Similar in concept and application to a hip, a valley is a part of a roof structure where two planes of the roof meet at an internal intersection. Valleys can be formed through the use of valley rafters, or through the use of lay boards. Both are suited to different applications and have their own strengths and weaknesses. Once these primary timbers are installed, cripple jacks/valley jacks are installed to continue the on-centre layout of the common rafters. These valley jack rafters are similar in concept to hip jack rafters, with the difference being that they span from the ridge to the valley, as opposed to from the plate to the hip.  

What are the criteria of a valley?

A valley follows all of the standard criteria that are applicable within a roof structure and the broader scope of carpentry. Accuracy in marking out must be achieved ensuring that the valley rises from the wall plate to the ridge at the correct pitch in relation to the pitch of the roof. The valley must also fall accurately along the geometric 45-degree diagonal plane in relation to the plates to ensure that the cripple jacks rafters fit correctly.