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Old 09-01-2013, 02:57 PM   #1
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Deck beams and joists


I have 12 inches of space to work within from the bottom of the beam to top of the joist for the deck (I won't get into the reasons why). Which of the two options for beam and joist construction is better using correct spacing for each option:
1) A beam consisting of 2 2x6's bolted together (secured to the tops of the deck posts) with 2x6 joists on top of the beams OR
2) A beam consisting 2 2x8's bolted together (secured to the tops of the deck posts) with 2x4 joists on top of the beams.

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Old 09-01-2013, 03:24 PM   #2
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Deck beams and joists


If you are building your deck according to code standards, that would likely drive your decision. My town (and many others) uses the Prescriptive Residential Wood Deck Construction Guide available at http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf.

If you download and read that guide, it will give you all sorts of useful information on sizing joists and beams, allowable connections, railings, posts etc. The required size of beams and joists has to do with the spans you need to cover, which you did not discuss in your post. Note that 2x4 is below the minimum allowable size for a joist, so would not be acceptable.

Of course, if you are not building to code, that is a different matter, and would be beyond DIY in my opinion.

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Old 09-01-2013, 10:05 PM   #3
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Deck beams and joists


Thanks for the link to code Deck Construction Guide. I noticed on page 7, figure 6, option 3 (http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf) that the joists appear to be attached within the perimeter of the beams (rather than on top of them). If I could do this I would be able to lower my deck platform by the height of the joist. How would I need to construct the deck to use this framing method pictured in option 3?

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Old 09-02-2013, 12:20 AM   #4
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Deck beams and joists


Consider using a flush beam instead of a drop beam. Can use a larger sized beam/joists that way.
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Old 09-02-2013, 06:19 AM   #5
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Deck beams and joists


To attach a joist flush with a beam, you use a joist hanger. Simpson makes a line of them, as do others. Joists can be supported above a beam, you need a different connector than for a flush beam. I suggest you discuss this project with your local building inspector, make sure you understand what is acceptable locally, assuming you need a permit for the project.
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Old 09-03-2013, 06:37 PM   #6
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Deck beams and joists


Frame with steel... With 12" to work with you could have joists that span 18' from ledger to beam, if you really needed to up it you can do some more engineering and probably get 25'+ span.

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