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Vermont Slate Quarry
Slate appears as a very beautiful unpolished dense smooth rock-
mostly grey in color. Carbonaceous matter from the decay of marine organisms on ancient sea floors, produces the black tint. The amount of iron or other organic material within the clay will produce other colors, such as red, purple, and mottled slates. The Slates of Virginia have a lustrous appearance because of their high mica amount.
Slate is formed over thousands or millions of years from a field of clay or mud residing in a basin
between convergent plate boundaries. This field of mud was compressed by pressure and heat produced by tectonic forces (movement of the earth’s crust) folded and refolded into dense smooth rock usually found on the sides of cliffs, underground and in pits. Slate can easily break into thin smooth sheets because of its foliation (a structure formed of thin layers). These thin sheets are great roofing material because they are moisture repellent and able to withstand freeze and thaw cycles.
Slate has been used as roofing since the 13th century and can easily last 175 years or more.
However, Slate durability between the regions of origin differs greatly. The passage of time has shown that Vermont and New York Slates will last about 125 years and Buckingham Virginia Slates about 175 or more. Whereas, Slate imported from China is not as durable as that of the USA and should not be used to repair historical buildings.
Splitting Slate Tile
Repairing Broken Slate Tile
Installing Copper Snow Guards
Inspection needs to be directed by experienced slate roofers
because they preform tests that require knowledge of what a sound slate tile sounds like when it is tapped on versus a decaying slate tile. However, if you see any white powder on the tiles, that gives indication minerals are leaching from the slate. Mineral leaching will indicate that the slate tile was inferior from the install or you truly have a very old roof that has spent decades weathering. A few years back some roofers were importing slate tiles from china which are often much softer than American slates. Sadly, people didn’t realize the distinction till the premature decay of their new slate roofs.
Most repairs of Slate Roofs
are for leaks caused by deteriorated flashing and broken and/or missing tiles (due to storm damage). Flashing will often deteriorate long before the slate itself and, in such cases, restoration will mean replacing all metal parts on the roof including damaged roof deck.
Categorically, Slate Roofs require an expert to service
them for several reasons: Slate tiles can’t be walked on so roof scaffolding or roof brackets must be used. Slates that are walked on may break apart immediately or suffer an invisible fracture that 5 to 10 years later cracks- giving rise to a roof that does not shed water. The roof decking must last as long as the slate, roughly, 150 years or more. Tried and proven material are lumber boards from ¾ inch to 1/5-inch-thick rough-sawn, planed or tongue-in-grooved from a variety of hard woods. Plywood, laminated woods, particle boards, and materials that rely on glue for structural integrity are sub-standard roof decking materials, for slate roofs, and should not be used as they will decay many years before the slate will. Also, the roof must be structured to handle the heavy weight (800 to 1,500 lbs. per 100 sq. feet) or roof collapse at some point will befall the inhabitants.
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