Rafter Bearing

[Mark K]

The AF&PA NDS says use a distance of D from the support. The NDS does not address the situation where the beam is installed on a slope.

Using the methodology proposed would underestimate the shear when there is a steep slope.. Check the shear when the slope is 1 to 1. Since the slope is 45 degrees the line would never intersect with the top of the beam.

I would recommend caution when dealing with a slopped roof.

 

[Paul Sweet]

"most old-school carpenters don't goof on rafters, because they were taught right and know what they are doing."

Unfortunately many old-school carpenters learned their trade when ceilings were uninsulated and snow melted off the roof before much could accumulate, at least here in the South.

 

[Inspector Gift]

A structural engineer that I trust told me to measure some distance away from the edge of the bearing point as I have depicted in the drawing. The explanation given is that dimensional lumber does not shear in a straight line. DL has fibers that bend and ultimately slide apart. The section derived at the point shown is the section to use for bearing properties.

View attachment 10331

ICE, I wouldn't approve what you illustrated above. The tapered rafter end is the limited to 25% (D/4) of the rafter. The Residential Code, Figures R802.7.1.2 and R802.7.1.1 illustrate the D/4 maximum allowable cut. What your illustrations shows is closer to 30% depth of cut measured at the inside face of bearing.

 

[ICE]

ICE, I wouldn't approve what you illustrated above. The tapered rafter end is the limited to 25% (D/4) of the rafter. The Residential Code, Figures R802.7.1.2 and R802.7.1.1 illustrate the D/4 maximum allowable cut. What your illustrations shows is closer to 30% depth of cut measured at the inside face of bearing.

What I drew is not to any scale nor is it a real world example. I am aware of the code and have posted the IRC illustration below.

What I have done is provide a picture and a description of the dimensions to the County engineer and find out if it will work. Also note that the illustration has a cantilevered rafter tail which I wouldn't attempt to justify beyond what the code allows.







R802.7.1.2 Ceiling joist taper cut. Taper cuts at the ends of the ceiling joist shall not exceed one-fourth the depth of the member in accordance with Figure R802.7.1.2.

Not to stretch this out too far but did you notice that the illustration has the heel of the notch not bearing?

 

[Paul Sweet]

A birdsmouth is the same as a notch or a cut. IRC R502.8.1 limits end cuts in sawn lumber to 1/4 the joist depth, and R802.7.1 limits cuts, notches, & holes in sawn roof framing to the same.

R802.7.2 prohibits cuts & notches in engineered lumber except "where permitted by the manufacturer's recommendations or where the effects of such alterations are specifically considered in the design of the member by a registered design professional."

 

[chief-inspector]

The shear capacity is reduced but the moment max is at center span so thats not effected. It is a frowned on practice unless the remaining depth can carry the load per the charts.