The load to be sustained by a roof-truss has been referred to in the previous three articles in general terms. It will now be treated more in detail. But first a few words regarding the slope of the roof. In a severe climate, roofs ought to be constructed steeper than in a milder one, in order that snow may have a tendency to slide off before it becomes of sufficient weight to endanger the safety of the roof. In selecting the material with which the roof is to be covered, regard should be had to the requirements of the inclination: slate and shingles cannot be used safely on roofs of small rise. The smallest inclination of the various kinds of covering is here given, together with the weight per superficial foot of each.

Material.

Least Inclination.

Weight upon a square foot.

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

Rise 1 inch to a foot.

5/8

to

1 1/4

lbs.

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

,,

1

,,

,,

,,

1

to

1 1/2

,,

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

,,

2

inches

,,

,,

4

to

7

,,

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

,,

3

,,

,,

,,

1 1/4

to

2

,,

Short pine shingles...............

,,

5

,,

,,

,,

1 1/2

to

2

,,

Long cypress shingles.....

,,

6

,,

,,

,,

2

to

3

,,

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

,,

6

,,

,,

,,

5

to

9

,,

The weight of the covering as here estimated includes the weight of whatever is used to fix it in place, such as nails, etc. The weight of that which the covering is laid upon, such as plank, boards, or lath, is not included. The weight of plank is about 3 pounds per foot superficial; of boards, 2 pounds; and lath, about half a pound.

Generally, for a slate roof, the weight of the covering, including plank and jack-rafters, amounts to about 12 pounds, as stated in Art. 215; but in every case, the weight of each article of the covering should be estimated, and the full load ascertained by summing up these weights.