5/12 Rafters With a Wall That Is Not Exactly 28 Feet
A crew is framing a gable roof on a nominal 28 ft wide building with a 5/12 pitch, using 2x10 rafters at 16 in. o.c. and a 1-1/2 in. thick ridge board. The plan shows a 12 in. horizontal eave overhang with a plumb-cut tail. When the crew strings the top plates, the wall-to-wall measurement comes out at 28 ft 3/8 in. One carpenter wants to lay out the common rafter from the 13 in. unit diagonal and cut all 30 rafters at once from that first pattern. Later, fiber cement lap siding will go over housewrap on the same building, and a porch roof dies into the gable wall at about 9 ft.
1. Compute the common rafter length for this roof, showing each step.
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Given: 5/12 pitch, actual span 28'-0 3/8", 12 in. horizontal overhang, 1-1/2 in. ridge.
- Run = half the span = 14'-0 3/16" (14.016 ft).
- Unit diagonal for 5/12: the square root of (12² + 5²) = 13. So the rafter line length is run x 13/12.
- Line length = 14.016 x 13/12 = 15.18 ft = 15'-2 3/16".
- Add the overhang. The overhang is given horizontally (12 in.), so convert it along the rafter: 12 x 13/12 = 13 in.
- Deduct half the ridge thickness, measured along the rafter: 3/4 in. x 13/12 = 13/16 in.
- Total: 15'-2 3/16" + 13" - 13/16" = about 16'-2 3/8".
Sanity checks: the rise is run x 5/12 = 5.84 ft, so about 5'-10" from plate to the top of the ridge - measure that at the ridge before cutting anything. The plumb cuts at both the ridge and the tail use the same 5/12 angle. Order stock a little long; you can always trim a tail, and you cannot add to it.
2. The carpenter wants to cut all 30 rafters from one pattern off the plan dimensions. What do you advise?
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Cut one pair, set them, and check them before batching - that is the whole point of a pattern rafter.
- The 3/8 in. span difference is only 3/16 in. per rafter and would probably never show. The real risk is what the string measurement does not tell you: plates out of level, walls out of plumb, and a building that is out of square. Check the diagonals of the wall box and the level of the top plates before layout.
- Test-fit the first rafter. If the ridge and the birdsmouth land right, then use it as the pattern and batch the rest, checking every tenth rafter against the pattern and against the building.
- Cut common rafters in groups and keep pairs together; label them.
- A pattern cut from plan dimensions alone, applied to 30 pieces, turns one layout error into 30 pieces of scrap.
- Also confirm the ridge board height and the rafter tail plumb cut before the ridge goes up - that is when corrections are still cheap.
3. At the exterior stage, how should the siding and flashing be handled where the porch roof dies into the gable wall?
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This is a water-detail judgment call, not a recall item: the wall above the porch roof takes the water and the roof-to-wall joint is where it fails.
Correct sequence and details:
- The weather-resistive barrier must lap over the step flashing - flashing is woven into the WRB in shingle fashion, not laid on top of it.
- Use step flashing at every course along the roof-wall intersection, plus a kick-out flashing at the bottom where the roof meets the wall, so water is thrown clear of the wall and into the gutter instead of behind the siding.
- Keep the bottom of the siding at least 2 in. above the roof surface (follow the manufacturer's requirement) and maintain a gap at the roof line - do not caulk siding tight to the roof.
- Hold the siding at least 6 in. above grade and keep the WRB continuous behind it.
- Back-prime or paint cut ends of fiber cement before installing; cement siding absorbs water at cut edges.
- Fasten through the siding into studs, not just the sheathing, and to the manufacturer's nailing pattern.
- Do not rely on caulk for the roof-to-wall joint. Caulk is a maintenance item; flashing is the waterproofing.