You find a slate in the flower bed after a windy night. It is whole. No crack, no chip, just a flat piece of stone lying face up on the mulch with two small holes near the top. Most people pick it up, decide the wind was unusually strong, and put it in the garage.
That intact slate is telling you something specific, and it is not about the wind. A slate that breaks has been hit by something or has worn out as stone. A slate that comes down whole was let go by its fasteners. The stone did its job right up to the moment it landed. The nails did not.
The difference matters because it changes what you are dealing with. Broken slates are usually a local problem with a local cause. Slipped slates are a symptom of something happening across the whole roof at once, on a timetable that has already started and will not reverse.
Why an intact slate on the ground is different news
Each slate on a roof is held by two nails near its top edge, and those nail heads are covered by the course above. The slate below it overlaps up under it. Nothing is glued. The whole assembly is a set of overlapping stones hanging on small pieces of metal, and it works beautifully for as long as the metal holds.
When you see damage on a slate roof, sort it into two piles before anything else:
- Broken slates, in a cluster. Cracked or missing corners, usually under a chimney, at the foot of a valley, below a dormer, or where a branch reaches the roof. Something concentrated water or impact in one place. The cause is local and the repair is local.
- Whole slates, sliding out. Intact pieces on the ground, or slates you can see from the driveway that have dropped an inch or two below the line of their course and sit slightly proud of the roof. The stone is fine. The fastening is gone. The cause is not local.
- Whole slates that stay put but rattle. The overlooked category. A slate can lose one of its two nails and remain roughly in place, held by friction and by the course above. In a strong gust it lifts and drops back. These are the ones that come down next.
If your pile is mostly the second and third kind, the roof is not failing as a roof surface. Its fasteners are failing, and that is a more predictable problem than most people expect.
Nail sickness, plainly
Most slate roofs in this region were originally hung on cut steel nails. Steel corrodes when it is repeatedly wet. The nail shank sits in a hole drilled through a roof deck board, and that board takes on moisture from below as well as from any water that works its way in from above. Over decades the shank rusts thinner in the middle of the board where it stays dampest, until it is a wire, and then until it is nothing.
That process has a name in the trade: nail sickness. It generally shows up somewhere around sixty to ninety years into the life of the roof, though it varies with the deck, the ventilation, and how wet that particular slope stays. It arrives long before good hard slate is worn out, which is why so many roofs that look finished are actually carrying perfectly serviceable stone.
Two things follow from that. First, the slate coming off your roof is probably worth keeping. Second, replacing individual pieces one at a time does not address the cause, because the nails holding the slates on either side of your repair are on the same corrosion schedule as the one that just let go.
The other way fasteners fail
Not every slipped slate is corrosion. Two other patterns show up regularly. One is overdriven nails, where the original installer set the head so hard it crushed the stone around the hole, leaving the slate hanging on a weak point that eventually tears out. The other is nails driven too high or too low in the slate, putting the fastener where it is not covered properly and letting water reach it directly. Both tend to appear in patches, because one person nailed one section on one day.
How the failure spreads, and where to look next
Fastener failure is not random. It follows moisture, and moisture on a roof follows predictable paths. When we look at a roof that is dropping slates, we expect to find the next ones in specific places.
Start with the slopes that stay wet. North-facing slopes dry slowly, hold frost longer, and grow moss and lichen at the head of the slates. Their fasteners go first, sometimes by a decade or more, on the same house with the same nails.
Then look at the water concentrations. The courses running alongside a valley carry the runoff of two slopes. The courses immediately below a chimney, a dormer cheek, or a plumbing stack take water that has been channeled by the flashing. If the flashing there is old galvanized steel that has rusted through, the deck under those courses has been damp for years and every nail in the area is further along than the rest of the roof.
Then look at the eave courses, where ice can back up in winter, and at the ridge, where wind uplift is strongest and a slate with one remaining nail is most likely to be worked loose.
Finally, pay attention to timing rather than location. One slate in a decade is maintenance. Three or four in a season, from different slopes, in different weather, is the roof-wide pattern. The clearest signal we see is slates coming down on calm days, because a fastener that fails without wind has nothing left at all.
What to do when you find one
Keep the slate. It is useful. Its size, thickness, color, and the position of the nail holes tell us what the roof is made of, and if the stone is sound it can often go straight back up. Set it somewhere flat where it will not get stepped on.
Then look at the gap it left. From the ground with binoculars you can usually see whether the slate directly above the gap has also dropped, and whether the felt or the bare deck board is now exposed. An exposed deck board is a leak waiting for the next rain, and the water will not necessarily appear inside anywhere near that spot.
Putting the piece back is a matter of hooking or bibbing it in, which is what slate tile replacement covers, and on a healthy roof that is the end of it. The real question is whether it is the end of it, and that is answered by checking fastener condition at several points on every slope rather than by looking at the one hole. Plenty of slate roofs around Big Island are old enough for their original nails to be at the end of their run, and on those houses the single slate in the yard is the first of a series.
What you should not do is have someone seal the gap with roof cement. It does not hold on stone, it traps water against the deck, and it makes the eventual correct repair more expensive because the tar has to come off first.
When one slate becomes a plan
The useful outcome of finding a slipped slate is not just getting it back up. It is learning where the roof actually stands, so you are not surprised in three years.
That means someone getting close enough to sound the slates, check how many are hanging on one nail, look at the condition of the valley and chimney metal, and inspect the deck from the attic side. We do that as a slate roof inspection, and the result is a straight answer about whether you have a one-slate problem or a fastener problem. Either way, the immediate fix is ordinary slate roof repair work and not a reason to start talking about tearing the roof off.
Common questions
Why did my slate fall off if it is not broken?
Because the nails holding it corroded through or tore out of the stone. Slate is held by two nails near its top edge, hidden under the course above. When that metal fails, the slate slides out under its own weight, usually intact. The stone itself may have many decades of life left.
Is one slipped slate a serious problem?
One slate on its own, on an otherwise tight roof, is routine maintenance, though the gap it leaves should be closed before the next heavy rain. Several slates over a season, from different parts of the roof, is a different matter and points to fasteners failing across the whole roof.
Can a slipped slate be put back, or does it need a new one?
If the stone is sound, it goes back. Tap it and listen: a good slate rings, a spent one thuds. A sound slate is refastened with a slate hook or with a copper nail and a metal bib over the nail hole. Only slates that are cracked, delaminating, or too damaged at the nail holes get replaced.
How long do slate roof nails last?
Original cut steel nails commonly run sixty to ninety years before corrosion becomes widespread, and the range is wide because it depends on how damp the deck stays. Copper and stainless fasteners used in modern work last far longer, generally as long as the stone above them.
Find out what the slate is telling you
A slate in the yard is worth ten minutes of attention now rather than a leak later. We will look at the fastener condition across the roof, put back what can go back, and tell you plainly whether there is more coming.
Call +15555555555 to schedule an inspection.
