Worn slate roofs - repair or replace?
Because slate is often called a "100-year-roof" homeowners sometimes assume that it doesn't need maintenance for a century. This is not the case.
Slate roofs (like all roofs) should be inspected annually. Cracked, broken, loose or missing slates should be replaced as soon as possible by a roofer experienced in slate work. Notice I said by an experienced slate worker.
The most common defect I encounter on slate roofs is inappropriate repairs. One example of this is chasing a leak with the black goop used by contractors unfamiliar with slate roofs.
Such "repairs" are not only unsightly; they can conceal easily repairable defects such as bad flashing or isolated broken slates. Ignore or cover up a minor defect and soon you'll find yourself needing a new roof.
Two-thirds of the buildings I inspect have a slate roof over at least the main portion. With a service life ranging between 75 to 200 years, slate is the most durable of all roof coverings. It's rare that I have to condemn a slate roof entire replacement.
Flashing and fastening
A chain is only as strong as its weakest link. For slate roofs, flashing is often the weak link. Terne coated sheet iron, lead coated copper, lead sheet, galvanized steel and copper are the most common flashing materials used with historic slate roofs.
Evaluation of these components does not differ from any other roofing system. But slate roofs commonly outlast the flashings. And if your flashing leaks, your roof will also leak. The solution is not to glob on tar or other sealers, but to replace your flashing.
Fasteners are also important in extending the life of a slate roof. Cut steel nails, copper wire nails and galvanized steel nails are the most common fasteners used with slate roofing. A less common French method of securing slates involves hooks that are partially exposed below the bottom edge of each slate.
On properly installed, well-maintained hard slate roofs, nail type fasteners can match the life of the roof. On a soft slate or a deteriorating roof, however, moisture absorbed by the slates can penetrate to the nails. This leads to rapid roof failure once the nails are no longer holding the slates in place.
When Replacement is Necessary
Even a slate roof that has been meticulously maintained will need to be replaced during someone's ownership.
When 20% to 30% of the slates on a roof need replacement, it is usually more prudent to replace the entire roof than to perform expensive annual repairs.
Broken or missing slates are easy enough to spot. But how does an inspector tell that if slate is at the end of its serviceable life?
Major delaminating (flaking) is a sign that a roof will need replacing soon. Powdering on the underside of the slates is another sign. Since most slates in my region are installed on spaced wood battens, I can often see the powdering from the attic.
Seeing is not always believing
The "knuckle test" is another method I use to assess the overall condition of a slate roof. This highly technical method involves tapping the surface of the slates in several areas with my knuckles.
When I hear a dull thud, I know replacement is in the near future. Intact, hard slates will emit aclear, solid sound when tapped.
I'll never forget evaluating a very large slate roof on a 220-year-old Pennsylvania Quaker meetinghouse. No significant delamination or ribbons of impurities were visible. The slates were installed over an earlier wood shingle roof, and so were not visible from the attic below. But a hands on inspection revealed that slates so porous that they felt like my son's sidewalk chalk that's been left out in the rain..
Replace slate with slate
When inspecting a building with a slate roof, I feel it is my responsibility to encourage the preservation of the historic character of this durable, natural material. I do this by educating building owners on effective maintenance and appropriate repairs of slate. Roofs are highly visible and an integral architectural element of historic buildings. If roof replacement is necessary, I encourage homeowners to replace slate with slate.
William Kibbel III is a home inspector and restoration consultant specializing in historic residential and commercial buildings. He is vice president of Tri-County Inspection Company, serving Southeastern Pennsylvania and Central New Jersey.