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Old 06-15-2011, 09:41 AM   #1
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can't stop deck sway


We have a 13 year old home with original deck. It's 12x12 and about 10 feet up. It's bolted to the house, then supported with 8"x8" legs on the far ourside end. See photo attached.

Originally, it had a piece of the decking tacked across the center bottom going the same direction as the decking above. I pulled that. Then I sured up the small Y supports at the top of the legs. Then I put two 45 degree 2x4s across the bottom towards the outside edge- you can still see one in the picture. No help.

So finally, I bought a long piece of decking lumber and ran it the whole length at the 45 (again, pictured) glued an double screwed. To do this, I had to remove the other 2x4 cross piece.

It helped some - but the deck still sways slightly - I'd say a half inch or inch. Might not seem like a lot, but like an earthquake - that's enough to rock a plate off the tray on the side of my grill.

So I'm open to suggestions.

I'm thinking my only solution is to put very large X supports between the legs themselves - but this blocks the view and is ugly. I also wonder if I could double the size of my present Y supports and bolt them on.

Note that there is a full cement patio under it, so putting another leg in the middle really isn't an option. However, the legs actually sit on poured cement footers in the grass - I suppose I could bring them in a foot to support a new deck-wide header - then they'd stand on the patio cement - might not hold the weight - then I'd need to cut out foot holes and pour new footers - might be too much to try.

Ideas? Also - I wonder how this exact deck would have been built to code if built today.
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Old 06-15-2011, 09:47 AM   #2
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can't stop deck sway


How are those angle braces at the top of the legs fastened? If they are only toe nailed I think I would fasten them with carriage bolts through the posts. You might also try going from top of post to bottom of post with cross bracing. Even top of post to midway on post might be better.

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Old 06-15-2011, 09:56 AM   #3
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can't stop deck sway


You know the problem but I think you're hoping to avoid it The braces on the underside of the deck might prevent twisting but the swaying problem rests in the height of the posts. I assume you don't want to drop the deck, so you need to add much larger bracing between the posts than you have. I've seen it done to look like beefed-up lattice work and people grow roses up it. Doesn't look as gaudy and out of place that way.
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Old 06-15-2011, 11:20 AM   #4
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can't stop deck sway


I like Joed's mock-up - I'd still be able to walk under. But I wonder if there is math for determining just how large the Xs must be (the cross pieces between the support legs). If they were larger - it would seem like that might be worse because they'd be less rigid.

Also - do you put both pieces of each X on the same side? If so, where they cross in the center, both would be bowed out slightly.

Or - do you put one arm of the X on the outside of the post, and one on the inside?
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Old 06-15-2011, 02:00 PM   #5
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can't stop deck sway


One on the inside and one on the outside. If it still sways add a block in the center and bolt them at the cross over as well.

The lower you drop the bottom the more effect it should have.
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Old 06-15-2011, 05:19 PM   #6
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The lower you drop the bottom the more effect it should have.
Agreed....

Id go all the way to the post base if its not going to be in the way. Otherwise go as low as you can without creating a headache.
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Old 06-15-2011, 05:26 PM   #7
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can't stop deck sway


What size are the joists, and how far apart are they. Also, can you post a shot of the deck attached to the house. Also, the 8x8 posts, how far apart are they, how are they secured in the ground, and are they just along the perimeter. Have you check with your local city hall or county to see if permits were ever pulled and plans entered. It is possibly that this deck may not be up to spec, if you are really want to make it safe, it may have to come down, and rebuilt from the ground up. Since the deck as you stated is possibly 13 years old, it is obvious that it has met its life expectancy, and needs to be rebuilt properly engineered within spec's as stated by your local AHJ.
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Old 06-15-2011, 08:24 PM   #8
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can't stop deck sway


The large diagonal bracing will definetly help but I have to wonder if some of the "sway" you feel is the flexing of the joists. If the joists are overspanned then they are flexing and you will feel it. If you are overspanned you need to add a girder mid-span. Also, if the deck has had movement you need to check the attachment to the house. Make sure that the ledger is properly attached to the rim board (box rim) of the house, through bolted with carriage bolts or lag bolted with a bolt that passes through the ledger, siding if they left it on, sheathing and substantially into the rim board. Google deck collapse, you will be surprised how common they are, typically due to improper construction or lack of proper maintenance. You put two dozen adults on the deck for your brother's surprise birthday party and you may have 4,000 pounds up there, like parking a car on your deck. Good Luck and stay safe and healthy.
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Old 06-15-2011, 10:10 PM   #9
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can't stop deck sway


TheCamper has a great point. You could be feeling movement in the joists. It doesn't look like your joists sit on any kind of a beam for support. It looks like a 2x? under the end of the joists that sit on the top of the posts, is that correct?

If so, joist movement is very possible. There should be a properly sized beam under the joists, which would sit on top of the posts. The joists should then either be toe-nailed to this beam, or connected with hurricane ties.

Also, how are the 45 degree braces attached to the posts? (screwed with deck screws?). They should be lag bolted with minimum 1/2" lag bolts as screws won't keep them from moving.

I'd also go to the city and see if permits were pulled when it was built to see if it was up to code. It may likely have to come down and be built up to today's code.

Here's a pic of the one I'm building right now. Not saying its perfect, but the building inspector said he wishes all decks were built this way. Everything has been over done (larger joists, triple 2x12 beam, all hurricane tied, etc).

Good luck.
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Old 06-15-2011, 10:46 PM   #10
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can't stop deck sway


dpach, someone stole your snowmobiles off of the trailer.
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Old 06-16-2011, 09:17 AM   #11
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can't stop deck sway


Just from what you're all saying, I can tell that things weren't done correctly from the start. But I'm not ready to build a new deck (just finished a basement) - so I'm hoping to sure-it-up to get a few more years out of it.

New pics attached.

The joists are 2x10s and 12 feet long. There is no support under them at all. The are capped/boxed at the outside end by a pair of 2x10s. Then there is a 2x2 lip on the inside bottom, and the 2x10 joists are notched to sit on it. Everything is toe-nailed.

Then this whole edge simply sits on the top of the three 8x8 legs and it's toe-nailed in.

At the house, there is a single 2x10 boxed on the end of the joists again with a 2x2 lip and notched joists, again all toe nailed. This is lag bolted to the house.

Note that I never trusted toe nailing. So I went back and lag bolted several L brackets securing joists to end headers. Not sure if it's enough - but I have a little higher level of confidence the deck is attached to the house.

The footers for the legs are poured concrete with a bolt sunk in the cement then a metal footer bolted on. I did this. The deck had sunk 6 inches on one side when we bought the house - I found cheesy-pre-fab footers in the dirt - one on its side. So I jacked the deck, removed the legs and old footers, and poured 18 inch footers 3 feet deep into the round molds at the right height for frost line.

I was wondering if the joists themselves were tall enough to cause a little sway, where perhaps they all just lean left/right in unison. Is that what a girder would help? Is a girder the small X between joists? And would I need them between all joists across the midline of the deck?

The Y supports are only attached with decking screws.

Yikes.
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Old 06-16-2011, 11:56 AM   #12
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can't stop deck sway


For your braces - use 6 X 6's (4 X 4's) instead of 2 X 4's.
As stated above - should be lag-bolted - not just screwed/nailed.
On the exterior end (away from the house - can't really tell from your pictures) - should be 2 - 2 X 12's.
Add at least one row of blocking (solid) between your joists - mid-span.
Yes, they can be "X" bracing - we would do solid blocking - at this
point: they'll be easier to install.
If you're going to add blocking - do every other joist bay - then fill the
remainders in - don't start from one end and proceed to install them -
you might end up with your perimeter joist(s) bowed.

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Old 06-16-2011, 12:57 PM   #13
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can't stop deck sway


I agree with Ross regarding the bridging. Non-bridged joists are a stability issue, but it's not what is causing your story sway problem. The horizontal bracing that you added under the deck will help stabilize the deck a little bit, but not much. It bothers me that your deck simply bears on your posts (as opposed to extending up and next to your framing), but that's not the end of the world. Your biggest problem is your knee braces (what you refer to as Y framing). They are a little small, but they are poorly fastened. The best thing would be to add a knee brace on the outside and the inside so that they lap on top of your rim board and column, which allows you to lag bolt through the knee braces and the rim/column. If you don't like the look of that, center new knee braces on the column and lag into the rim and the column. In any event, I believe most of your flex is coming from the connection between the knee braces and the columns. The loads that are causing the sway are small and are predominantly a stability issue. When the deck sways back and forth, some knee braces go into compression and some in tension. The ones that go in tension have little to nothing in their joints to resist that tension. Get up on a stepladder and have someone make the deck sway and see if those joints open up with the sway. I would also make the knee braces longer - maybe 4' down the column. Joed's bracing is ultimately more stiff and knee braces, but I doubt it would make much difference with the framing and loads that you have.


I don't like how the joists are attached to the rim/ledger boards either. You need joist hangers. How are the deck boards fastened? You need two or three nails/screws in each location. If you only have one fastener per location, then you have a stability problem which would allow you deck to rotate out of square a little bit.
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Old 06-16-2011, 12:59 PM   #14
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can't stop deck sway


One more thing, while you are at it, you might as well add knee braces in the other sides. The ledger board at the house should keep the deck from leaning away from the house, but knee braces will take some of the load off of the ledger board and the joist connections to the ledger.
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Old 06-16-2011, 01:17 PM   #15
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dpach, someone stole your snowmobiles off of the trailer.
Actually, they are on the trailer; I just installed a cloaking device so you can't see them....heck of a theft deterant. It pays to sled with some Klingons!! No, my sleds are always stored indoors. Just need a place to keep the trailer and with a 6ft high deck, it works nice.

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