Rain. Acid rain. Not the kind that burns holes through umbrellas like in cartoons, but the slow, pH-tampered sorrow that sneaks up on historical slate roofs and eats away at stuff like time with uneven teeth. If you’ve ever seen a 19th-century church roof looking like someone scraped it with angry steel wool, you might guess—this ain’t just age. This is chemistry misbehaving.
Look, slate’s tough. Mostly. Fought wildernesses, fought hurricanes, once fought a raccoon population in Maine. Held out. But acid rain? That’s a sneak thief. Not loud, not showy, but persistent like a dripping faucet in a motel room you can’t get refunded. That kind of bad roommate of the sky.
Slate: Not As Stoic As It Pretends
Slate would like you to believe it’s noble. All stoic and unmoved. Nope. It’s a diva in disguise. The moment the rain’s got more acid than a punk band’s third album, the slate’s out there flaking like a croissant with self-esteem issues. Not dramatically, not suddenly. Think long-term betrayal. One shingle at a time. Spalling, they call it. Bits flaking off like… actually, no metaphor necessary. It just flakes off. And slowly, horribly, water gets in. Expansion, contraction, a cold night, pressure builds, pop, there it goes. That’s your grandmother’s schoolhouse roof gone weird and buckly.
Slate is a metamorphic rock, which means it has changed state from one type of rock into another. In the case of slate, mudstone was the original material, and the change is brought about by temperature and pressure over many years. The majority of slate is currently mined in Europe and Brazil. The American mines are primarily located in New York, Pennsylvania, and Vermont. Once mined from the ground, the slate is split into layers by hand or by machine, and subjected to inspection and further refinement before being used in landscaping or as roofing tiles.
https://www.bravarooftile.com/blog/what-is-a-slate-roof/
A Word About pH, Then Probably Another One
pH of rain, in normal times, should hover around 5.6. Biology class told us that. On paper, fine. Out in the wild where coal smoke still haunts and traffic coughs out sulfur dioxide like bad excuses, rain can get down to pH 4.3 or worse, depending on where you are standing and how angry the sky feels toward local industry. That may not sound like much of a drop—but in chemical terms, it’s like turning down the heat from “warm bath” to “boiling oil.” Slight shift, big punch. Slate, made of compressed clay and volcanic whispers, can’t handle much of that before layers untangle like wet socks in a blender.
The pH scale measures how acidic an object is. Objects that are not very acidic are called basic. The scale has values ranging from zero (the most acidic) to 14 (the most basic). Pure water has a pH value of 7. This value is considered neutral—neither acidic or basic. Normal, clean rain has a pH value of between 5.0 and 5.5, which is slightly acidic. However, when rain combines with sulfur dioxide or nitrogen oxides—produced from power plants and automobiles—the rain becomes much more acidic. Typical acid rain has a pH value of 4.0. A decrease in pH values from 5.0 to 4.0 means that the acidity is 10 times greater.
https://www3.epa.gov/acidrain/education/site_students/phscale.html
Restorers, They Try
Walk too fast on a worn-down slate roof and you’ll step through history with your boot. So they come—restorers, artisans, sometimes odd folk who love ladders but hate gloves. They show up with gentle hammers and skeptical stares. They patch what they can. Swapping out degraded slate pieces with new ones—or salvaged ones, if they’re deep into authenticity and have the patience of sepia-toned monks. Some will even argue about which quarry offers the right “color match”, which is an argument you can lose friends to.
But even then, they’re playing catch-up with weather. A Band-Aid on a bone fracture, kind of fix. You can try sealing. Some folks brag about copper sulfate-based buffering applications—as in, they coat the slate with compounds that push back against acid penetration—but those work about as well as politely asking a tidal wave to stop thirty feet offshore.
Wait—Why Not Just Replace It With Something Modern?
Because that’s cheating. Also, because you’d lose that whisper-of-the-past vibe. Modern roofs pretend to be noble, but you look at them and hear the plywood underneath. Historical slate, though—it sings a grumbly, uneven tune when rain hits it just right. Makes you think of iron nails, of workers smoking hand-rolled cigarettes, of wood smoke and 1882. If you want to wallpaper over that with asphalt shingles? Well then, maybe don’t restore old buildings. Maybe go design airports or something.
Preventative Tactics or Just Wishful Thinking
Could coat the slate. People try. Some use breathable silane-based sealers. Fluoropolymers. The whole periodic table if you let them. Adspeak always promises hydrophobic layers that keep acid out the way Tupperware keeps meatloaf fresh. Here’s the kicker—most of them wear off. UV light laughs in chemical innovation’s face. Two winters, sometimes three, and we’re back in leaky hell.
Gutters matter too. Keep the water off properly and it might not sit long enough to do damage. But then it becomes about maintenance, which most building owners remember slightly less often than their third cousins’ birthdays.
Let’s Not Even Talk About Flashings. But We Will
Galvanized flashings. If they rust, which of course they do, the water’s now bringing a whole new pH party. Iron oxide soup flows across slate, staining it, exacerbating decay like insult slathered on injury peppered with neglect. Copper flashings work better, but cost the same as emotional therapy and three cappuccinos a week for twenty years. Still, worth it. Maybe.
You Probably Want Closure. There’s None
This ain’t a plucky story with a triumphant note. You do your best. Historical slate gets old and acid rain doesn’t stop to read plaques. You monitor, replace what must go, and sometimes write long reports that no one reads. You hope some future caretaker gives enough of a damn to do the same.
Oh, and if you ever walk across a flaky slate roof in springtime with the ice just melting? Walk slow. Shoes with soft soles. Pretend it’s sacred ground—because, in a way, under the rot and chemistry, it still is.
