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I just bought a new house that has a deck that sits about 20 feet off the ground. it is roughly 10ft by 20 ft. it has two rows of 4x4 posts as support spaced about 5 feet apart. one row in the back and one row in the front. there is no middle row. when walking on it you can really feel it sway. i don't see any other bracing like a 45degree brace or anything else. my question is what can i do to limit the sway based on these conditions.
 

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I just bought a new house that has a deck that sits about 20 feet off the ground. it is roughly 10ft by 20 ft. it has two rows of 4x4 posts as support spaced about 5 feet apart. one row in the back and one row in the front. there is no middle row. when walking on it you can really feel it sway. i don't see any other bracing like a 45degree brace or anything else. my question is what can i do to limit the sway based on these conditions.
maybe some 45 degree bracing. post some pics please.
 

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Sounds scary to me. A row of post next to the house, is the deck not connected to the house with a ledger?

You can’t support a 20’ deck with 4x4. You can’t even buy 4x4 that length in my area just for that reason. Are you sure there not 6x6?

You defiantly need lateral bracing and probably much more.

Can you post a picture of this deck?
 

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If your deck is 20 feet in the air and not fastened to your house and no bracing it is not safe. You need a home inspector, a pro carpenter or engineer come out there and check it out before someone gets hurt.
 

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Civil Engineer
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A 4x4 douglas fir/larch select structural column in wet service (outdoors) that is 18 feet long can support 1,612 lbs by computation without danger of buckling. Your deck has a total area of approximately 250 square feet, and ten columns. Assuming 50 psf loading (dead plus live), this would yield a total load of 12,500 lbs. Your code may require higher or lower loading. Assuming equal distribution of the load (this is not correct, but is a starting point), you would have 1,250 lbs per column, meaning you are close to maximum.

If your column is taller than 18 feet, the allowable loading goes down, if shorter than 18 feet, the allowable loading goes up. If your wood is less strong than douglas fir, your allowable loading goes down, and if the grade is not structural the allowable loading goes down. Adding bracing to a column substantially increases allowable loading by reducing the free length of the column. Adding bracing at the midpoint is therefore best, but is not always possible.
 

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Thank you Daniel that is very helpful. I am wondering about the dimensions. i do not believe the posts are 4X4. i should have measured but eyeballing it from the picture it looks more 4x3 or 6x4. definitely not square. I added some more pics if that helps. looks like there are 8 beams and they are more like 15 feet long.
 

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Framing Contractor
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I just bought a new house that has a deck that sits about 20 feet off the ground. it is roughly 10ft by 20 ft. it has two rows of 4x4 posts as support spaced about 5 feet apart. one row in the back and one row in the front. there is no middle row. when walking on it you can really feel it sway. i don't see any other bracing like a 45degree brace or anything else. my question is what can i do to limit the sway based on these conditions.
Post a picture of the deck front and side views. That will help getting an answer for you.
 

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The deck is about 10 feet off the ground.
Is this your house?
Ron
 
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Thanks again to everyone. Good info. To reply to a couple of the last posts. This is all the pics i have. it shows the deck from all angles.

This is my house and i measured and these posts are more like 12 feet not 20.

Thanks again and excuse my ignorance on this matter.
 

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looks like 8-10' to bottom of band. is there a beam sitting on top of the posts at the outside of the deck where the floor joists end or are they attached to the band?
 

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This deck would not pass inspection in our area. Was the deck permitted? For one, the band only appears to be a single joist, followed by a composite material on the outside (no visible fasteners). Second, the posts should have cross beams (double 2x12 girder) with the joists supported on these girders (rather than by the band).

Refer to the earlier picture in this thread. This deck has the cross beam (girder is the best description) PLUS additional 45 degree supports for each post. Much stronger! The girders provide a LOT of side support for each post, plus distributes the vertical load across multiple posts, particularly if the girder is cut into the posts ("let in"). For this reason, I always use 6x6 support posts, as letting in weakens each post slightly. "Letting in" ensures the loads are supported by wood, not your fasteners...fasteners hold up the girders, not your deck.

Last, since your deck boards are on 45 degree angle, you cannot use standard (5/4) deck boards with 16" joist spacing, but are restricted to 2x6 deck boards (if PT). I would have built it with joists 12" in center, even with 2x10 joists. I can't tell what you have; composite materials are even weaker that PT.

It's done now, so question is what to do...I'd refer to load tables, pull a permit if required, and add adequate posts and cross beams...you can do this without rebuilding the entire deck. If you don't know how to read load tables, get someone who can (you can still do the manual work, but make sure it is designed and engineered well...do it right!).

Visualize what us happening without girders...a post is only supporting the vertical load of ONE joist, plus whatever load is transferred back to it at the end through the band boards at the end. Currently, your middle support posts are essentially only supporting ONE joist! Add girders across each set of posts...all vertical loads are shared across multiple posts, plus much more lateral support (no sway) for the posts. Girders also make it MUCH easier to build the deck, as you just lay and space the joists on the girder(s) as you build it!

Good luck with it...I've seen much worse in terms of underconstructed decks. On my previous new home, the builders deck wouldn't pass code...but the inspectors apparently focus on other issues when inspecting new homes.

Hope this is helpful.
 

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Wouldn't pass muster around here. Requirements around here are 6x6 posts minimum, 14 foot max post length, and diagonal bracing is required.

I'm harboring some doubt that the deck was built with a permit. Did your home inspector pick that up during the inspection?
 

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I agree with Aggie67; no way. After additional review of the pictures, you may need to start over...granted, not what you want to hear.

1) joists (visually) appear to be 24" on center; marginal if 2x6 deck boards, but yours are at a 45 degree...exceeding a 24" max span. Have you verified spacing? Deck board size?
2) it appears that there used to be a concrete patio under the deck that was cut away...with some of the posts resting on the old patio as a pad! No way...you must dig footers to code specified depth for EACH post.
3). Posts are undersized and unevenly spaced, and supported by ??? Some seem to be only resting on concrete. Secured how? Concrete in or on wood will rot, PT or not.
4) no evidence of double bands.
5). No girders (support beams) for the joists.
6) no blocking between joists.
7). How is the ledger fastened to the house band? Flashed properly? Bolted or lagged? Many areas will not allow lag bolts, but require through bolts.

These are not minor issues...think of the prying load being placed on the ledger board at the house when the deck sways (plus the dead load)! That's a mighty long lever (length of 2 decks)...and that's without a significant live load.

Get it inspected and engineer it right.
 

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After additional review of the pictures, you may need to start over...granted, not what you want to hear.
I think starting over is a bit much. It’s would be pretty easy to temp up that deck add joist and rework the post and beam to meet code.
 

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This is an existing deck, which for all we know may have met code at the time it was constructed, in which case it would presumably be grandfathered. I agree that discussing rebuild is way overkill at this point. The OPS started out wondering about bracing, and somehow this thread morphed into a forensic examination of all the problems with the deck. Possibly of interest to the OPS, possibly not. As for the bracing, it is certainly possible to add bracing to this deck, typically it would be done using 2x4 PT or 2x6 PT elements at approximately 45 degrees between posts, see Figure 22 in the "Prescriptive Residential Wood Deck Construction Guide" IRC 2006 edition for a full discussion about how to install the braces.
 

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It's not up to me...the question is whether it can be made to meet code by just addressing the posts and beams. It would still fail in my area due to joist spacing and inadequate banding (if confirmed).

I'd double check everything, given the shortcuts noted just from the pictures. This deck is a good example why code inspectors often don't like dealing with DIY's. I doubt a permit was ever pulled, and shame on the home inspector who didn't highlight these issues during the inspection prior to purchase by the new homeowner.

There are deck codes for a reason; code is the minimum requirement. People underestimate the loads on a deck, particularly with a large group if people using it. Failure of a deck can have disastrous consequences; it happens.
 

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It "morphed" as any structural changes to the deck structure in most areas would REQUIRE a permit, regardless of when the deck was built; it would have to be brought to current code.

The question was why there was excessive sway and how to address it. Causes are multiple, and addressing it will require structural changes. If a permit is required, the OPS might not like the answer.
 
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