Lee Valley Tools Woodworking Newsletter
Vol. 3, Issue 3
January 2009
 
What Is It?
 


Slate chisel

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 construction.

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 150 years.

By 1920, asphalt shingles had started to replace slate and clay tiles.
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.

 
 
         
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