How long a roof actually lasts

A roof is not one thing with one lifespan. It is a covering, the nails that hold it, the battens those nails go into, a layer of underlay, and mortar or lead at every edge and joint. Each has its own clock. The roof is finished when the shortest clock runs out.

That is why a slate quarried in 1890 can be perfectly sound in the skip. The slate did not fail. The nail did, or the batten did, and every slate on that slope had to come off to reach them.

So the useful question is never how old the roof is. It is which layer has gone, and across how much of the slope.

These are working lifespans on London houses rather than manufacturer figures. A sheltered rear slope facing north-east will beat them. A front slope facing south over a main road, with a plane tree dropping on it every autumn, will not.

A slate is not fixed down. It hangs on two nails driven through holes near its head, and the slate above covers those nails. Nothing else holds it on the roof.

Victorian and Edwardian slaters used iron or galvanised nails. Iron rusts. Galvanising is a thin zinc coat that corrodes first, on purpose, to protect the steel beneath — and once the zinc has gone the steel has nothing left. Either way the shank thins in the damp air of the roof space until it can no longer carry the weight of the slate plus the lift of a gust. Then the slate slides out, almost always during the first proper winter gale.

That is nail fatigue, and the point about it is that it does not happen to one nail. Every nail on that slope went in the same week, from the same keg, and has had the same hundred and thirty winters since. When one goes, the rest are exactly as old.

So the pattern of the drops tells you more than the date on the deeds, and the questions below set out how to read it. What matters here is that the clock is a shared one: the moment a slope starts shedding sound slates in more than one place, every remaining nail on it is exactly as old as the ones that went.

The stone, though, is usually fine. Because the slate outlives its fixings, most of a re-slate goes back on the roof as your own slates, on new graded battens, a new membrane and copper or stainless nails that will not corrode the same way.

Battens are the timber the slates and tiles nail to. Until the 1970s they were whatever sawn softwood came off the lorry, often ungraded, often full of sapwood, and rarely treated. They fail at the nail holes, because that is where damp sits longest, and a batten that has gone soft around a fixing will not hold a nail no matter how good the nail is. The lowest metre of a slope goes first, since it stays wet the longest.

Battens today are graded to BS 5534, treated, and dyed blue or green so anyone can see at a glance that the correct timber went on. It is worth looking for that colour in the photographs a roofer sends you.

Most London houses built before about 1930 have no underlay at all. The slates were the waterproof layer, and the gap beneath was sometimes torched — lime mortar buttered along the underside of the slates to stop wind-blown snow. Torching crumbles with age, and by now a good deal of it is on your loft floor.

Roofs from the 1930s to the 1980s have bitumen felt underlay, hanging in a shallow drape between the rafters. Over thirty or forty years the bitumen loses the oils that keep it flexible, goes brittle, and tears along the fold at the bottom of the drape. You can often see that tear from inside the loft with a torch. Once it has gone, any rain that gets past a tile has nothing to catch it.

Bedding mortar on a ridge or a hip is sand and cement. It shrinks as it cures, then freezes and thaws every winter for twenty years, and twenty years is a decent run. Cracked bedding is the single most common source of a leak that has nothing at all wrong with the tiles.

The standard has required ridge and hip tiles to be mechanically fixed since 2014, not simply bedded. A dry ridge system screws them down over a ventilated roll. It costs more on the day and removes a job you would otherwise pay for again in two decades.

Lead gets blamed for failing when it was really fitted wrong. Lead moves a lot with temperature: a long piece grows several millimetres on a hot afternoon and shrinks back overnight. Fix it at both ends and make it too long, and it cannot move, so it works itself thin along one line and splits — a straight crack running the length of the sheet, in the same place every time. The answer is not thicker lead. It is shorter pieces with proper laps, which is why the standard limits a single length of flashing to about a metre and a half.

Lead that is the right code, the right length and properly dressed will see out the roof it sits on. Lead patched with mortar fillet, mastic or a tin of black paint will be back within a few winters.

Felt rarely dies of one thing. It blisters first and goes brittle afterwards, and on a roof of about the right age the two arrive together. One blister is a half-day repair; a slope of them is the deck below saying it is wet.

Felt also goes brittle. Bitumen holds oils that keep it flexible and sunlight drives them off. The mineral chippings on the top layer exist to shade it, so once they have washed down into the gutter the felt below has a few summers left. It splits where the deck flexes most, usually along a joint between the boards.

Rubber and fibreglass do not go brittle the same way, but they are only as good as their edges. Nearly every EPDM or GRP failure we attend is a seam, an upstand where the roof meets a wall, or the trim along the front. Ponding on its own rarely kills a modern flat roof, but a standing puddle does tell you the falls are wrong, and it will find any weak detail there is.

Very little of this is weather. Most of the roofs that die early were helped along.

Nobody can date a roof accurately from the pavement, and nobody honest prices a re-roof from a photograph. From below we can read the covering, the ridge line, obvious slippage and the state of the mortar. What decides the job is underneath: whether the battens still hold a nail, whether there is any underlay left, and how much of the covering can go back on.

That needs a look in the loft, or a few slates lifted, and it takes about half an hour. It is also the difference between a repair and a re-roof.

If someone tells you the roof needs replacing without having been in the loft or on the slope, get a second opinion before you agree to anything.