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Old 05-16-2011, 03:12 PM   #1
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Building 24'X16' deck, Beam question


I am building a 24'X16' deck, obviuosly I can't find a 24' beam. How would I run my beams across to get to the 24' length and attach them to my posts? I have 4X4 posts and want to sandwich two 2X8's on each side of the post. how would I attach the beams to the posts?

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Old 05-16-2011, 03:28 PM   #2
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Building 24'X16' deck, Beam question


You can actually get lumber that long. However, you can also splice the beam on the post and install extra bolts. I would use a 4x6 instead of the 4x4 which will give you more surface area to splice on.

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Old 05-16-2011, 03:52 PM   #3
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Building 24'X16' deck, Beam question


how far apart are your 4x4 posts going to be and what is your joist span? (that will determine whether you need 2-2x8's or 2-2x10, or even 3-2x10's). My deck is 14.5 x 20 feet wide with a 12.5 foot joist span with 2 foot cantilever and 2x10 joists and I went with a triple 2x12 laminated together that sits on top of 6x6 posts, which are approx. 5.5 feet apart. The beam is attached to the posts with Simpson brackets.

I personally don't like the sandwich idea for a beam since it relies on the shear strength of the method you are attaching them (lag bolts, carriage bolts, etc). The only way I'd use a sandwich method is with a 6x6 post and notch the beams into the post so they sit at least on part of the post.

I'd rather have the beam sitting on top of the posts and supported laterally. But that is just my preference.

Please check with local building codes to see what you are required for the permit and to pass the inspections. They will tell you max. joist span, post spacing, post size, depth of footing (always go deeper), beam size, method of connections, etc.

Good luck.
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Old 05-16-2011, 03:56 PM   #4
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Building 24'X16' deck, Beam question


I have already put 4X4's in the ground. Codes says I can use 2X8's. Can I sandwich them on each side?
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Old 05-16-2011, 03:58 PM   #5
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Building 24'X16' deck, Beam question


dpach, If I set them on top of the post how would I get all the boards on there? What size in length boards would you use for the header an 16fter and 8fter.
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Old 05-16-2011, 04:02 PM   #6
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Building 24'X16' deck, Beam question


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Originally Posted by bowanna03 View Post
I am building a 24'X16' deck, obviuosly I can't find a 24' beam. How would I run my beams across to get to the 24' length and attach them to my posts? I have 4X4 posts and want to sandwich two 2X8's on each side of the post. how would I attach the beams to the posts?
From my area, code requires that you notch the posts (which should be 6x6s), sit the beams in the notch, and attach them using 1/2" through-bolts with washers (attaching beam to either side of posts is not allowed here). To get the length you need, you split the beam at the middle of a post, keeping each split less than 8 feet long.
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Old 05-16-2011, 04:28 PM   #7
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Building 24'X16' deck, Beam question


My code says exactly the same as the code Oatlord quoted. Specifically, my town used the 2006 International Residential Code, and supplies a guide called the Design for Code Acceptance when you pull your permit.

The reason that this code does not permit attaching beams to posts using the sandwich technique is that the beams would be supported entirely in shear on the bolts, which can lead to distortion of the bolt and/or the wood surrounding the bolt. As noted by Oatlord, the code specifically requires that beam splices be made over the post, which as he correctly noted must be a minimum of 6x6, which allows adequate support for beams after notching.

You have already installed the 4x4 posts apparently, so I suggest you discuss your situation with the building inspector, if you have one. At his discretion, you may be allowed to sandwich the beam. If so, you may need to install a support block under each beam.
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Old 05-16-2011, 04:34 PM   #8
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Building 24'X16' deck, Beam question


Daniel,
I have a permit and it was approved by the town inspector. He never questioned that at all. My other question is how do I attach the beams to the face of the post? Do I offset each one so 2" of each board secure to the post and then the next board attach 2" of that.
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Old 05-16-2011, 04:45 PM   #9
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Building 24'X16' deck, Beam question


Quote:
Originally Posted by bowanna03 View Post
dpach, If I set them on top of the post how would I get all the boards on there? What size in length boards would you use for the header an 16fter and 8fter.

First question: did you cement the posts into the ground? (if so, you have no choice but to use 4x4's now). If you set them on footings using post connectors, can you change the post connectors and posts to use 6x6 posts? (that would allow you to knotch out the posts on each side for your sandwich method).

Strongly recommend checking out:
http://www.strongtie.com/ftp/fliers/F-DECKCODE09.pdf

If you have to use the 4x4's, you should use the beam-on-post method since it is more reliable than mounting them to the side of the post without the knotch support (4x4's too narrow to knotch on each side). Use simpson strong tie beam-to-post connectors (I used the LPC Post Caps) and make sure the splits in the beam are directly above the posts (use flat connector plates over the joints in the beams to add strength). Plus, use a good construction adhesive to laminate your beam boards together.

Of course, if you decide to go this way and set your beam on top of the posts, you will have to cut your posts shorter by the height of the beam so the top of the beam (when mounted on the post) is the same height as the bottom of your ledger board on the house. That way you can set your joists on the beam and be level from the ledger board.

If you decide to use the sandwich method (since your inspector approved it without the knotching), it could be tough since a 4x4 post is really 3.5x3.5 so you would only have 1.75 inches of beam on the post at each split. I would definitely put another support block (12" or so) attached to the post directly under the beam joint for added support so 6"s would support each side of the beam joint (as Holzman mentioned).

Also, if you are using ACQ pressure treated lumber (uses copper rather than arsenate CCA wood), make sure your fasteners (nails, screws, joist hangers, etc) are ACQ approved. I actually used the below-ground quality pressure treated which is the old CCA stuff (actually injected for better preservation rather than dipped like most ACQ lumber that only has the outside 2-3 mm's treated). It's covered with cedar deck boards so we won't be in direct contact with it. If you don't use ACQ approved fasteners, they will react with the ACQ and rust/corrode very fast.

Last edited by dpach; 05-16-2011 at 05:49 PM.
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Old 05-16-2011, 04:51 PM   #10
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Building 24'X16' deck, Beam question


I guess I am a bit confused. You pulled a permit, presumably you showed the inspector your plans. At least when I pulled my deck permit, I showed the inspector a full set of plans, and he showed me the Design for Code Acceptance booklet, which includes details on how to splice a beam.

So since the inspector did not question your use of 4x4 posts, I assume he had something in mind for splicing a beam over such a post. You probably should discuss this matter with him prior to making the attachment, as he might have a strong opinion on an acceptable way to do it.

The problem with sandwich splicing a beam using a 4x4 is that there is only 1-3/4 inches bearing available for each beam across the post if you bolt through the post (based on 3-1/2 inches wide for a 4x4 post). This means that if you center the 1/2 inch bolts, you only have 5/8 inch clear either side of the bolt, which is pretty marginal. If you can install a bracket over the post and attach the beam that way, it would be a lot stronger, but perhaps it is too late, as you may have cut the posts already.
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Old 05-16-2011, 05:02 PM   #11
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Building 24'X16' deck, Beam question


We have not yet cut the posts so we do have the ability to place the bracket on top of the posts and secure the beam that way. So in that case I just butt up the two ends of the beams inside the bracket and secure it

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