General Considerations

The importance of having an absolutely weather-proof roof for shop buildings is evident without argument. The kind of roof covering employed determines in a large measure the possible pitch or slope of the roof. A roof with a steep slope sheds rain and snow more efficiently than one which is more nearly flat, but it has the disadvantage of a greater area and consequently of being heavier and also of presenting a larger surface to wind pressure. All metal roofs are lightning-proof and because of their smooth surfaces are more easily kept clean by the wind and rain and the rainwater from them is likely to be more pure than that off a shingle or gravel roof. With these brief general remarks attention will be turned to the various forms of roof coverings.

Slate Roofing

Roofing slates are usually from 1/8-in. to 1/4-in. thick and of various sizes. The minimum slope of roof recommended for slate covering is one with a 6-in. pitch. If the pitch is less than this water is likely to be driven through the joints in beating rains. If, however, the joints are laid in cement, the pitch may be decreased to 4 ins. or 5 ins. to the foot. Cement joints are advantageous in any case since they prevent the slates from breaking and make the building warmer in winter and cooler in summer. A few courses of slate, with cement joints, are always advisable at the eaves and ridges and around chimneys. If the roof is exposed to the action of the corrosive gases, as is the case in chemical works, cement joints are imperative because any kind of nails will be destroyed after a time.

Slate when well laid have a longer life probably than any other form of roof covering; they will last for 50 years or more. Slate make a fire-proof roof covering, but they will crack when exposed to heat and also if they are walked upon. Hard slate of a shiny appearance are the best; those that absorb water will be destroyed by frost. Slate may be laid on boards, on lath or directly on iron purlins. When laid on wood they are held in place by two nails, one in each upper corner. When laid on iron purlins they are held in place by copper wire. For roofs of small pitch a lining of roofing felt will help to make the roof watertight.

The cost of nails for slate roofing varies with the market, but the following table is a fair average:

3d. galvanized slate nails, per keg.........

$5.50

4d. " " " .........................

5.00

3d. tinned " " .........................

5.75

4d. " " " .........................

5.25

3d. or 4d. polished steel wire nails, per keg.....

4.00

Copper nails, per lb.....

.20

Slaters felt in rolls of six squares costs $1.25 per roll; two-ply tar roofing felt costs $1 per square, and three-ply $1.25 per square. Slaters' cement in 25-ib. kegs costs 10 cts. per pound.

Shorter slates must be used for the first course at the eaves and the final course at the peak. To give the first course at the eaves the same inclination or slope that the succeeding courses will have, a thin lath must be laid under the slate at the edge of the eaves. A lap of 3 ins. is the amount usually allowed and it should not be decreased. Slate does not make a cheap roof covering, because it is heavy and requires a stronger framing to carry it, and be-cause the steep pitch required makes the area to be covered large. At present the Brownville and Monson slates of Maine and the Peach Bottom slate of Pennsylvania are the best and also the most expensive.

The weight of slate per cubic foot is 174 lbs., hence the weight per square foot of different thicknesses of roofing slate is as follows:

Thickness.

Weight, lbs. per sq. ft.

1/8-in.

1.81

3/16-in.

2.71

1/4-in.

3.62

Thickness.

Weight, lbs. per sq. ft.

3/8-in.

5.43

1/2-in.

7.25

An experienced roofer will lay about two (10 ft. x 10 ft.) squares per day of ten hours. The price of the best slate on board cars at the quarries is from $5 to $7 per square, according to size and color. Red slate costs from $10 to $12 per square, and ordinary slate, black, purple, or of mixed colors, cost from $2 to $4 per square. These prices include punching and countersinking the nail holes. Table VI. gives the number of slate per square, using 3-in. lap for various sizes of slate.

Table VI. Showing Number of Roofing Slate of Different Sizes and 3-in. Dap Required per Square of 10 x 10 ft.

Size, ins.

No. in each square laid.

6x12

533

7x12

457

8x12

400

7x14

374

8x14

327

9x14

291

8x16

277

Size, ins.

' No. in each square laid.

9x16

247

10x16

222

9x18

214

10x18

192

10x20

170

11x20

154

12x20

142

Size, ins.

No. in each square laid.

11x22

138

12x22

126

12x24

115

13x24

106

14x24

98