20 x 20 deck with no interior posts

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Old 09-06-12, 11:27 AM
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20 x 20 deck with no interior posts

Hi,

I have a 10 x 20' deck currently attached to my house on the 20' side. I am wanting to replace it and go out 20' instead of 10' to make it 20 x 20. Is it possible to use beams large enough so that I do not have to put any posts on the interior space underneath the deck? It will be attached to the house. If possible, how many 6x6 posts will i have to use and what beam sizes?

Thanks
 
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Old 09-06-12, 11:36 AM
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Welcome to the forums.

Current thought around here is to build free-standing decks; to not attach them to the house at all.

So, what you're asking, though, is whether you can build this by having it attached to the house and then no posts between there and the outside edge of the deck, right?
 
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Old 09-06-12, 12:00 PM
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Its already attached and has the vinyl siding installed nicely around the ledger board, so im going to keep it that way.

Here is a picture of what I am talking about. My uneducated thought was to have 2 - 2x12s bolted together for the beam that connects the posts that run perpendicular to the house to support the weight. I drew these beams in the picture for reference.

This deck is over my walk-out basement door and I want to use the space underneath it and not have any posts in the middle of that space.
 
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Old 09-06-12, 12:05 PM
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Ah, I get it - support at the perimeter, just nothing inside that.

Hang tight, we have pros here that know a lot more about what kinds of spans can be accomplished.
 
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Old 09-06-12, 12:09 PM
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Yessir,

I should add that the deck is about 9' high at the farthest edge from the house.
 
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Old 09-06-12, 02:34 PM
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What are your present joist sizing and spacing? May be critical as to the spacing of posts. If they are sizable enough you can get by with perimeter post and one in the middle. But you will need to leave the post set up you have presently since the deck is sitting on them. Let us know the joist/rim size so we can go from there.
 
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Old 09-06-12, 02:55 PM
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Chandler,

I will be building an entirely new deck. I can build it with whatever size and spacing it needs to be. I will be tearing down the current deck. The post size will be 6 x 6
 
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Old 09-07-12, 02:59 AM
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You will still need a midspan support, at the point you have, now. Do you have sonotube pours presently for your vertical supports? How far apart are they? You can get away with a single support in the middle as well as one on each outside band, but it sure would be better to have two in the middle. Where is your door in relation to the deck position? I would use 2x10's at 16"oc.
One of our members posted this a while back and I took the liberty of saving it. It makes for great reading on methods of deck building. Take time to read over it, as it will answer most of your questions. Those questions not addressed, please post back and let us help with them. You may even have questions regarding the link, and if so we're glad to help.
http://www.lancova.com/deckinfo.pdf
 
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Old 09-09-12, 08:55 AM
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Wanting a 20' free-spanning interior is asking for problems. Such as the spanning members sagging over time. You should seriously consider using properly-sized steel instead of wood.
 
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Old 09-10-12, 01:03 PM
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Bridgeman is right, a steel beam running down the middle of the deck parrallel with the house would be best and then run your 2x10 joist the full 20' dimension if possible because a multiple span joist is stronger than two 10' long single span joists. Of course you will need steel posts to support the beam and some way to stablize the entire thing so make it rigid in all directions.
 
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Old 09-11-12, 10:01 PM
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@Bridgeman45

When you say "...properly-sized steel instead of wood...", are you referring to steel joists?

@spiper-

I am confused.

I think OP wanted a deck that did not have any mid-span footings. Besides the fact that a 20 ft long, unsupported span joists will flex and eventually sag, why would you need a "steel beam running down the middle"? Why not a doubled or triped 2x8, 2x10, 2x12?

Also, can you explain why you would need steel posts vs a regular PT 6x6 posts?
 
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Old 09-12-12, 12:37 AM
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My reference was for a single steel beam, 20' long and running parallel to the house at midway between the house and outside row of columns. If properly sized, it will easily support the 2 spans of 2 x 10 joists running perpendicular to it. I never said anything about requiring steel columns, but they would have the advantage of not burning when welding the steel beam's flanges to them. And on a more serious note, steel columns could be embedded in the concrete footings, making for very rigid connections in all directions which would never rot.
 
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