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Old 03-11-2011, 06:44 PM   #1
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Another deck beam span question


The deck I am planning to build is a little different than most. First of all, my deck will be 20' x 20' attached to the 2nd story of my house. The deck will also be covered using a shed style roof off the house. I also plan to have a 2' cantilever and the posts for the shed roof will rest on the rim joist at the end of the cantilever portion. So, the joists, which are 20 foot 2 by's, will rest on the beam at the 18 foot mark.

Questions:
1.) What size joists? 2"x10"x20' or 2"x12"x20'
2.) use 16" OC or 12" OC?
3.) What size beam? (2)-2"x10", (2)-2"x12", (3)-2"x10", or (3)-2"x12"
4.) What 6"x6" post spacing should I use? 6', 8', or 10'

Mucho thanks!

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Old 03-11-2011, 07:01 PM   #2
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Another deck beam span question


Your deck design is off the standard deck design manual for a couple of reasons. First, it is rather tall, and the posts are likely to be taller than the standard maximum height in the IRC design guidelines. Second, you have a roof, which will add load to the structure not accounted for in the standard design guidelines. You are attaching to the second floor, which may change standard framing practice, but without a plan I can't say.

The cantilever sounds like it is within normal design guidelines, but combined with the roof, is probably not covered by your standard design guide. As for resting posts on the rim joist, that is definitely not standard practice per IRC.

In short, while you may be able to do all of this, you probably should hire an architect or structural engineer to check all the structural element sizes and connection details, especially if you are planning to pull a permit (mandatory in most jurisdictions) for this project. Without a detailed structural investigation, it would be reckless for anyone on this forum to size structural members for you.

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Old 03-11-2011, 07:49 PM   #3
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Another deck beam span question


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Originally Posted by Daniel Holzman View Post
Your deck design is off the standard deck design manual for a couple of reasons. First, it is rather tall, and the posts are likely to be taller than the standard maximum height in the IRC design guidelines. Second, you have a roof, which will add load to the structure not accounted for in the standard design guidelines. You are attaching to the second floor, which may change standard framing practice, but without a plan I can't say.

The cantilever sounds like it is within normal design guidelines, but combined with the roof, is probably not covered by your standard design guide. As for resting posts on the rim joist, that is definitely not standard practice per IRC.

In short, while you may be able to do all of this, you probably should hire an architect or structural engineer to check all the structural element sizes and connection details, especially if you are planning to pull a permit (mandatory in most jurisdictions) for this project. Without a detailed structural investigation, it would be reckless for anyone on this forum to size structural members for you.
Luckily, my posts are not taller than the 14' maximum listed in the IRC Deck guide on page 7. For the ledger, I may have misled about the second story. My house has a walkout basement. So, the ledger will, in fact, be attached to the house band joist. For the posts that will support the shed roof, I plan to use 6"x6" post located on the outer edge of the deck. This will place them on the cantilevered portion of the deck. I guess I could just a easily place the posts directly over the beam portion of the deck.
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Old 03-14-2011, 11:33 AM   #4
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Another deck beam span question


Anybody know the answers?
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Old 03-14-2011, 11:38 AM   #5
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Another deck beam span question


you need to take this to someone who can design loads and shear. I dont think you will find 18' clear span exterior material for the loads you need
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Old 03-14-2011, 11:48 AM   #6
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Another deck beam span question


...all the span tables are here. When in doubt, seek the advise of a structural engineer.

http://www.awc.org/publications/DCA/DCA6/DCA6-09.pdf


and no you cant put roof posts on the cantilever.
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Old 03-14-2011, 05:15 PM   #7
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Another deck beam span question


I do not have the IRC deck guide, but the following points popped up in my mind when I read your post! I do not have answers other than "have a structural engineer review your plans before you buy any materials"!

1. 18 feet feels like a VERY long span. I think you may need engineered beams for a span this long! Of course, loading, especially with the roof adds to your problem. The extra load on your house wall could also be a factor!

2. You will need some kind of foundation for your support posts. Exactly what you need to do here will depend on soil conditions in your area. I am guessing at minimum 3' square footings on really solid soil. Your local building code will dictate. Again this is another 'engineering' item.
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Old 03-14-2011, 07:11 PM   #8
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Another deck beam span question


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Originally Posted by tpolk View Post
you need to take this to someone who can design loads and shear. I dont think you will find 18' clear span exterior material for the loads you need
That is wrong....check the joist span tables.
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Old 03-14-2011, 07:15 PM   #9
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Another deck beam span question


Quote:
Originally Posted by 12penny View Post
...all the span tables are here. When in doubt, seek the advise of a structural engineer.

http://www.awc.org/publications/DCA/DCA6/DCA6-09.pdf


and no you cant put roof posts on the cantilever.
That is wrong also.....specify your source. You can put load bearing walls on a cantilever, which is what the roof posts are doing....bearing the load of the roof. However, for typical construction I thought it was no more than 1' cantilever. I figured since I was just supporting a light roof that perhaps I could go greater than the 1'.
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Old 03-14-2011, 07:24 PM   #10
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Another deck beam span question


Quote:
Originally Posted by handy andy View Post
I do not have the IRC deck guide, but the following points popped up in my mind when I read your post! I do not have answers other than "have a structural engineer review your plans before you buy any materials"!

1. 18 feet feels like a VERY long span. I think you may need engineered beams for a span this long! Of course, loading, especially with the roof adds to your problem. The extra load on your house wall could also be a factor!

2. You will need some kind of foundation for your support posts. Exactly what you need to do here will depend on soil conditions in your area. I am guessing at minimum 3' square footings on really solid soil. Your local building code will dictate. Again this is another 'engineering' item.
1.) I don't think they make engineered beams that are ACQ treated.....this is an outside deck. I agree that the roof adds to my problem. It would really be easier if I could just use the IRC guide.
2.) IRC Guide list 32" sq footings for an 18' joist. Four more inches never hurt anybody......at least, that is what my wife tells me.
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Old 03-15-2011, 09:24 AM   #11
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Another deck beam span question


...why dont you just put your posts over the deck beam? No engineering, code enforcement will be happy and with the overhang the roof edge will line up with the edge of the deck.

Something like this.
Attached Thumbnails
Another deck beam span question-s5300711.jpg  

Last edited by 12penny; 03-15-2011 at 09:40 AM.
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Old 03-15-2011, 09:41 AM   #12
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Another deck beam span question


sounds like you have it already figured out to do the way you want so why the question bug?
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Old 03-15-2011, 04:40 PM   #13
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Another deck beam span question


The ICC residential code book is a great resource to find answers to questions like the one above.
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Old 03-15-2011, 05:01 PM   #14
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Another deck beam span question


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Originally Posted by kcremodeling View Post
The ICC residential code book is a great resource to find answers to questions like the one above.
I have not heard of the ICC; only the IRC. I will check it out. THANKS!
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Old 03-15-2011, 05:01 PM   #15
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Another deck beam span question


Quote:
Originally Posted by 12penny View Post
...why dont you just put your posts over the deck beam? No engineering, code enforcement will be happy and with the overhang the roof edge will line up with the edge of the deck.

Something like this.
A picture is worth a 1000 words. That really helps me out!!!!!

THANKS!

The deck portion will be almost identical to what I have planned for the cantilever. However, what would be the harm in placing the roof support posts toward the rim joist to give more area under cover? Just curious.

Also, one more important question....what is your roof slope and what roofing material are you using....certainly not shingles?

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Last edited by Bugman1400; 03-15-2011 at 05:07 PM.
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