In a timber-framed house, one of several such rafters that extend from the ridge of the roof down to the wall plate; somewhat heavier than a common rafter; often located at a corner post, story post, or chimney post and framed into a tie beam. Principal rafters, together with the principal purlins, form a roof framing system having considerable stability. Also called a blade.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.