Slate Roof Repair in Cincinnati: Repair or Replace?

Most slate roofs should be repaired, not replaced, as long as the slate itself is still sound. The National Park Service says slate roofs "will last 60 to 125 years or longer," and that the nails and flashings often fail long before the stone does. Its rule of thumb: if fewer than about 20% of the slates are broken, missing, or sliding, look at repair. At 20% or more on a roof or slope, replacement is usually the cheaper path.

Slate still tops many older homes and churches in Cincinnati's older neighborhoods. If you own one, here is how to think about repair, maintenance, and when it is time to replace.

How long does a slate roof last?

It depends on where the slate was quarried. The National Park Service's Preservation Brief 29, The Repair, Replacement, and Maintenance of Historic Slate Roofs, gives these life spans:

The brief says these figures are "only a general guide." Roof pitch matters too. Steep slopes shed water faster, so their slate tends to last longer. Eaves and valleys that carry the most water and ice tend to wear out first.

Why do slate roofs leak?

The stone is rarely the first thing to go. Brief 29 lists these common causes:

• Rusted nails. The brief says plain steel and galvanized nails "will usually rust out long before the slate itself begins to deteriorate." Slates then slide out of place.
• Worn flashing. "Because slate is so durable, metal flashings often wear out before the slate does." The brief adds that "even the smallest pinhole" in flashing can let in a lot of water.
• Broken slates. Falling limbs, ice dams in gutters and valleys, and "the weight of a workman walking on the roof" all break slates.
• Weathered slate. Old soft slates absorb water, flake, and split into layers. That is called delamination.
• Gutters and valleys. The brief says not to assume the slates are leaking: "Gutters, valleys and flashings are more likely candidates."

If your house has built-in gutters or metal valleys, look there first when a slate roof leaks.

Why you should never walk on a slate roof

Slate is stone, and it can crack under a person's weight. The brief notes that weathered slates lose strength and are "more prone to breakage." It says "slate roofs should not be walked on if at all possible."

The Slate Roofing Contractors Association of North America (SRCA) says the same in its repair guidelines: "Workers shall not damage slates by walking on them." It calls for roof ladders, hook ladders, staging, or lifts to keep weight off the slate.

This matters for anyone who goes up there: painters, satellite installers, gutter cleaners, and chimney sweeps. Before anyone works on or near a slate roof, ask how they will get up and move around without stepping on it.

How is a broken slate repaired?

A skilled slater can replace a single slate without disturbing the ones around it. Brief 29 describes the usual method:

1. A tool called a slate ripper slides under the broken slate and cuts or pulls its nails.
2. A new or salvaged slate that matches "the size, shape, texture, and weathered color" slides into place.
3. One nail goes through the joint between the slates above.
4. A piece of copper, roughly 3 by 8 inches, slides under the slates above and covers that nail. This is called a bib.

The brief notes that one shortcut, a copper strip with a tab folded over the new slate, "is not recommended for northern climates where snow and ice" can bend the tab open. Cincinnati gets enough snow and ice that this is worth asking about.

Roof cement is not a slate repair. Brief 29 says mastic or sealant on damaged slate "should not be considered a viable repair alternative." It hardens, cracks, and lets water in, and it makes later repairs harder. The SRCA guidelines say broken or missing slates "are not to be repaired with mastic or caulk."

Should I repair or replace my slate roof?

Brief 29 includes a repair-or-replace guide. Here it is in short form:

1. Compare the roof's age to its expected life for that type of slate.
2. Count the damaged and missing slates. Under about 20% on a roof in generally good shape points to repair.
3. Find the real source of any leak. Check gutters, valleys, and flashing before blaming the slate.
4. Check the wood underneath. In the attic, look for stains and soft spots in rafters and sheathing. Old slates that hold moisture can rot wood even with no visible leak.
5. Look for sliding slates. Many slipped slates often mean rusted nails, not bad slate. The brief says those slates can often be salvaged and re-laid.
6. Inspect the flashing closely. It may be the only part that needs replacing.
7. Check whether the wear is even. Sometimes one slope needs replacing and the others can be repaired, which spreads the cost over years.
8. Tap a slate that has fallen off. A "full, deep sound" means good slate. A "dull thud" suggests poor slate.
9. Ask whether matching slate is available. Even if replacement is needed, repairs may buy time to find the right slate.

What about replacing slate with something else?

For historic buildings, the Park Service recommends replacing slate with new slate. It notes that a synthetic slate roof costs "only marginally less" up front, because labor, flashing, and tear-off cost the same either way. Over the long run, it says, natural slate "tends to be a better investment."

For homes that are not historic, owners sometimes choose synthetic slate or other materials. On a typical Cincinnati home, metal, synthetic slate, or tile roofs often run $25,000 to $60,000 or more. When any slate roof is torn off, the brief says rotted sheathing should be replaced and the rafters checked for signs of stress. If your home is in a historic district, check with the local historic review board before you change roofing materials.

How should I maintain a slate roof?

Brief 29 recommends:

• Inspect it once a year and after severe storms. Do it from the ground with binoculars, or from a lift. Note cracked, missing, or sliding slates and any loose flashing.
• Check the attic. Look for water stains and rot near the eaves, valleys, and hips.
• Clean the gutters at least twice in the fall and once in early spring.
• Replace damaged slates promptly. One missing slate lets water reach the wood under it.

Keep a file of every repair, with photos and receipts. It helps the next person who works on the roof.

If a storm knocks off many slates, the brief says a temporary cover should go on right away. It notes that plastic tarps are hard to secure on slate and can blow off. Our guide to emergency roof tarping covers temporary protection after a storm.

How Shamrock can help

Slate work takes specialized skills and tools. Brief 29 says repair should be "entrusted only to experienced slaters." We offer a free roof inspection and will tell you plainly what we see: whether the slate is sound, whether the gutters or flashing are the real problem, and whether you need a slate specialist. The SRCA keeps a contractor directory if you want to compare. You can also read about our roofing services.

FAQ

Can a slate roof be patched with roof cement? No. The Park Service and the SRCA both say mastic is not a proper slate repair. It cracks and traps water.

Can I clean my own gutters on a slate roof house? From a ladder against the house, carefully, yes. Never step onto the slate to reach a gutter or valley.

Does homeowners insurance cover slate roof repair? Storm damage, such as slates broken by hail or a falling limb, may be covered. Normal wear, rusted nails, and old flashing usually are not. Our Ohio storm claim guide explains the process.

How often should a slate roof be inspected? Brief 29 recommends once a year and after severe storms. Do it from the ground with binoculars or from a lift, not by walking on the roof.

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