A Roof That Only Leaks When It Really Rains

Some roofs sit through a week of drizzle without a mark on the ceiling, then drip into the hallway the moment a real downpour arrives. If your roof leaks only in heavy rain, that pattern is a clue worth paying attention to.

It narrows the fault down to a short list, because light rain and heavy rain do not test a tiled roof in the same way.

A tiled roof is not a sealed surface. It is a system of overlapping pieces that sheds water downhill faster than the weather can deliver it. When the weather starts delivering faster than the roof can shed, the weak points show.

What changes when the rain gets heavy

Three things change at once, and each of them tests a different part of the roof.

The volume rises. A tile can only carry so much water down its face before the film of water gets deep enough to reach the edges of the tile rather than running down the middle of it. Depth is what matters here, not how wet the roof looks from the street.

The wind picks up. Heavy rain in Melbourne rarely arrives straight down. It comes across, and a roof that handles vertical water beautifully can be defenceless against water arriving sideways.

The duration stretches out. A short shower wets the surface and stops. An hour of steady rain saturates the mortar, fills the valleys and keeps every join under pressure long enough for slow paths to become obvious ones.

Wind driven rain and where it gets in

This is the single most common answer to the question. Wind driven rain does not obey gravity, and a tiled roof is built almost entirely around the assumption that water runs downhill.

When wind pushes rain up the slope, water is driven under the bottom edge of each tile and into the lap where one tile sits over the one below. On a still day nothing gets in there. With enough wind behind it, water climbs that overlap and comes out on the underside.

The same thing happens at the edges of the roof. Ridge and hip lines, gable ends and the verge where tiles meet the wall all rely on mortar and on the tile sitting tight. Wind finds any gap in those lines and drives water straight through it.

There is a useful tell here. If the leak only shows when the weather comes from one particular direction, you are almost certainly looking at wind driven rain rather than a hole. A roof with a hole in it leaks whenever it rains.

Note the direction the weather is coming from next time it happens, along with the date and roughly how far into the rain the drip started. That is genuinely useful information, and you can collect all of it from inside the house.

Water travelling between and under tiles

It surprises a lot of homeowners to learn that a small amount of water gets under the tiles of a healthy roof in most decent storms. That is expected, and it is not a fault on its own.

What matters is where that water goes next. On a roof with sarking, the layer under the tiles catches it and carries it back out to the gutter line. On many older homes there is no sarking at all, and anything that gets past the tiles lands on the battens, the ceiling joists or the plaster.

That is why two roofs in the same street can behave completely differently in the same storm. The tiles are doing the same job on both. Only one of them has a second line of defence underneath.

Add a cracked or slipped tile to a roof with no sarking and the arithmetic changes again. In light rain, the trickle through a hairline crack is small enough to run along a batten and dry out before it reaches anything.

In heavy rain that same crack delivers far more water than the timber can absorb, and it finds the plaster.

Broken pointing along the ridge works the same way. Cracks that shed a drizzle happily will take in water steadily once the rain is heavy enough to pool against them.

Valleys and flashings that cannot cope with the volume

The valleys are the channels where two slopes of roof meet, and they carry the water from both of those slopes at once. They are the busiest part of any tiled roof and the first place volume becomes the problem rather than the weather.

A valley narrowed by debris still works in light rain, because light rain gives it very little to carry. Fill that same valley with leaves, silt and broken down moss, then put a downpour through it, and the water backs up, rises above the edge of the valley and runs sideways under the tiles on either side.

Mortar is a common culprit too. Where pointing has been run too far into the valley, or tiles have been cut and bedded too tightly against it, the channel is narrower than it was built to be. That restriction costs nothing in a shower and matters a great deal in a storm.

Flashings tell a similar story. Anywhere something breaks through the roof line, a chimney, a skylight, a vent, the join between that object and the tiles has to be sealed and lapped correctly. A tired flashing will hold back a light rain and let a driven one straight past.

What the repair involves depends entirely on what is found up there, which is why an inspection comes before any answer about the work.

How a leak that only appears in a storm gets traced

The awkward part of this kind of leak is that it refuses to perform on demand. Someone can stand on a dry roof and see nothing wrong at all.

So the tracing starts with your observations rather than the roof. The date it leaked, the direction the weather came from, how long the rain had been going, where the water appeared and how fast. Photograph the ceiling or the wall while it is wet, because a damp patch looks very different once it has dried out.

Make those observations from inside the house, in daylight, or from the ground and an upstairs window. Do not go up to look during rain, straight after it, or in the dark. A wet tile is slippery, tiles crack underfoot, and nothing you could see up there in those conditions is worth it.

From there the roof gets checked properly in dry, calm weather: the pointing along every ridge and hip, the valleys and what is sitting in them, the flashings at each penetration, cracked and slipped tiles, and the state of the sarking if there is any.

The roof space gets looked at as well, because water leaves trails and staining on timber that show the path it took.

One thing worth knowing before you start hunting yourself: water runs along battens and beams before it drops, so the stain on your ceiling is rarely sitting underneath the actual fault. Working back from the stain is a job for someone who can read the roof from both sides.

Where the picture is still unclear, sections of the roof can be tested with water under controlled conditions, one area at a time, until the entry point shows itself. It is methodical rather than clever, and it beats guessing.

If any of this sounds like your house, the practical next step is to write down what you noticed during the downpour and have someone look at the roof while it is dry.

Our page on tracking down and repairing a roof leak sets out how we approach it, and the master guide to managing leaky roofs covers the wider picture.

Spic and Span Roof Restoration is a family owned business based in Cranbourne, working on tiled and terracotta roofs across Melbourne’s south east. More than 30 years in the trade, over 1,000 roofs restored, residential and commercial.

Repointing only jobs start at $1,500 and restorations start at $4,500, with most complete jobs landing between $4,000 and $9,000 depending on size, pitch and access. Full roof restorations carry a 7 year warranty.

Quotes are free. Call 0435 104 623, open Monday to Friday, 8am to 5pm. We also cover roof repair in Clyde and the surrounding south eastern suburbs.

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