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How to Keep the Top Plane of Common and Hip Rafters in Line

Joseph Skoler | Posted in General Questions on

I hope it’s okay to post a question to all your brilliant and experienced people that doesn’t specifically have to do with green building techniques.

I’m building a wrap around covered porch.  House is 34′ wide.  Porch will extend 12′ past side wall and 12′ along side wall, so outside edge of porch will be 46′ long on the front and 12′ long on the side (and 12′ deep).  6×6 posts along the outsider perimeter of the porch.  2×6 rafters.  4/12 roof pitch.  2×8 ledger board for rafters.

My confusion is in building the hip rafter, coming out of the corner of the house to meet the outside corner of the porch.  That is, the hip rafter will cover off the corner of the house at 135 degrees from the front face and side face of the house, and will be 17.5′ long (4′ rise on a 17′ run (square root of 12 squared plus 12 squared)).  I would like to use double 3×10 (yes, 3″; but really 9.25″ high) for the hip rafters.

So here’s where I get lost (although it’s certainly possible I’m lost already):

I need to keep the top plane of the 2×6 common rafters in line with the top plane of the hip rafter so that the sheathing lies flat.

I am planning to use hurricane ties for the 2×6 common rafters to the top plane of the rafters will be 5.5″ above the outer corner of the double 2×8 girders between the 6×6 posts.

That means I need a birdsmouth cut into the double 3×10 hip rafters, right?

If so, then the heal cut would need to be approximately 4″ into the 9.25, right?

Is this okay?

Here’s a picture.

I don’t claim to have any knowledge/skill/experience, so please don’t hesitate to tell me I’m wrong about everything (assuming I actually am).

Thank you very much!

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Replies

  1. plumb_bob | | #1

    Birds mouthed standard rafters require 1.5" of bearing where the horizontal cut lands on the wall plate or beam. Your hip rafter will have a different bird mouth profile due to the 16.97 (17) run. The code I am familiar with requires the hip rafter to be one size larger (deeper) than the standard rafters. So without checking the math I think you are on the right path.

    1. Joseph Skoler | | #2

      Thanks very much for sharing that info.

      I hope the code requires AT LEAST one size larger so I can use the 3x10.

      My concern is that that in order to keep exactly 5.5" above the beam at the outer point, and keep a 4 pitch roof, the cut needs to be pretty deep (3-15/16" in the diagram above). That's about 42% of the 9.25" hip rafter.

      Thanks!

  2. plumb_bob | | #3

    I do not see a problem with the shaping of the hip rafter, the location of the cut is not where the rafter will see the maximum stress, that is at mid-span. The theory behind roof framing is to take the natural spreading force that the roof wants to exert to the exterior walls and transfer this to vertical loading. Depending on what you plan for a roof overhang and rafter tail length, the stress on the bird mouth sections of the rafters should not be too bad.
    Probably a good idea to run this by your local inspector as he/she is familiar with the local snow/wind/seismic loads for your area, which can affect design.

  3. John Straus | | #4

    Generally you want to limit the birdsmouth cut to no more than 1/4 of the rafter dimension -- in your case about 2 5/16". If I am looking at your drawing correctly, your birdsmouth would be a little big at ~ 3 3/4". You can proceed with the conditions as drawn by adding a hanger on the interior side of the plate to support the hip. This is one of many approaches to a common problem with low slope hip roofs. I suggest buying a copy of "A Roof Cutter's Secrets" by Will Holladay if you want to better understand roof framing.

  4. plumb_bob | | #5

    Keep in mind that if you lay out the hip rafter correctly it will be the center line of the top of the hip that will be in plane with the 2 roof planes you are trying to join, and the square edges will actually be above the roof plane. To account for this most guys drop the hip by increasing the cut at the seat, or you can plane off the corners of the hip.

  5. Joseph Skoler | | #6

    Do I understand correctly that if I sister another piece of wood to the inside of the supporting 6x6, I'll be okay? Like this -- shown in red:

  6. Expert Member
    Michael Maines | | #7

    You should have full bearing at the seat cut. The best way to do this is to drop the hip rafter down into the wall framing. You will have to cut through the top plates. As Plumb Bob mentions, you can also extend the seat cut to the interior, but it doesn't meet code because the rafter can split, and if the hip rafter is exposed it's an ugly detail. Better to "drop the hip" down into the wall framing.

  7. Expert Member
    Akos | | #8

    I think a better way to frame the roof is how lot of the older houses around me are framed.

    Extend your ceiling joists past your top plate and set the rafters on this. This would give you much more space above your top plate for insulation and also avoid needing any bird mouth cuts.

    Depending on the gap, the rafters can be supported above the top plate by a horizontal 2x on edge or 2x4 cripples cut to size.

    1. Expert Member
      Michael Maines | | #10

      That is a better approach, Akos, in many cases and one I've seen in a lot of 19th century homes. But it's not always possible. I'm framing a hipped-roof addition now that has a vaulted ceiling so we have to drop the hip.

  8. Joseph Skoler | | #9

    First, thank you all so much!

    I suspect it's irrelevant, but just in case: This is a covered porch, so there are no walls -- only 6x6 support posts and 2x8 girders connecting them to each other. And, the design is without a horizontal ceiling to the porch -- only the pitched underside of the porch roof. So, no ceiling rafters.

    Also, I'm shooting for a 2' overhang for all rafter (past or farther out from the house face from the 6x6 supports).

    And, I don't have a top plate -- only double 2x8 girders between the 6x6 along with the actual 6x6 -- to support the rafters.

    I think I understand that if I dropped the structural piece that the hip rafter sits on, then a small birdsmouth cut can be made thereby achieving the goal is maintaining the 1/3 code requirement for structural integrity of the hip rafter.

    But, I'm not sure I understand how I would drop the hip rafter in this case.

    Am I making sense?

    Thank you!

    1. Expert Member
      Michael Maines | | #11

      Joseph, your options are limited, though with timber-framing techniques you could still make it happen, it would require different techniques than stick-builders are used to.

      Although a rule of thumb and perhaps some prescriptive codes advise using a hip rafter one size larger than the common and jack rafters, from an engineering point of view you can use a wider hip rafter instead of a deeper one. Properly sizing hip and valley rafters is a bit tricky, and not something covered in the IRC. One of the first ones I designed, many years ago, is nearby at my now-mother-in-law's house and every time I'm there I notice how much sag there is in the hip.

  9. Joseph Skoler | | #12

    Michael,

    Thank you so much for sharing your knowledge and experience.

    I went ahead and used two 3x10 (doug fir) pieces for the hip rafter, supported at the house side with a joist hanger as well as a 6x6 directly underneath it all the way down to concrete.

    On the outermost point, the cut I made to keep the top of the hip and common rafters in the same plane leave just over 6" of 'meat' -- so there is 6" of height and 6" of width of wood.

    While I can't put a center support, I can (if I really needed to), put another 6x6 under the full height (9.25") at the outermost corner.

    But, it sounds like the most likely 'failure' is sag in the middle, which I can't do anything about now.

    Right?

    Thanks!

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