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Deck is sagging

Posted by carterinms (My Page) on
Sat, May 30, 09 at 8:55

DH is installing deck boards and has found that 6 of the 7 the deck beams sag 3/4" at the center. The beams are 2x10 that span approximately 12'. The intermediate supports weren't installed well - they are nailed in, but there are lots of gaps.

The easiest fix would be to jack up the beams and fix the intermediate supports so that they tie in better, but we're not sure that would solve the problem. Other options are to leave the beams alone and shim the deck boards, take the beams down and flip them over, or replace the beams with 2x12's.

Any advice would be greatly appreciated. Here are some pictures of the beams:
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Follow-Up Postings:

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RE: Deck is sagging

My span tables say for a 2 x 10 joist, the max length between supports depends on the distance between joists on-center...
12" OC 19'6" max
16" OC 16'5" max
24" OC 13'4" max

The joists look wider tha 10" to me, which would be approx 9 1/4"? That's what they are?

So the next question is what is your joist spacing OC?


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RE: Deck is sagging

Are they realling sagging?

When installing joists like that they should be installed with the "crown" up. Rarely will you find a 2x lumber that isn't slightly curved towards one side. To deal with this they should be installed with this crown or bow on the top so with additional weight and time they will flatten out naturally. Did he check each board and install crown side up - if not, that is probably what you are seeing.

I wouldn't worry too much about the cross bracing, those aren't intended to and don't add load bearing capacity, they are there to keep the joists from wanting to twist.

If he doesn't have too much of the decking on those joists yet I would remove it and flip them over to put the crown side up. If that's not doable then yes you could shim under each board as needed but that is not ideal and quite frankly will be a royal pain.


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RE: Deck is sagging

They are on 16" centers, and I confirmed the 12' span. One joist is installed crown up, another is crown down. It's hard to tell about the other 5. Nothing would surprise me with the idiots that framed the house.

Some of The ends of the joists are 1/4" lower than the 2x12's they attach to - it may be that they have shrunk at a different rate.

Thanks for the span tables - I was pretty sure that I had checked that when they were installed, but couldn't remember for sure. I'll talk to DH - he was considering adding another double 2x12 to the deck to shorten the 2x10's to 6' spans. Sounds like that would be overkill.


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RE: Deck is sagging

Those span books have the material at Max before failure and they are ment for inside framing those 19'/13' spans would get strongbacks and blocking installed in a house.

Yes well installed blocking will tough up the span and with decks will take out a lot of bounse. Blocking installed like that will do nothing but add to the weight in the center of the span. All the old wood framed wharehouses had blocking or x bracing runs in the joist span to support the next story this is common Carpenter know how nothing new.

A 2x10 12' span on a deck will sag,crown up or crown down,crown up will not produce a dip in the joists it will produce a hump and no it will not flatten out naturaly it will always be there.

Fix.... take out the stupid installed blocking, carefully jack up the joist span a foot or so from center do this with a string across the top of the span. Side note>>make sure all the joist hanger holes are filled with a teko nail<< Find a place while running those two jacks where the top of the joists are close to netural go slow your not killing snakes this is a good job for both of you put on a halter top/red shorts and smile a lot when he starts yelling.

Using new material reinstal the blocking in a very close manor in a stagered line so you can end nail both sides, or install x bracing this is really the best far as strength goes and is what was normal in those old wharehouses you probley have see it in a redone piza house or a weastern dance bar. Anyway it can be done with pt 1x4s and a nail gun find a leangth thats common and cut them all the same.. Just be sure the lay out stays close to right its real easy to just go on with it and change the lay out check it as you go. As you go you both will be really happy because you will feel the thing stifing up.

Keep the jacks on until your finished, let them down slowley. If the joists still have humps/dips out of tolarnace again using a string get a makita 4'' power hand planer and take out the humps.

Thats how the big boys do it but its not really all that hard and wont take all that long. J.


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RE: Deck is sagging

As usual, John nailed it. I like the idea of 1x4 bridging vs. solid blocking because the 1x4's hit at the top and bottom of the joisting. With solid blocking, often times the joist has a "cup" in it causing the block only to hit in the center and throws off the layout if there are enough of them, unless you scribe the block to the cup, but who does that? The 1x4's will give you a better center strength. Why do you see that framing in those old barns, warehouses, even old covered bridges still straight as an arrow. They used bridging! Lumber was primo in dem days as well which helps.


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