Framing A Hip and Valley Roof
Hip and valley roof rafters are load bearing roof members that run at an angle – (usually 45 degrees) to the support walls and also run at a pitch.
The difference between the two is a hip is at the top of the rafters that hang on it, and a valley is at the bottom of the rafters that hang on it.
Both usually fall at a relative factor of 1.4 to the common pitch. If the common pitch is 8 in 12, the hip and valley roof pitch will be 8 in 17.
A hip attaches to a common ridge and may or may not land on a support wall. A valley that does not land on a common wall is called a “dead valley”. This is because it will shed water onto some other portion of the hop and valley roof and must be dealt with properly to prevent major water problems.
When installing a hip and valley roof, it is easiest if you first install all common ridges. Extend the common ridges a couple of feet longer than the dimensions call for. This will simplify the hip & valley installation. After the hips & valleys are placed, you can cut off any excess common ridge.
Hips & valleys connect to theoretical center points, and must be adjusted accordingly. The upper end should attach slightly lower than the edge of the common ridge, and the lower end should attach slightly lower than the edge of the framing member. This is to provide that the center point of the plane meets the center point of the other members.
Hip and Valley rafters should be at least one lumber size larger than the rafters, and preferably two lumber sizes larger, especially if you have no way to brace under them.
To build a hip roof end:
STEP 1: Install the common ridge and the end two common rafters on either side.
STEP 2: Install a third, identical common rafter directly on the end of the common ridge, running straight in line with the ridge.
STEP 3: Cut the two hip rafters. Use 17 as the pitch factor. (8 in 12 becomes 8 in 17) The height of the bird mouth needs to be ¾” higher than the common bird mouth height.
STEP 4: Install the top end in the corner between the two common rafters. Nail to the ridge and both commons.
STEP 5: If there is a question as to where the lower end connects, pull a string across the commons, where the plane intersects the lower ridge.
STEP 6: The lower end of the hip connects where the common plane intersects the ridge.
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