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Rafters 4 feet on center

I

Inspector 102

Silver Member
Joined
Oct 22, 2009
Messages
357
Location
N/E Indiana
An owner wants to put a lean-to onto an existing building. He has proposed rafter at 4 foot on center with 2x4 flat purlins for roof decking. Since there are no guidelines in the Residential Building Code, is there any available standards for this. Seems senseless to hire engineer for a 10 foot by 24 foot lean-to. Understand saving money, but also suggested going 24 inch on center to avoid hassles.
 
I would not be comfortable walking on flat 2"x4" decking spanning 4 ft. unless it was tongue and groove. That might be hard to find. Even with T&G ....at most the span should be limited to 36". There's not many fat roofers but there's plenty of fat inspectors.
 
Our Canadian codes work with several structural assumptions, including light framing with repetitive members spaced no more that 24" O/C. Otherwise you are into engineering. I would assume the other major codes are the same.
Rafters spanned 3' or 4' O/C is pretty standard, with structural T&G decking for timber frame or other non-standard construction. But again, to be calculated by an engineer.
I agree engineering for a 10' x24' lean-to seems overkill, so the owner should frame with 24" O/C to avoid the pain.
 
Inspector 102 said:
An owner wants to put a lean-to onto an existing building. He has proposed rafter at 4 foot on center with 2x4 flat purlins for roof decking. Since there are no guidelines in the Residential Building Code, is there any available standards for this. Seems senseless to hire engineer for a 10 foot by 24 foot lean-to. Understand saving money, but also suggested going 24 inch on center to avoid hassles.
Click to expand...
Swami says this ain't anywhere that has snow...
 
WHAT IS THE SPAN RAING OF THE SHEATHING?

2020 Indiana Residential Code
Section R107 General Workmanship
All construction methods shall be accepted practices to ensure livable and safe housing and shall demonstrate acceptable workmanship.

Four feet is not Generally accepted practice, not conventional framing, therefore shall be engineered.
 
mark handler said:
Four feet is not Generally accepted practice
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Yet nominal 3/4" sheathing typically has an APA span rating of 48/24, meaning that it can be used as roof sheathing with supports 48" on center (strength axis to run perpendicular to the supports as usual). So seems to me that the APA considers it generally accepted.

Cheers, Wayne
 
bill1952 said:
Even though TABLE R803.1 permits 48 to 72" with 1 1/2 t&g depending on grade/species? Seems like 72" is not hard.
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Proof that the code is the minimum allowed. Remember that the OP wants to use 2"x4". Of course the table does not specify a minimum width so perhaps 1.5" T&G is the deciding factor irrespective of any other dimension.

Note that I said that the span "should" be limited to 36"...not that it "is" limited to 36". Granted that is semantics and to be honest, I didn't trouble myself with what the code said about it. Having been on many 1.5" T&G roofs with wide spans I have no problem with 2'x6" pine sheathing. There's even the 4" thick crude board which I do not like.

Screen Shot 2023-07-18 at 8.34.45 AM.png
 
wwhitney said:
Yet nominal 3/4" sheathing typically has an APA span rating of 48/24, meaning that it can be used as roof sheathing with supports 48" on center (strength axis to run perpendicular to the supports as usual). So seems to me that the APA considers it generally accepted.

Cheers, Wayne
Click to expand...
3/4 sheathing can span 48" under what loading? With all respect, the fact that sheathing can span X distance does not make the spacing of rafters or joists standard. This is my view from snow country.
 
For clarification, the rafters are intended to be 2x10 spaced 48" on center, with 2x4 purlins nailed on top of the 2x10 rafters, then 1/2 OSB decking applied to the purlins.
 
If there is no prescriptive option in the code, the builder must "pick his poison". Either select a prescriptive option, or hire an engineer.
 
Plumb-bob said:
3/4 sheathing can span 48" under what loading?
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Good question. APA publishes Q225, which gives the load ratings in psf for various types of plywood with various support spacings.

APA Publication Search - APA – The Engineered Wood Association

Search for publications, videos, CAD details, images, and other materials in the APA Resource Library.
www.apawood.org www.apawood.org

So for example, if you have a 10 psf dead load, and a 50 psf snow load, with a L/240 deflection criterion, then for supports 48" o.c., you couldn't use "Plywood Sheathing 48/24" (only 39 psf in bending), but could use "Plywood Structural I Sheathing 48/24".

Cheers, Wayne
 
wwhitney said:
Good question. APA publishes Q225, which gives the load ratings in psf for various types of plywood with various support spacings.

APA Publication Search - APA – The Engineered Wood Association

Search for publications, videos, CAD details, images, and other materials in the APA Resource Library.
www.apawood.org www.apawood.org

So for example, if you have a 10 psf dead load, and a 50 psf snow load, with a L/240 deflection criterion, then for supports 48" o.c., you couldn't use "Plywood Sheathing 48/24" (only 39 psf in bending), but could use "Plywood Structural I Sheathing 48/24".

Cheers, Wayne
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I have never seen it and it would make me uncomfortable, but maybe it would work. But with our code it would require engineering based on spacing of structural members.
 
Inspector 102 said:
For clarification, the rafters are intended to be 2x10 spaced 48" on center, with 2x4 purlins nailed on top of the 2x10 rafters, then 1/2 OSB decking applied to the purlins.
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On the rafters, what is the dead load and what is the snow load?

If the dead load is 10 psf or less, and the snow load is 20 psf or less, then IRC Table R502.3.1(2) tells you than any of the species in #2 or better can span over 10' with 20 psf dead, 40 psf live, 24" o.c. spacing, and L/360 deflection limit. If you halve the loading and double the spacing, the load per member is identical. So the 2x10 rafters with 10 psf dead, 20 psf live, and 48" o.c. should span 10' (and be stiffer than typically required, as the deflection limit is L/360, not the L/240 or L/180 of the rafter span tables).

If the snow load is higher, then you'd need to check something like the WFCM to see if it has an applicable span table, with or with out the load doubling trick.

Spanning between the rafters is a separate check, and the info so far in this thread has been that 2x T&G decking can span 48", or that sufficiently thick plywood or OSB could work. But I'm not aware of a prescriptive table that covers the combination of some 2x purlins plus thinner OSB sheathing.

Cheers, Wayne
 
48" rafter spacing exceeds numbers set in section R802.4.
Section R802.4 states: "Rafters shall be in accordance with this section."

It exceeds the conventional framing parameters set forth in the text and in the tables.
What is the load-carrying capability and deflection of a 2x4 w/ span and a 48" spacing?
What is the grade of the lumber?
What live loads do you have in your area? Snow? wind?
Species of wood? All species are not created equal.
Lumber grade? Modulus of Elasticity?
Deflection?
 
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In CA this would be a moot issue: R105.2 Work Exempt From Permit:
one-story detached accessory structures, provided that the floor area does not exceed 120 square feet.
 
mark handler said:
48" rafter spacing exceeds numbers set in section R802.4.
Section R802.4 states: "Rafters shall be in accordance with this section."
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OK, but 2021 R802.4.1 says " For other grades and species and for other loading conditions, refer to the AWC STJR." https://awc.org/codes-and-standards/span-tables/

That document provides rafter span tables for loading up to 20 psf dead and 50 psf live, with a choice of L/180 or L240 deflection criterion.. And Appendix A.3 of that document tells you that the allowable span is based on, among other things, the load per unit length on the rafter, which it says is based on the psf loading and the member spacing. Which makes it clear that if you double the member spacing and halve the load, you'll get the same load per unit length and the same allowable span.

Use of span tables from the WFCM is allowed per R301.1.1, although I haven't checked if it has an prescriptive tables that would be helpful for 48" o.c. rafter spacing. https://awc.org/publications/2018-wfcm/

mark handler said:
What is the load-carrying capability and deflection of a 2x4 w/ span and a 48" spacing?
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I agree that the flat-wise 2x4 purlins are outside the scope of the prescriptive design allowances I'm aware of and would likely need to be eliminated to prescriptively design the roof in the OP, in favor of one of the prescriptive options per Table R803.1 or Table R503.2.1.1(1).

As to the rest of your questions, those are all things you need to know to apply the span tables found in the prescriptive allowances.

Cheers, Wayne
 
Guessing purlins are 16" on center?

Give the owner/builder the option of building it as planned and load up a bay with tube sand or other weight to design snow loading. Or 25 or 50% over. You can measure deflection with a straight edge and a tape measure.
 
I am amazed at the knowledge on this site and the willingness to get into spirited debates. After discussion with the owner, he determined that he would follow the "Kings Domain" theory and take the chance on 4 foot centers. I told him my inspection notes would indicate the modification and not receive approval. At the end of the day, with 30# snow loading, I believe the deflection will be within acceptable ranges. The owner agreed to allow me to do some load testing as mentioned, just to find out what the results are. I will try to remember to post any results obtained.
 
Inspector 102 said:
I am amazed at the knowledge on this site and the willingness to get into spirited debates. After discussion with the owner, he determined that he would follow the "Kings Domain" theory and take the chance on 4 foot centers. I told him my inspection notes would indicate the modification and not receive approval. At the end of the day, with 30# snow loading, I believe the deflection will be within acceptable ranges. The owner agreed to allow me to do some load testing as mentioned, just to find out what the results are. I will try to remember to post any results obtained.
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I think you're taking a risk.

If you're looking at 30psf snowload on a 10-foot span, my handy-dandy span table book for Canada (where we kinda know snow) says the bare minimums for rafters are
2x4: 12" spacing
2x6 or better: 24" spacing.
 
Inspector Gadget said:
I think you're taking a risk.

If you're looking at 30psf snowload on a 10-foot span, my handy-dandy span table book for Canada (where we kinda know snow) says the bare minimums for rafters are
2x4: 12" spacing
2x6 or better: 24" spacing.
Click to expand...
Keep in mind the grading rules for US lumber is lower than Canadian, so they might have to further decrease the spans/spacing.
 
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