Valley rafters are structural timbers that span from the internal corner of a set of wall plates up to the ridge board. From a layout perspective, valley rafters are very similar, though not identical to standard hip rafters. Both feature the same dual edge bevel at the plumb cut, with the pitch of each also being the same. Once installed, the valley rafter supports the valley jack rafters in order to continue the on-centre layout of the roof structure. These valley jacks span from the ridge board to the side of the valley, with a common rafter plumb cut and a jack edge cut on them respectively. These valley jacks are cut in consistently decreasing lengths, in accordance with the jack rafter diminish principle.  

When a hip rafter is installed, without a dihedral backing bevel, the edges of the timber meet the ridge intersection. This is because the edges are in line with the true plane of the roof. The part that is missing is the extra amount that the backing angle would add, which allows the top point of the hip rafter to meet the apex of the hip intersection. When looking at valley rafters, the edges of the timber must still meet the top of the ridge and valley intersection, as this falls in line with the true plane of the roof. In this arrangement, the top of the unbacked valley rafter rises slightly above the geometric intersection and must be cut off. This is quite normal, and the standard valley rafter does not cause any issues for the installation of typical felt and batten.

In scenarios where the roof is being sheathed with sheet material, this standard valley can cause a deviation in the true plane of the roof. Whilst the edges of the valley rafter are in the line with the true plane of the roof, the centre of the rafter sits higher. To overcome this scenario, the valley rafter can be lowered so that the centre of the rafter is in line, or the reverse of the hip rafter dihedral backing angle can be cut on the valley rafter. When the top edges of the valley are kept up to the top of the valley intersection, the edge cut on the bottom of the valley cripple jacks can meet the edge of the valley. If we’re going to overcome this excess material without cutting out the hollow/reverse backing angle, then we must drop the valley down so that the apex of the dual plumb cut meets the valley intersection. However, if we do this, the bottom edge cut of the valley cripple jacks can no longer come into the edges of the valleys rafter, as they are now too low. A line needs to be struck on the cut face of the valley jacks to space them appropriately above the valley rafter. When the valley rafter is dropped down so that the top point meets the ridge intersection, this distance must also be cut out of the seat cut, so that the angle is maintained. Due to the length of the valley rafter, it is easy to make a slightly incorrect length valley rafter fit.