12. In planning a roof, the first thought must be given to the pitch, which depends upon the climate, against which it is to act as a protection; the second consideration is the devices of construction to aid the free flow of water from the whole external surface when covered; and the third is the determination of the materials best suited to both the pitch and climate.

Design And Construction Of Roofs General Considera 238

Fig. 15.

13. Fig. 15 is a diagram of comparative pitches, from 1/6 to 1/2 pitch, and the following table gives a list of roof materials and the least pitch permissible with each:

Table 1

Material.

Pitch. Inches to the Foot.

Asphalt and composition.........

1/2

Tin...................

3/8

Zinc........................................

3/8

Corrugated iron.............

1/4

Sheet iron................

1/2

Copper......................................

1/2

Lead..................

3/8

Thatch.................

6

Shingles................................... ................

4

Slate..................

4

Tiles, terra cotta or copper.......................

4

14. Although there are no invariable rules governing pitches, it has been the custom to construct slopes suitable to different localities, and to use the materials which are best adapted to meet the climatic changes.

In hot countries, the lack of rain makes it unnecessary to give the roof more pitch than enough to shed the water; but the roof must be well constructed and tight, for when the rain falls in hot regions it does so in great volume.

Rapid evaporation is a great help in preserving the roof, for the moment the rain ceases the roof is dried, and the conditions tending towards corrosion or decay of the materials are soon removed.

In cold climates, where the air holds the moisture, and the rain and snow are driven by high winds, the pitch must be steep, to prevent the loosening of the shingles, slate, or tile by the wind, and to cause snow and moisture to be quickly shed from the roof.

In temperate climates, the pitch may be varied as the locality approaches either the warm or the cold zones.

In very wet, damp, or cold situations, the roof is given a pitch of from 40° to 60°; in dry climates, where the roof is quickly cleaned of water, a moderate pitch of from 20° to 30°; and in hot climates, only sufficient to shed the water.

This must, of course, be varied with the material used. Thus, shingles, slate, or tiles could not be used in a hot climate, unless the pitch were sufficient to keep the water from driving under them, or overflowing their edges during a rainfall. Neither could lead be used on a steep-pitched roof, as it would crawl off by expansion.

15. The roof should be carefully prepared for the covering; and all pockets, where ice, snow, or water can accumulate, must be studiously avoided in the construction.

On steep roofs, the backs of chimneys, or other vertical walls cutting the roof plane, must be fitted with a saddle or cant board.

All angles made by corners or flashings against walls should be avoided, and a tilt fillet or cant board must be used, to allow the flashing to take an easy bend. In slate, shingle, and tile roofs, this observance will assist the passage of water and snow and prevent its accumulation. On metal roofs, the sand, dust, or other refuse blown thereon will not so readily lodge. Where such accumulations do lodge on the roof, they keep it damp, rotting and corroding the most vulnerable parts.

16. The boarding should run one way, so that the shrinkage may be uniform and not pull and tear the joints of the roofing material. With metal roofs, it is best to have the boards laid from the eaves to the ridge, as this, in case the boards warp, brings the ridges thus raised in the roof in line with the current or pitches. This is not always practicable, but in any case it is possible to lay the boards diagonally. The nails driven from the surface of the roof must be countersunk at least 1/4 of an inch below the top of the boards, to prevent corrosion and wear of the covering.

17. Double gables, or dormers, in which the roofs form an should not be erected, it being impossible to keep them clear of snow in winter; for the same reason decks should not be used, unless unavoidable.

Table 1 239