In recent years, especially since hurricane Katrina, used slate tiles have become more expensive due to the high demand for repairs on existing structures where the entire roof is not replaced. The result is that the cost of used slate tiles are about the same as new. In the case of unfading Gray/Black such as PA Peach Bottom and Buckingham, Welsh slate tiles and Munson slate tiles , the price of reclaimed slate roofing is sometimes more than new matching slate.
Even before this was true, we always recommended that new slate tile be used on full replacements or new roofs for the simple reason that used slate tiles do not have the same useful life as new. That factor is difficult to determine with some used slate roofing unless the origin quarry and original date of manufacture are know. Because of the long lifespan of slate roofing and the fact that slate tiles from many different quarries can be visually indistinguishable once aged, but have significantly different expected service life, there is always an unknown risk taken when reclaimed slate tiles are used.
There is also a good reason to use reclaimed slate roofing in the case of partial replacements and repairs. Since the original source of a particular slate involving a repair is usually unknown, the best way to match existing weathered slate is with reclaimed slate. Even slate from the same quarry produced years apart can weather in different ways. Slate is a natural stone with varying amounts of minerals contained in the slate from day to day. The variation is less with unfading black slates, but with Vermont unfading greens, grays and especially semi-weathering or weathering slates the variations in weathering can be substantial.
So the bottom line recommendation remains the same regardless of price. When you are replacing an entire roof or installing a new roof, new slate tiles are the best option. When patching or repairing an existing slate roof where the old slate roof is to be reused, it is best to source reclaimed slate.
If reclaimed is to be used, the material should be evaluated to be sure it is suitable for reuse. The “ring test” and visual inspection are the best methods in lieu of knowing the original source quarry. When held by a corner tip, the slate should resonate sound or ring when struck on the lower portion with a hard object. If the resulting sound is a “thud”, the slate tile is beyond the useful life. It may last 5 or 10 years, but the cost of labor and installation negates the feasibility from a cost standpoint. Slate with a high moisture absorption used in cold climates can show visual signs of delaminating even if it still “rings”. Many of the historic Pennsylvania slates were S-2 Grade with a 40 to 50 year service life and act in this manner. So if the slate face can be easily flaked away, the slate is not suitable for reinstallation. The best advice when using reclaimed slate tiles is to purchase it from, or have it inspected by, someone knowledgeable and experienced in slate roofing and reclaimed slate in particular.
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