How To Identify Your
Roof Slate
There are many types of roofing slate and they each have their own particular qualities and idiosyncrasies. It is imperative that people who own or work on slate roofs know the different types of roofing slate, their origins, longevities, and qualities, and be able to identify the slate on the roof in question. Presently, in the US, roofing slate is still being quarried in Virginia, Pennsylvania, New York, and Vermont. However, a century ago there were hundreds more American slate quarries than there are today, including in Maine and Georgia. The differences between the slates from the various quarries were sometimes striking, so a knowledge of the history of slate quarries is valuable for people who work with traditional slate roofs.
Note: Life expectancy estimates are only estimates. Life expectancy can vary greatly depending on the exact source of the slate. For example, some sea green slates are starting to get soft after 110 years, others are still very hard after 120 years, depending on the quarry of origin. You can judge the quality of your slate very simply by looking at the exposed surface. Is it flaking and crumbly looking or is it smooth, maybe even shiny? Smooth is good, and smooth surfaced slate roofs should be preserved. Flaky ones can be preserved too, although flaking is usually a sign of deterioration. It should also be noted that environmental conditions such as pollution will change the appearance of slates over time. NY red slate roofs in Pittsburgh are black after a century of soot. You can see the original color of the slate by breaking a piece and looking at the inside. There is much more information about the types of slate in the Slate Roof Bible.
SOURCES OF SALVAGED ROOFING SLATES
The main types of slate that still exists on older slate roofs in the US today are:
1) Sea green slate (below), which is the color of the sea when first quarried but changes to light gray with buffs and tans after it weathers. It's known as a "weathering" or "fading" gray-green slate today. This is a hard slate from Vermont with a life expectancy of about 150 years on average. This is a very common slate in the American northeast and these roofs are highly restorable. Below is a sea green roof that is typical (light gray with buff overtones).
Above: A darker Vermont Sea Green slate roof mixed with Vermont purple slates.
2) Purple slate (below), also from Vermont remains dark purple throughout its life. Some are variegated or speckled with green. This is an excellent slate with a life expectancy of 150+ years and these roofs are highly restorable. Photo immediately below shows purple slates with horizontal sea green bands and NY red slate florets.
Photo below shows Vermont purple slates blended with other Vermont slates.
Above: NY Red andVT Unfading Green, on a North Country (Canadian) Unfading Black background.
3) Unfading green slate (below), from Vermont is an excellent slate, light green or light gray, often shows no wear after 120 years (as below). This slate could last 200 years or more.
4) Pennsylvania black slate (below) from the Lehigh Northampton slate district tends to be the "soft" slate with a life expectancy of 75-125 years. This is a generalization, as some of these slates fail sooner, some last longer. If the surfaces of the slates are smooth, then the roof is probably still serviceable. One of the more common of the PA black slates is "Bangor" slate from Bangor, PA. It is a smooth surfaced finely grained slate that lasts about 90 years. The worst of the PA black slates is ribbon slate, which contain a band of soft carbon material that causes the slates to fall apart in time. Ribbon slate roofs, once they start to fall apart, are hopeless. Many of the softer black PA slate roofs are now reaching the end of the line and must be replaced - they cannot be restored. They should be replaced with slate, and PA slate is a good candidate for a replacement material if it is still available (after all, it did last 90 years the first time around). Chapman slate, from the Lehigh Northampton district, lasts about 100 years, maybe more. It is easily identified by the diagonally striated patterns (below).
Chapman slate (above)
Bangor slate (above)
PA Cathedral Gray slate (above). One of the best.
Old Pennsylvania black slate (above).
Old Pennsylvania black slate (above).
Old Pennsylvania black slate (above).
5). Peach Bottom slates (below) are dark black, hard, and long lasting. They are sometimes hard to distinguish from Buckingham slates. From Pennsylvania and Maryland, these slate roofs should be preserved. Life expectancy is 150-200 years or more. Light slate (right photo) is a poorly chosen repair slate.
6) Monson slates (below) are also a dark black slate. Quarried in Maine, this is an excellent, long lasting slate with a 150 year, or more, life expectancy.
Old Monson roof in the Boston area (above). Note the smooth surface.
7) Buckingham slates (below), from Virginia, are a gray-black slate with a life expectancy of about 150-200 years, maybe much more. You will see the glint of countless tiny silica crystals in this slate when viewed in the sun, a unique characteristic of Buckingham slate. The roof below was 132 years old when photographed. The slates still rang like a bell.
8) NY red slate (below) is a very high quality slate with a life expectancy of 150-200 years. Below is a new installation.
Above: Old purple and green slate from Newfoundland, Canada, a high-quality slate.
Above: New Vermont black slates, "extra heavy," should last a century or more.
Above: New Spanish slates, German style.
Above: New Chinese black slates in Manhattan.
Below: New Chinese green slates in Louisiana.
China Multi-Color Slates (below). Easily damaged by hail and freeze-thaw.
Above: New Brazilian green slates on a house in Louisiana.
Above: An asbestos roof, NOT SLATE!
Above: FAKE slate, not the real thing!
Roof consultation with Joe Jenkins via Email
Learn about some of the mistakes people make when installing slate roofs.
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