Rafter Bearing

[BN4537]

It's usually not bad framing, aside from this violation.

The main issue that I had when I posted this originally in 2021 was that the builder was telling us that all these other jurisdictions were approving it, the engineer was telling him that we were overreacting because there is not much load on hip rafters, and the builder was convinced that he was doing everything correctly and we were the problem in the equation. This thread was primarily an effort to demonstrate that this is a very normal national standard we are enforcing.

 

[north star]

@ > > >

Now in `23, ...what does your Bldg. Official,
or the city leadership want to do ?

< < < @

 

[BN4537]

The stamp on the lumber is specific to the grade, it does not regulate the cutting of the lumber. If a No.2 DF 2x10 were reduced in dimension, the grade is still the same. As for removing the stamp, if the inspector is wanting to see the stamp, be sure to cut so as to leave it on the rafter. Or be sure to safe the pieces with the stamp to show the inspector. The Building Official is the one who has ultimate decision on the matter. (R104.11)

This was said some time ago, and it is mostly true. However, I heard recently that part of a piece of lumber's grade has to do with the distance between the edge of the board and any knots in the board. If you rip the lumber down, you get a lot closer to some of those knots, which may reduce the grade. A DF #2 2x10 might become a DF #3 2x6 if you get too close to a knot, which is relevant to the span in some instances.

@ > > >

Now in `23, ...what does your Bldg. Official,
or the city leadership want to do ?

< < < @

Let sleeping dogs lie. If the builder prefers to pay an engineer every time instead of building prescriptively, it is not our choice or our business.

 

[ICE]

Does it come down to whether there is sufficient section of modulus to resist shearing at the support?

 

[Mark K]

This contractor uses PSL sometimes, regular SPF other times, but notches/cuts like this all of the time. He told me that it was cheaper to use shorter lumber cut this way and pay an engineer than to get longer lumber and bear prescriptively. But he's getting tired of paying an engineer to come look at every house he builds...

So, I was always told that bearing on the toe of the cut and not on the heel is non-compliant, but what code section says that rafters have to have full bearing? I'm kind of stuck for a code section that specifically addresses this problem, I have been using R801.2 (Roof construction need to be strong enough to support all the loads), and 806.2 just says that the ends of each rafter shall have 1 1/2" of bearing.

All of the pictures shown have 1 1/2" bearing, just not where it supports the full rafter.

If someone here knows a better section or sections to reference, I would appreciate it - I would like something a little more substantial to work with.

The way I understand the IRC is if the engineer can address the concern using the provisions in the IBC and the standards referenced from the IBC there is no need to comply with the prescriptive provision in the IRC.

 

[Mark K]

This was said some time ago, and it is mostly true. However, I heard recently that part of a piece of lumber's grade has to do with the distance between the edge of the board and any knots in the board. If you rip the lumber down, you get a lot closer to some of those knots, which may reduce the grade. A DF #2 2x10 might become a DF #3 2x6 if you get too close to a knot, which is relevant to the span in some instances.

Let sleeping dogs lie. If the builder prefers to pay an engineer every time instead of building prescriptively, it is not our choice or our business.

While there is an element of truth in the concern it might make sense to consider: Are you willing to require that whenever a piece of lumber is cut that it be regraded. Also consider all of the buildings that were constructed with lumber whose grade was changed as a result of the way it was cut. I suggest that this issue be very very low on your enforcement priorities.

 

[Mark K]

While there is an element of truth in the concern it might make sense to consider: Are you willing to require that whenever a piece of lumber is cut that it be regraded. Also consider all of the buildings that were constructed with lumber whose grade was changed as a result of the way it was cut. I suggest that this issue be very very low on your enforcement priorities.

If the member is engineered lumber then the grading rules used to assign a grade to sawn lumber do not apply.

 

[ICE]

If the member is engineered lumber then the grading rules used to assign a grade to sawn lumber do not apply.

True enough. What say you to engineered lumber that is re-sawn or ripped?

You being an engineer, I was expecting a reaction to the statement that I made in post #28 (Does it come down to whether there is sufficient section of modulus to resist shearing at the support?).

Perhaps you would be willing to educate us on what and at what location that measurement is taken in relation to the support. Of course that would require multiple case scenarios as engineered lumber lacks the structure of sawn lumber.

Understand that I am not twisting your nose here. No, not at all. I am offering an olive branch with the hope that you will embrace a magnanimous attitude and help us to understand.

Many dozens of times I have placed the speed square to find the location and then presented the finding to the County engineer.. However I am hoping that you will join the team and explain the procedure so that the rest of the inspectors will know what, where and why.

A simple request from a simple inspector.

 

[Mark K]

True enough. What say you to engineered lumber that is re-sawn or ripped?

You being an engineer, I was expecting a reaction to the statement that I made in post #28 (Does it come down to whether there is sufficient section of modulus to resist shearing at the support?).

Perhaps you would be willing to educate us on what and at what location that measurement is taken in relation to the support. Of course that would require multiple case scenarios as engineered lumber lacks the structure of sawn lumber.

Understand that I am not twisting your nose here. No, not at all. I am offering an olive branch with the hope that you will embrace a magnanimous attitude and help us to understand.

Many dozens of times I have placed the speed square to find the location and then presented the finding to the County engineer.. However I am hoping that you will join the team and explain the procedure so that the rest of the inspectors will know what, where and why.

A simple request from a simple inspector.

Re-sawn or ripped members have the same material properties as the original members.

Section modulus is a property of a section that is used to access the bending stresses and is not used to access the shear at the support. Shear capacity is based on the area of the section.

The treatment of notches at the ends of members is complex. Rather than trying to provide sound bites which can be easily misinterpreted my recommendation is that you take a course in wood design. Buy a copy of the "National Design Specification" (NDS) which the IBC wood provisions reference. Also buy a copy of the AITC "Timber Construction Manual"

When faced with notches greater than addressed by the NDS the notch may be justified if the stresses are low enough or if the section is reinforced. Sometimes special connectors are fabricated that eliminate the need for notches. These special connections cannot be addressed in the space available

 

[ICE]

Re-sawn or ripped members have the same material properties as the original members.

What do you mean by "same material properties"? Does that mean that the member will react the same with less capacity to resist?

Section modulus is a property of a section that is used to access the bending stresses and is not used to access the shear at the support. Shear capacity is based on the area of the section.

Thanks for straightening me out on the use of the word modulus. Now that you have clarified that a section area is identified as the basis for the member's capacity to resist shear could you explain the presumed location of the section in relation to the support?

The treatment of notches at the ends of members is complex.

While I respect the sentiment, a basic explanation of this one facet of the design could go a long way in demonstrating that the amount of lumber needed to resist shear is demonstrably different than that which is required to resist bending.....that being the only topic of this thread. Trust me on this....nobody is going into business with this limited amount of instruction.

my recommendation is that you take a course in wood design. Buy a copy of the "National Design Specification" . Also buy a copy of the AITC "Timber Construction Manual"

That's swatting flies with a shotgun. Hundreds of pages of stuff that I wouldn't understand searching for that one page that I would.

When faced with notches greater than addressed by the NDS the notch may be justified if the stresses are low enough or if the section is reinforced. Sometimes special connectors are fabricated that eliminate the need for notches. These special connections cannot be addressed in the space available

You went off on a tangent. First of all I don't justify anything and I don't suggest that anyone other than an engineer fill those shoes. What I do is measure the "section" and provide that information to the engineer. I was hoping to engage you in meaningful discussion as a way to show others the possibilities.....and it would mean so much more coming from an engineer.

 

[bill1952]

I believe vertical shear is highest at the edge of the bearing. Just draw a vertical line up from inside of top plate. But that's just what I recall from structures classes - in which I used a slide rule since calculators were not common.

 

[ICE]

I believe vertical shear is highest at the edge of the bearing. Just draw a vertical line up from inside of top plate. But that's just what I recall from structures classes - in which I used a slide rule since calculators were not common.

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?

 

[bill1952]

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?

I'm not an engineer either, but I associate the fibers sliding with horizontal shear, which is greatest at the center (uniformly load).

What I recall from doing the calcs is that in wood construction, vertical shear is rarely a limitation. I guess in heavily loaded very short spans, where the cross section may be small because bending moment is small, vertical shear may be an issue.

Relatively more important in steel iirc.

 

[ICE]

I'm not an engineer either

This is why I tried to enlist an actual engineer to help with understanding how lumber fails at the support. I just asked the wrong engineer. So go with what you know.

 

[bill1952]

Well, when in doubt build it stout.

We don't seem to have a big problem in this country with structural failures in wood framed houses. Most today are built much more robustly than the still standing 100 year old ones. It seems that fasteners are at fault more than the properties of and given wood structural member.

 

[Mark K]

What do you mean by "same material properties"? Does that mean that the member will react the same with less capacity to resist?


Thanks for straightening me out on the use of the word modulus. Now that you have clarified that a section area is identified as the basis for the member's capacity to resist shear could you explain the presumed location of the section in relation to the support?


While I respect the sentiment, a basic explanation of this one facet of the design could go a long way in demonstrating that the amount of lumber needed to resist shear is demonstrably different than that which is required to resist bending.....that being the only topic of this thread. Trust me on this....nobody is going into business with this limited amount of instruction.


That's swatting flies with a shotgun. Hundreds of pages of stuff that I wouldn't understand searching for that one page that I would.


You went off on a tangent. First of all I don't justify anything and I don't suggest that anyone other than an engineer fill those shoes. What I do is measure the "section" and provide that information to the engineer. I was hoping to engage you in meaningful discussion as a way to show others the possibilities.....and it would mean so much more coming from an engineer.

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.

 

[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.