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Old 08-18-2008, 07:15 PM   #1
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Help - Question on deck rebuild?


The previous owner did something I nor anyone else I know had ever seen. He pored a concrete elevated deck. Anyway I'm tearing it out because I'm getting some wood rotting due to the concrete. I will need to replace the header/beam/girder (lots of rot), the long beam that spans the from pillar to pillar. The span is 14 feet. That beam was a left over log from the house, 6"x7" (it's a log house). See picture below.

Anyway I have calculated I can double up 2 2x10x16's and make the span work. My question is this I have a bunch of 2x8x16's, and would like to use those. Plus the 2x8's would look better for a number of reasons, additionally I have no idea how I'm going to get the 2 2x10x16's to the house. I friend picked up the other decking, but now he's out of town for the next 3 weeks. My question is this, can I triple up 3 2x8x16's? If I can't, I would be curious as to why I can't.
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Last edited by missin44; 08-18-2008 at 07:25 PM.
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Old 08-20-2008, 09:05 AM   #2
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bump....

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Old 08-20-2008, 09:19 AM   #3
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Is any of the lumber that you plan to use pressure treated?
It's difficult to spec a beam on the internet. I think the double 2x10 is stronger than the triple 2x8. You're LLY (local lumber yard) may be able to spec that beam for you.
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Old 08-20-2008, 06:46 PM   #4
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It is pressure treated...My local lumber yard is Lowes and Home Depot...both worthless.
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Old 08-20-2008, 07:34 PM   #5
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Quote:
Originally Posted by missin44 View Post
...My question is this, can I triple up 3 2x8x16's? If I can't, I would be curious as to why I can't.
NO, please do not.

Such a structure requires a building permit.

Additionally, do you know if those stone/pillar supports are built properly and are strong enough to support a new structure. You will need to confirm that for the permit.

Last, assuming that the stone pillars are strong enough, consider an engineered beam, weather resistant, to make the span.
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Old 08-21-2008, 07:29 AM   #6
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OK, I'm convinced I can't triple the 2x8's, the question I have now is why not? The question of a permit isn't the answer, I just want to know why the 2x8's being tripled wouldn't work. The pillars that are there aren't the problem. They supported a 8'x15'x5" concerte slab before I tore it out, and that had an approved building permit. I now assuming the the tripled 2x8's will not be approved...why?

Also any idea what an engineered beam would cost, ballpark figure?
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Old 08-21-2008, 12:16 PM   #7
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I'm not sure if you realize this, but contractors are not engineers. You can bump this thread all day long but what it comes down to is you need to get an engineers' stamp on the plans for the beam calcs to be correct. No engineer is going to look at a pic online and throw out an opinion on what size beam you'll need either. It's a liability issue.

It might be possible to take the plans to your city permit office and have them calc the beam size - you could call and see if they do that.

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Old 08-21-2008, 04:17 PM   #8
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to greatly simplify it, from a mechanical strength standpoint, the important equation is (b*h^3)*constants
For sake of simplicity, lets assume a 2x8 and a 2x10 are really 2" x 8" or 10"
2 2x10s -> 2* (2*10^3) = 4000
3 2x8s -> 3* (2*8^3) = 3072

You would actually need 4 2x8s to match the strength of 2 2x10s (note, I am not saying its ok to use 4 beams). This is a very simplistic explanation making some simplifying assumptions about wood. For the real story, you would have to talk with a structural engineer, or an architect.
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Old 08-21-2008, 04:38 PM   #9
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You are right that Lowes and HD are worthless for this but if you have them in your area you also have a real lumber yard with an engineer nearby. Find it and ask him to spec you the beam and buy it from them. I'm guessing somewhere between $100 and $200 and you'll be going home with the right beam that you could tie down in a regular long bed pickup bed.

How's the ledger and flashing?
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Old 08-22-2008, 12:21 PM   #10
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BuiltByMac, I'm not trying to bump anything. My piont is when I hear that something can't be done, I like to know why. Not because I'm trying to figure a way around the issue, but because I like as much knowlege as I can get. I have a curious mind, simple as that. Thanks daxinarian and jogr for answering my question.

joger - the ledger was the reason I started this project, it had lots of rot due to the concrete. It has since been replaced. Will be installing new flashing.

Thanks agian.

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Last edited by missin44; 08-22-2008 at 12:25 PM.
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