof covering and close off its path behind the sheet metal. The arrangement of the aprons, the connection to the chimney and the underlay layer are what matter. Sealant alone does not replace these elements. The detail is selected according to the chimney's position on the roof slope, the height of the trapezoidal profile and the chimney's construction. Particular care must be taken with prefabricated system chimneys, which must not be cut arbitrarily. Measure the width and length of the chimney and the distance from the ridge. Check the roof pitch, the sheet profile and the condition of the substrate. A chimney located close to the ridge requires different water management than a wide chimney in the middle of a long roof slope. Behind its upper wall, a larger stream of rainwater and sliding snow may accumulate.

Before choosing the covering, establish how the building's shape and span will translate into the arrangement of common rafters, hip rafters and shorter elements at the ridges. Check where supports and bracing are needed, how drainage and ventilation should run, and how to insulate the corners without gaps. With a complex form including dormers, valleys and additional ridges appear, changing the sheet-metal cutting plan and the amount of labour. Give the contractor floor plans, sections and roof pitches so the quote separates structure, covering, insulation and flashings.