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Rafter thrust

Simonsays

Simonsays

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Joined
Mar 2, 2010
Messages
72
After removing a flat gypsum board ceiling and the supporting ceiling joists to expose the roof rafters to create a "cathedral ceiling", how can I explain to the applicant that without dealing with the resulting rafter thrust, I won't approve the permit application? They simply don't believe that the wall will begin to move apart. 2019 Residential Code of Ohio (based on 2018 IRC).
 
I have had to demonstrate this several times.

In the field I had a rafter thrust issue over a bonus room. The rafters were supported y a 5' knee wall, framed on top of a floor. Upon my inspection I showed the contractor the obvious bow in the temporary ridge support. I asked him if he thought the bow was there when they framed it. He said no. I then had him measure the floor at the base of the knee wall, then at the top of the knee wall. It was 2" different. I asked if he framed his wall 2" out of plumb. He said no. I asked him if he could figure out what was going on. He said no. I enlightened him.

Other times, I have simply folded a piece of paper in half, to make a little tent, and sat it on my desk. Then I gently push down on the crease as we watch the bottom of the paper kick out.

But in my experience it takes an ability to admit ignorance and accept instruction that some don't have. So I don't try to prove it, I just cite the code.
 
Sifu said:
Other times, I have simply folded a piece of paper in half, to make a little tent, and sat it on my desk. Then I gently push down on the crease as we watch the bottom of the paper kick out.
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THIS^^^^^I usually show with my index fingers and thumbs....
 

R802.5.2​

Where ceiling joists run parallel to rafters and are located in the bottom third of the rafter height, they shall be installed in accordance with Figure R802.4.5 and fastened to rafters in accordance with Table R802.5.2(1). Where the ceiling joists are installed above the bottom third of the rafter height, the ridge shall be designed as a beam in accordance with Section R802.3. Where ceiling joists do not run parallel to rafters, rafters shall be tied across the structure with a rafter tie in accordance with Section R802.5.2.2, or the ridge shall be designed as a beam in accordance with Section R802.3.
 

R802.3​

A ridge board used to connect opposing rafters shall be not less than 1 inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter. Where ceiling joist or rafter ties do not provide continuous ties across the structure as required by Section R802.5.2, the ridge shall be supported by a wall or ridge beam designed in accordance with accepted engineering practice and supported on each end by a wall or column.
 
I hear about this kind of issue, and I immediately wonder how many other bat brained bone headed deficiencies I'll find.
 
Simonsays said:
After removing a flat gypsum board ceiling and the supporting ceiling joists to expose the roof rafters to create a "cathedral ceiling", how can I explain to the applicant that without dealing with the resulting rafter thrust, I won't approve the permit application? They simply don't believe that the wall will begin to move apart. 2019 Residential Code of Ohio (based on 2018 IRC).
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You could show them the section of the code that says if they don't have rafters at the plate height, the ridge has to be designed as a beam.

That would be sections R802.3, R802.5.2 and R802.5.2.2

The ceiling joists have already been removed? Winter's coming. Does it still snow in Ohio? You might consider really getting their attention by condemning the structure.

What's the roof pitch?
 
Thanks for all your responses. Here's what I'm writing the applicant:

Submit sufficient technical data to substantiate the proposed design and to prove that the exterior walls’ performance meets the intent of this code per Section 106.5.3 RCO.

Alterations shall conform to code requirements for new construction and not be made to an existing residential building which will cause the existing building to be in violation of any provisions of this code per Section 113.4 RCO.

Buildings and parts thereof shall safely support all loads including dead loads, live loads, and roof loads as prescribed by this code per Section 301.1 RCO.

List on the drawings the minimum live loads for the existing roof as indicated in Table 301.6 or the snow load as indicated in Table 301.2(1) based upon Figure 301.2(6), whichever is greater per Section 301.6 RCO.

The allowable deflection of the existing exterior walls under the live load listed in Sections 301.5 and 301.6 shall not exceed the values in Table 301.7 per Section 301.7 RCO.
 
If this is intended as a prescriptive design I might keep it simple and cite the prescriptive requirements. Don't have the Ohio building code, and not sure if Yankee is citing a version of the IRC or the RCO but looks like the citations would be the equivalent sections of the IRC I have. If the RCO has the same general requirements as the IRC I would cite the sections that REQUIRE rafter ties or a ridge beam and let them decide if they want to get an engineer to evaluate the design, and provide sealed drawings and calculations to validate whatever they come up with.

No matter what your concerns are valid and I wouldn't mess around. Engineered design, or prescriptive. Prescriptively, they really have no choice but to do one or the other unless they can conjure up some equivalent without an engineer which you would then need to approve (2018 IRC provides this option in 802.5.2.2 but I likely wouldn't go that route).

Following this, would like to read about the outcome so keep it updated.
 
Simonsays said:
Thanks for all your responses. Here's what I'm writing the applicant:

Submit sufficient technical data to substantiate the proposed design and to prove that the exterior walls’ performance meets the intent of this code per Section 106.5.3 RCO.

Alterations shall conform to code requirements for new construction and not be made to an existing residential building which will cause the existing building to be in violation of any provisions of this code per Section 113.4 RCO.

Buildings and parts thereof shall safely support all loads including dead loads, live loads, and roof loads as prescribed by this code per Section 301.1 RCO.

List on the drawings the minimum live loads for the existing roof as indicated in Table 301.6 or the snow load as indicated in Table 301.2(1) based upon Figure 301.2(6), whichever is greater per Section 301.6 RCO.

The allowable deflection of the existing exterior walls under the live load listed in Sections 301.5 and 301.6 shall not exceed the values in Table 301.7 per Section 301.7 RCO.
Click to expand...

But you didn't mention any part of section R802, and that's where the requirement is spelled out.
 
Simonsays said:
Thanks for all your responses. Here's what I'm writing the applicant:

Submit sufficient technical data to substantiate the proposed design and to prove that the exterior walls’ performance meets the intent of this code per Section 106.5.3 RCO.

Alterations shall conform to code requirements for new construction and not be made to an existing residential building which will cause the existing building to be in violation of any provisions of this code per Section 113.4 RCO.

Buildings and parts thereof shall safely support all loads including dead loads, live loads, and roof loads as prescribed by this code per Section 301.1 RCO.

List on the drawings the minimum live loads for the existing roof as indicated in Table 301.6 or the snow load as indicated in Table 301.2(1) based upon Figure 301.2(6), whichever is greater per Section 301.6 RCO.

The allowable deflection of the existing exterior walls under the live load listed in Sections 301.5 and 301.6 shall not exceed the values in Table 301.7 per Section 301.7 RCO.
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The contractor is going to read this and still not know what he needs to do or why. You skipped over the most important section, R802.
 
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