This is the single most common misunderstanding we are called about, and it is the reason a lot of people end up paying twice. The stain is where the water finally gave up and fell. The hole is somewhere uphill of it.
How water actually travels through a roof
Once water gets past the roof covering it does not fall straight down. It meets sarking, or a batten, or the top edge of a rafter, and it follows whichever of those slopes away fastest. It will run along a batten for several metres before reaching a fixing hole or a joint, and only then does it drop onto the plasterboard below. On a low-pitch roof it travels further, because it has less help leaving.
The distance between the entry point and the stain is set by the framing, not by the leak. On a tiled roof with battens running across the fall, water regularly travels three to five metres before it shows itself.
Why the roof can look perfectly sound from the ground
Most of the failures that cause interior damage are invisible from a driveway. Cracked bedding mortar under a ridge cap sits below the cap itself. A pinhole in a valley iron is under the tiles on either side of it. A lifted lap in flashing around a chimney or a vent is a few millimetres wide. None of that changes the look of a roof from below, and all of it will wet a ceiling.
The things that usually turn out to be responsible
- Ridge and hip bedding that has gone chalky, letting water past the capping. This is the most common cause on Federation and interwar tile roofs across the Inner West.
- A rusted or blocked valley, especially where street trees drop into it, which pushes water sideways under the tiles rather than down the valley.
- Flashing around a penetration: chimney, vent pipe, skylight, or where a later addition meets the original roof.
- A blocked or undersized box gutter behind a parapet, which has no overflow path except into the building.
- Sarking that has been torn by a trade working in the roof space, removing the second line of defence that was catching minor leaks.
Why patching the stain does not work
Sealing or painting over the mark hides it until the next sustained rain. Worse, a stain that keeps being covered rather than traced lets the water keep running across framing, and timber that stays wet through a couple of winters stops being a roofing job and starts being a carpentry one. The cost of finding the entry point properly is almost always lower than the cost of the damage that accumulates while it is not found.
How the entry point gets traced
From inside the roof space first, where the tracking is visible as staining on the underside of the sarking or along the top of a rafter, and that trail leads uphill to where the water came in. Then from on the roof, to confirm it and see the condition of everything around it. A stain that has been there a while is easier to trace than a fresh one, because the path has marked itself.
