Replacing a Slipped or Broken Slate

Pick the slate up before you look at the roof

It is lying face up on the grass about a metre out from the front wall, and it is not broken. That is the first useful fact of the morning and almost everybody walks straight past it to get the ladder off the van. A slate that has left a roof carries the reason it left in two places — the condition of the slate itself and the condition of its two nail holes — and both are gone the moment somebody stacks it behind the shed with the bits of broken paving.

So handle it first. Whole slate, clean round holes, nothing torn: the fixings let go and the slate simply slid out from under the courses above. Now look for the nails. Still through the holes, sheared or wasted to a brown stub, and the answer is already in your hand. Holes empty, nails still up there in the batten, and something else let go — the head of the nail pulled through, or the hole itself wore oval and rode off. If instead the slate is snapped clean across and part of it is still in the gutter, it was loaded rather than corroded: a foot, a ladder stile set down on the covering, a branch, or the tail levering itself in a gust. Those fixings may be entirely sound.

Then do the thing a slater does without being asked. Balance it across two fingers and tap it with a knuckle. A sound slate rings. One that has taken up water and begun to open along its own bedding gives a dull knock, and if the edges crumble under a thumbnail the material has failed rather than the fixing. No standard backs that test and none needs to; it costs ten seconds, and it decides whether you are pricing an hour's work or the end of a roof covering.

Everything below is the same question asked in progressively more expensive ways: did two nails fail, or have all of them started to. The awkward part is that the evidence sits where you cannot see it. The nails that held this slate are buried under the heads of the courses above the gap — two of them on head-nailed work, one on centre-nailed, as the next section sets out — which is exactly why nobody inspects them and exactly why the repair has to work blind.

Two nails, and the several ways they die

Each slate on that roof is held by two fixings and nothing else. Where those two holes sit along the slate is a decision somebody made a century ago and it governs your whole repair, because it decides how deep under the covering the old nails are. Head-nailed work puts the holes an inch or so down from the top edge; on a double-lapped roof that position ends up buried under two slates, which is generous cover for the hole and a long unsupported tail below the fixing. Centre-nailed work puts them at roughly mid-length, where the lever arm is short and the tail cannot flutter, but where the arithmetic leaves only one slate over the hole rather than two. Neither is wrong. Both are still out there, often on the same street, and the ripper you are about to slide up the gap has to hunt in a different place for each.

What kills the fixing is almost never load. Slate outlasts everything fastened through it, so a covering fails from its nails upward. Plain steel nails were used freely on cheap work and on repairs, and they corrode until the shank parts; galvanised nails last longer and then fail at the same speed once the coating is gone, which is why the trade calls it sickness rather than wear — nothing happens for decades and then the roof starts shedding on a schedule of its own. Copper, aluminium alloy and stainless fixings do not go this way and a roof laid with them fails elsewhere, usually at the battens. Where the nails were copper and the slates are still coming off, look hard at the timber: a batten that has taken thirty years of a leaking valley lets a sound nail pull straight out of it.

The loft is the diagnostic room and hardly anybody goes up there. On a roof with no underlay — normal enough on pre-war work, where the slate backs were parged with lime torching instead — you can see the batten tops, the nail shanks and daylight, all at once. Rust runs staining the batten under every fixing, shanks that snap when you flick them with a screwdriver, or a scatter of nail heads lying on the ceiling joists are not indicators of a problem; they are the problem, counted. On an underlaid roof you get less, but you still get the torn-felt map of where water has been arriving and how long it has been doing it.

Finally, read the pattern rather than the incident. Slipped slates that cluster on one elevation, and specifically on the elevation that takes the prevailing weather, are behaving like a fixing failure driven by exposure. Slates coming off both pitches, at both ends, at an accelerating rate over three or four winters, are a roof telling you the fixings have reached the end together — which is what you would expect, because they went in on the same day out of the same keg. A single slate off the middle of an otherwise unbroken plane, with everything else tight and the neighbours' roofs untouched, is an incident.

Nothing left to nail through, and four honest ways round it

Here is the structural awkwardness of the whole job. The new slate has to sit in the same plane as its neighbours, with its head under the two courses above and its own nail holes therefore in a position no hammer can reach. You cannot nail it where the original was nailed without taking off the courses that cover that line, and those courses are sound, are nailed themselves, and will break if you try. Every method below is a way of holding a slate that has no reachable fixing of its own, and the difference between them is how long the hold lasts and how much of it you can see from the pavement.

First, clear the ground. A slate ripper is a long flat blade with hooked barbs at the tip; it goes up the gap under the slates above, is felt onto the shank of each old nail in turn, and is then jerked down so the barb either draws the nail or shears it. That is two nails to find, and where they are depends on whether the roof is head-nailed or centre-nailed, which is why the fallen slate in your hand is worth reading before you climb. A shank left proud will hold the new slate off its bed and show as a lifted tail from the street, so it is worth going back with the ripper a second time rather than accepting a slate that sits high.

The classic fixing is a tingle, also called a tab: a narrow strip of copper or lead, nailed into the exposed batten through the gap, folded up over the head of the new slate as it goes in, and then folded back over its tail once the slate is home. It works, it is reversible, and it has two well-known faults. Lead creeps, so a tingle cut too thin or made too long will slowly straighten under the weight of the slate and let it go again, usually in the third or fourth winter. And the folded tail is visible — a bright line on a dark roof that dulls in a season or two but never disappears. Copper strip holds its fold better than lead of a comparable stiffness and stains to something quieter; purpose-made stainless tingles are sold for the job and are stiffer again.

Slate hooks are the alternative and they are a proper fixing rather than an improvisation. A stainless hook is nailed or driven into the batten in the vertical joint below the gap, the slate is slid up over its shank, and the hook's turned end catches the tail. They hold far better than a folded strip in an exposed position, they are the standard field fixing across much of continental Europe rather than an emergency measure, and they are sized to a particular slate thickness and gauge, so they have to be bought for the roof rather than out of the bottom of the toolbox. Their drawback is the same as the tingle's and slightly worse: a hook is a visible spot at the foot of every repaired slate, and a roof that has collected forty of them over twenty years looks exactly like a roof that has collected forty of them.

The remaining two are worth knowing because they answer different questions. A copper rivet — a disc on a shank, driven through the vertical joint between the two slates below and turned up — gives a stiff, small, unobtrusive catch and is the traditional answer where appearance matters on a visible elevation. And where several slates have gone in one area, the right move is not four tingles but to strip back deliberately: lift the sound slates above in order, keep them, re-nail everything properly with copper or stainless into sound batten, and re-lay. That is an afternoon rather than an hour, it leaves nothing visible, and on a roof whose fixings are otherwise good it is the repair that lasts as long as the covering does.

1. Read the fallen slate for nailing position and material, so you know where the ripper is hunting before you go up.
2. Set the access, and get a roof ladder over the ridge rather than standing anywhere on the covering.
3. Slide the ripper up the gap, find each of the two old shanks in turn, and draw or shear them cleanly.
4. Check the batten through the gap with a screwdriver — soft timber means the fixing has nothing to bite and the job has just changed.
5. Fit the tingle, hook or rivet into sound batten first, sized so it will not creep under the weight it is about to take.
6. Slide the replacement up until its head is level with its neighbours, then close the fixing over the tail and sight the course from the ground.

Matching a slate nobody kept the paperwork for

A replacement has to match in four dimensions, not one: length, covering width, thickness and colour, and the roof will forgive an error in the last of those far more readily than in the first three. Length is set by the gauge already on the roof and cannot be fudged — a slate short by 20 mm loses that much headlap for good, in the one position on the roof you have already proved is exposed. Thickness matters because a slate noticeably thicker than its neighbours will not bed down and lifts the course above it into a visible ridge. Colour is the one everybody worries about and the one that fixes itself: a new slate reads bright for a couple of years and then weathers back, and a well-chosen new slate ages better than a badly-chosen reclaimed one.

Reclaimed slate is the obvious answer and comes with a catch that gets skipped. Every second-hand slate already has two holes in it, and unless they happen to fall where your gauge wants them the slate has to be re-dressed and re-holed — which shortens it, moves the fixing line, and puts the redundant original holes somewhere they must still end up covered. A merchant selling reclaimed slate by the pallet has usually not done that work. Ask whether the slates are graded and re-holed or sold as they came off, and price accordingly, because sorting a pallet on the drive to find the fifty that match your roof is real time that lands on somebody's bill.

For new slate, ask for a classification instead of an adjective. ASTM C406 sorts roofing slate into grades by expected service life, and BS EN 12326-1 codes it against thermal cycling, sulphur dioxide exposure and water absorption, which between them are the three things that actually take a slate apart. A supplier who can quote you the code is selling a specified product; one who says the slate is good quality is selling you a word. And take a moment over the fixings while you are on the phone: putting a plain steel nail into a repair on a roof that is slipping because of plain steel nails is a joke that gets told on a lot of roofs.

Write down where it came from

Finish by recording the thing that turns the next slipped slate from an incident into a data point: the date, which elevation, and roughly where on the plane — third course up, four slates in from the left-hand verge. A photograph from the pavement with the gap visible takes five seconds and is better than any description. None of this helps today. All of it is the entire basis on which somebody decides, in four winters, whether the rate is flat or climbing, and whether the losses are scattered or walking steadily across one pitch.

That record is the only thing standing between a homeowner and the two failure modes at either end of this job: paying for a roof that had ten good years left in it, and paying four times over for repairs to a covering whose fixings had already gone. Neither is caused by bad slating. Both are caused by nobody keeping count.