Ordinarily, as in roofs, the load is not concentrated, it being that of the framing itself. Here the amount of the load will be in proportion to the length of the rafter, and this will increase with the increase of the angle of inclination, the span remaining the same. So it is seen that in enlarging the angle of inclination to the horizon in order to lessen the oblique thrust, the load is increased in consequence of the elongation of the rafter, thus increasing the oblique thrust. Hence there is an economical angle of inclination. A rafter will have the least oblique thrust when its angle of inclination to the horizon is 35o 16' nearly. This angle is attained very nearly when the rafter rises 8 1/2 inches per foot, or when the height B C (Fig. 32) is to the base A C as 8 1/2 is to 12, or as 0.7071 is to 1 . 0.