Doubled 2 x 8 joists vs one 4 x 8 - pros and cons ?

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Old 08-30-13, 12:51 AM
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Doubled 2 x 8 joists vs one 4 x 8 - pros and cons ?

Why are wood decks sometimes built with double or even tripled 2 x 8 joists here and there, and is that OK vs why isn't a 4 x 8, or 6 x 8 used at those locations? I understand the need for a nailing surface where the deck boards have joints where one board ends and needs to be butted up against the next board, but wouldn't either arrangement provide that regardless? Is there an issue of moisture getting between the joists if they are doubled or tripled, and rotting the wood, even if it's pressure treated, and which would be avoided if a thicker 4 x 8 or 6 x 8 were used at those locations? Or are two 2 x 8's stronger than one 4 x 8 or is some other factor at play, like greater stability against twisting with double 2 x 8 joists vs a thicker 4 x 8 beam?
 
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Old 08-30-13, 02:41 AM
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Welcome to the forums! The practice of doubling framing lumber has a great deal to do with cross sectional stability. Two boards will give more stability if fastened and supported than a single grain board. Finding 4x8 and 6x8 lumber may be an oddity in itself. Do you have a source for such lumber in pressure treated for a reasonable price?
 
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Old 08-30-13, 05:38 AM
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Also far less likely to twist, sag and curl.
 
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Old 08-30-13, 07:22 PM
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According to my AITC manual, the section modulus of two 2 x 8s is 2 x 13.14 = 26.28 cu. in. The section modulus of a single 4 x 8 is listed as 30.66 cu. in., making it slightly (16.7%) stronger than the two 2 x 8s bolted together. Assuming identical species and grade of wood, of course.

If you can find pressure-treated long stock in the 4 x 8 configuration, I suspect it will cost more than twice as much as 2 single 2 x 8s in the same length.
 
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Old 09-01-13, 01:16 PM
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I'm with BridgeMan, I think cost is the main concern here; the price increases quickly with the size.
 
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Old 09-01-13, 10:06 PM
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Doubled 2 x 8 joists vs one 4 x 8 - pros and cons ? Moisture between boards?

OK, thanks for the helpful comments, but still wondering if there is an issue of moisture getting between the joists if they are doubled or tripled, and rotting the wood, even if it's pressure treated?
 

Last edited by Nemra; 09-01-13 at 10:08 PM. Reason: Typo (missing word "is") correction
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Old 09-02-13, 06:26 AM
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As long as the seam is vertical the water will find its way through and exit the bottom, there won't be a problem.
 
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Old 09-02-13, 04:10 PM
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Around here, using 4x8 is almost unheard of since they really aren't a stock item... they are almost always special order and definitely cost more. If you live near the trees and the mills, that might be another story.

As far as moisture getting between the boards, I know that some guys (the really anal ones) will cover the tops of their joists and beams to prevent trapped water that occurs from wood to wood contact (whether vertically on doubled/tripled beams or horizontally on joints between the decking and the joists). I think they use 3"+ wide strips of EPDM or something similar to lay on the joists. Looks like hell from underneath, but it does do the job.
 
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Old 09-06-13, 12:33 PM
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All decks are build with 2x? lumber because of availability. If you use pressure treated lumber, there should be no problem. Just double or triple them up.

The rotting of joists comes from dirt, leaves sitting in cracks in deck and holding moisture. To avoid this, before putting down decking, place strips of 30# felt on top of all surfaces. If you want to, Grace sells a vycor product that is made for this.
 
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