Rafter Bearing

[Mark K]

I was taught a different location that involves a 45° angle. It has to do with the fibers sliding as opposed to failing in a vertical line. But hey now, I'm no engineer so what do I know?

The 45 degree angle from the support is often used when there is a concentrated load near the support, to indicate when horizontal shear is of lesser concern and where it may be necessary to check bearing stress perpendicular to the grain.

 

[ICE]

These code provisions were developed by engineers and not by carpenters. Thus, to understand the code provisions it is necessary to understand how engineers use the various terms. The previous comments would suggest that we would have to start at the beginning. This is why I suggest taking a course in wood design.

This forum is not the place to provide such material. If I were to provide the requested equivalent to sound bites I fear that what I said would be misinterpreted. The confusion regarding the terms "material properties" and capacity support my belief that more background material is needed.

I find it interesting to contrast the previous statements to the effect that engineers are not necessary with the need for an engineer's opinion.

More fundamentally is the question of what is the inspector's role? The inspector's role is to verify compliance with the approved permit documents. While the inspector may encounter situations where it appears that a code provision has been violated such situations should be brought to the attention of the plan checker. Except for very limited circumstances an inspector will not have the information nor the background to make a definitive determination that the notch at a support is a code violation.

Telling me that I am not smart enough to understand a real answer equates to you not being smart enough to get the information across.

 

[Mark K]

Telling me that I am not smart enough to understand a real answer equates to you not being smart enough to get the information across.

Somebody is defensive.

I am not saying you could not learn. I am simply stating that you do not currently have the necessary knowledge and that this forum is not the place to address that issue.

 

[ICE]

Somebody is defensive.

I thought that I would try that rather than the usual offensive. Didn't make a difference.

 

[Inspector Gadget]

I've dealt with this a few times. Canadian Codes are a bit different, but the way I've dealt with this is that (a) the initial point of bearing must be full width, ie: the birdsmouth doesn't start until the top plate; and if this is not the case then (b) if the birdsmouth begins before the top plate, I'll take the width of the rafter at the point it starts at the top plate and calculate for a contiguous rafter of that size.
In other words, if you hack a 2x6 down to 2x4 size at the start of the top plate then I'll calculate a snow load based on a 2x4 throughout.

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

My concern with with the original post is that the lintel bearing the weight of the rafters appears unsupported.

 

[Energystar]

The solution I have seen used in this situation is to nail a chunk of LVL material with 45 degree cuts on each end across the corner on top of the walls for the hip rafter to bear on.

 

[Inspector Gadget]

The solution I have seen used in this situation is to nail a chunk of LVL material with 45 degree cuts on each end across the corner on top of the walls for the hip rafter to bear on.

I wouldn't accept that, for several reasons.... Not the least of which is that my Codes don't specify this as suitable for bearing loads, but also the fact that now you're trying to carry loads across two sets of connections with a rotational element that will ultimately apply force in a plane roughly parallel to the shaft of the nail. Nails work better in a shear plane. Supports work best at 90 degrees to the (in this case) horizontal element.

 

[ICE]

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.

 

[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."