Repair is usually the first choice for a slate roof. The National Park Service says slate commonly used in the late 1800s and early 1900s lasts 60 to 125 years, and rusting 19th-century cut nails are a common cause of slate loss. When 20 percent or more of a slope's slates are broken, cracked, missing or sliding, replacing that slope is usually less expensive than patching it.
Key takeaways
- The National Park Service gives 60 to 125 years for the slate most commonly used in the late 1800s and early 1900s, and says some lasts as long as 150 years.
- Rusting 19th-century cut nails are a common cause of slate loss, so the fasteners are the first thing to check.
- Solid copper or stainless steel slater's nails should always be used, because plain steel and galvanized nails usually rust out before the slate wears out.
- Using one metal family across flashing and gutters helps avoid galvanic corrosion where the metals meet.
- Roofing mastic and sealant are not a viable repair for slate, because they harden, crack and let water in.
- When 20 percent or more of a slope is broken, cracked, missing or displaced, replacing the slope is usually less expensive than individual repairs.
How long does slate last on an older Springfield house?
The slate most commonly used in the late 1800s and early 1900s lasts 60 to 125 years, according to the National Park Service. Its Interpretive Technical Study No. 32 says slate can last as long as 150 years. Age alone is not a reason to replace a slate roof.
Many of the city's Victorian houses have slate roofs, and the business description for this area points to the slate-roofed Victorians in the McKnight and Forest Park Heights districts. McKnight's houses were built largely between 1870 and 1900, and Ridgewood's largely between 1890 and 1910. Slate from that era is durable. The National Park Service recommends repair rather than replacement whenever possible, which is the principle this guide follows throughout.
Life depends on what has been done to the roof since it was laid. Nail type, flashing, gutter metal and a few broken slates in a bad spot all shape the years ahead. A slate roof is judged slate by slate and slope by slope rather than by one age figure. Ask the crew to show you the nails and the flashing, not only the slates, because those are the parts that decide how much life is left.
What usually causes slate to fail?
Most slate loss traces to the fasteners and the metal around them. Rusting 19th-century cut nails are a common cause, and failed flashing lets water reach the nails and the decking below.
Slate is held in place by nails, and the nails are often the part of the roof that has aged worst. Many older roofs were fastened with cut nails, which rust over the decades. When a nail corrodes, the slate it holds can slip, crack or fall. The slate itself may still be sound, which is why a slate roof with missing pieces is often a fastener problem rather than a material problem.
Water is the other route. Flashing at chimneys, valleys and walls is the transition between slate and the rest of the building, and it is where many leaks start. A slipped slate can send water onto the deck, and a wet deck can rot the nails that hold the next row. The result looks like a slate problem, but it may have begun at the flashing or at a nail. Check both during any inspection, and do not treat a leak as a slate problem until the flashing has been looked at.
Which nails and metals should a slate repair use?
Use solid copper or stainless steel slater's nails. Plain steel and galvanized nails usually rust out before the slate wears out. Keep the flashing and gutters in the same metal family to avoid galvanic corrosion where the metals meet.
The National Park Service says non-ferrous slater's nails, meaning solid copper or stainless steel, should always be used in slate work. The reason is simple. A nail that rusts through lets go of its slate long before the slate reaches the end of its life, and the repair then has to be done again. Choosing the right nail is one of the few decisions in a slate repair that is hard to revisit later, so it deserves a direct question before the work starts.
Galvanic corrosion is the reason to keep metals consistent. When two different metals touch in the presence of water, one of them corrodes faster. The advice is to use one metal family for the flashing and the gutters as well as the fasteners, so that the pieces around the slate do not fail at different times. Ask the crew which metal it plans to use and why, and check that the answer covers the flashing and gutters as well as the nails.
Is roofing mastic a quick fix for slate?
No. Roofing mastic and sealant are not a viable repair for slate. They harden and crack over time and let water in, which turns a small leak into a larger one.
Mastic is tempting because it is quick and easy to apply. The National Park Service is clear that it is not a repair for slate. Once it hardens it stops moving with the slate, and the cracks that form in it let water into the same spot it was meant to seal. A patch that looks tidy from the ground can hide a nail that has already failed underneath it.
If you see mastic on a slate roof, do not assume it is holding anything. Ask whether it was used to seal a slate or a flashing joint, and plan to repair the flashing properly rather than sealing over it. A repair that uses the right nail and the right metal will last longer than a sealed patch, and it is easier to inspect the next time the roof is looked at.
When does replacing a slope beat repairing it?
When 20 percent or more of the slates on a slope are broken, cracked, missing or sliding out of position, replacing the slope is usually less expensive than individual repairs. Below that point, careful spot repair is generally the better choice.
The National Park Service gives a threshold that is useful for planning. Count the damaged slates on a slope, including those that are cracked, missing or sliding, and compare that count with the slope's total. At 20 percent or more, replacing the slope is usually less expensive than repairing it piece by piece. That is a comparison of cost and effort, and it is the reason the threshold is worth knowing before a crew quotes the job.
Below that threshold, a repair that replaces the broken slates, sets the nails correctly and fixes the flashing is usually the right answer. The count is a planning tool, not a formula. A crew that finds a small share of damage in one place and a much larger share on another slope should assess each slope separately, and it should explain its count in writing. Ask to see the slopes and the count before you agree to any work.
How do historic district rules affect slate work?
Springfield has local historic districts, and the Historical Commission must approve exterior work before it begins. Ordinary maintenance and work not visible from a public street or park can go by certificate of non-applicability, with no public hearing.
McKnight is a local historic district, created in 1976 and listed on the National Register that year, and its Victorian houses were built between 1870 and 1900. Forest Park Heights is the second district, created in 1975, and was listed on the National Register in 1982. Exterior roof work on a house in either district is reviewed by the Springfield Historical Commission before it begins, and planting is not regulated. The Commission's material says it has seven mayor-appointed members and meets on the first and third Thursdays.
There are two routes. A certificate of appropriateness covers conforming changes. A certificate of hardship is a separate route that requires supporting material. Ordinary maintenance, and work not visible from a public street or park, can go by certificate of non-applicability with no public hearing. Tell the Commission what you plan to do, and ask it directly whether a specific address sits in a district, since this guide cannot say that for any individual property. The historic district roofing service page covers the review step in more detail.
How should a homeowner decide between repair and replacement?
Ask the crew to show the nails, the flashing, the slopes and the damage count, then decide slope by slope. Repair is the first choice when the nails and metal are sound, and replacement makes sense when a slope has crossed the threshold.
Start with a walk of the roof if it can be walked safely. A roof that is steep, wet or icy is a reason to stay off it and let a qualified crew inspect it. The aim is to count the damaged slates on each slope, look at the nails where they are accessible, check the flashing at chimneys and walls, and note the condition of the gutter metal. That gives you the information for the threshold and for the repair or replacement decision.
Get the answer in writing. A written scope should name the slopes, the slates to be replaced, the nail and metal to be used, the flashing work, and whether a permit or historic review applies. Massachusetts requires a written contract for residential work over $1,000, and that contract should carry the scope in detail. If the decision is repair, the work should use the right fasteners. If it is replacement, the slope count should appear on the page. A slate roof repair job should be scoped that way from the start.
Frequently asked questions
Can slate roofs in Springfield be repaired instead of replaced?
Usually, yes. The National Park Service recommends repair rather than replacement whenever possible. A slate roof that is sound apart from a few missing or cracked slates, or from corroded nails, is generally a good candidate for repair. Replacement becomes the better option when a slope has crossed the 20 percent threshold.
Why do slates slide off an older roof?
Most often the nails have corroded. Rusting 19th-century cut nails are a common cause of slate loss, and when the nail fails the slate slips out of position. Failed flashing can also let water reach the nails and decking below. Checking the nails and the flashing is the first step before any slate is replaced.
Is copper or stainless steel really necessary?
The National Park Service says non-ferrous slater's nails, solid copper or stainless steel, should always be used for slate work. Plain steel and galvanized nails usually rust out before the slate wears out. Paying for the right fastener is one of the few slate decisions that is hard to revisit later, so ask about it before the work begins.
Can I seal a leaking slate roof with roofing mastic?
No. Roofing mastic and sealant are not a viable repair for slate, because they harden and crack and let water in. A leak is a sign that a nail, the flashing or a slate needs attention. Sealing over it often hides the problem for a time, and the water keeps moving underneath.
Does a historic district affect a slate repair?
It can. Springfield has local historic districts, and the Historical Commission must approve exterior work before it begins. Ordinary maintenance and work not visible from a public street or park can go by certificate of non-applicability. Ask the Commission whether a specific address is in a district before any work starts.
How long does a slate roof last?
The National Park Service gives 60 to 125 years for the slate commonly used in the late 1800s and early 1900s, and says some slate can last as long as 150 years. The remaining life of a particular roof is judged on the roof itself, meaning the slates, the nails, the flashing and the decking underneath.
This guide is general information for Hampden County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.