Porch ridge beam

[Sifu]

Please explain. The table in the IRC may leave a lot to be desired, but it is there to provide a prescriptive path for porch beams. It does a poor job of directing which beams, and it is reasonable to explore how it applies. The way I see it, the table is prescribing beam sizes based on 1/2 the tributary load of a given porch. In the general context of the beams per the table, why would there be a difference between a ridge beam supporting 1/2 the load, and a boundary beam supporting 1/2 the load. You advised I should draw it, so I did below. Given a 12' x 12' porch of both configurations, the green beam in each drawing supports 1/2 the tributary load. What am I missing?

 

[redeyedfly]

Please explain. The table in the IRC may leave a lot to be desired, but it is there to provide a prescriptive path for porch beams. It does a poor job of directing which beams, and it is reasonable to explore how it applies. The way I see it, the table is prescribing beam sizes based on 1/2 the tributary load of a given porch. In the general context of the beams per the table, why would there be a difference between a ridge beam supporting 1/2 the load, and a boundary beam supporting 1/2 the load. You advised I should draw it, so I did below. Given a 12' x 12' porch of both configurations, the green beam in each drawing supports 1/2 the tributary load. What am I missing?

View attachment 9526

That's honestly a more complicated way to look at it than the actual calcs. Yes, that might work. But, it is certainly easier to just add up the loads and look up a table that gives you the max load per foot at the required span. The beam doesn't really care where it is, it only cares about the span and the loads (for the most part). Uniformly loaded simple span beams are about as straightforward as it gets. Some of the better load tables, e.g., Microllam, give you the max loading whether it is controlled by strength or deflection to make it even easier.

 

[Mr. Inspector]

Shouldn't the table say it's for shed roofs and not for porch roofs? Does it matter if it is a porch or a carport or sunroom or shed?

 

[Glenn]

I posted a clip from my 3-hr On-Demand Course "Porch Roof Framing" on YouTube. Here you go.

 

[Sifu]

I should have known you had a tutorial. Not quite the same methodology I was using but I think the result is the same. To be frank, I am pretty sure most inspectors/reviewers don't use the table, I have very rarely, because it is a little vague and as stated, very limited. And, very, very few have the need or desire to use 2x material for a ridge beam, as it isn't the aesthetic they are looking for. It is a tool, but not one in the top of the toolbox.

 

[Glenn]

I should have known you had a tutorial. Not quite the same methodology I was using but I think the result is the same. To be frank, I am pretty sure most inspectors/reviewers don't use the table, I have very rarely, because it is a little vague and as stated, very limited. And, very, very few have the need or desire to use 2x material for a ridge beam, as it isn't the aesthetic they are looking for. It is a tool, but not one in the top of the toolbox.

I don't disagree at all, but it's all the code I had available too teach. I have it on my wish list to expand the provisions for prescriptive porch roof construction similar to what we have for deck in 507.

 

[Genduct]

I seem to remember a discussion about the lack of definitions for such things as decks, porches, patios etc. at a code hearing. Not sure it was ever resolved. A porch with a gable is what I was imagining based on the question. All other considerations aside, I still think the table spans and sizes of the beams work for a ridge beam as opposed to a shed type roof. Take a 8' deep x 12' wide shed roof. The outer beam would carry 1/2 the tributary area, and the ledger would carry the other 1/2. Same size gable structure, the ridge beam carries 1/2 the tributary area, and the outer beams carry 1/4 the load each (the table sizes would be oversizing for the outer beams in this case). My guess is that the beams are sized for 1/2 the load, based on a shed configuration. Just depends on whether you want to guess here. If I am right, the table for this porch would require a 2-2x10 ridge beam at a minimum. If I am wrong, sorry for wasting your time. You could go conservative and upsize it. Note that the problems may not be solved. A ridge beam needs support at both ends, and unless a column carries the outer end of the ridge beam down to the foundation, another beam would be required to transfer it, and a beam with a concentrated load would push you out of the table. Disclaimer: as stated, this is interpreting and possibly contorting the code. I am not an engineer, and am not calculating a beam, merely trying to put a somewhat square peg into a somewhat round hole. Just a quick sketch to illustrate the configuration (I ran out of room so I put the section view for the shed inside the plan view).

View attachment 9501

10# dead load + 30# Snow load ft in my area (is about the same for bedroom live load) per SF X 4ft = 160# per lin ft framing
SO what side Headers or Beam would you use to hold it up with a span of ?
You are probably not concerned about deflection value

Does that begin to define the problem

 

[Genduct]

10# dead load + 30# Snow load ft in my area (is about the same for bedroom live load) per SF X 4ft = 160# per lin ft framing
SO what side Headers or Beam would you use to hold it up with a span of ?
You are probably not concerned about deflection value

Does that begin to define the problem

use 1/360 for some extra safety factor

 

[Mr. Inspector]

I don't disagree at all, but it's all the code I had available too teach. I have it on my wish list to expand the provisions for prescriptive porch roof construction similar to what we have for deck in 507.

They need a definition of a "porch roof" before they add anything more to the code.
I don't know why it could not be used by other things besides porches.