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"Slate that splits easily into thin and uniform sheets and that resists weathering and discoloration is suitable for roofing. The quarry blocks may be conveyed directly to the splitting shanties, where they are split and broken into convenient sizes. "
Oliver Bowles
The Technology of Slate, p.63
While the slate industry produced a significant number of products made from slate, by far the largest was roofing slates
Traditionally the splitting and trimming of roofing slate occurred in small wooden buildings roughly 10 by 10 feet in plan. These were the "shanties" and they were placed some distance from the "factory", where structural slate was made, so as to spread out the waste from the two processes. These buildings, being simply constructed, could be moved or raised as desired.
"The usual thickness to which roofing slate is split is 3/16 of an inch. Standard specifications, however, provide for the following additional sizes 1/4, 3/8, 1/2, 3/4, 1, 1-1/2, 1-3/4 and 2 inches."
"The usual thickness to which roofing slate is split is 3/16 of an inch. Standard specifications, however, provide for the following additional sizes 1/4, 3/8, 1/2, 3/4, 1, 1-1/2, 1-3/4 and 2 inches."
Blocks sent from the mill or directly from the landing to the roofing slate shanties had to be reduced to dimensions that were manageable for easy handling. The saw used for this purpose was generally 36 inches or more in diameter, and was about 3/8 inch thick; the teeth were fixed in one piece with the blade. Some blocks were transferred directly to the shanties if they were taken from the quarries at a size and shape that were manageable.
Image Source: Bowles, Oliver. 1922. The Technology of Slate, by Oliver Bowles. U.S. Government Printing Office, p.90
A plan of the mill dumps and quarry of the North Bangor Slate Company in Bangor. Notice that the main mill and the shanties for making roof slate are separated from each other by most of the yard and that a simple track system joins in the center, leading to the dump located at the bottom of the map .
Splitting
After the slate had been reduced to pieces about 1.5 x 2 feet in area and 5 inches or less in thickness, it was carried to the shanties. At each shanty there was generally a splitter and two assistants. The process began with one of the assistants splitting the blocks into slabs of roughly two inches in thickness and a length and breadth slightly greater than the slates being produced. Here the splitters swabbed the blocks with water, keeping them damp to allow them to split more easy. A thin, wide bladed, and very flexible chisel was then worked into the slate along cleavage cracks by gentle tapping with a mallet. When the chisel was finally well inserted, another was commonly inserted in a similar way, prying apart the two cleavage surfaces. Gentle tapping and deeper forcing of the chisels finally induced the slate to part along the desired plane.
Image Source: From Rock to Roof
Above, men in the shanties using tools that were designed specifically to split the slate into sheets. Below, a drawing taken from the 1932 Architectural Graphic Standards. Graphic Standards has been the principle resource for architects for identifying methods of installing materials.
After the rock was split to the thickness of roofing slate, it was trimmed under a heavy steel blade which was fixed at one end and operated by a treadle. The blade was made to swing by a spring pole, placed outside the shanty, or by a coil spring. This instrument cut the larger pieces of thinly split slate into the sizes desired for roofing. This became an antiquated device when a more modernized design was introduced which had a power driven machine.
In general a piece of slate was cut out to the largest size possible consistent with standard roofing sizes. A set of metal plates attached to the trimming machine was arranged to permit the rapid gauging of the dimensions to which the piece was best adapted. It was said that the treadle must be worked more slowly for more brittle and softer slate.
Image Source: Joseph Elliott
A contemporary photograph of a trimming machine which is still in use today. This machine has the metal stops attached allowing pieces to be rapidly cut to a specific dimension. (visible just below the blade extending out towards the chair in the view)
"Many of our readers will be interested in the statement that flat slate roofs have been in use for a number of years and are said to have given entire satisfaction. The impression sometimes prevails that slate is not adapted for flat roofs and with the idea of refuting this impression, the North Bangor Slate Co Bangor Pa. is distributing a booklet entitled Flat Slate Roofs which aims to show that flat slate roofs have stood the test of time and uphold the record for durability economy and general satisfaction that has given Genuine Bangor Slate its reputation. The booklet contains numerous illustrations of buildings with flat slate roofs and also testimonials from architects roofers and owners as to the value of this type of roofing."
"Many of our readers will be interested in the statement that flat slate roofs have been in use for a number of years and are said to have given entire satisfaction. The impression sometimes prevails that slate is not adapted for flat roofs and with the idea of refuting this impression, the North Bangor Slate Co Bangor Pa. is distributing a booklet entitled Flat Slate Roofs which aims to show that flat slate roofs have stood the test of time and uphold the record for durability economy and general satisfaction that has given Genuine Bangor Slate its reputation. The booklet contains numerous illustrations of buildings with flat slate roofs and also testimonials from architects roofers and owners as to the value of this type of roofing."
Building Age, Volume 39, 1917
Punching
The nail holes in slate shingles were made with a machine also operated by a treadle. Opinions often differed among operators as to the desirability of punching slate before shipment. Some asserted that the loss suffered was appreciable. Others maintained that the sevice was better if punching was done at the quarry because the full quantity of slate ordered was then available to the consumer, no losses being sustained by them after receipt of the shipment. This they believed resulted in better feeling between producer and consumer and more than balanced the financial loss through occasional breakage in punching.
Modernized Procedures
The industry changed and with it so did the processing methods. While the earliest approaches to splitting slates for roofs involved shanties, in plants of more "modern" design the splitting and trimming was done in one large building, through which a hand or horse car was drawn to haul away the waste slate. A plan of the Jackson-Bangor Slate Company roofing mill above shows a hatched perimeter representing the outline of a single structure where the roof tile process no longer employs individual shanties.
Storage
Roofing slate was commonly stored outdoors. When exposed to the weather the slates generally became a shade lighter in color and producers preferred that the preliminary exposure or "seasoning" should take place at once, prior to shipment. In the Pen Argyl region the practice of indoor storage was followed. Portable storage racks were in use locally, but more generally the finished slates were stacked on edge in "blocks", a course of lumber laid upon them, and then another ''block" stacked on top of the first.
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