Roof flashingsSmall strips of metal doing the hardest job on the roof.

Where are the flashings on your roof?

Ridge and hip cappings run along the top and the outside corners. Barge flashings finish the sloping edges at the gable ends, above the barge boards. Apron flashings seal the joint where a lower roof runs into a wall.

Valley flashings, often called valley irons, line the channel where two slopes meet and carry water from both. Penetration flashings go around anything poking through: flues, vent pipes, solar mounts, aerials and heat pump pipes. Chimney and skylight flashings wrap the bigger openings, usually in several overlapping pieces.

Why do flashings fail?

The wrong metal. New Zealand Steel says lead isn't compatible with ZINCALUME or COLORSTEEL: contact between them, or rain running off lead onto the roof, causes corrosion. For penetration flashings it suggests neoprene or silicone rubber, EPDM, aluminium or soft zinc instead. Copper causes trouble too, especially where water runs from copper pipes onto coated steel.

Salt and shelter. The NZ metal roofing code notes that spots rain rarely washes, such as under overhangs and in sheltered corners, collect salt and dust and corrode faster. Flashings tucked against walls are often exactly those spots.

Sealant doing a flashing's job. A bead of silicone ages in the sun. Properly folded metal sheds water by its shape and uses sealant only as back-up.

Low pitch. NZ metal roofing makers say low-pitched roofs need more attention to flashing details, because the side and end laps become more critical.

Snow. In snow-prone areas, flashings and chimneys need extra fixings so sliding snow doesn't drag them loose.

Later trades. Holes cut for aerials, heat pumps or solar panels and finished with a dab of sealant are a classic leak. The COLORSTEEL warranty excludes damage from badly installed air conditioning or solar systems.

Reseal, repair or replace?

Resealing makes sense when the metal is sound and only the seal or fixings have tired. If the flashing has rusted through, is the wrong metal or was badly shaped in the first place, replace it.

New flashings should suit the roof they sit on. NZ Steel's ZINCALUME brochure says flashings and ridge capping should be made from the same coating system as the main roof, and Dimond recommends cladding and accessories from one manufacturer so the paint weathers evenly.

Some flashings are easy to reach and some sit under wall cladding or roofing. E2/AS1 expects exposed flashings to last at least 15 years and hidden or inaccessible ones 50 years. If yours is one of the hidden ones, the job grows, because other materials come off first.

Does living near the coast change what's used?

Yes. E2/AS1 requires metal roofing and flashings to be chosen for the site's exposure zone. As BRANZ summarises NZS 3604, zone D, the harshest in that standard, covers everywhere within 500 metres of the coast including harbours, and within 100 metres of tidal estuaries and sheltered inlets. E2/AS1 adds a zone E for breaking-surf beachfronts.

Geothermal areas are tougher again, and E2/AS1 says metal roofing and flashings there need specific design. Ask the roofer which zone they've allowed for and why, rather than assuming whatever's on the shelf suits your street.

Do flashing repairs need a building consent?

Most don't. Schedule 1 of the Building Act allows repairs using comparable products, and cutting a hole of up to 300 millimetres across for a pipe, cable or duct, weatherproofing included, through an existing house of up to 3 storeys. The exception is a flashing that failed early and let water in. Repairing that isn't covered by the exemption, so check with your council first.