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Old 03-31-2012, 06:36 PM   #1
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Roof deck framing and strength?


Hi and thanks in advance.

*** original post has been edited to delete some unrelated info and ease some concerns that were voiced regarding some of the questions I had asked so some of the answers below may not make sense****

What you see in this picture is a cross section of the upper floor back half of our house ..

The "Ripped 2x12 @ 16" OC" surface is a deck .. the deck is 16' wide by 60' long running the lenth of the house..

The first and second floor of the home has 2x10 floor joists and that got me thinking..

He have had large parties at the house but never on the roof deck as we tore out the interior stairs that were used to get to it when we remodeled the inside ....We are now stripping outside to studs and part of that involves exterior stairs to the roof.

My question is as follows:

How do you calculate the strength of ripped boards (ie the deck floor) to understand how they compare to standard size lumber?

We have had 20 people in our living room supported by 2x10 floor joists and obviously the house did not cave in ...

I am trying to figure out how the ripped deck floor would compare to 2x10 floor joists in terms of holding weight?

I hope that makes sense and thanks for any feedback







Last edited by beachfront71; 04-01-2012 at 12:19 PM.
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Old 03-31-2012, 07:48 PM   #2
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Roof deck framing and strength?


Seriously, this type of question is way beyond a DIY chat forum. The only people on this forum qualified to address such a difficult question are the registered professional engineers and architects on here, and we are all legally prohibited from discussing this matter with you due to professional ethics clauses in our licenses that prohibit us from performing engineering in states where we are not licensed, and for people with whom we do not have a contractual relationship, and where we have not actually seen the project.

This leaves you with the folks on here who are not professionally qualified to answer such a difficult structural question. So if one of them answers, are you really going to take their advice? And if your party turns into a catastrophe, are you planning to sue them for giving you bad advice? You need to discuss the load capacity of your roof deck with the original builder, or perhaps the local building inspector, or hire an engineer or architect to perform a site visit.

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Old 03-31-2012, 09:26 PM   #3
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Roof deck framing and strength?


I can see a bunch of flaws, I agee you need someone on site to look this one over.
Is the 1/2 subflooring already in a finished part of the building, or is it yet to be built?
1/2" is to thin for a proper subfloor. Adding the thicker plywood over it will do little good.
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Old 03-31-2012, 09:48 PM   #4
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Roof deck framing and strength?


Thanks, and yes I understand this may go above and beyond what a forum would provide..

We do have architects, a contactor and a SEngineer working on the project as we speak..

I was just looking for opinions to compare and take notes from.

I have come here in the past and recieved good information to be used as discussion topics when dealing with the local people actually working on the home ... As you know, there may be more than one way to accomplish different tasks and the person you/I hire may not always have the best solution.

Last edited by beachfront71; 04-01-2012 at 12:17 PM.
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Old 03-31-2012, 09:52 PM   #5
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Roof deck framing and strength?


I'm talking about the subfloors in the main house not the roof sheathing.
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Old 03-31-2012, 10:00 PM   #6
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Roof deck framing and strength?


THanks,

The subfloors are actually 3/4 ply .... we know this because we installed hardwoods and had to replace some of the subfloor on both levels.

The picture I included was not the final approved by the city but one of the few that show how the roof deck was structured ...

Last edited by beachfront71; 04-01-2012 at 11:58 AM.
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Old 04-01-2012, 11:15 AM   #7
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Roof deck framing and strength?


[quote=joecaption;889392]
1/2" is to thin for a proper subfloor.

Huh. I've got nothing against over-engineering, but purely going on code, I believe 1/2" subfloor is technically sufficient for 16" spaced joists. See table R503.2.1.1 in IRC (2006).
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Old 04-01-2012, 11:49 AM   #8
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Roof deck framing and strength?


What I have found is by using 1/2 first it ends up acting like shims on top of the joist and sags or flexs between them.
With at least 3/4" you can use T & G and end up with a far stronger floor.
It seemed to be a populer way to build houses in the 70's. I've seen linolium floors that looked like waves, hardwood floors that squecked like crazy from all the movement under the floors.
Just made no since to me to install the weakest subfloor first then the strongest on top of that. Seems backwards to me.
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Old 04-01-2012, 12:18 PM   #9
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Roof deck framing and strength?


Tried to revise the original topic to ease some of Daniels concerns.

Also have confirmed the roof deck joists have a 3/4 plywood subfloor on top of them .. the picture I included has the wrong info regarding this..
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Old 04-01-2012, 02:41 PM   #10
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Roof deck framing and strength?


I am still puzzled why you ask such a technical question on an internet chat forum, when you state that you have two architects and a structural engineer working on your house. Clearly they are the people who should be addressing your question.

I will address the technical issue of how you compute the strength of a tapered structural element under bending load (that is what you have). By the way, I am NOT going to do the computations for you, I have already explained why. I am simply going to tell you the procedure. This is definitely way beyond DIY, the method is hard enough to explain, it definitely requires structural knowledge to apply.

1. Compute the moment along the tapered beam (joist). This requires detailed knowledge of point loads and distributed loads, and the end conditions of the joist. Usually you get the required loads from code, but in some cases the actual load to be applied will exceed code, and you ALWAYS use the more restrictive case.

2. Determine if Bernoulli-Euler (BE) bending moment theory is applicable to your beam. This is determined by the taper of the beam. If BE theory is applicable, go on to step 3, else you have to perform finite element analysis on the beam.

3. You are here because BE theory is applicable. You need to determine the section along the beam where the extreme fiber stress is at a maximum. This is probably NOT going to be at the midpoint of the beam, even though the moment is maximum there, because the moment of inertia decreases rapidly as you move from the maximum depth part of the tapered beam to the minimum depth part of the beam. The most severe stress is likely to occur partway between the midpoint of the beam and the thinnest tapered side of the beam. You must compute the moment and stress at several points along the beam, and figure out where the worst case is.

4. Once you have computed the worst case, you determine if the beam can handle the worst case stress.

This is what your engineer is going to do. They may also perform a worst case computation based on the thinnest part of the beam, however that would be very conservative, and not very accurate.
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Old 04-01-2012, 03:08 PM   #11
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Roof deck framing and strength?


Quote:
I am still puzzled why you ask such a technical question on an internet chat forum
The same reason everyone else comes on here which is to get feedback and opinions.

I do have the architects working on it and prob wont hear back from them for a few weeks so I asked the question.


I am basically asking if a ripped 2x12 is stronger or weaker than a 2x10 ..

If no one knows the answer or does not feel like throwing out an opinion or experience that is fine ..

Last edited by beachfront71; 04-01-2012 at 03:10 PM.
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Old 04-01-2012, 03:16 PM   #12
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Roof deck framing and strength?


As I understand it, the cantilevered portion would be one of/the point loads Daniel said in his Point #1. Beachfront71, also you need to figure the uplift connector at the cantilevered wall support and add solid blocking there and a full-depth rim joist to meet minimum code....
Notice the span charts for a deck, with and without a cantilever, what a reduction using 2x8, Chart #2: http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf

You really need professional help on this. They will take full liability for the floor loading, not us- not you, to satisfy your H.O. Insurance carrier; if ever a claim.

Gary

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