The struts are needed in the roof frame to transfer the loads from underpurlins and other beams to the load bearing walls. Struts should be adequately supported over studs in timber walls. They should have anti-slip blocks, where necessary, to prevent them sliding horizontally.
Struts are generally more efficient the closer they are to the vertical. Struts must be either vertical, perpendicular to the rafters or at an angle to the vertical which does not exceed 35o. They are 'birdsmouthed' or halved to fit underpurlins as shown below. (Birdsmouthing is so called because the cut looks like a bird's open beak.)