0

Are there good options for attaching roof eaves/overhangs essentially as cantilevered beams with a fixed support? This is a monoslope (lean-to) roof with a 2:12 pitch (~9.46 deg), unheated structure, and I'd like to "bury" the rafters so that they hang more like ceiling/floor joists instead of sitting on top of the carrying beams.

The design roof load is 90 psf (dead + snow load) and the rafters are spaced 24" O.C., overhangs are 2', thus each eave has a load tributary area of 4 sq ft and needs to carry 360 lb.

Can I use something like this rafter connector? Will that meet code?

enter image description here

Another option I thought about is attaching the eave rafter with a couple of bolts through the carrying beam:

enter image description here

Is there a name for this type of connection? I'm not quite sure how to evaluate the stress.

Improve this question
5
  • Do you already have a shed and want to add to it or are you designing the shed? My impression is that the SS Tie you linked to is not at all suited for cantilever stress. I don't get the alternative one in the 2nd diagram.
    – Jim Stewart
    Mar 10, 2017 at 13:53
  • I get it now, the diagrams are for the high end of the shed. But I stlll question the strength of the cantilevered section. Why do you want to hang the rafters on the beam rather than having them pass over it?
    – Jim Stewart
    Mar 10, 2017 at 15:53
  • No existing structure, just a design so far. I'm trying to create a lower profile for the roof assembly - 2x8s sitting on top of 4x12s seems high. This is car port so the roof framing will be visible. How much wood do I need to leave per code when making a seat in the rafters? (finding conflicting answers.. 2", 3.5", d/3)
    – Serguei
    Mar 10, 2017 at 17:45
  • If it were me I would notch both the beam and the rafters so they key together. Perhaps double up the 4x12 to compensate, then notch it down 4", then notch the 2x8s up 3". That will leave a 0.5" gap between the top of the beam and the bottom of the sheathing for ventilation. Or you can set it right down to 3.5" notch if you do not want a gap. Pretty sure 2ft cantilever with 360lbs wont hold. that's like 2,160lbs pulling on those screws.
    – Trevor_G
    Mar 10, 2017 at 18:43
  • 2
    ". . . The IRC allows the ends of rafters to be notched as much as one-fourth their depth (R802.7.1), but on a shallow-pitched roof this can result in a seat cut that's wider than the top plate. When that happens, roof loads are carried by the toe rather than the heel of the rafter, reducing the rafter's bearing capacity . . . " So figure out what notch cut, n, to make so that the seat, s, in the rafter is exactly at full bearing on the 4x12 beam. I think this is n = s (2/12), so if the beam is 3.75 in, then the notch is only 5/8".
    – Jim Stewart
    Mar 10, 2017 at 18:53

2 Answers 2

Reset to default
1

We don't know the distance from the 4x12 beam over to the next support. So, we don't know the load (tributary load in pounds per linear foot) on the 4x12 and therefore we don't know how much, if any, the 4x12 can be notched.

However, assuming you have calculated the Total Load (LL + DL) correctly at 90 psf, then a No. 1 Doug. Fir 2x8 at 24" o.c. with a load of 90 lbs. per square foot (180 plf) can span a maximum of 9'-0" with a bending stress of fb = 1500 and modulus of elasticity = 1.4. If you need the 2x8 joist to span further, you could span 11'-0" at 16" o.c. Or span 12'-0" at 12" o.c. (This does not account for "short duration loading" (snow that lasts less than 30 days) or "impact loading" (snow that lasts less than 7 days) like where I live.

Therefore, to determine if the 4x12 beam is adequate, I calculated the maximum tributary load on the 4x12, which is half the 9' span plus the 2' overhang, which is: 4.5' + 2' = 6.5' x 90 psf = 585 plf The maximum load on a 4x12 spanning 10' is about 700 plf (Therefore, the 4x12 is slightly over sized and can be notched...if the 2x8's only span 9', which I'm guessing they are over stressed.)

So, I think there are 3 options:

Option 1: The 2x8 joists are in "reverse-bending" as they pass over the 4x12 beam. So, the top half of the joist is in tension and the bottom half is in compression. If you carefully notch the bottom one-third of each 2x8 so that the joist fits tight over the 4x12, then the overhang won't sag or fail.

Option 2: if you lower the 4x12 enough to let the 2x8 joists to sit on the 4x12, there would be no need to notch the joists.

Option 3: The 4x12 is slightly over designed, you could notch the 4x12 beam a maximum of 2" (making the beam an equivalent of a 4x10).

Note: In your original statement you indicated that the total load is 90 psf. This seems extremely high. Did you mean 90 plf on the 2x8 joists. If so, that would make the snow load 40 psf, which is high, but reasonable. (Meanwhile, I'd double check the span of those 2x8's.)

Improve this answer
1
  • Yup ground snow load is 80 psf. Courtesy of the local regulators and site elevation. In reality it should be x2-4 less but this is what we must deal with.. The beam carries ~600plf (D+S load combo) and in the worst case the 4x12 is at 96% bending load - not much to spare for notching. The rafters are OK - span <7' and spaced 24" OC.
    – Serguei
    Mar 16, 2017 at 6:48
0

If it were me I would notch both the beam and the rafters so they key together.

Perhaps increase the 4x12 to three 2x12s to compensate, then notch it down 4".

Notch the bottom side of the 2x8s up 3". That will leave a 0.5" gap between the top of the beam and the bottom of the sheathing for ventilation. Or you can set it right down to 3.5" notch if you do not want a gap.

Cuts should be accurate so they fit snuggly together.

I'm pretty sure a 2ft cantilever with 360lbs won't hold with your current plan. That's like 2,160lbs pulling on those screws, nails and bracket.

Improve this answer
8
  • The 4x12 spans 10' were just enough to support the 90psf design load. I haven't checked the numbers pretty sure a 3-2x8 carrying member won't be enough (8" unnotched beam depth, since beams can't be notched in 1/3 midspan).
    – Serguei
    Mar 10, 2017 at 19:42
  • @Serguei, it depends how OVER designed they are. That is, if say you really only needed a 4x10 to match your loading requirements.. the top 2 inches is superfluous and open to cutting as long as you don't damage the 2 by 10 that's IN the 2 by 12.
    – Trevor_G
    Mar 10, 2017 at 19:48
  • 1
    It's pretty tight, 4x12 is just enough. I'd need a 4x16 beam to notch it 4" - might as well just set the rafters on top of the beam proper and add blocking. The design snow load (80 psf) is in reality overkill but that's the local regs that have to be met.
    – Serguei
    Mar 10, 2017 at 20:25
  • Is that a composite beam? 4x12 by 10ft is a lot of lumber to twist and warp over time. That's another good reason to nail 2xs together.
    – Trevor_G
    Mar 10, 2017 at 20:31
  • BTW according to that R802 thing, you can reduce the 2x8 to a 2x4 in the cantilevered part if its under 2' and just rest it on top in hurricane brackets. That would bring it down a bit and save lots of notch cutting.
    – Trevor_G
    Mar 10, 2017 at 20:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.