PERTHRoof penetration flashing: where sealant fails and formed flashing works
Flashing roof penetrations on metal roofs in Perth: the AS 1562.1 clause 4.8 drainage rule, why over-reliance on sealant causes ponding and leaks
Every hole cut in a roof is a place water wants to get in, and on a metal roof the way that hole is flashed decides whether it stays out for thirty years or leaks in three. Building and Energy published guidance on this because the wrong method, over-reliance on sealant, quietly fails across Perth roofs.
What a roof penetration is
A penetration is anything that comes up through the roof surface: a flue, a vent pipe, an evaporative cooler base, a solar mounting, a whirlybird, a plumbing stack. Each one interrupts the roof's water-shedding surface, and each one has to be flashed so water runs around and past it rather than into the hole.
On a corrugated or profiled metal roof this is harder than it looks, because the surface is not flat — it is a series of ridges and pans, and water runs down the pans. A flashing that blocks a pan, or that dams water on the high side of the penetration, creates ponding, and ponding finds a way through.
The method that works, and the one that fails
Building and Energy's guidance addressed exactly this. The failure mode is over-reliance on flexible sealant to make the high side of a penetration watertight, which dams the pans and causes ponding. The better method, which manufacturers of steel roofing now supply, is a flue or flashing with a pre-shaped upper edge that fits the contours of the corrugated sheeting, so the installer can mechanically fix and seal it into the pans, allowing the pans to drain fully.
The Building Commission's view is that this method complies with clause 4.8, Projections through roofs, of AS 1562.1, the standard for design and installation of metal roof cladding, because it provides adequate flashing. The principle that governs all of it: the high side of any projection must provide for adequate drainage of all pans or corrugations. Water must never be dammed above a penetration.
Why sealant alone is a warning sign
Sealant has a service life measured in years, not decades. A penetration that relies on a thick bead of silicone to stay watertight is relying on something that will crack, shrink and let go, usually long before the roof itself needs replacing. When we see a flue or pipe surrounded by a mound of sealant rather than a properly formed flashing, that is a leak on a timer, and it is one of the more common sources of roof-space water staining in Perth homes.
What an inspection looks at
From inside the roof space, water tracking down from a penetration shows as staining on the underside of the sheeting, on the framing, or on the insulation below. From the roof exterior, where there is safe access, we look at how each penetration is flashed — whether it is a formed flashing draining the pans or a sealant patch waiting to fail. Under AS 4349.1 we report the visible evidence and the roof-space consequences; where a penetration is clearly relying on sealant over a proper flashing, that goes in the report as a defect to address before it becomes water damage.
This is one of the things that only shows from inside the roof void, which is why an inspection that never enters the roof space, covered in our guide on roof tie-downs, misses it entirely. A pre-purchase inspection that gets into the roof catches the water path before it reaches the ceiling.
Why penetrations are where roofs leak
A roof is very good at shedding water across its unbroken surfaces, where the tiles or sheeting do exactly what they are shaped to do. It is at the interruptions that roofs leak, A hole is a weakness. A penetration, anywhere something passes through the roof, a vent pipe, a flue, an aerial mount, a skylight, a solar mounting bracket, is an interruption. Each penetration is a hole deliberately made in the weatherproof surface, and the flashing around it is what restores the weatherproofing that the hole broke. When the flashing is done well, water runs around and past the penetration and off the roof as if the hole were not there. When it is done poorly, the penetration becomes a path for water into the roof space and the structure below.
This is why penetrations get attention out of all proportion to their size. A vent pipe is a small thing, but the flashing around it is doing a critical job at one of the most vulnerable points on the whole roof, and a failure there admits water directly into the building. Roofs rarely leak through the middle of a sound sheet of iron or an intact field of tiles; they leak at the penetrations, the valleys, and the junctions, which is exactly where a careful inspection concentrates its attention.
The right way and the wrong way to flash
There is a right way to flash a penetration and a wrong way, and the difference is durability. The right way integrates the flashing into the roof so that it physically directs water around the penetration and onto the roof surface below it: the flashing is lapped correctly with the roofing material, dressed to the shape it needs, and relies on the geometry of how water flows rather than on anything that degrades. Done this way, a flashing lasts as long as the roof, because it works by shape and lap rather than by a seal that can fail.
The wrong way relies on sealant to bridge the gap, a bead of silicone or similar smeared around the penetration to keep water out. Sealant has its place as a secondary measure, but as the primary means of weatherproofing a penetration it is a warning sign, because sealant degrades. Sun, heat, and the thermal movement of the roof work on it, and within a few years it cracks, shrinks, and lets go, and once it does the penetration leaks. A penetration weatherproofed with sealant instead of proper flashing is a leak waiting for the sealant to fail, and finding sealant doing a flashing's job is one of the things an inspection reads as a sign of work done quickly rather than properly.
What an inspection looks for, and its limits
When an inspection looks at a roof, the penetrations and junctions are a focus, and there is a great deal to read from them. From within the roof space, where it is accessible, the underside of penetrations can be checked for the staining and dampness that betray a leak, often the clearest evidence because it shows where water is actually getting in regardless of how the flashing looks from above. From the roof exterior, where it can be safely viewed, the flashings themselves can be assessed: whether they are proper flashings or sealant, whether they are lapped correctly, whether they show signs of failure or corrosion. The combination of the two views, the evidence of leakage from inside and the condition of the flashing from outside, is what lets an inspection form a picture of whether the penetrations are sound.
There are limits, as with all of it. Access to parts of a roof exterior can be restricted by safety and pitch, and a flashing that looks sound may still leak under wind-driven rain in ways a dry-day inspection cannot reproduce. An honest report is clear about what could and could not be accessed and viewed. But penetrations and flashings are one of the highest-value things a roof inspection assesses, because they are where roofs actually fail, and a penetration weatherproofed with degrading sealant, or a flashing showing signs of failure, is exactly the kind of finding that predicts a leak before the ceiling below is stained. Catching it then is the point. Not after the water is inside.
Common questions
How should a roof penetration be flashed?
On a metal roof, with a formed flashing whose upper edge fits the sheeting contours and drains the pans, mechanically fixed and sealed. The Building Commission views this as complying with clause 4.8 of AS 1562.1. The high side must always drain all pans.
Why is sealant around a flue a problem?
Sealant has a service life of years, not decades. Over-reliance on flexible sealant to waterproof the high side of a penetration dams the pans, causes ponding, and cracks over time — a leak on a timer.
What does a leaking penetration look like inside?
Water tracking from a penetration shows as staining on the underside of the sheeting, the framing, or the insulation below — which is why an inspection has to enter the roof space to find it.
This comes up on every pre-purchase building inspection and building stage inspections we carry out.
Read next: metal roofs act like a wing. Tie-downs are what stop them leaving. and water penetration: the root of the defect family tree.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.