9d • General discussion
How to Set Out Hip and Valley Rafters
1. Lock In Your Key Measurements
Before touching a saw, confirm:
- Span (outside to outside of plates)
- Run (half the span)
- Pitch (roof angle)
- Plan layout (hip or valley location, building shape)
These control the entire hip/valley system.
2. Use the Hip/Valley Multiplier
Hip and valley rafters are longer than common rafters because they run diagonally.
Each pitch has a hip/valley multiplier:
Pitch Hip/Valley Multiplier15°1.03520°1.06422°1.07825°1.10330°1.155
Hip/Valley Length = Run × Hip/Valley Multiplier
Example (22° roof, 3 m run): 3 × 1.078 = 3.234 m
This gives you the length from the ridge to the birds mouth.
3. Mark the Plumb Cut
Hip and valley rafters use a different plumb cut angle than common rafters.
Why? Because they sit at 45° to the building, so the angle changes.
Use your speed square:
- Set the square to the hip/valley plumb angle (found on the hip/valley scale)
- Mark a clean plumb line
- Cut accurately — this determines how the hip meets the ridge
Tip: Most speed squares have a dedicated “HIP/VAL” scale for this.
4. Mark the Birds mouth
The birds mouth on a hip/valley rafter is the same concept as a common rafter, but:
- The seat cut is longer because the hip sits diagonally across the plate
- The heel cut is steeper due to the hip angle
Steps:
- Measure down from the plumb cut the calculated length
- Mark the heel cut (plumb line)
- Mark the seat cut (horizontal line)
- Keep the depth within one‑third of the rafter
5. Allow for Overhang
Hip and valley rafters also need overhang, but the length is different.
Use the hip/valley multiplier again:
True Overhang = Horizontal Overhang × Hip/Valley Multiplier
Example (450 mm overhang, 22° roof): 0.45 × 1.078 = 485 mm
Mark the tail cut using the seat‑cut angle (90° – pitch).
6. Cut One Perfect Pattern Hip/Valley Rafter
Just like commons and jacks:
- Cut one hip/valley rafter perfectly
- Test it at the ridge
- Test the birds mouth on the plate
- Check fascia height and overhang alignment
Once perfect, use it as your pattern.
7. Transfer All Marks From the Pattern
Lay each blank rafter under the pattern:
- Trace the plumb cut
- Trace the birds mouth
- Trace the tail cut
- Cut clean and consistent
This keeps the entire roof geometry tight.
8. Install and Check Alignment
When installing:
- Set the hip/valley into the ridge
- Check the center line of the hip is straight
- Ensure jack rafters land cleanly along the hip
- Confirm fascia and soffit lines match the commons
A straight hip makes the whole roof look professional.
Tomasi’s Builder Tip
“Hip and valley rafters are just common rafters on a 45° — multiplier, plumb cut, birds mouth, pattern. Keep it systematic.”
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How to Set Out Hip and Valley Rafters
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