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Thread: How to properly sister rafters/joists in my attic (pic)

  1. #1

    Default How to properly sister rafters/joists in my attic (pic)

    Here's a pic of my attic:

    Counting the rafters/joists from left to right, number them #1, #2, #3, #4, #5.

    I'm going to cut #3 & #4 (both rafters/joists) to install a 44 1/4" wide (rough opening) skylight. I need to double-up joists & rafters #2 and #5

    My questions are:
    1. Can I double-up rafter #2 by adding a new one to the RIGHT of it? As you can see, it would not be able to be a "full length" rafter like the existing #2 is because joist #2 is "in the way"....so the rafter would have to "rest" on joist #2, is that ok? Or do I NEED to double-up to the left of the existing rafter with a full-length 2x6?

    2. I need to double-up joist #2 to the LEFT of it, otherwise my rough opening for the skylight will be compromised. How can I double-up joist #2 to its left? Can I add 2 2x6's, one that won't be full length (because the rafter is in the way) and then one that WOULD be full length (because it would be to the left of rafter #2)?

    Hope I described it well enough for you to picture it and understand my dilemma. Let me know if you need clarification or more pictures. Thanks!

  2. #2
    Join Date
    Oct 2003
    Living in, not from North Carolina


    Why are you trying to sister the rafters? You just need to cut #3 and #4 and install headers across from #2 and #5.

  3. #3


    Because at a different forum, when I indicated that I'd be cutting 2 rafters & 2 joists, I was given this info:

    "Under 4' wide, you need to double the sides, bearing to bearing, and add header joists. Over 4, dbl sides, and headers. 6 all with hangers: http://www.cciccweb.org/files/Framing.pdf Easier to double existing sides, add headers, add skylight framing."

    I don't want anything to sag or not be to code or have drywall cracks show up a year from now, etc... Is the above incorrect?

  4. #4
    Join Date
    Oct 2003
    Living in, not from North Carolina


    The diagrams your quoting are for floor framing, there are a whole lot more load amounts on floors.


    If you really want to head down the double rafter option then add the double rafter on the left side of #2 and left side of #5 then install your headers (top and bottom) then reinstall the tail rafters. make sure you use joist hangers and fill all the nail holes with #10D gal. nails. Then to make your R/O the correct size add lumber to correctly build it out.
    You could do the same for your ceiling joists just add the new joists to the right of the existing ones and frame headers off these again use joist hangers.

    Do you have you enough access in that attic to pass up 2x6's that length ?

  5. #5


    Yes, attic stairs opening gives plenty of room to get 14' long 2x6's up there.

    If I can avoid doubling the rafters & joists, but just double the headers, then that's fine. But a few folks seemed to indicate that cutting 1 joist/rafter is ok, but once you cut 2 side-by-side like I'll be doing, they were concerned and recommended doubling everything...

    If I do what you suggest, doubling joist to the right, doubling rafters to the left, then my rough opening is too small.

    R/O is 44 1/4".
    On-center, #2 & #5 are 48", but the measurement between the insides of the rafters (not the centers) is 3/4" less on each. That leaves me with 46 1/2". If I double the #5 rafter on the left. I'll be at 45". That's fine.

    However, if I THEN double joist #2 on the RIGHT, then my R/O shrinks another 1 1/2" down to 43 1/2" (because the drywall needs to come vertically down on the sides, I'm only sloping the front/back of the skylight tube, not the sides). So 43 1/2" is less than the required skylight R/O. So that's why I posed my initial question up above.

    I guess it kind of leads to a side question which is when you double up joists/rafters, must they be "full length" doubling? Or can you just double up a portion of the full length? I was told in the past, when I framed my attic stairs, that when I doubled my joist, it should go over both the inner & outer load-bearing walls in my house (I have the 2 outside load bearing walls, then just 1 in the middle of the house). That's why I thought everything needed to be full-length when doubling, including rafters I'm assuming...

  6. #6


    As for my last question up above, about full-length sistering, figure 7.8 here may answer that:


    It looks like non-full-length sistering is allowable as long as you put in lag bolts every 12" for added rigidity, versus just nails if it was a full-length sister. Make sense? Is that valid?

  7. #7


    You are not required to double up on the joists on a framed opening until your distance between rafters exceed 48". See copied section of the minimum standard rule below. If you want to play it safe not being required to double the joists you can just sister on the same size board getting as close to the end of the rafter as you can. You will be exceeding the rule for minimum. A single rafter on each side is allowed per IRC rules up to 48" between the two supporting rafters. Hope this helps Wg


    R802.9 Framing of openings. Openings in roof and ceiling
    framing shall be framed with header and trimmer joists. When
    the header joist span does not exceed 4 feet (1219 mm), the
    header joist may be a single member the same size as the ceiling
    joist or rafter. Single trimmer joists may be used to carry a single
    header joist that is located within 3 feet (914 mm) of the
    trimmer joist bearing. When the header joist span exceeds 4
    feet (1219 mm), the trimmer joists and the header joist shall be
    doubled and of sufficient cross section to support the ceiling
    joists or rafter framing into the header. Approved hangers shall
    be used for the header joist to trimmer joist connections when
    the header joist span exceeds 6 feet (1829 mm). Tail joists over
    12 feet (3658 mm) long shall be supported at the header by
    framing anchors or on ledger strips not less than 2 inches by 2
    inches (51 mm by 51 mm).

  8. #8


    Perfect! Great to know that I'll be starting at compliant with singles and then exceeding by going double (as close to full length as the structure allows). That'll give me both the compliance & peace of mind I seek in all of my home projects.

    I knew I came to the right place! Thanks again.

  9. #9
    Join Date
    Oct 2003
    Living in, not from North Carolina


    As well if you still want, you can double up #5 by adding the sister on the right side hard against the ceiling joist, because this will leave 1 1/2" space you will need to add a member between the original joist and the new sister, kinda like triple raftering but you will need to bolt these three together.

  10. #10


    Keeping in mind that from Joist #2 to Rafter #5, it's 45" already. The outside frame of the skylight is 44 3/4" so I can't double-up anything to the right of Joist #2 or the left of Rafter #5. With those restraints, my plan is (bolts & nuts will be used for all sistering):

    1. Sister rafters #2 & #5 to their right, new rafters will extend to end of existing joists #2/#5 ("resting" on top of it)

    2. Sister joists #2 & #5 to their left, new joists will extend until it hits existing rafters #2/#5 (since the rafter prevents full length joist)

    Not sure that I want to triple anything just because of all the extra work involved (and it's not really required at all)....but I'll probably buy the extra wood & bolts just in case I change my mind and then return them if I decide against it. But, yes, if I decide to triple, they will be with full-length members adjacent to the plan I described above. Then if my roof rips off during a hurricane, at least my skylight won't budge .

    I'll try & post pictures when I complete it probably within a couple of months.

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