[poolecw]

I'm getting ready to start building a rather large deck and wanted to bounce my pier layout by you guys. Joists will be 12" OC due to running PT pine deck boards on a diagonal. The deck will not be tied to the house, but rather free standing. The yellow dashed lines in the drawing represent the perimeter of the deck. I'm concerned about the 12'8 span between the two beams. But at the same time, I don't want to a beam and piers between the two if its not needed?

What say you?

 

[Daniel Holzman]

The allowable span depends on the code you are operating under, so you need to check with your local building inspector if you have not already. If you operate under the Prescriptive Residential Wood Deck Construction Guide (many towns do) based on the 2009 IRC code, you can span up to 13'1" with 12 inch joist spacings and no overhang, or up to 10'1" with a maximum overhang of L/4 where L is the maximum allowable joist span. If this does not work, you can span using 2x10 PT lumber. See the guide for information about construction methodology http://www.awc.org/codes-standards/publications/dca6.

 

[poolecw]

Thanks for the info. From the document, I follow the 13'1" being the max span for 2 x 8 pine. However, I can't seem to arrive at your 10'1" number.

 

[ron45]

There's no where near enough info to say....
Do yourself a favor and forget about the " 13'1" being the max span for 2 x 8 pine."

 

[Daniel Holzman]

The table that covers allowable span for lumber is Table 2 on page 2 of the Guide. If you are using southern pine, which is typically the case for PT lumber, you look on the first line of the table, which is for 2x8 lumber. You will see that the figure for joist spacing 12 inches used to be 13'8", which is now crossed out (2013 revision), and has been reduced to 13'1". If there is an overhang, you move to the right and you will see figures for an overhang not to exceed Lj/4, where Lj is the maximum joist span. That figure is 10'1" for joists spaced 12 inches OC.

If you read the footnotes, you will see that the tables are derived based on 40 psf live load and 10 psf dead load, plus additional deflection assumptions and service characteristics of the lumber. So there is adequate information to state that IF the OPS is allowed to use the Prescriptive Residential Wood Deck Construction Guide, then based on the OPS description of the deck as using 2x8 PT lumber (assumed to be southern pine), and with an overhang, the maximum allowable span of a joist is 10'1".

If the OPS is required to use a different code, then then results may be different.

 

[jlhaslip]

If there is an overhang, you move to the right and you will see figures for an overhang not to exceed Lj/4, where Lj is the maximum joist span. That figure is 10'1" for joists spaced 12 inches OC.

Does this table still apply for overhangs at BOTH ends? or would that be a different calculation? or would the total of both overhangs have to be max L/4?