Where chimneys, flues, stairs, etc., occur to interrupt the bearing, the beams are framed into a piece, b (Fig. 42), called a header. The beams, a a, into which the header is framed are called trimmers or carriage-beams. These framed beams require to be made thicker than the common beams. The header must be strong enough to sustain one half of the weight that is sustained upon the tail-beams, c c (the wall at the opposite end or another header there sustaining the other half), and the trimmers must each sustain one half of the weight sustained by the header in addition to the weight it supports as a common beam. It is usual in practice to make these framed beams one inch thicker than the common beams for dwellings, and two inches thicker for heavy stores. This practice in ordinary cases answers very well, but in extreme cases these dimensions are not proper. Rules applicable generally must be deduced from the conditions of the case - the load to be sustained and the strength of the material.