There are two issues to consider when specifying roofing slate:
1.) the geology and characteristics of the deposit from which the material will be quarried, and
2.) the quarrying and fabrication processes of the company that will actually produce the material.
All slates do not share the same properties. Deposits vary tremendously in terms of quality, water absorption rates, strength, color variation, weathering characteristics, acid resistance, cleft texture, durability, etc. Our slates will last over 125 years on a roof; others may last only 20 years.
We are committed to producing the highest quality roofing slate. By quarrying and fabricating our own products, we control production at every stage. For strength, we produce our own Vermont roofing slate "on the grain", meaning the grain direction runs parallel to the longest dimension of the shingle. Our splitting (unless otherwise specified), trimming and punching is consistent and accurate. We are attentive to quality and sort out inferior pieces at several stages of production. And we will certify a particular slate's weathering characteristics and that it was produced "on the grain".
We encourage you to get to know the performance characteristics of a certain deposit through test data, track records and references. And visit the quarry. Meet the people who will be responsible for the production of your order. Inspect the quarry and roofing slate production process.