Fix for bouncy composite deck

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Old 07-24-10, 04:09 PM
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Fix for bouncy composite deck

I had a deck built last summer with composite decking and treated framing. It is 14’ perpendicular to the house and 20’ wide. 6X6” treated posts and cojoined 2X12 beams support it at 12’ from the house Joists are 2X8 and 12” OC. 2X8 blocking is at 6’ from the house and 2x6 blocking is at 3’ and
9 feet from the house. It meets code but it still bounces. After it was completed he put in a 2x10 supported by 4X4s at a 45 degree angle up from the 6x6 posts along the width at
9’ from the house to try to fix the bounce. It didn’t do much to alleviate any bounce. He has suggested steel “L” shaped framing on the underside of every other joist to prevent the joists from bowing. The steel would need some sort of coating to prevent rust. Does this sound like a reasonable ‘fix’? He admits that he should have used 2x10.

His other suggestion was treated plywood covering the entire understructure—with holes drilled to let moisture and snow drip through. We don’t get a lot of rain in Western Montana but it still must drain away whatever comes down. This sounds like a lot of dead weight under the deck and I am not sure this would be the best solution either.

If blocking should be the same size as the joist, would adding
2X8 blocks up against the two rows of 2X6 blocks (at 3’ and
9’) help? Or would simply adding blocking between the current three rows be a better choice—that would put blocking at about every 1 ½ feet.

I would appreciate any advice on which method, or any combination of fixes, will stop or at least minimize, the bounce. I have the skills to manage any of the above but would also welcome a simpler and/or better fix.

Thank You.
 
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Old 07-24-10, 08:45 PM
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I would just guess that you need another 2x12 header beam installed the length of the deck, parallel to the other one, but directly underneath the blocking that is located 6' from the house. The header would obviously need to be supported, perhaps with 4 more 6x6 posts... one on each end, and the others on 80" centers.
 
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Old 07-25-10, 12:13 PM
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Welcome to the forums! 2x8's are minimal for that distance, although probably approved. If he admitted he should have used 2x10's, it solves it for me. As XSleeper says, an additional beam the length of the deck with proper support may be a solution. How are the 6x6's installed? Curious.
 
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Old 07-25-10, 12:56 PM
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Deck

How high is the deck above the ground?
Is some of the movement lateral due to insufficient bracing?
 
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Old 07-25-10, 03:05 PM
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He originally suggested that I put another parallel header under the center of the deck. There is a concrete pad ( 9 x 24) under the deck and we use that area quite a bit. I was looking for a solution that didn't cut into the space under the deck.

The 6x6's are on 12" concrete pillars sunk about 4 feet into the ground. The surface of the deck is 9" above the concrete pad below. There is no lateral movement that I can detect, just bounce. (I fixed a lateral sway on our old wood deck with two 2x8's screwed to the underside from corner to opposite corner. It extended its life by 3 years but I finally relented and had a composite built.)

Is there an alternative solution to minimize the bounce?
 
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Old 07-25-10, 07:05 PM
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If the deck is only 9" above the slab, how is that area useful?
 
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Old 07-25-10, 08:04 PM
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LOL! I'm guessing he/she meant 9'! I think you will have to bite the bullet and put in the beam. It is the only real solution, aside from tearing the deck down and starting over. Sorry if that isn't what you want to hear.

No amount of additional blocking or even adding plywood onto the bottom (dumb idea) will help beef up an undersized joist.
 
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Old 07-26-10, 04:50 AM
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I could not think of the word "dumb" for the plywood. My words were much more stern, but dumb suffices. Yeah, I thought 9' was correct, but I wanted to make sure. Your posts may can extend to either side of the pad and not infiltrate your area. Check into it.
 
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Old 07-27-10, 11:35 AM
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Do you suspect your bounce is coming from the flexing 2x8 joists? If so, I would recommend sistering all of them and maybe sandwiching some exterior plywood between the main and sister joist.

Somewhat different situation, but I once did this in a bathroom where I needed(wanted) to install ceramic floor tile, but couldn't increase the joist size or structure below(basement). There was noticeable bounce before I did the above mentioned method... afterwards it was SOLID!
 
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Old 07-27-10, 12:54 PM
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If it were me I would try doubling joists. If it works it's a cheap and easy fix and you won't use any of your slab space. I would probably start with every other joist.

I'm still trying to wrap my head around how plywood might stiffen the floor. I'm not there yet.
 
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Old 07-30-10, 03:34 PM
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Thank you Baha and Wayne for the suggestion.

When sistering joists do I use commercial adhesive between the joists as well as screws to attach them to each other?

Is it also necessary to screw the deck boards to the sistered joist?

(His reasoning for the marine or exterior plywood underneath was to tie it all together which could help increase the blocking. It was just one of many ideas he was thinking outloud.)
 
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