The simplest covering for a house would at first seem to be beams laid from wall to wall, forming a flat roof. This is in use in some countries, but it has many practical disadvantages.

The rain and snow are not thrown off, and will leak through the slightest opening.

In consequence of this, the material to cover a flat roof must be one (such as lead or copper) in which there are no open joints.

Such roofs must be restricted to small spans; if not they will require very heavy timbers, or expensive girders.1

Pitch of Roofs.1 - In order to throw off the rain and snow, it is necessary to tilt up the sides of the roof, giving them a slope as shown in Figs. 316, 318, etc.

The inclination of the sides of a roof to the horizontal plane is called its "pitch" and is described either by the number of degrees contained in the angle of inclination to the horizon, or by the proportion which the height, from the springing line to the apex, called the "rise" bears to the span.

Thus, a roof whose sides slope at 26 1/2° to the horizon, has a rise equal to 1/4 of the span, and is called a roof of 1/4 or 26 1/2° pitch. The best angle for the slope of the sides of a roof depends upon the material to be used to cover it, the climate, etc. (see "Roof Coverings," Part II.)

This course includes the consideration of slated roofs only, and the pitch found by experience to be the best for slates of different sizes is given at page 207.

A Lean-to Roof2 has only one side or slope which is fixed between two walls one higher than the other. Tig. 316 is a section of such a roof. The rafters (cr) are nailed to the wall plates (wp), the higher of which is well secured to the wall by bolts, so that the stress upon the rafter may not pull it out.

A Couple Roof3 is one formed by the meeting of two beams or rafters (R R) fixed at an inclination. Their feet are nailed and frequently also notched upon a wall plate imbedded on the top of the wall, and their heads meet upon a ridge board (r) to which they are secured by nails.

Fig. 316. Lean to Roof.

Fig. 316. Lean-to Roof.

1 Latterly, in densely populated districts where space is much required, flat roofs have been largely used, but they are generally constructed of,iron, concrete, and asphalt, and do not come within the scope of this chapter.

2 So. Too-fall. 3 Sc. sometimes spelt Cupple.

In such a roof there is nothing to prevent the rafters from spreading out and thrusting over the walls as shown in dotted lines.

This form of roof may, however, be adopted for spans up to 18 feet.

Scantlings for Single Span ok Couple Roofs1 of Baltic Fir Timber, with rise 1/4 span; slated with countess slates on boarding.

Fig. 317. Couple Roof.

Fig. 317. Couple Roof.

Span from centre to centre of Wall Plates.

Rafters, R.

Ridge Board, r.

it-Ceiling Joists, (if used).

Remarks.

8

feet.

3

by

2

7

by

1 1/2

4

by

2

* From wall to wall 2" wide by J" deep per foot run of span.

With these spans 14" brick walls built in ordinary lime mortar would be too weak, and must be strengthened or relieved of the thrust of the roof.

10

,,

3 1/2

by

2

7

by

1 1/2

5

by

2

12

,,

4

by

2

7

by

1 1/2

6

by

2

14

,,

4 1/2

by

2

7

by

1 1/2

7

by

2

16

,,

5

by

2

8

by

1 1/2

8

by

2

18

_. _ .

,,

5 1/2

by

2

8

by

1 1/2

9

by

2