Coatings for metal flashings: matching the finish to indoor and outdoor exposure

Include parts that are easy to overlook in the exposure schedule. Starter profiles at the plinth, internal corner trims and flashings around openings may face conditions unlike those at a roof ridge. Describe how each area is used rather than relying on a generic building description.

Galvanised sheet has a zinc coating; aluminium-zinc-coated sheet has a metallic coating containing both metals. Colour-coated flashings also have organic layers. Distinguish the steel substrate, metallic protection and outer finish. A colour designation tells you nothing about the thickness or composition of those layers.

A data sheet may give sheet thickness in millimetres, organic coating thickness in micrometres and metallic coating mass per unit area. These are three different quantities. Label each value and its unit in the specification so that quotations can be compared on the same basis.

Check materials at the points where they meet. Verge trims and gutter aprons receive runoff from the roof; corner trims and wall covers may touch sealants or the underlying substrate. Compatibility at these contacts matters even when the visible faces look the same.

A shared colour number does not guarantee matching gloss or texture. Differences can be conspicuous on broad flashings across a large façade. Compare physical panel and sheet samples. If the order is split into phases, retain the finish specification and the earlier batch details.

Once folded, the reverse face may look into the building or be visible from below. Its properties need not match the decorative face. Mark the orientation on the folding drawing. Reversing a complex section can expose the wrong protective finish as well as the wrong colour.

This matters particularly where opposite faces see different environments. Mark both sides of bespoke flashings unambiguously. Assess chimney flashings and trims at service penetrations individually; a coating suitable for a straightforward façade corner may not suit these locations.

Incorrect cutting or folding can damage a coating that was well chosen for its exposure. Use tools and bend radii suited to the sheet. Do not drag long sections across sharp edges. Inspect adjacent panels too: swarf and scratches may mark them even if the flashing looks intact.

Use an approved cutting method for coated sheet. Abrasive cutting generates heat and hot particles that can harm the protection. Clear waste and swarf after installation. Follow the coating supplier’s instructions for local repairs: applying an arbitrary paint does not restore the original protective system.

Include flashings and dirt traps in the maintenance plan. Use cleaning products compatible with the surface and keep to their specified concentrations. Increasing the strength to speed up cleaning may also affect neighbouring joints and seals differently from the coated steel.

Retain the material documentation and a record of repairs. Reassess exposure if the building use changes. The service life of the finish depends on fabrication, installation and maintenance suited to that use; choosing a coating does not make the flashing maintenance-free.