Material.

Breaking Stress in Tons per square inch.

Working Stress in Tons per square inch.

Tension.

Compression.

Tension.

Compression.

Cast Iron.....................

9

48

1

8

Wrought Iron.............

23

18

5

4

Steel .........................................................

32

32

6

6

Timber, Fir .....................

4

3

1/3

Oak . ..........

6

4

2/3

Copper is found in the metallic state or is smelted from ores.

It is red in colour, not easily oxidised, very malleable, and has a greater tensile strength than any metal except wrought iron and steel.

It is used by the builder chiefly for slate nails, bell wires, and lightning conductors, also for dowels, bolts, and fastenings in positions where iron would be corroded or rusted, and sometimes for covering roofs (see p. 62).

Lead is reduced from ores. It is an extremely soft and plastic metal - very malleable, fusible, heavy, and very wanting in tenacity and elasticity.

It is used for covering flat roofs, for flashings, pipes, bedding girders, etc.

Sheet Lead is to be purchased in two forms - cast or milled; both are described according to their superficial weight. Thus 7 lb. lead means lead weighing 7 lbs. per square foot.

Cast Lead is thicker, heavier, and with a harder surface than milled lead, but subject to flaws and sand-holes, and of irregular thickness. It is cast in sheets from 16 to 18 feet long and 6 feet wide.

Milled Lead is rolled out thinner than the other, is more uniform in thickness, bends easily and makes neater work, but is not so durable as cast lead.