Black smears around a chimney. A thick bead squeezed along a valley. A broken slate held in place with a trowel-load of something that looked like it would stop the leak. If your slate roof has any of that on it, someone treated a stone roof like an asphalt shingle roof, and the repair is going to cost you more than the original leak would have.
Roofing tar, roof cement, plastic asphalt cement, and the various tubes of black sealant sold for flashing work all behave more or less the same way on slate. They stick for a season or two, they fail, and they leave behind a mess that has to be removed before anything correct can be done. The stone is not damaged by the tar in a chemical sense. The damage is mechanical, and it comes from what happens at the boundary between a flexible petroleum product and a rigid piece of rock.
The good news is that every job tar gets used for on a slate roof has a proper solution that is not much harder and lasts decades instead of months.
Why tar ends up on slate roofs at all
Nobody sets out to ruin a roof. Tar gets used because it is what is on the truck. A general roofer who works on asphalt shingles all week has roof cement in his hands every day, and on a shingle roof it is a legitimate material for certain details. Faced with a slipped slate or a leaking chimney apron, the instinct is to seal it.
There is also a genuine difficulty behind the shortcut. Replacing a single slate in the middle of a roof is not obvious work. The fastener is hidden under the course above, so you cannot simply nail the new piece in place the way you would a shingle. Someone who does not know the slate hook or the bib method has no way to make the repair hold, and tar looks like the only option available.
The result is the same either way. A few years later the tar has hardened, cracked, and let water back in, and now there is a second problem sitting on top of the first.
Four ways asphalt sealant fails on stone
Thermal movement
Slate barely moves with temperature. Asphalt cement moves a great deal, softening in summer heat and going brittle and contracting in a hard freeze. Bond a material that moves to a material that does not and the bond line takes all the strain. In this part of Virginia a roof can run through dozens of freeze and thaw cycles in a single winter, and each one works the edge of the patch a little looser. The failure starts as a hairline separation at the perimeter, which is exactly where water runs.
Trapped moisture
A slate roof is not a sealed membrane and was never meant to be. It works by shedding water down overlapping courses, and it relies on air moving through and around the stone so that everything dries between rains. A patch of tar blocks that. Water gets behind it, through the cracked perimeter or by capillary action up under the course, and then it cannot get back out. It sits against the underside of the slate and against the nail. That is how a small leak becomes a rotted sheathing board and a set of corroded fasteners in the surrounding area.
This is the failure mode homeowners underestimate. The visible symptom often improves for a year or two while the damage underneath accelerates.
Staining
Asphalt bleeds oil. On a hot slope, the lighter fractions in roof cement run out of the patch and down the stone below it, leaving a dark streak that does not wash off. On gray or green slate it is obvious from the street. On a purple or red roof it is worse. The staining is not structural, but it is permanent for practical purposes, and it is the reason tar repairs are so easy to spot on an otherwise handsome roof.
The cost of taking it off
This is the part that surprises people. When we come to do the repair properly, the tar has to come off first. Hardened roof cement grips the stone and has to be worked off with scrapers and solvent, slowly, without cracking the slate underneath. On a valley that has been coated end to end, it can take longer to clean than the actual repair takes to install. Slates that have tar wedged into the headlap often have to be removed and replaced outright because the material cannot be cleared from underneath.
So a tar patch does not just fail. It converts a one-hour slate roof repair into a half day of cleanup plus the repair.
What to use instead
Every situation where someone reaches for a caulk gun on a slate roof has an established answer. None of them are exotic.
- Slate hooks. A stainless or copper hook is driven into the joint between the two slates below, the replacement slate slides down over it, and the hook catches the bottom edge. It holds mechanically, it is nearly invisible from the ground, and there is nothing to dry out or crack. This is the standard method for replacing an individual broken slate in the field of a roof.
- Copper nails and a metal bib. The alternative method. The replacement slate is nailed through the vertical joint above it, then a formed copper or stainless strip is slid up under the course above to cover that nail hole. The bib sheds water over the nail permanently. It takes more skill than a hook but it is the more traditional repair.
- Soldered metal at the details. Chimneys, valleys, and sidewalls are not sealant problems. They are metalwork problems. Properly formed copper step flashing, counterflashing let into a mortar joint, and a soldered valley pan will handle water without any adhesive at all. If a chimney has been leaking and someone has been sealing it annually, what it actually needs is slate roof flashing repair done in metal.
- Salvaged slate for matching. New stone rarely matches weathered stone in color or thickness. Where appearance matters we use reclaimed slate of the same type, which is one of the reasons a correct repair disappears and a tar repair announces itself.
The common thread is that all of these solutions work the way the roof works, by shedding water over an overlap, rather than by trying to glue the water out.
If your roof already has tar on it
Do not panic and do not start scraping. Tar on a roof is not an emergency, and amateur removal breaks slates. What matters is knowing how much there is, where it is, and what it was hiding.
Start by getting a real count. A roof with three patched slates near one chimney is a small job. A roof where every valley has been coated and a previous owner sealed forty slates over twenty years is a different scale of work, and it usually turns out that the tar was covering a fastener problem or a failed valley rather than isolated breaks.
Where tar has been applied heavily across a roof, the work generally gets handled as slate roof restoration rather than as a series of patches: clean the stone, replace what cannot be saved, rebuild the metal in copper, and leave the roof in a condition where routine repairs are possible again. The older houses around Amherst often have slate roofs carrying decades of accumulated patch work, layered by successive owners who were each told it was the cheap fix.
One thing worth checking before any of that: whether the leak the tar was meant to stop is still happening. Often it is, in a different place, because the water was never coming from where the patch went.
Common questions
Can you put roofing tar on a slate roof?
You can, and it will hold briefly, but it is not a repair. Asphalt cement moves with temperature while stone does not, so the bond cracks at the edges within a season or two and water gets behind it. The tar then traps that water against the slate and the nails, which does more harm than the original leak.
How do you fix a broken slate without tar?
With a slate hook or with a copper nail and a metal bib. Both methods hold the replacement slate mechanically and keep the fastener covered so water runs over it. Neither relies on adhesive, and both are standard practice on slate roofs.
Will tar stains come off slate?
Mostly not. The surface deposit can be scraped and solvent-cleaned, which improves the appearance considerably, but asphalt oils soak into the stone and leave a shadow. Assume the mark will be visible after cleaning, just much less obvious.
Is it worth removing old tar repairs if the roof is not leaking?
It depends on what is underneath. Tar that is sitting on sound slate and not holding anything up can often wait until the next round of work. Tar that is acting as the only thing keeping a slate in place, or that is bridging a valley, should be dealt with, because when it lets go the roof is immediately open. We have found plenty of slate roofs in Bedford relying entirely on old sealant without the owner knowing it.
Get the tar off and the repair done properly
If your slate roof has been patched with roof cement, the sooner it is assessed the smaller the eventual job. We will tell you what the tar is covering, what can be cleaned, and what has to be replaced.
Call +15555555555 to schedule an inspection.
