How to flash a chimney on a trapezoidal steel roof

Check whether the proposed detail fits beneath the chimney cladding. Its thickness and fixing method determine how the seal can terminate. On an existing roof, examine old holes and sealant residues. Old layers may prevent new components from seating properly. Resolve questions about the flue before roof work begins.

Allow space behind the chimney for water draining from the upper roof. A larger catchment increases the importance of diverting runoff to either side. A wide chimney or one set at an unusual angle may need a separate diverter. Have the designer or an experienced roofer select the detail.

Leave the upslope side accessible for inspection and cleaning. Leaves can obstruct flow even when the sheet is intact. Keep the drainage space clear of improvised fillers. Trace where water on the underlay will discharge. The layer under the metal needs its own detail at the penetration.

Rib height and pitch determine the flashing line across the sheet. A flashing formed for another profile may leave gaps or dam water unnecessarily. Base its dimensions on measurements and the installation detail. The width of a flat strip alone says little about the finished junction.

Allow for hems, bends and laps in the cutting dimensions. Mark the coated face. A compatible tape or profiled seal can form part of the assembly but cannot provide missing fall. Check materials against the coating and service temperature. Avoid combining unidentified offcuts from earlier jobs.

Include an end-on photograph of the profile with the sketch. A distant view of the chimney does not show the flanges to which the flashing must fit.

Roof and chimney can move differently. The sheet responds to temperature changes, and the roof structure carries varying loads. The detail must accommodate the expected movement. A rigid joint made by bonding all the pieces together can crack at the interface.

The upper termination must keep water from getting behind the flashing. It may require separate counterflashing, fixings or substrate preparation. Any chase depth depends on chimney construction. Do not drill into the flue simply to make a termination strip easier to fix.

Each higher component should shed water onto the next lower one. This applies to aprons, sides and joints with the roof covering. Decide the sequence before closing the roof. Adding a missing part later may require removing neighbouring sheets.

Position fasteners so they avoid unnecessary holes where water collects. Spacing depends on the chimney and profile. Cut the metal with tools suitable for its coating and remove all swarf afterwards. Loose particles can rust and stain an otherwise sound roof.

Request photographs of the underlay and pieces hidden beneath the metal. Inspect the corners, back of the chimney and lower termination from the outside. Look for ponding, incomplete laps and distortion. Check the chimney walls for damage caused by the fixings.

Record the materials and work in the handover notes. Agree on a check after the first heavy rainfall. A dry interior at a sunny handover can conceal a defect. Even a correctly formed junction needs periodic inspection. Plan access for the chimney sweep and roofer from the outset.