Your roofer is correct: All slate is not created equal. Slate is sedimentary rock composed of compressed and hardened layers of clay or volcanic ash. Slate varies in color, denseness and purity -- and serviceable life as a roofing material.
The color of slate
This country's slate quarries, located on or near the Appalachian Mountain chain, provide a variety of colors due to local geological conditions. The color of the slate provides clues to where it was quarried:
- Black slate comes from Pennsylvania, Maryland and Maine.
- Blue/Gray slate is from Pennsylvania and Virginia.
- Green, Gray and Purple slate comes from New York and Vermont (Slate Valley).
- Red slate can only be found in New York.
Hard and soft slate
Local geology is also the primary factor in the average service life of a slate roof.
Varying amounts of impurities (minerals) are contained in a vein of slate. When exposed to weather, these minerals absorb moisture quicker than pure slate. This causes flaking, or delaminating on the surface of the slates. When the surface begins to flake, the slate absorbs more moisture. The slate then becomes soft and inner layers delaminate with freeze/thaw cycles.
Porosity and hardness are also factors that affect the life of a slate roof. Even with significant amount of impurities, dense slate weathers much slower than more porous slates.
Some Pennsylvania slates have a random stripe across the individual slates. These are called ribbons, and are caused by carbon, calcite, iron or quartz that were present in the sediment from which the slate was formed. Slates with these ribbons have the shortest service life - about 75 to 90 years.
Slate from different mines has different average slate roof life expectancies (with proper installation and regular maintenance):
- Vermont and New York slates: 125 to 175 years.
- Virginia slates: 175 years or more.
- Pennsylvania/ Maryland slates: 100 to 200 years.
- Pennsylvania or ribbon slates: 60 to 90 years.
Other factors in roof longevity
The pitch of a roof can also be a factor in life expectancy. Generally, slates on steeper slopes will outlast those on lower pitches. Steeper pitches shed rain and snow more readily, allowing the roof slates to dry thoroughly. For example, an almost vertical mansard roof shingled with slate can significantly outlast a low-slope hipped or gable roof constructed at the same time with the same slate.
Flashing, maintenance and repairs can also affect the life of a slate roof, and I dealt with these issues in another column. See the links at the bottom of the page.
Slate in America
Slate was a common roof covering in Europe centuries before it ever appeared on buildings in American in the 1800s. Some of the earliest known slate roofing can be found on Roman forts in Wales and on Welsh castles from the 14th century. I've actually had the privilege of seeing slate roofs in Wales that are documented to be more than 300 years old.
In the late 1600s and early 1700s, some Welsh settlers in Pennsylvania imported slate from North Wales to roof their primitive farmhouses. In 1734, two Welsh farmers discovered slate on their farm in York County, Pennsylvania. The first commercial slate quarry in America was established at this same location in 1785. Due to primitive transportation, the use of slate from this quarry was limited to the surrounding area.
By the 1840s, canal systems allowed mined slate to be transported to seaports that supplied coastal cities. The establishment of railroads in the 1870s allowed distribution across the country. By this time many other slate quarries had been established in the mid-Atlantic and New England. By 1900, more than 200 slate quarries operated in 13 states. Manufactured roofing products, which were cheaper to produce and distribute, became more popular than slate a few decades later.
Slate is still being used on new and historic homes and buildings. It remains among the most beautiful and durable of roof coverings, and I encourage owners of historic homes to preserve or replace their slate roofs.
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.