Deck swaying too much...help needed

Seven333 · 08-09-2011, 08:04 AM

I completed my deck last summer and noticed some sway when we have a lot of people on it. Some info on the deck:

-Deck is 20x28 feet

-2x12 ledger is attached to the rim board with lag bolts

-6x6 posts (4 of them) are 18 feet away from the house, inset 2 feet from the ends of the deck

-2x12 joists are spaced 12 inches apart

-2x12 beam is carriage bolted through the posts

-Composite decking (MoistureShield) is run perpendicular to the joists

-Double joist on one end, single joist on the other but 8 feet of it is lagged to the house (family room jut out)

-Deck is approximately 6-7 feet above grade

-Deck passed inspection

-Can probably attach a diagram if needed

We considered a 2nd set of posts about 10 feet out but wanted to keep the area under the deck clear for a future patio. I've installed 2 sets of blocking between the joists but that seems to have had minimal impact on the side sway. I'm having a large gathering in about a month and would like to tighten things up. Any ideas? Thanks!

Daniel Holzman · 08-09-2011, 08:25 AM

Since your deck passed inspection, I assume it was built to local code. You did not mention installation of diagonal bracing (sometimes called sway bracing). For a large deck like yours, with only four support posts, diagonal bracing is essential to minimizing deck movement. Check out the Prescriptive Rediential Wood Deck Construction Guide, written by the American Forest & Paper Association, which is based on the International Residential Code (IRC). When I pulled my deck permit, the building inspector gave me a copy of this guide, and said "follow it", which I did. Good diagrams, good discussion about every aspect of deck building.

On pages 14 and 15 of my guide (which is the 2006 IRC version) there is a full discussion about diagonal bracing, how to install, where to install. If you don't have diagonal bracing, that could be the source of your issues.

Ron6519 · 08-09-2011, 08:34 AM

Post photos of the deck, diagrams aren't too much use.
Agree with Daniel about the bracing. That size deck, that far off the ground, with that weight potential would need significant lateral bracing.

SteelToes · 08-09-2011, 03:04 PM

Also your composite decking...

Is it screw down type or grooved hidden fastener type ?

Seven333 · 08-09-2011, 04:21 PM

Decking is screwed down. I'll try to get some pics once the ground dries a bit. Thanks!

Seven333 · 08-10-2011, 01:27 PM

Bump for pictures...does this help at all? Thanks!

Ron6519 · 08-10-2011, 02:10 PM

I'd put diagonal bracing from each 6x6 to the carrying beam above.

Seven333 · 08-10-2011, 05:07 PM

Thanks, Ron. Are the diagonal braces more effective than the metal "T" braces that span the beam and post? Would horizontal bracing under the joists be even more effective?

Ron6519 · 08-10-2011, 05:17 PM

Quote:

Originally Posted by Seven333

Thanks, Ron. Are the diagonal braces more effective than the metal "T" braces that span the beam and post? Would horizontal bracing under the joists be even more effective?

You're getting movement from the base of the post because it's basically sitting on the concrete. I would think the diagonal bracing from the posts to the beam would be most effective.
You can try whatever you want and see if it helps. I can't see the horizontal bracing doing anything, as it's the posts that are swaying.

AGWhitehouse · 08-10-2011, 05:29 PM

I second Ron that the most effective measure would be the diagonal bracing at each column.

Daniel Holzman · 08-10-2011, 06:03 PM

The previous two posts are correct, horizontal bracing will do very little because you lack the triangle effect. Diagonal bracing from post to beam is effective because it creates a triangle, and any movement of any element of a triangle forces another element to go into tension or compression. Since wood is very strong in tension and compression parallel to the grain, and stiff in that directions, the effect is to minimize movement.

With horizontal bracing, the entire deck can move, assuming each joint rotates. Since the joints are typically nailed in, rotation is easy, and you can get whole deck movement. For details on how to install the diagonal bracing, see the Prescriptive Guide I mentioned, it has diagrams and a description. I am uploading a photo of my deck showing the diagonal bracing for your information as well.

Joe Carola · 08-10-2011, 09:13 PM

Quote:

Originally Posted by Seven333

Thanks, Ron. Are the diagonal braces more effective than the metal "T" braces that span the beam and post? Would horizontal bracing under the joists be even more effective?

Do it like this.

Seven333 · 08-10-2011, 09:16 PM

Wow, thanks everyone! So is a 2x4 as shown the preferred material? I've seen people use 4x4s but attached them to the inside of the posts, not front and back like the guide and in the attached pic. Seems like front and back would provide more surface area. How long of a piece should I use? And this may sound silly, but won't the bolts (carriage or lag) weaken the posts somewhat? Thanks again! This puts my mind at ease. I'm really amazed that the inspector didn't say anything...

Ron6519 · 08-10-2011, 09:58 PM

Quote:

Originally Posted by Seven333

Wow, thanks everyone! So is a 2x4 as shown the preferred material? I've seen people use 4x4s but attached them to the inside of the posts, not front and back like the guide and in the attached pic. Seems like front and back would provide more surface area. How long of a piece should I use? And this may sound silly, but won't the bolts (carriage or lag) weaken the posts somewhat? Thanks again! This puts my mind at ease. I'm really amazed that the inspector didn't say anything...

I'd use 2x4's for the bracing. The bolts for the bracing won't weaken the 6x6 any more then the top beam bolts.
The longer the bracing, the more stable the deck.

Joe Carola · 08-10-2011, 10:31 PM

Quote:

Originally Posted by Seven333

Wow, thanks everyone! So is a 2x4 as shown the preferred material? I've seen people use 4x4s but attached them to the inside of the posts, not front and back like the guide and in the attached pic. Seems like front and back would provide more surface area. How long of a piece should I use? And this may sound silly, but won't the bolts (carriage or lag) weaken the posts somewhat? Thanks again! This puts my mind at ease. I'm really amazed that the inspector didn't say anything...

I just drew that real quick. Nail the 2x4's on the back side of the 6x6's so that you don't see them as much.You don't have to bolt them.