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Old 05-25-2011, 10:19 AM   #1
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new deck sways


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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Old 05-25-2011, 10:27 AM   #2
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Quote:
Originally Posted by bidderbl8 View Post
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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Old 05-25-2011, 10:34 AM   #3
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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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Old 05-25-2011, 11:11 AM   #4
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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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Old 05-25-2011, 02:23 PM   #5
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I had the same issue until I did the 45 degree bracing. now, a truck couldn't move it.
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Old 05-25-2011, 02:44 PM   #6
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First of all thanks to everyone for the input. I was off on the dimensions. it is more like 14X18. I have attached some photos for clarification.

The original deck is indeed fixed to the house. this is a secondary deck. it is the part below the steps in the first picture.
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Old 05-25-2011, 03:46 PM   #7
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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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Old 05-25-2011, 05:53 PM   #8
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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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Old 05-25-2011, 08:26 PM   #9
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Quote:
Originally Posted by bidderbl8 View Post
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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Old 05-25-2011, 09:31 PM   #10
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The deck is about 10 feet off the ground.
Is this your house?
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Last edited by Ron6519; 05-26-2011 at 11:41 AM.
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Old 05-26-2011, 09:48 AM   #11
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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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Old 05-26-2011, 11:11 AM   #12
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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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Old 05-27-2011, 10:44 PM   #13
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tpolk,
thanks. they are attached to the band.
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Old 05-28-2011, 06:14 AM   #14
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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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Old 05-28-2011, 09:08 AM   #15
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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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