Birdsmouth Limit

Started by North Sask, January 29, 2014, 12:03:38 AM

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North Sask

If I am not mistaken, the old IRC specified that the birdsmouth cut on a rafter should be limited to 1/3 of the rafter depth. I think the new IRC is 1/4 of the rafter depth. Can anyone elaborate on why this depth must be limited? Is it a strength of rafter tail issue? If so, I assume a boxed in eave with lookouts would eliminate the issue. It seems to me that matching the seat cut to the wall thickness (2x6 wall with 2x10 rafter, 9:12 pitch) shouldn't be an issue. In reality, this wouldn't comply with the 1/3 requirement, let alone the 1/4 requirement.
It would be greatly appreciated if you stopped by my thread and left your two cents.
Great Northern Saskatchewan Adventure...Round 2

Don_P

That is one section of code that has been misinterpreted to the point where those writing the laws have confused themselves. Hopefully it will get straightened out in the next few code cycles. You are correctly backing up and looking at the physics involved... the real laws.

First, there are limits on notching bending members. The provision for notching the ends limits the notch to 1/4 depth on the bottom, tension, edge. In the NDS, where this comes from, there is a drawing of a joist with 1/4 of the bottom edge notched out for bearing over a girder. In other words 1/4 of the depth of the joist is unsupported and it is bearing on the 3/4 height end of the joist. In the birdsmouth the entire heel of the cut is, or should be, bearing on the wall. This is not a notch in the sense that the NDS is describing, there is nothing "hanging" in tension perp to grain. The latest writing of code stems, I think, from a poorly written article in a trade magazine.

In older versions the limit of the birdsmouth was that for an overhang of up to 2' you need to leave at least 3-1/2" of undisturbed wood above the birdsmouth... basically, leave a 2x4 over the wall to support the tail. Don't make the birdsmouth so deep that the level seatcut extends inboard of the wall, this induces splitting. If you've followed along, you could technically extend the seatcut until 1/4 of the rafter depth is hanging inboard but don't do it, as I've said that induces splitting.

Again quite correctly, you've realized that by boxing the overhang you've trussed that area, strengthening the projecting tail well beyond the code minimum.


North Sask

Thanks, Don. That nicely summarizes and clears up the birdsmouth situation. I suppose it is becoming much less of an issue as most projects now use an engineered truss system.
It would be greatly appreciated if you stopped by my thread and left your two cents.
Great Northern Saskatchewan Adventure...Round 2