how to calculate the maximum birdsmouth depth?
how to calculate the maximum birdsmouth depth?
(OP)
How do you calculate the minimum depth of rafter required above the top plate in the attached detail? I'm currently using NDS 3.4.3.2 to come up with a solution. (NDS 2005 edition) I'm not sure if this is the correct approach.






RE: how to calculate the maximum birdsmouth depth?
BA
RE: how to calculate the maximum birdsmouth depth?
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: how to calculate the maximum birdsmouth depth?
This 1/4 rule you speak of... is that in the code somewhere or is that just a rule of thumb?
RE: how to calculate the maximum birdsmouth depth?
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: how to calculate the maximum birdsmouth depth?
BA and Woodman are right on the money with their comments. Look to minimize the birds mouth cut in this situation, not what can the max. be. Otherwise you are introducing cross grain tension in the rafter, right where the horiz. shear is also max., a sure recipe for horiz. splitting up the length of the rafter. This turns the rafter into two independent 2x4's the bottom one being unsupported at the wall. Make the vert. cut 1.5" to match the top pl. of the double top pl. on the stud wall, then a higher vert. cut on the rafter at the outside face of the wall. That’s a very complicated detail. You’ve got enough blocking in there to build another house. Why the double 2x strapping for the interior sht. rock? What are you doing to manage the rafter lateral thrust on the ext. bearing walls?
RE: how to calculate the maximum birdsmouth depth?
Mike McCann, PE, SE (WA)
RE: how to calculate the maximum birdsmouth depth?
dhengr:
Thanks for your input. I'm new to wood design. This detail was approved by my boss so I just decided to go with it. The architect wants to raise the top plate without raising the rafter, thus cutting into the rafter even more than before, that that's when I got nervous and turned to you guys. This is actually the architects detail. I do not know why he needs 2x strapping. We have a steel ridge beam bearing on steel pipe columns at either end. Therefore there is no thrust in the top of the wall, correct?
RE: how to calculate the maximum birdsmouth depth?
To mitigate this, I would add strapping over the top of the ridge to connect the rafters on either side.
Mike McCann, PE, SE (WA)
RE: how to calculate the maximum birdsmouth depth?
There will still be trust to the ext. bearing walls to the extent that the rafters settle in their hangers or the ridge beam deflects, just imagine the geometry, the rafters don’t shorten. And, don’t make a full width/long horiz. seat cut on the rafter at the top of the wall, that is no better. You are still applying the bearing load in a way which imparts cross grain tension to the top half of the rafter, at about mid depth. Archs. do all kinds of crazy things becuase their pencils tell them to do it or their CAD programs make them do it, they have no real sound reasons, they need to fill the space with something. You have to be a bit of an educator and gently explain the errors of their ways. I would lower the double top plate to provide the 1.5" deep birds mouth on the rafter and then apply something akin to a 2x10 (2x8?) rim joist out at the outer face of the stud wall, bevel ripped on the top to match the roof SIPS. I would run the wall SIP’s up to within 1.5" of the underside of the gutter, and install a single 2x member on top of the SIPS to tie them all together, and eliminate the three 2x blocking members at the top of that wall section. You better be very confident in your gutter installer’s capabilities or you could be growing mushrooms in those walls.
RE: how to calculate the maximum birdsmouth depth?