The Avonhead house had been repainted in autumn 2022, and by the following August the owner had a wet patch on the lounge ceiling and a firm belief that the paint had failed. I was asked to look because it was my product on the roof. The stain was 1.2 metres downhill of a brick chimney and 600 mm to one side of a 2005 skylight. The paint was fine. What I found was a chimney with a back flashing made entirely of sealant, and a skylight whose head flashing had been lapped the wrong way, feeding water straight underneath. Two textbook failures, both older than the paint by years, both invisible from the ground.
What a flashing is actually doing
A flashing takes water from a place it cannot drain, the junction of a roof and something that sticks up through it, and carries it back onto the roof surface. It works by gravity and lapping, never by sealing. Every part of a good flashing laps over the part downhill of it, the way the sheets themselves do. The moment a flashing depends on sealant, it has a working life measured in summers, because sealant on a roof is cooked, frozen, stretched by thermal movement and hit by UV, and in Canterbury’s dry summers and frosty winters it cracks in three to five years.
The Building Code’s requirements and the acceptable solution for minimum flashing covers and upstands are published on building.govt.nz. For corrugate at a typical pitch the head flashing should lap the sheet by at least 130 mm, and any upstand against a wall should be at least 75 mm, with 150 mm where snow or leaves can bank against it.
How chimney flashings fail
A chimney needs four flashings: a back flashing on the uphill side, two side flashings, and an apron on the downhill side. Chimneys wider than 600 mm also need a saddle behind them that splits water around each side. The failures I see, roughly in order:
- Sealant instead of metal. The back flashing at Avonhead was three generations of silicone. Somebody had chased a leak by adding sealant every few years instead of fitting a flashing.
- No saddle. Water and leaves pond behind a wide chimney and eventually climb over a back flashing with too little upstand.
- Corroded galvanised flashings. Old galvanised steel flashings rust from the underside and show nothing until they perforate.
- Lead on Zincalume. Lead flashings cause galvanic corrosion of the steel sheet below. Lead belongs on tile roofs, not steel.
- Mortar failure. Side flashings chased into mortar joints let water in once the mortar cracks, which in Christchurch after 2011 is nearly every chimney.
Re-flashing a chimney properly in new Colorsteel or aluminium with a saddle and soakers costs NZ$900 to NZ$2,400. The sealant repair offered instead costs NZ$150 to NZ$300 and lasts one or two winters.
How skylight flashings fail
Skylights fail for two reasons: the kerb is too low, or the flashings were fitted out of sequence. A kerb of 100 mm above the roof is the minimum, and 150 mm is better on a low-pitch roof where rain is driven uphill. The head flashing must go under the sheet above and over the skylight upstand; the apron must go over the sheet below. At Avonhead the installer had put the head flashing on top of the uphill sheet and dressed sealant over the join. For seventeen years that sealant had held, more or less, and then it had not. Re-flashing a skylight runs NZ$650 to NZ$1,600; replacing the unit with a new flashing kit is NZ$1,800 to NZ$3,500 depending on size.
Finding the entry point
Flashing leaks rarely show up directly below the fault. Water enters at the chimney, runs down the underlay or along a purlin, and drops where the underlay sags or is torn, which can be two metres away. On this roof it took twenty minutes with a hose to show that the chimney leaked under a steady flow and the skylight leaked only when water was directed at its head. The method for tracing water that only appears in certain conditions follows the same sequence described in finding a leak that only shows in heavy rain.
Both were re-flashed in September 2023 for NZ$2,850, and the ceiling was patched afterward. The roof coating, the original suspect, needed nothing.
Frequently asked questions
How much does it cost to re-flash a chimney in Christchurch? NZ$900 to NZ$2,400 for a full set of new metal flashings with a saddle and side soakers on a typical house chimney, depending on chimney size, roof pitch and access. A sealant-only repair is NZ$150 to NZ$300 and usually lasts one to two winters.
Why does my skylight leak only in some storms? Most often because the head flashing sits over the uphill sheet instead of under it, or the kerb is under 100 mm, so wind-driven rain from a particular direction climbs over the upstand. Condensation dripping on frosty mornings without rain is a separate issue and not a flashing fault.
Can I fix a leaking flashing with sealant? Only as a stopgap for a season. Flashings work by lapping and gravity, not sealing, and roof sealant in Canterbury’s UV and frost cracks within three to five years. A flashing that needs sealant to stop water is missing a lap or an upstand and should be refitted in metal.