slate (for roofing)

United States

In consultation with industry, in 1924 the U.S. Dept. of Commerce published a standard set of roofing slate sizes (Simplified Practice Recommendation No. R14-28, all in inches):

Lengths Widths Expo­sure Thick­nesses
6 7 8 9 10 11 12 14
10 checkmark checkmark checkmark           3/16, ¼, ⅜
12 checkmark checkmark checkmark checkmark checkmark      
14   checkmark checkmark checkmark checkmark   checkmark  
16     checkmark checkmark checkmark   checkmark   3/16, ¼, ⅜, ½, ¾,
1, 1¼, 1½, 1¾, 2
18       checkmark checkmark checkmark checkmark  
20         checkmark checkmark checkmark checkmark
22           checkmark checkmark checkmark
24             checkmark checkmark 10½

The main difference between this set of sizes and those in earlier catalogs was that some of the wider widths in the larger sizes, such as 24 × 18, were eliminated.

The most common thickness is 3/16. Run of the quarry 3/16 slates are entirely usable but vary in thickness. It is possible to specify “not less than 3/16,” which will be more expensive. For slates ¼-inch thick and thicker, variations in thickness may only err on the positive side.

For architectural reasons some slate roofs are graduated. In such a graduated roof, random widths are used throughout, but thickness and length change. For example, ¼-inch slates may be laid on the top quarter, ½-inch slates on the second quarter, ¾-inch slates on the third quarter, and the bottom quarter covered with 1-inch thick slates. Now divide the same ridge-to-eaves distance into a greater number of lengths, say, in this case, 5: 16, 18, 20, 22 and 24. Lay the top fifth with 16-inch long slates, the second fifth with 22-inch long slates, and so on. Obviously, there will be more courses per foot near the ridge than near the eaves.

Slate roofs are generally sloped at not less than 6 in 12. If driven snow is to be expected in the area, an even greater slope, of 8 in 12, is recommended.

United Kingdom

Wales was the largest producer of slate. The thousand of slates was 1200 slates (see mille). The imperial and larger sizes were sold by the ton, not by count, and called “ton slates.” Traditional names of the sizes were

Name Dimensions,
Queens 36 × 24  
Rags 35  × 24  
Imperial 30 × 24  
Princesses  24 × 14 from Port Madoc only
Duchesses 24 × 12  
Small Duchesses
22 × 12  
Marchionesses 22 × 11  
Small Marchionesses 20 × 12  
Countesses 20 × 10  
Wide Countesses 18 × 10  
Viscountesses 18 × 9 from Port Madoc only
Ladies 18 × 10  
16 × 10  
16 × 8  
15 × 8 not made at Port Madoc
14 × 12  
14 × 8  
Plantations 13 × 11,
some say 13 × 10
Doubles 13 × 18  
12 × 20  
13 × 6 not made in Port Madoc
12 × 8 not made in Port Madoc

Imperials were 18 to 36 inches long; Queens were 26 to 33 inches long; widths varied.

Sources: John T. Hurst, A Handbook of Formulas,...(1865), page 203; H. L. J. Warren (1898).


American exporters found a 24 x 16 size was desired.




That cleavage and bedding are independent of each other plays a most important part in mining slate blocks. When the term vein is used, in accordance with popular custom, it means, or should mean, that portion of the rock in which the cleavage is most perfect and workable, though frequently it means something different. The joints, or splits, which separate the vein are normal to the stratification. In mining the blocks the quarrymen aim to get them out up to the split. Bands, or ribbons, a fraction of an inch to an inch and more wide, of darker or lighter color than the main body of the slate, mark lines of bedding. Usually the slate deposits are covered with earth and gravel 10 to 40 ft. deep, and the first step in prospecting for a slate quarry, after noting the surface indications, is to strip off the top and the weathered outcrop of the rock. If all continues favorable a horse derrick is brought into service soon after compact rock makes its appearance. As depth is gained the horse is replaced by a hoisting engine, and later, should a profit-paying vein be opened, — for a good slate quarry is nearly as hard to find as a good gold mine, — an inclined cableway is installed. The cableway is both a hoisting and conveying device, the carriage riding on a suspended overhead cable as a trackway. Its introduction in open-pit mining, about 10 years ago, is the foremost economic improvement in modern slate making. Some quarries have three to eight cableways, each operated by a separate engine, which are ordinarily spoken of as hoists or derricks.

In quarrying slate, holes are drilled at right angles to the cleavage planes so as to loosen large blocks. Black powder is used, as the force desired is a dull heaving one which will displace the rock without unnecessarily shattering it. The holes are placed in such positions and the blasts so adjusted that the blocks are hardly more than freed from the bed and never moved but a trifle. These large blocks are divided into smaller blocks, a foot or more thick, two or more feet wide, and 6 to 12 ft. long, which are hoisted out of the quarry, loaded on cars, and trammed to the shanties where the slates are made. Each block is marked as one of four grades before it is loaded on the cableway; viz., three or four straight marks indicates a No. 1 block; “L” is a “lookat,” for the blocker on top to decide its merits; “B” means that it is from the big bed, or free from ribbons; “H,” that it is a “hollabobber,” or practically worthless. The discarded “lookats” and “hollabobbers” are sometimes worked over by rubblers, who make such slates as they can from material that is destined for the rubbish dump.

One is amazed at the amount of waste rock in slate mining and the rapidity with which the rubbish mounts into large piles. In fact the disposing of the waste is one of the most serious difficulties contended with, and a large and ample dump is as necessary as in hydraulic gold mining. Not infrequently it occurs that a mountain of rubbish is stacked on ground that proves to carry strong beds of good slate, and it becomes a perplexing question whether to move the rubbish or abandon the ground. It is said that 1/6 to 1/20 is about the proportion of finished products to the total rock removed, though some quarries do better. In Wales, where the stock is worked more closely than in the United States, for they make everything at all workable into some sort of marketable product, this proportion is rarely above 1/8, and in one of the best paying quarries only 3½ tons of marketable slate is made of every 100 tons broken.

Power drills and channeling machines are used where they are found to be of advantage. When the angle of cleavage is not too steep, as in the large Bangor quarries, the channelers often come well into service. Frequently they reduce the percentage of waste very materially and enable the getting out of a huge block, which affords slabs of the largest dimensions required for structural purposes, more readily than could otherwise be done.

The blocks having been delivered to the shanties where the finished roofing slates are made, the next operation is the making ready of the small blocks, or slabs, for the splitters, which is done just outside the shanties sometimes by one man and at others by two working together. First the blocker, the head one if there are two, who is the foreman of the shanty crew, determines how each large block will divide with the smallest possible waste, avoiding flaws, ribbons, bends, knives, spar seams, etc., sometimes by bringing them to the ends of the slate that they may be cut off in the dressing. If the block is wide it is split lengthwise, to suit two slates of the same or different widths. This is done by making a notch at one end with a chisel at the point desired to start the lateral split, and, aided by the grain cleavage, a block 6 to 8 ft. long will ordinarily be divided in two by dexterously turning the split from this end. At times it is necessary to make a hole, somewhere midway on the line of split, in which two iron wedges are inserted and a third driven between them before the division is perfected. These reduced blocks are split into thinner blocks, or slabs, about 2 in. thick, and these again are broken across into such lengths as the stock will best make up. The transverse division is made by cutting a notch on one side at the point where the break is desired, turning the slab so that this notch is down, and then striking the opposite side a sharp blow with a heavy wooden mallet, when the break occurs between the notch and the point struck by the mallet. These reduced blocks, or slabs, are ready for the splitters. It is generally necessary to keep the ends of the blocks moist, as most slate will not split readily when dry, and the blocker uses a swab freely for the purpose. A helper piles the 2-in. slabs in the shanty near the splitter, who is seated but a little above the ground, with the blocks at his left side. Placing a block against his left thigh, with the straightest and smoothest end up, he divides it in halves by a long thin chisel 2 to 3 in. on the end, called a splitter. In doing this he makes two or three cuts along the center line, using a wooden mallet, the blow from wood being better adapted for slate making than one from steel. The middle one is sent further into the slab, which begins to divide, when a second splitter is inserted and moved backwards and forwards, extending the split down till the division is complete. The process is repeated, dividing each piece into halves, till they are reduced to the required thinness usually five to seven to the inch. Each time greater care is required, and when the material is fractious tact and judgment are needed to humor the split, so as not to destroy good material. Finally the splitter roughly shapes each piece, catting out flaws, etc., to about the size slate it will make, when it is ready for the last trimming. A boy takes these embryo slates and piles them up near the dresser. Cleavage is greatly aided by buttering or greasing the splitters.

 The dresser works by a window in the shanty, standing in front of the trimming machine, which has gauges at one side for the different sizes of slate. In Northampton County, Pa., the squaring is done by a treadle contrivance bringing down the knife attached to a sapling, one end of which is made fast in a horizontal position overhead. A better-appearing, patented, trimmer is used in Vermont and elsewhere in which the knife is part of the machine, which is also worked by a foot treadle. In splitting and dressing roofing slates care is taken that the grain is parallel to the long way of the rectangle. Slate grain, though never so well marked as that of timber, has a like influence upon the strength in different directions; slate, for example, being more liable to crack lengthwise than transversely. The dresser knows at a glance, as he picks up each piece, what size slate it will make, and with rapid, deft handling and four strokes of the knife a finished slate is complete. On a shelf in front, or rather to the right, each is put with the others of the same size. Roofing slates range in size from 24xl4 in. to 12x6 in.

Two splitters and one dresser commonly work in a shanty, though some slates split so easily that one splitter is sufficient. Bangor quarrymen say the reason they do not use the more modern trimming machine is that its motion breaks the slates. In Wales circular saws are used to shape the blocks into sizes before they are split. The slate saw has been introduced by the Eureka Co., Fair Haven, Vt., but others affirm it is more costly than the method in vogue.

At Bangor and Pen Argyl all work is generally done by contract, on the “in and out” system; i.e., the men “in” the quarry, who mine and get out the blocks, are in partnership with the men “out" on the banks, who divide up the large blocks and convert the stock into slates. They work in crews of six or eight men, two in the quarry and the others on the surface, and are paid so much per square for the slates made. The company exercises a general superintendence, hoists the blocks from the quarry, delivers them to the shanties, and removes the waste; the contractors supply their own tools and divide the work among themselves as they see fit. In some instances powder is furnished by the company and when channeling machines are used these are operated by the company. Sometimes two separate contracts are made, one “in” and the other “out,” or the rock is mined under one contract and the slates are made under another. In this region the cost is about $2.50 per square of slate, which includes the company's outlay for engineers, firemen, trammers, helpers, boys, etc. In Vermont, New York, and Lehigh County, Pa., there are no contracts, and all men are paid by the hour, working ordinarily nine hours a day in winter and ten hours in summer. Wages in Vermont and New York range from 12c. to 22c. per hour, the average for the quarrymen being 18c.; in Lehigh County they are about the same or a little lower. The belief seems to prevail in Vermont that there is greater waste by contracting, and in other ways it is less satisfactory from their viewpoint. At the “sea-green” quarries they count on making a square a day per man employed, while in the unfading green and red it runs three to four squares to every five to eight men. In the Vermont region practically all the slatemakers and most of the pitmen are Welsh; in and around Bangor there are fewer Welsh and some Italians,and in the Lehigh region the Pennsylvania Dutch are easily in the majority. The last, around Slatington, demonstrate that it is not essential to be a Welshman to understand the quirks and curls of slate from top to bottom. Probably the largest output per shanty is that of the Hazel Dell quarry, near Slatington, where 14 squares are commonly made in a day. Around Bangor less than half this number is considered a good average, and the same is true elsewhere.

H. L. J. Warren.
Richard P. Rothwell, ed.
The Mineral Industry, vol. 6.
New York: The Scientific Publishing Co., 1898.

want more?

Joseph Jenkins.
The Slate Roof Bible. 2nd edition.
Joseph Jenkins, Inc., 2003.

National Roofing Contractors Assn.
The NRCA Roofing Manual: Steep-slope Roof Systems—2009.
NRCA, 2009.

ASTM C406-10/C406M-10. Standard Specification for Roofing Slate.

A picture of the colors of slate quarried on the New York-Vermont border, which is the principal producer in the United States, is shown at the Slate Valley Museum Web site.


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