Can Wind Lift Roof Slates Without Breaking Them?

What Happens in the Gust

A slate is held by two nails, and the courses above cover them. In a gust, the suction over the roof lifts the tail of the slate - the free lower edge - and flexes it upward.

If both nails hold, it drops back and nothing has happened.

If one nail has already corroded to the point of failure, that gust finishes it. The slate is now held by one nail. It drops back into position, looks exactly as it did, and is now free to pivot on that single fixing.

If both go, the slate slides down the roof and either stops on the one below or ends up in the garden - and that is the version everybody notices.

The one-nail version is the one that matters, because nobody notices it at all.

Why It Does Not Show

From the ground, a slate held by one nail looks identical to a slate held by two. It is in the right place, in line with its neighbours, at the right angle.

It may creep down a few millimetres over the following months, which reduces the headlap - the amount the slate above covers it by - and that is the first visible sign. A course sitting slightly low, a line that has stopped being straight.

Long before that shows, though, the roof is already less watertight than it was, because a slate that can lift in the wind lets water past it in driving rain even when it settles back down afterwards.

So the sequence is: storm in November, nothing visible, damp patch appearing in February during a wet spell with no wind. And the February weather gets the blame for damage the November gale actually did.

The Wallasey Connection

This is one of the four reasons a leak keeps coming back, and it is the one that is hardest to argue with a customer about.

Somebody comes out, finds and fixes a defect, and the leak returns. It looks like a failed repair. What has often happened is that the storm loosened several slates and only the one that visibly moved was dealt with - so there was a second source all along.

On the front at New Brighton, where exposure is as high as anywhere on the peninsula, that is common enough to be the first thing to suspect on a recurring leak that started after a named storm.

And the older Wallasey and Liscard terraces raise it in another form: their slate roofs are frequently nail-sick already, so a gale does not need to be exceptional to take out a population of fixings that were nearly done anyway.

How It Is Actually Found

Not from the ground. The two ways are the loft and the roof.

In the loft, in daylight with the lights off, a slate that has lifted often shows as a thin line of light or as a patch of fresh damp on the underside that was not there before.

On the roof, a slate held by one nail can be felt. It moves under a hand when the ones around it do not. That is a slow job across a whole elevation and it is why a proper post-storm inspection on a slate roof takes longer than people expect.

And where nothing shows either way but the leak is real, a water test - wetting one area at a time from the bottom upward with somebody watching inside - is the honest answer.

What It Means for the Repair

A slate held by one nail is refixed the same way as a replacement: with a lead or copper tingle folded over its bottom edge, because the original nail positions are covered by the courses above.

The more important point is scope. After a storm on an older slate roof, the sensible instruction is not "replace the slate that fell" but "check the elevation the slate fell from". If one fixing has gone, its contemporaries are close behind - and finding three loose slates while somebody is already up there costs a fraction of finding them one at a time over the next three winters.