Comments for More Joists or Beams for More Strength for Deck?

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May 20, 2011
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Informative one
by: Crystal Beach

This was very very informative, however I am trying to come up with a cantilevered (angled) 8' wide deck 20' long extension off some 12" by 12" pilings can it be done and how... Oh the deck will be 20' off the ground. Thanks in advance.

Nov 06, 2010
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Helpful Deck Building Calculator Tools
by: Editor - Rich Bergman

Great question.

I am going to tell you to go to our Deck Building Calculator Tools page and see all the cool helpful tools you can work with to point you in the right direction.

Then to tackle your issue directly I would go to the Joist Span Calculator Tool and work with it.

First of all, the tool is not substitute for confirming anything like this with either a local building inspector or a structural engineer. But all of the data in this calculator comes from the American Wood Council based on their recommendations. So it is a great starting point.

I did a bit of an analysis using the information you provided and you should too - and then play around with different scenarios. Here is what I did:

Lumber: southern pine
Beam size lamination: 3-2x12
Joist size: 2x12
Joist spacing: 12" o.c. (on center)
Result: 18'

This says joists of all of those specifications could safely span 18', two feet more than you currently have. But - that is not the end of it. You have to now see if the support beam or beams are built strong enough to carry all that.

You can pass through all of this data to the Beam Span Calculator Tool by clicking the link below on the joist tool page.

I then added the following specializations you provided to see if your existing beam can span 16' from each 8x8 support post you mentioned. Seemed long to me.

Lumber: southern pine
Beam size lamination: joist hanger beam configuration

I chose that because it was the closest arrangement to what you had described

Joist span: 16' - I changed it to 16' because 18'was carried over from the joist calculator but you said you only have two 8x8 support posts at each corner - therefore they are 16' apart

No Overhang

Result: 10'-8"

What does this mean? Based on all this data the initial analysis suggests you should have another 8x8 support post under that long beam. It says you should not span that supporting beam more than 10-8" from one post to the next.

So there you go. Try it yourself and print out all the results and work from there. Then talk to a local building expert!

Do not assume this is the Bible but it is a great starting point. Only when you speak to a local official or engineer will they be able to view what you really have going on at your site and make a professional assessment. OK?

But you will surely find the tools helpful.

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