Struts. The position and use of a strut are shown in Fig. 86 at a and b.

A strut is often used as the support for a purlin, as at c, in which case it is cut short enough to allow for the thickness of the purlin (c) which rests upon it, and of the plate (d) upon which it rests.

A strut is frequently set vertically, and used as studding for the sides of a room in the attic.

The length of a strut may be found upon a steel square, placed upon H (Fig. 87) to the constants of the rise and run of the house.

Slide the square along to the given distance, R, upon the floor line. R equals the base of a right-angled triangle, of which A is the altitude, or length of the strut, measuring from R to the common rafter H, the hypotenuse, or the line of the common rafter, and gives the distance of the strut from the outside of the plate. The steel square formula is as follows: -

Fig. 86. - The Strut.

Fig. 87. - Method of finding the Length of a Strut.

A = length of the strut.

H = pitch of the roof.

R = distance upon the floor from the plate line.

Formula 27. Slide Bl. on floor line to H. Reading of

To. at H = A.

If a strut like b, Fig. 86, is to be used, the thickness of the purlin, c, and of the plate, d, must be subtracted from A. If a strut like a is being cut, the length is usually found after the rafters and floor joists are in place; to this length must be added enough to provide nailings at each end. This latter form of strut is often used, as at e, to form a truss.