How should a chimney be flashed on a metal roof?

The chimney interrupts the roof slope and its drainage path. The stack and metal covering can move relative to one another as temperatures change and the structure deflects. A rigid junction or an incorrectly arranged lap can open up over time.

Tile-effect sheets have transverse steps; T18 and T35 sheets channel water along their ribs. A flat flashing detail will not fit each profile equally well. Check where water leaves the chimney area and whether a raised edge could trap it on the way.

Identify the chimney casing and finish on the drawings. Render and cladding present different fixing surfaces. An outer finish may not be able to carry the intended fasteners. Inspect the chimney itself before starting work on the roof covering.

The lower apron discharges onto the roof below, while the side flashings carry runoff along the walls of the stack. The back flashing intercepts water from above and diverts it sideways. Its shape depends on the width and position of the chimney and the roof area draining towards it. A short flat strip may be insufficient.

The upper edge against the stack needs a termination suited to its substrate and system. Do not make arbitrary cuts into a prefabricated chimney or damage its casing. Coordinate the fastening and sealing method with the instructions for both the chimney and the roof covering.

The roofing underlay also needs a proper junction at the penetration. Provide a drainage route for water that reaches this level. A new surface flashing may leave the leak unresolved if the underlay connection is missing, particularly when replacing an older roof covering.

Observe the specified clearances and protection around parts that get hot. General-purpose expanding foam is not suitable for every gap around a chimney. Insulation materials differ in their behaviour in fire. The joint must meet the chimney system’s requirements even where the gap will be out of sight.

Select flashing material for the roof exposure and check compatibility with adjacent metals. Consider corrosion protection as well as colour and texture. A broad flashing in a different shade can stand out more than small colour variations between roof sheets.

Use sealant as part of a detail that already sheds water. Match it to the substrate, temperature range and outdoor exposure, and prepare the surfaces as instructed. Adding sealant over a dirty, detached joint obscures the fault and gives an unreliable repair.

Arrange laps so water passes over each lower piece. Keep unplanned penetrations out of concentrated flow paths. Cut the surrounding sheets to leave room for the designed flashings. A tight fit restricts movement, while excessively wide gaps are harder to cover reliably.

Remove swarf after cutting and inspect the coating. At corners, several flashing pieces meet; an incorrect fold or lap sequence can admit water. Include the ridge-facing side in the inspection, even if it cannot be seen from the yard. Keep a photograph for later checks.

A damp patch beside the stack does not pinpoint a fault in the lower apron. Water may travel from higher up the roof, enter through a cracked casing or come from another leaking detail. Trace its path before repairing the flashing. Where needed, the roofer should also inspect beneath the sheets.

Once the cause is known, replace damaged parts or revise the detail as required. Repeated applications of sealant can postpone an effective repair. For a quotation, provide photos from every side, identify the covering and describe the chimney. Arrange safe access on the roof for a complete inspection.