In roof structures where a valley rafter is used, the maximum amount of usable space inside the roof is maintained for storage or living purposes. This is opposed to the alternative form of framing a valley - a lay board.  

Valley lay boards, also known as crickets in North American framing, are an alternative solution to framing a valley, without the use of a valley rafter. Where valley rafters are implemented, a much larger portion of the roof structure is formed from valley jack rafters bearing on the valley rafter. As such, the structural requirements for the valley rafter are typically greater. Also, the process for framing a valley with a valley rafter is more involved due to the implementation of the valley rafter. An alternative option to overcome this is the use of a valley lay board. This timber lays flat over the top edges of the common rafters of the primary roof structure. The lay board is cut with appropriate angles to fit tightly into the correct location, spanning from the wall plate to the ridge board. The correct alignment for the lay board sees the top edge of the timber in line with the common rafters of the secondary roof, as well as in line with the top edge of the ridge board. This allows for the valley jack rafters to be installed with their top edge in alignment with the top edge of the lay board. The positioning of the lay board in comparison to a valley rafter changes the shape of the valley jack rafters.  

This form of valley allows for the main structure of the roof to be framed with common rafters first, with the secondary roof laying over it. This enables much of the weight of the secondary roof to bear down on the primary roof, and directly onto a structural bearing. It is also an excellent choice for framing valleys when extending an existing roof, as it removes the need to chop into the existing roof. That being said, the usable space within the roof structure is limited by the common rafters of the main structure that sail down to the wall plate inside the structure of the roof underneath the new valley.  

The angles for the lay board are as follows. Set the framing square to the layout settings of the common rafter, with the relevant pitch and pivot point set. Measure the distance between the two points - i.e the length of rafter per “”””””” of run. Reset the square to match the pivot point on the tongue, and this new distance on the blade. These new angles are the top and bottom cut of the valley lay board. The tongue edge displays the bottom cut of the lay board, with the blade edge showing the top cut. If the lay board extends all the way down to the wallplate of the secondary roof then a compound angle can be cut to allow it to sit flush. This angle is just the pitch of the primary roof. Valley Jack /Cripple rafter

Valley jack rafters are a form of jack rafters that are used in the assembly of a valley. Whilst similar in concept to a hip jack, the valley jacks span from the ridge board to the valley rafter or lay board to continue the layout of the roof once they can no longer bear on the wall plate. These jack rafters are cut to length according to the diminish, as is the case with regular hip jacks. Valley jacks that receive into the side of a valley rafter feature a regular jack edge cut, but in reverse. For valley jacks that bear down on a lay board, the presentation of the edge cut is altered to accommodate the change in layout of the lay board. The top of the jack rafters features a common rafter plumb cut that receives into the ridge board. Here is how the different angles for the valley jack rafters are found.