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"Birdsmouth" Depth - Code vs Local Code Officer


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In a project I am considering using 12" rafters that sit on 8x8 beams at the side-wall (it is a 10:12 roof), but the rafters end at the outside face of the beam (the rafters are not part of the overhang) and this is why "birdsmouth" is in double quotes in the thread title. After reading through the residential building code, it would seem to me that the max depth that one can cut into the rafter at the top plate is 1/4 of the rafter depth (in this case, 12"/4 = 3"). First of all, is this correct?

However, the confusion is that my local code enforcement officer is saying that I can do a full bearing on the 8x8 side-wall beam. This would make the depth cut into the rafter just over 5" (more than 3"). See the picture below.

Click to Enlarge
tn_2015425134738_birdsmouthdepth.jpg

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Am I misunderstanding something? Is there some sort of exception when the rafter itself does not extend past the top plate? I thought the main concern with cutting into the rafter at the top plate was related to potential shearing along the length of the rafter.

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Hi,

I think what's confusing you, is where you have the cut out drawn.

Take a pencil and extend the bottom edge of the rafter past the beam. That's where the birds mouth will be. Not where it's drawn. Whether you cut off the end or not, shouldn't matter.

Another edit. The one you have drawn is backwards. Now, you should see it.

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I apologize for not being clear. I did not intend for the two lines drawn on the rafter that meet at a right angle to be interpreted as cuts, but rather simply as lines perpendicular and parallel to length of the rafter in order to indicate the depth into which the rafter is cut (depth perpendicular to the rafter length)

EDIT: I'll clarify again. Is the dotted white line here not the measure that must be limited to no more than 1/4 the rafter's depth?

Click to Enlarge
tn_2015425143428_birdsmouthdepth.jpg

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I apologize for not being clear. I did not intend for the two lines drawn on the rafter that meet at a right angle to be interpreted as cuts, but rather simply as lines perpendicular and parallel to length of the rafter in order to indicate the depth into which the rafter is cut (depth perpendicular to the rafter length)

EDIT: I'll clarify again. Is the dotted white line here not the measure that must be limited to no more than 1/4 the rafter's depth?

Click to Enlarge
tn_2015425143428_birdsmouthdepth.jpg

17.05 KB

Yes.

Now, I think I see the problem. You're dealing with an 8x8 which is the reason the cut is deeper than if it were a ?x6 or ?x4, correct?

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In a project I am considering using 12" rafters that sit on 8x8 beams at the side-wall (it is a 10:12 roof), but the rafters end at the outside face of the beam (the rafters are not part of the overhang) and this is why "birdsmouth" is in double quotes in the thread title. After reading through the residential building code, it would seem to me that the max depth that one can cut into the rafter at the top plate is 1/4 of the rafter depth (in this case, 12"/4 = 3"). First of all, is this correct?

However, the confusion is that my local code enforcement officer is saying that I can do a full bearing on the 8x8 side-wall beam. This would make the depth cut into the rafter just over 5" (more than 3"). See the picture below.

Click to Enlarge
tn_2015425134738_birdsmouthdepth.jpg

18.07 KB

Am I misunderstanding something? Is there some sort of exception when the rafter itself does not extend past the top plate? I thought the main concern with cutting into the rafter at the top plate was related to potential shearing along the length of the rafter.

Looks fine to me. Since the rafter isn't extending past the top plate, you're not notching anything. You're overthinking it.

PS: remember that a 2x12 is only going to be 11-1/4" wide.

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I apologize for not being clear. I did not intend for the two lines drawn on the rafter that meet at a right angle to be interpreted as cuts, but rather simply as lines perpendicular and parallel to length of the rafter in order to indicate the depth into which the rafter is cut (depth perpendicular to the rafter length)

EDIT: I'll clarify again. Is the dotted white line here not the measure that must be limited to no more than 1/4 the rafter's depth?

Click to Enlarge
tn_2015425143428_birdsmouthdepth.jpg

17.05 KB

Yes.

Now, I think I see the problem. You're dealing with an 8x8 which is the reason the cut is deeper than if it were a ?x6 or ?x4, correct?

If I were using a smaller beam, then yes this could reduce the depth of that cut (perpendicular to the rafter length) to less than or equal to 3 inches, but I also have the option of addressing it by raising the rafter higher (vertically, perpendicular to the ground).

However my concern is whether or not the local code enforcement officer is wrong about what was depicted in the first picture passing code. If it turns out that I can have a full bearing on the 8x8 like that, it makes other aspects of my design substantially simpler.

In a project I am considering using 12" rafters that sit on 8x8 beams at the side-wall (it is a 10:12 roof), but the rafters end at the outside face of the beam (the rafters are not part of the overhang) and this is why "birdsmouth" is in double quotes in the thread title. After reading through the residential building code, it would seem to me that the max depth that one can cut into the rafter at the top plate is 1/4 of the rafter depth (in this case, 12"/4 = 3"). First of all, is this correct?

However, the confusion is that my local code enforcement officer is saying that I can do a full bearing on the 8x8 side-wall beam. This would make the depth cut into the rafter just over 5" (more than 3"). See the picture below.

Click to Enlarge
tn_2015425134738_birdsmouthdepth.jpg

18.07 KB

Am I misunderstanding something? Is there some sort of exception when the rafter itself does not extend past the top plate? I thought the main concern with cutting into the rafter at the top plate was related to potential shearing along the length of the rafter.

Looks fine to me. Since the rafter isn't extending past the top plate, you're not notching anything. You're overthinking it.

PS: remember that a 2x12 is only going to be 11-1/4" wide.

I see, if it's not a notch it won't be an issue here. Thanks! EDIT: Also, forgot to note that I'm using true sized lumber.

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I've cut 2X6 rafters to fit on 2X4 walls, at pitches up to 12/12 while bearing fully upon the 2X4 plate. The ratio of 12 to 8 is about the same as 6 to 4, plus you're only at a pitch of 10/12 which further reduces the penetration of the bird's mouth into the rafter so I don't see any issue with cutting a 7 1/4 (level cut) bird's mouth into that 2X12 rafter.

If your rafters are without tails then you can cut the rafter bottom just like you've drawn it. Just make sure the lower side of the rafter bears upon the top of the plate (beam) and not the side facing the interior of the house.

You're not using engineered lumber are you?

Marc

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I've cut 2X6 rafters to fit on 2X4 walls, at pitches up to 12/12 while bearing fully upon the 2X4 plate. The ratio of 12 to 8 is about the same as 6 to 4, plus you're only at a pitch of 10/12 which further reduces the penetration of the bird's mouth into the rafter so I don't see any issue with cutting a 7 1/4 (level cut) bird's mouth into that 2X12 rafter.

If your rafters are without tails then you can cut the rafter bottom just like you've drawn it. Just make sure the lower side of the rafter bears upon the top of the plate (beam) and not the side facing the interior of the house.

You're not using engineered lumber are you?

Marc

. . . Also, forgot to note that I'm using true sized lumber.

What's "true sized lumber"? Are we talking engineered lumber?

Rough cut lumber, not engineered. But I guess rough cut isn't very true at all.

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'Sawn lumber' is the general term, I believe.

Marc

You mean "locally milled" lumber? [:)]

Yes you can cut the rafters to full width, and here, you call it a custom home and do whatever with the rafter tails including trimming them down to about 3" wide like a swallow tail. Maybe that is a Craftsman style design.

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