How Roof Tiles Are Fixed Down, and Why Some Still Lift

A storm blows through overnight. In the morning one house in the street has three tiles off and a tarp on the way, while the place next door has nothing wrong with it. So how are roof tiles fixed down, and why does wind find one roof and leave another alone?

The answer is mostly in how the roof was built and what has happened to it since. A tiled roof is held down by a combination of weight, overlap, mortar and mechanical fixings. When one part of that combination weakens, the wind does the rest.

Why wind lifts a tile rather than blowing it off

Wind does not push a tile off a roof. It lifts it.

Air moving across a roof speeds up as it goes over the ridge and around the edges, and faster moving air presses less firmly on the surface it passes over. The air inside the roof space stays closer to normal pressure, and that difference pulls upward on the tiles.

Which is why damage starts at the edges rather than in the middle. The ridge, the hips, the gable ends and the eaves line see the strongest lift. A storm takes its first tile from a corner or along the ridge, not out of the centre of a roof plane.

It also explains the neighbour problem. A tile sitting flat, properly overlapped and held by sound mortar or a clip gives the wind almost nothing to work with. A tile already sitting slightly proud, with a gap under its leading edge, hands the wind a lever.

Two roofs of the same age in the same street can differ at exactly that level of detail. Our write up on what extreme winds did to Melbourne roofs covers what this looks like at the severe end.

How tiles and ridge caps are held down

Four things hold a tiled roof in place, and they share the load unevenly.

Weight comes first. A concrete or terracotta tile is heavy, and across the main field of the roof that weight does a great deal of the work on its own. It is why a tiled roof can get away with fewer fixings than a lighter covering would need.

Then the overlap. Each tile sits over the head of the tile below it and interlocks with or butts against the tile beside it, so every tile is partly held by its neighbours. The overlap is also the waterproofing, which matters later.

Then the mechanical fixings. The perimeter rows, the tiles along the ridge and hips, and the ones cut into valleys are the tiles that get mechanically held, using nails, screws, clips or wire ties depending on the profile and how the roof was built.

Tile clips are a metal component of a tiled roof and a normal part of one. Then mortar: ridge and hip caps are bedded onto mortar and pointed over the top, and on many older homes the verge tiles along a gable end were mortared in as well.

That mortar is not decorative. It is what stops the caps moving. Which is why ridge mortar matters out of all proportion to how much of the roof it covers, and the caps sit on the most exposed line of the whole roof.

Why older roofs were fixed to a lower standard

Fixing practice has changed a great deal over the working life of a tiled roof. On many older homes across Melbourne’s south east, only a modest number of tiles were ever mechanically fixed, and the rest relied on weight, overlap and mortar.

That was ordinary practice at the time, and plenty of those roofs have done well. The catch is that the two things carrying most of the load, the mortar and the tiles’ own weight, are also the two that degrade.

Mortar cracks and washes out. Tiles get walked on by other trades, crack, lose their surface and slip. Newer roofs generally have more fixings as a matter of course, and more of the perimeter mechanically held.

Whether a particular roof meets the requirements that apply to it is not something anyone can judge from the ground or from a photograph, and it is not something we can tell you either. That is an engineering assessment, and a building surveyor is the right person to ask.

What we can tell you is the condition the fixings and the mortar are in, which for most homeowners is the more useful question. A roof built with plenty of fixings that has lost all its ridge mortar is still a roof with a problem.

What happens after a few tiles move

A tile does not have to be broken or missing to cause trouble. It only has to be out of position.

Once a tile lifts and drops back slightly off where it was, the overlap that had been shedding water downhill now has a step in it. Wind driven water gets under the tile instead of over it.

That is the kind of leak that only appears in heavy rain and often will not show in an ordinary shower. The second problem is that movement spreads: every tile is partly held by its neighbours, so a gap at one loosens the ones around it.

Third, breakages. A tile that lifts and lands back on a batten edge rather than flat will often crack, sometimes with a hairline you would never pick up from the ground. Those let water through quietly before anything appears on a ceiling.

On a roof with no sarking laid under the tiles there is nothing to catch what gets past them.

If strong winds have been through and you want to know whether anything has shifted, check from the ground in daylight. A phone camera zoomed in along each roof plane, or binoculars, will show a tile out of line, a gap along the ridge or a cap sitting crooked.

An upstairs window is a good vantage point for a lower roof section. Do not climb up to look and do not put a ladder against the gutter. The same ground level approach applies after a hailstorm, which we go through in how hail damages roof tiles.

Putting it right without re-roofing

The useful thing about a tiled roof is that it comes apart one tile at a time. A roof that has lost a few tiles, or has a run of loose caps along the ridge, very rarely needs replacing.

Tiles get replaced individually and matched to the profile already on the roof. Slipped or loose tiles get lifted, laid back in position and fixed properly. Ridge and hip caps get rebedded and repointed so they are held down again.

The valleys and the flashings get checked at the same time, because those are the places water concentrates. There is more on that approach in repairing tiles without replacing the whole roof.

We work on tiled and terracotta roofs only, and we do not replace roofs. Where water has already been getting in, the work starts with finding the entry point rather than the wet patch, which is what tracing and repairing a leak on a tiled roof involves.

Where a roof has broken tiles, failed ridge mortar and a worn tile surface all at once, a full restoration is usually better value than chasing each fault separately. Restorations start at $4,500 and repointing only jobs start at $1,500.

Most complete jobs land between $4,000 and $9,000 depending on size, pitch and access, and full roof restorations carry a 7 year warranty.

Spic and Span is family owned and based in Cranbourne, with over 30 years in the trade and more than 1,000 roofs restored, residential and commercial. We cover Melbourne’s south east, including roof restoration in Hallam.

You can see what the work involves on our roof restoration service. If something looks like it has moved up there, get someone to put eyes on it before the next front comes through.

Ring 0435 104 623 for a free quote. We are open Monday to Friday, 8am to 5pm, and we will tell you whether it is a handful of tiles or something that needs more.

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