The valley is a very common feature of domestic roof structures. Some of the common roof shapes are illustrated in Fig. 3.9 with full valleys, i.e. a valley running from eaves to ridge in parts A and B, with shorter valleys in parts C and D.
If one considers the hip to be an external mitre of the roof, then the valley is an internal mitre. The easiest way to consider a structural solution is to imagine one roof being wholly or partially imposed upon the other, and this is most easily illustrated in Fig. 3.9B where the top part of the roof can be imagined to run through undisturbed as a normal gable to gable roof, with the leg of the T imposed upon it. Figures 3.9A, C and D show other valley situations.
Figure 3.10 shows a typical solution with valley jack rafters imposed upon a valley board which is itself supported by the main roof common rafters. The solution does of course assume that the common rafters will themselves be supported by a wall plate or beam immediately beneath the valley area. The alternative solution where this support is not provided is shown later in the solution for valleys on attic roof structures.
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