Slate roof repair on a commercial or institutional building

Slate is stone, and stone does not wear out on the timescale of a building’s ownership. A slate roof that is leaking has almost never run out of slate. It has run out of nails, or flashings, or sheathing, or it has been repaired badly enough times that the repairs are now the problem.

That distinction decides a great deal of money on an older commercial, institutional or multifamily building. Churches, schools, courthouses, libraries and nineteenth century commercial blocks carry slate roofs that are perfectly capable of another fifty years, and they are regularly replaced with something with a twenty year life because nobody separated the stone from the things holding it on.

What actually fails, in order

A slate roof is a system, and its components have wildly different lifespans.

Read that table from the bottom up, because that is the order in which a slate roof gives trouble. The sealant somebody put on in 1998 fails, water gets behind the valley metal, the sheathing beneath it softens, the nails in that area corrode, slates slip, and the owner is told the roof is finished.

Nail sickness, and why it is not the end

When the nails holding a slate roof corrode to the point of releasing slates across an area, the condition has a name: nail sickness. It presents as slates slipping for no apparent reason, often after wind, and often in patches rather than evenly across the roof.

It is a fixing failure, not a slate failure. In patches it is repaired by refixing the affected slates with hooks or tingles. Where it has gone across whole slopes, the answer is relaying, which is a different proposition from replacement and worth being precise about:

1. The slate is carefully removed and sorted. Sound slates are kept; delaminating ones are discarded.
2. The sheathing or battens are repaired where water has rotted them.
3. New underlayment goes on, and new flashings go into the valleys and around every penetration.
4. The same slate goes back, with new copper or stainless fixings.
5. Shortfall is made up with salvaged or matching new slate.

The building keeps its roof and its appearance, which on a listed or institutionally significant building is frequently a requirement rather than a preference, and the cost sits well below a new roof in new material.

How a slate is properly replaced

This is the detail that separates competent slate work from the kind that shortens a roof’s life, and an owner can check it from the ground with binoculars.

A slipped or broken slate is removed by lifting the slate above it rather than breaking it. The old nail is cut or withdrawn. A new slate of matching length, width and thickness is then held in place by a copper or stainless hook over the batten, or by a lead tingle folded over the slate below.

What should not happen is a nail driven through the face of the new slate and the hole sealed with mastic. That puts a hole in the water shedding surface and relies on a sealant with a short life to keep water out of it. When that sealant fails, which it will, the leak is back and the slate now has a hole in it.

So after any repair, look at the roof. Visible blobs of black or grey sealant are the signature of the wrong repair, and they are a reliable indicator of what else was done while nobody was watching.

The three things that destroy slate roofs

Foot traffic. Slate is strong in bending and brittle under a concentrated load. A person walking a slope cracks slates, and the cracks often do not show until water finds them a season later. Proper access is by ladder boards, roof ladders or staging that spreads the load, and it costs more than walking, which is precisely why it gets skipped.

Mortar, mastic and tar. Mortar fillets at abutments crack, then hold water against the slate and the wall behind it. Mastic and roofing tar spread over a valley or a flashing make the next repair harder and sometimes impossible, because the materials cannot be cleaned off and nothing bonds to them.

Coatings. A slate roof does not need a surface treatment of any kind. A coating prevents the slate from drying out, holds moisture against the nails, and peels in sheets taking weathered surface with it. It also has to be removed before any genuine repair can be made. An owner being sold a slate roof coating is being sold the thing that will end the roof, and the honest alternative is a survey.

The flashings are usually the actual problem

On most leaking slate roofs the slate is fine and the metal is not. Valleys, chimney and parapet abutments, dormer cheeks and any penetration are all weathered with metal, and metal has a far shorter life than stone.

Valleys are the common culprit, because they carry the volume of two slopes and because they are where leaves and grit collect. A valley that has corroded through will leak persistently and will look, from inside, exactly like a roof that has failed. Replacing it means taking back the slate on both sides, fitting new metal and relaying the slate, and it fixes the roof for decades.

The same logic applies at chimneys, where the step flashings and the back gutter behind the stack are the usual failure points and the hardest details on the whole roof to build properly.

Reading the roof from the ground

Most of what an owner needs to know about a slate roof can be established without going anywhere near it, which is useful because going near it is how slate gets broken. A pair of binoculars and half an hour will tell you a great deal.

Look first for slipped and missing slates, and note whether they are scattered or clustered. Scattered losses after a storm are ordinary wind damage. A cluster in one area, appearing gradually, is the signature of nail failure in that area, and it tells you the problem is the fixings rather than the weather.

Look next at the slates themselves. Slate that is sound looks like stone. Slate that is failing looks layered at its lower edge, flaking in thin sheets, and sometimes slightly swollen. That delamination is the only genuine end of life condition for the stone, and a roof showing it across whole slopes is a roof whose slate cannot be reused.

Then look at the metal. Valleys, the flashings at every chimney and wall, and the metal around any vent or stack. Corrosion, lifted edges, patches of sealant and anything that has been bent back into place are all visible from below, and they are where most leaks on a slate roof originate.

Finally, look for repairs. Mismatched slates in size or color, bright new metal in one valley only, and visible sealant all tell you the roof has a history, and the history is usually more informative than the current condition. A roof that has been patched by whoever was cheapest for twenty years needs a different conversation from one that has simply been left alone.

Underlayment, and the question of what is under the slate

Older slate roofs were often laid over battens with no underlayment at all, relying entirely on the slate and the laps to shed water. Others have a felt layer that has long since gone brittle. Either way the slate is doing the whole job, which it is capable of doing as long as the laps are correct and the slates are all present.

This matters at relaying, because that is the one opportunity to add a modern underlayment beneath the stone. It is worth taking. An underlayment gives the roof a second line of defense for the first time in its life, and it costs very little when the slate is already off.

It also changes the risk profile of a missing slate. On a roof with no underlayment, one slipped slate in the wrong position puts water directly onto the sheathing. On a relaid roof with underlayment beneath, the same slipped slate is an appearance problem for a few weeks rather than a leak that afternoon.

What a survey of a slate roof should produce

Before any work is agreed, an owner should have a document that answers five things.

1. The condition of the slate itself, by slope, with an estimate of what share is sound and reusable.
2. The condition of the fixings, and whether nail failure is local or widespread.
3. The condition of every flashing, named and located rather than described generally.
4. The condition of the sheathing or battens wherever water has been getting in.
5. A recommendation with a reason, repair by area, relay by slope, or replace, and what evidence supports it.

A survey that recommends replacement without addressing the first two points has not done the work. That is the single most useful thing to insist on, because the answer decides between a repair budget and a capital project.

Finding the right contractor

Slate is a separate trade from membrane and shingle work. It uses different tools, different fixings and different access methods, and a general roofing crew doing its best will usually leave a slate roof worse than they found it.

Two questions sort the field. How will you access the slope. And how will you fix a replacement slate. The right answers involve staging or ladder boards, and hooks or tingles rather than face nailing. Ask to see slate work completed on comparable buildings, and ask to speak to the owner of one of them.

Budgeting slate work over time

Slate rewards a maintenance budget more than almost any other roof, because its failures are local and slow. A roof inspected every other year and repaired in small amounts rarely needs a capital project at all, and the annual cost of that approach on most buildings is modest against the alternative.

What breaks the arithmetic is deferral. A slipped slate costs very little to refix. The same slate left in place for three winters lets water onto the sheathing beneath, and that area then needs the slate off, the boarding replaced and the slate back on, which is an order of magnitude more work for an identical outcome. Multiply that across a roof left alone for fifteen years and the relaying conversation arrives, not because the slate failed but because the repairs did not happen.

So the useful budget line on a slate building is not a reserve for replacement. It is a small recurring amount for inspection and minor repair, with the understanding that valleys and chimney flashings will need replacing once or twice within a normal ownership while the slate itself will not need replacing at all.

Where to go next

If the leak is active and the roof is unfamiliar, tracing where water is actually entering comes before any decision about slate, because on a slate roof the entry point and the stain are often on different slopes. If the question is the larger one of whether to keep repairing, repair against replacement sets out how that comparison should be made.