If your house has a slate roof, you probably already know it is not like other roofs. Slate is stone. Good slate can stay on a roof for generations, and plenty of Lehigh Valley homes still carry the slate they were built with. So when a slate roof starts leaking, the question is rarely whether the slate is worn out. It is usually what has failed around the slate, and how much of it.
Why slate roofs leak
Slate roofs tend to fail at their weakest parts first:
- Nails. Each slate hangs on two nails. Older steel nails rust through, and the slate slides out of place. A roof can lose dozens of slates this way while the stone itself is fine.
- Flashing and valleys. The metal at chimneys, walls and valleys often wears out before the slate does. Valley metal carries a lot of water and corrodes over time.
- Broken slates. Falling branches, ice and people walking on the roof all crack slates.
- Poor past repairs. Face-nailed slates, roofing cement smeared over cracks and mismatched replacement pieces often cause more leaks than they fix.
- Slate that is genuinely worn out. Some slates soften and flake apart (delaminate) as they age. When that is happening across the whole roof, repairs stop making sense.
The 20 percent rule of thumb
The National Park Service’s Preservation Brief 29, written for historic slate roofs, offers a useful starting point: count the damaged and missing slates. If they come to less than about 20 percent of the roof and the roof is generally in good condition, it should be evaluated for repair rather than replacement.
The National Slate Association describes the same rule from the other side. When 20 percent or more of the slates are broken, cracked, missing, sliding, badly delaminated or suffering from poor earlier repairs, replacement is often more practical. That assumes the damage is spread across the roof. If it is concentrated on one slope, replacing that slope alone may be enough.
Signs your slate roof is a good candidate for repair
- Most slates are hard and flat, with crisp edges
- Damage is scattered or limited to one area
- Leaks trace back to a chimney, valley or wall flashing
- Missing slates have slid down from rusted nails rather than broken in place
- The roof boards under the slate are dry and solid
Signs replacement may be the better spend
- Slates flaking, crumbling or soft across much of the roof
- Slipped slates from failing nails all over the roof, not just in one area
- Large areas of earlier patching with roofing cement or mismatched slate
- Rotted roof boards or repeated leaks in many places
Even then, replacement does not have to mean giving up on slate. Sound slates taken off one slope can sometimes be reused to repair another, and new or salvaged slate can be matched to the original.
What a good slate repair looks like
A well done repair is hard to spot from the street. Replacement slates should match the originals in size, thickness and color. They should be fastened without exposed face nails, using methods such as slate hooks or a concealed nail covered by a small metal bib. Flashing and valleys should be replaced with long-lived metal rather than patched with tar. And nobody should be walking directly on the slate, which breaks underfoot.
What you can check from the ground
You do not need to climb up to gather useful information. With binoculars, look for gaps where slates are missing, slates hanging out of line, rust streaks at the valleys, and dark smears of patching compound. Inside, check the attic after a rain for stains on the roof boards, especially around chimneys and valleys. Write down what you see and when the leaks happen. That makes any roofer’s assessment faster and more accurate.
The bottom line
Most slate roofs that leak need repair, not replacement. The decision turns on how much of the roof is affected, whether the slate itself is still sound, and whether earlier repairs have caused new problems. If you own a slate roof in Allentown or nearby, our slate and tile roofing page explains what we look at, and you can call (610) 432-9870 to arrange a visit.
