roofing slate circa 1890, 22" x 11"
Roofing slates are about 1/4" thick and come in different sizes, ranging from 24"x14" to 8"x4." There could also be color variations for the slates ranging from black to green to slate.
This is a spare slate from my old house on North Street in Slatington.
Roofing slate is usually sold in an increment called a square, i.e., the amount of slate needed to cover 100 sq. ft. For 22"x11" slates, it took 138 pieces to make a square since there was some overlap of slates when put on a roof. That would equal a little over 700 lbs. for the square.
Since a roof might end up weighing a couple of tons, it was important that the truss structure for the roof had to be pretty strong to support that weight.
The slate region in Pennsylvania, running from roughly Slatington/Slatedale in Lehigh County to Bangor/Pen Argyl in Northampton County, yielded both "hard' slate suitable for roofing slates and "soft" slate used for blackboards and school writing slates.
There are still a very few quarries operating in the region.