The common method of framing wooden roofs is illustrated by Fig. 75. The timbers which support the boarding are called rafters; they are supported at or near their lower end by the wall plate, and at their upper end by the ridge pole or by another rafter. At all ridges or valleys larger timbers are placed to receive the ends of the roof boarding, and also of the short pieces of rafters. The timbers under the hips and valleys are called hip and valley rafters, respectively, and the rafters which cut against them are called "jack rafters." When the length of the rafters is over 18 feet it is desirable to support them somewhere near their centre. In dwellings this is generally done by means of studding or partitions, and sometimes by "collar" beams; in larger buildings the centre support is furnished by means of purlins resting either on trusses or on posts set over the interior walls or columns.