{Contributed by T. H. Bishop, A.R.I.B.A.)

There are many circumstances which necessitate that roofs shall be flat, and covered with a material that is easy to handle and impervious to wet. Lead is generally used, and, when skilfully laid, makes an excellent job, and stands a considerable amount of traffic. Its range of expansion and contraction, however, makes it necessary that the work be well done.

Should the question of expense preclude the use of lead, zinc is the next material in favour. It is light, and the labour on it is not so expensive as that on lead. The noise made on it by the rain, and the ravages of the domestic cat, cause its use to be limited generally to the Speculative Builder.

Where cost has not to be considered, copper is an excellent roofing material. Its lightness, small range of expansion and contraction, and good appearance when oxidised, cause it to be used in the very best work.

The following table, taken from Sutcliffe's Modern House Construction, gives a comparison of copper, lead, and zinc: -

Copper.

Lead.

Zinc.

Specific gravity

8.85 to 8.94

11.37

6.86 to 7.21

Atomic weight.

63

206.4

65

Weight per square foot 1/16 in. thick .

4.6 lbs.

5.9 lbs.

3.7 lbs.

Melting - point in degrees Fahr........

2012°

633°

773°

Conductivity (silver being 100) ......

73.6

8.5

Linear expansion between 32° and 212o Fahr .......

.00171

.00285

.00297

Relative linear expansion ...

58

96

100

Weight per square foot used for roofs

1 to 1 1/8 lb.

6 to 8 lbs.

1 to 1 1/2 lb.

Lead Flats form strong and impervious roof covering, and are well adapted for roofs subjected to great wear.

The woodwork on which lead is laid consists of joists or purlins, covered with boards at least 1 inch thick. The boards should be tongued, and laid in the same direction as the flow to prevent warping of the boards retarding the flow of water. In practice, however, the boards frequently have only their edges shot to form a close joint. Lead, owing to its expansion and contraction, must not be nailed to the boards, but be allowed free play. To permit of this freedom, wood or hollow rolls are used. These are usually 2 or 2 1/2 inches diameter, nailed to the boarding and fixed from 1 ft. 6 in. to 3 ft. apart, but the divisions should never exceed 3 ft., and smaller ones are preferable. The maximum width of a sheet of lead is 7 ft. A sheet of this size can be cut into two strips each 3 ft. 6 in.

Lead Zinc Copper And Asphalt Roofs 580Wood Rolls

Wood Rolls.

Lead Zinc Copper And Asphalt Roofs 582Fig. 298. Hollow Roll.

Fig. 298. Hollow Roll.

wide, so that if 3 in. is allowed along the edge of each sheet to form the overcloak on a 2 1/2-inch roll, and 8 in. along the other edge is dressed up to form the over-cloak and splash lap, the flat portion of the lead will be 2 ft. 7 in., the distance apart of the rolls 2 ft. 9 in. from centre to centre.

Fig. 298 shows three different forms of rolls, two of which, marked X and Y, are wood rolls; the third, marked Z, is called a seam or hollow roll.

The seam or hollow roll is formed by dressing up one edge, marked A. Tacks or tingles of lead, or thin copper, B, are then nailed to the boarding at intervals of from 3 to 4 feet, and turned over A. The edge of the sheet, marked C, is set up and turned over until it reaches half-way down the undercloak. The two edges are then dressed over, as shown at D, and finally bent to form a roll. Should lead tacks be used, sinkings should be made in the boards to receive them. Lead flats are usually laid to a fall of 1 1/2 inch in 10 feet, but 2 to 3 inches in 10 feet is not excessive.

In some cases felt is used under the lead, but it is difficult to dress the lead on this material. At the upper end the lead is dressed over the roll and turned up 4 inches to receive the cover flashings, whilst at the lower end it is dressed over the end of the roll and turned down 2 inches. As sheets of lead should never be laid in lengths exceeding from 7 to 9 feet, drips are necessary. These are formed in various ways, and should be from 2 to 3 inches deep; when made 2 inches, a groove should be formed to resist capillary attraction.

Lead Zinc Copper And Asphalt Roofs 584Lead Zinc Copper And Asphalt Roofs 585Fig. 299. Drips.

Fig. 299. Drips.

Fig. 299 shows the various forms of drips.

Nosings

These are formed where the flats adjoin sloping roofs. A wood rounded nosing about 2 inches diameter is fixed on the left of the boarding, and the lead is dressed over and secured by lead clips.

Lead Zinc Copper And Asphalt Roofs 587Fig. 300. Hosings.

Fig. 300. Hosings.

Nosings may also be formed by a welted joint with copper clips the width of welt, and the two edges of the lead and copper welted together, and copper nailed (see Fig. 300).

Zinc Flats

Zinc flats are used on account of lightness and economy; zinc having a small range of expansion and contraction as compared with lead.

The wood framing is formed in a similar manner to that for lead flats; the boarding should be deal (not oak), and the heads of the nails should be punched in.

Rolls, drips, and falls can be formed as in leadwork. The drips should be 2 1/2 inches and 7 feet 6 inches apart. Drips are not required when the fall exceeds 1 in 8, the usual fall being 1 in 30.

The wall flashings should be finished with a bead to stiffen the lower edge, the upper edge turned 1 1/2 inch into the wall and pointed in cement. The zinc for flats should be 14, 15, or 16 gauge, 14 gauge being only used in the cheapest work.

In laying zinc flats, the roll and cap system is the most general, the rolls being spaced 2 feet 10 1/2 inches centre to centre. The sides of the zinc are turned up against the wood rolls in the width of the sheet, and the folding laps in the length. This method allows the sheets to expand and contract freely.

Holding Down Clip.,

Holding Down Clip.

Fig. 301. Zinc Roll.

Fig. 301. Zinc Roll.

The wood rolls are covered with zinc caps, which are fixed by holding down clips (see Fig. 301).

In some cases the Italian corrugated method is used. This effects a great saving of cost in large spans, as the boarding can be dispensed with and the purlins fixed as far apart as 10 feet (see Fig. 302).

Fig. 302. Italian Roofing

Fig. 302. Italian Roofing.

Copper Flats

Copper is an excellent material for covering roofs, on account of its toughness, lightness, and durability, ready manipulation and small range of expansion and contraction, while it can be stamped, stretched, or hammered without damage. It can be laid with rolls in a similar manner to that described for zinc, the rolls being about 2 feet 4 inches centre to centre. The caps should be welted to the sheets and left open at the sides to allow for expansion; and the saddles and stop ends welted in a similar manner.

This method of laying to rolls is preferable to the old method of welting, as the welts are damaged by traffic; and although the metal is practically unworn, the damaged welts necessitate the relaying of the roof.

When the sheets are welted, drips are unnecessary; but, if used, they can be as far apart as 15 feet.

The following is a table of gauges and weights of copper: -

B.W. Gauge.

Weights.

Per Square Foot.

Per Sheet.

Lb.

Oz.

Lbs.

20

1

12

14

22

1

6

11

24

I

0

8

26

13

6 1/2

28

10

5

30

8

4

The best gauge for flats is 20 B.W., and for flashings and caps 24 B.W. gauge.