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Porch ridge beam

Mr. Inspector

Mr. Inspector

SAWHORSE
Joined
Nov 28, 2009
Messages
4,114
Location
Poconos/eastern PA
On plan reviews I get a lot of porches (covered decks) with only a ridge beam, no rafter ties or ceiling joists. I always ask for engineering for the roof ridge beam but I hate to do this for a small project. Is their a way to figure this out myself?
 
Unless it is prescriptively sized correctly as a beam that supports the roof load, you best avoid designing.
 
In the past I was also a plan checker. Simple stuff like a porch didn’t get to our engineers. You reach a point when you know what will work. It is easy to over engineer most things. If I had a concern about a beam I would ask for a beam that I knew would be sufficient. If the applicant balked I would suggest that they have a choice between the honking big beam that I asked for or hire an engineer.
 
The 3 local lumber stores (not the big box stores) around here have the software programs from the manufacture of the engineered products they sell and will provide the documentation for the size and type of beam required. We just verify the information used within the input of the software program is correct.
Now if they are going to use dimensional lumber that will require an engineer. We also use StruCalc https://www.cesdb.com/strucalc.html to verify sizes but never design or provide a corrected size for them.
 
I understand the idea and desire to serve the applicant in the best and most responsible way possible. You could possibly use 2018 IRC t602.7(3) for porch girders with a little contortion if you were so inclined. Since the table is for girders presumably loaded with 1/2 the roof for a non-beam roof, I could probably work in some cases. It might provide you with enough justification to pass on something oversized. Of course, the next question is how to figure a timber equivalent, which is what most folks want to use for these type of porches. For that, you could probably figure that a 2-2x6 is probably weaker than a 3x6 timber of equivalent grade and species and go from there.
 
Sifu said:
For that, you could probably figure that a 2-2x6 is probably weaker than a 3x6 timber of equivalent grade and species and go from there.
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Maybe just let the engineers do it. 2-2x6 is stronger than a 3x6. That's how LVLs work but with thinner laminations.
The porch roof beam table could work but you would need to double the size of the beam listed. You certainly cannot double the span listed. Using that method all your ridge beams would be a minimum of (4) laminations.
 
I had the same though but I'm not so sure. If the girder is presumed to carry one half of a non-ridge beam rafter span, it would be the same as a ridge beam carrying half the load on either side of the ridge beam. My thoughts were the same though, but instead of doubling the laminations, if you interpreted it that way you could simply cut the allowable span in half. So a 2-2x6 would be good for a 3'9 span on an 8' roof. Same result, just different application. Not sure I would consider much difference in the built-up beam vs. the solid timber though. It is true that a properly laminated beam might be stronger, but in practicality I don't think many get properly laminated. This is all subject to a lot of flexibility, so if not sure you could always use some reduction factor.
 
Sifu said:
I had the same though but I'm not so sure. If the girder is presumed to carry one half of a non-ridge beam rafter span, it would be the same as a ridge beam carrying half the load on either side of the ridge beam. My thoughts were the same though, but instead of doubling the laminations, if you interpreted it that way you could simply cut the allowable span in half. So a 2-2x6 would be good for a 3'9 span on an 8' roof. Same result, just different application. Not sure I would consider much difference in the built-up beam vs. the solid timber though. It is true that a properly laminated beam might be stronger, but in practicality I don't think many get properly laminated. This is all subject to a lot of flexibility, so if not sure you could always use some reduction factor.
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Draw a picture of the tributary area. The ridge beam is carrying half the span on either side, so double what the header on the wall side is carrying.
Cutting the allowable span in half would be extraordinarily conservative. Beam strength is proportional to the square of the span and the deflection (which very often controls the design in wood) is to the fourth power of the span.
Relatively few fasteners are needed to get full strength of the laminated beam and if the rafters are bearing on top of all of laminations the fasteners aren't really necessary.

I am so sure.
 
Sifu said:
I understand the idea and desire to serve the applicant in the best and most responsible way possible. You could possibly use 2018 IRC t602.7(3) for porch girders with a little contortion if you were so inclined.
Click to expand...

This is what I teach in my On-Demand Course "Porch Roof Framing". You can definitely adapt that table to use for VERY LIMITED applications of ridge beam design.
I have been asked by many in the deck industry to create new prescriptive design codes for various porch framing designs, specifically enhancing the porch girder table and adding a center ridge beam to it. However, I know of no one working on that.

The cool thing about prescriptive design in the codes is it's only limited to what people propose for it. And what's approved... of course!
 
Under the the table for porch girder spans it says the porch depth is measured horizontally from the building face to the centerline of the header.
1. What is a "building Face?
2. By center line of the header do they mean the header center's length or the width?
3. Also why are spans different for porches than the girder spans for exterior walls tables for houses?
4. Are you allowed to use the girder spans for exterior walls tables for porches if the measurement from the header to the building face is more then the maximum of 14' on the porch table?
 
Rick18071 said:
Under the the table for porch girder spans it says the porch depth is measured horizontally from the building face to the centerline of the header.
1. What is a "building Face?
2. By center line of the header do they mean the header center's length or the width?
3. Also why are spans different for porches than the girder spans for exterior walls tables for houses?
4. Are you allowed to use the girder spans for exterior walls tables for porches if the measurement from the header to the building face is more then the maximum of 14' on the porch table?
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I saw the "building face" as well and realized they may be applying the table to a shed type roof beam at the outer edge only. The ridge beam question sent me in a gable style roof direction, which is what I was imagining. There is a lot to unpack with this table, I think it would be the location of the ledger on which the rafter hang...but if so, what about a ledger connection? Just use the deck ledger tables? Like Glenn said, the applications are limited, and probably take more imagination than it is worth in some cases.
 
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).

1663162872097.png
 
Sifu said:
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
Click to expand...
Table R602.7(3) sizes the header for the bottom picture.
 
redeyedfly said:
For the walls? Yes.
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For all. In a shed roof design, the outer beam carries 1/2 the tributary load. For a ridge beam design, isn't the ridge beam carrying 1/2 the tributary load? So if it is agreed the table is designed foe a shed roof outer beam carrying 1/2 the load, would it not also stand to reason that a ridge beam carrying 1/2 the load could also be covered by this table?
 
Sifu said:
For all. In a shed roof design, the outer beam carries 1/2 the tributary load. For a ridge beam design, isn't the ridge beam carrying 1/2 the tributary load? So if it is agreed the table is designed foe a shed roof outer beam carrying 1/2 the load, would it not also stand to reason that a ridge beam carrying 1/2 the load could also be covered by this table?
Click to expand...
In the ridge beam arrangement above, each of the walls are carrying 1/4 of the total roof load, the ridge 1/2.
 
I would personally be hesitant to require engineering for a simple ridge beam unless the span or loading were outside of my design guides.
The Canadian Wood Council has a handy guide called "the span book" and has detailed prescriptive requirements for joists, rafters, lintels, floor beams, ridge beams, etc. that gives much more detail than we can get from our code books. It is recognized by our codes.
Is there not something similar down there?
 
Exactly. So if the table is meant to apply to a beam carrying 1/2 the load, it should work for either configuration.
 
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