A birdsmouth is the little triangular cutout in the bottom of a rafter that provides a flat area so the rafter can rest solidly on—and be solidly attached to—a wall top plate.
Why is it called a birdsmouth?
It is an indentation cut into the rafter which consists of a “seat cut” (the face of which rests on the top plate) and a “heel cut” or “plumb cut” (the face of which lies parallel to the supporting wall), forming a shape resembling a bird’s mouth.
What is a birdsmouth seat?
The Birdsmouth Joint (also called the birds beak cut or birds beak joint) is a small triangular shaped cut out at the base of a roofing joist that allows the joist to sit nicely on top of the wall plate (top of the wall before the roof starts).
Do you need a birdsmouth on a rafter?
Structurally, a rafter sitting on an angular bearing point (the inclined plane of the ripped strip) would require that the rafter-plate connection deal with the horizontal and vertical components of the force differently than with a birdsmouth.
How do you nail a birdsmouth rafter?
At the birds’ mouth cut of a rafter connecting to the top plate we use three nails per side, two down through the top of the birds’ mouth into the top plate and one (on either side) of the rafter’s vertical portion of the bird’s mouth cut into the face of the top plate.
How deep can a birds mouth cut be?
The two International Residential Code (IRC) standards that apply to cutting a birdsmouth in a roof rafter are: 1) “Notches at the ends of the member shall not exceed one-fourth the depth of the member” (IRC R502. 8.1). That would be a max of 1-3/8” for a 2×6, 1-3/4” for 2×8, 2-1/4” for 2×10 and 2-3/4” for 2×12.
How big should birdsmouth cut be?
In a properly-cut bird’s mouth, the length of the horizontal or level line (below my two red arrows in the page top photo) should be about equal in length to the width of the top plate.
What is hap on a rafter?
Height Above Plate (HAP) with Heel Cut Depth
You make the height above plate measurement from the wall’s top plate to the top of the rafter along the plumb line. HAP height depends on rafter depth, the heel cut, and the roof’s pitch. As the pitch of the roof increases, so will HAP.
What is the difference between a ridge board and a ridge beam?
Ridge beam sizing is based on the span of the beam between supports, and the amount of roof load supported by the beam. A ridge board is a non-structural member that serves as a prop for opposing rafters to rest against and connect to. Ridge boards can only be used in roofs with slopes from 3 in 12 up to 12 in 12.
What are the rafters fastened to on the top of the wall?
In a sloped roof, common rafters run from the peak of the roof down to the top of the exterior walls of the building. At the peak, the rafters are nailed to a ridge board, and at the top of the wall, rafters are attached to the horizontal top plate of the wall framing.
Do I need blocking between rafters?
Rafter blocking is an essential part of the roof framing process in that the blocks provide a stop for ceiling insulation and a continuous surface for the roof sheathing to be edge nailed to. Each block is 2×4 or 2×6 depending on the size of the rafters, and is secured to the top plate.
What prevents rafters from spreading?
Rafter ties prevent spreading
Located in the bottom third of the roof structure, rafter ties effectively resist the horizontal forces caused by gravity loads that would otherwise cause the roof to pancake, pushing the sidewalls out.
