In layperson's terms, slate is essentially a fine-grained
rock produced by extreme pressure on mud compositions left
by the receding oceans from the earth's earliest times. Almost
all slate has 55% to 65% silica as a major ingredient. The unique
combinations of other minerals and inclusions give each region's
slate distinctive coloration and texture. Workable slate deposits
have been found and mined on all continents. Used as a building
product, slate in various forms has been employed as roofing and
siding material, blackboards, countertops and, in thicker
sections, as architectural accents in institutional and residential
Slate roofing is produced from a suitable block that may be
blasted or sawn from the quarry bed. The quarryman may reduce
the block to a more manageable size before it is sent to be
finished. Depending on the product,
it is then further manipulated by specialists. To create a
roofing slate, the splitter takes a smallish block and reduces
it by cleaving thinner sections.
In practice, these slates are riven from 3/16" to 1/2'
thick, the thinner being the norm. The tools used are a suitably
weighted mallet or sledge and at least two chisels of the
type shown. A sharp blow inserts one chisel along the edge
and as the slate opens, a second chisel is inserted causing
a full splitting off of the required piece. These pieces are
then transferred to the dresser, who uses choppers and piercing
tools to create a uniformly sized roofing slate and who may
provide the nail hole for installation. Today's manufacturers
offer over 30 different sizes. A slate-covered roof, depending
on the material source, can be expected to last from 50 years to
By 1920, asphalt shingles had started to replace slate and
Per square (100 sq. ft.), slate at 3 /16 thick weighs 700
lbs. to 850 lbs;
the same coverage in asphalt-type shingles weighs 225 lbs.
to 325 lbs.
This material modification changed the amount of heavy framing
required and reduced the member sizing of the roof structure.
A slate roof requires an engineer's calculation to determine
the proper sizing of the roof framing.