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Old 06-02-2010, 11:38 PM   #1
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Another underbuilt floor issue


I suspect people see this fairly often. Anyway, I have been googling around and whatnot but wanted to ask my somewhat specific question.

I recently bought a house that has one sagging/bouncy floor, in one room. This is a second story bedroom above the dining room.

The span is about 15'7" and when I tore up the ceiling I discovered the joists are 2x6. The house is about 50 years old and these are genuine 2x6" joists. Well, they are obviously bouncy as hell.

I'm not trying to achieve perfection here, and I'm not worried about removing the sag, just supporting the floor to a reasonable level.

I have started by installing a 5/8 subloor just to even out the weight a bit.

Next I plan to install blocking. I don't think this will really do the job, so I'm planning on following that with a post/beam support through the middle to help support the joists.

The biggest issue is that the ceiling is already very low - about 6'8, so I'm loath to have a large beam running across the room. I intend to put the posts into the room somewhat, so the span for the beam will be about 10'.

Basically what I'm hoping to get some advice on is what materials should I use? Oh, one of the posts will be near an exterior wall, the other I will support from below in the basement with another post in line.

I have a friend who is a very good and respected contractor who is sort of guiding me through this, but as he is already investing time/energy in another project on the house I really want to be able to tackle this mostly on my own. He has looked it over though and thinks a post/beam will work well and do the job.


So, I'm hoping to lose the least amount of headroom possible while still supporting the floor reasonably well. Any suggestions on what type of materials and sizes I should be looking at for the beam? What do people typically use for the posts - are we looking at just 4x4 wood beams or am I over simplifying?

Is there anything that is only about 4-5" I can use over a 10' span that will do the job?

Thanks to all for any advice on this at all, I appreciate it.

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Old 06-03-2010, 09:45 AM   #2
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Another underbuilt floor issue


I would recommend a steel I beam. You can get them 3" to 4 " tall. This is the only thing that will give you the support you need with the height restrictions you have. It can be supported on the ends by double 2x4's of 4x4's. Posts should be set in existing walls on plates. Don't just sit them on the floor unless there is a joist directly under them. Don't want your supports to break through the floor. The steel beam will also straighten the floor above. What I do is set one end of the beam at the proper height then jack the other end until all joists are touching the top of the beam. Lock the beam in place on the ends with 2x4's on either side of beam lIl and supports below.

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Old 06-03-2010, 11:02 AM   #3
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Another underbuilt floor issue


Cool, that's what I will start looking for then - a steel Ibeam and some plates.

When you say the posts should be built into the wall...is there an issue structurally with my plan to set them maybe 18" into the room. I figured that would reduce the span and I will integrate them into the room so I don't mind them being there.
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Old 06-03-2010, 02:30 PM   #4
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Another underbuilt floor issue


could you slap up some 2x4s or 1x6s (glue and screw) onto the bottom of the 2x6, making it half an I-beam?

that would only lower you ceiling the thickness of the boards

you might even put some angle iron up instead. It wouldn't even have to span the whole area, even the middle 8' I bet would make a big improvement.

That being said, a big I-beam the efectively cuts the span in half would be better.
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Old 06-03-2010, 02:49 PM   #5
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Another underbuilt floor issue


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Originally Posted by forresth View Post
could you slap up some 2x4s or 1x6s (glue and screw) onto the bottom of the 2x6, making it half an I-beam?
That wouldn't do much, if anything. The vertical member of an Ibeam is what gives it stiffness. Just sistering another 2x6 to the floor joist would be more effective, but putting posts in the center is going to give the best results without losing too much height.
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Old 06-03-2010, 03:23 PM   #6
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Another underbuilt floor issue


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That wouldn't do much, if anything. The vertical member of an Ibeam is what gives it stiffness. Just sistering another 2x6 to the floor joist would be more effective, but putting posts in the center is going to give the best results without losing too much height.
That's what I've been thinking. I was going to put some verticals along the bottom after doing the subfloor and blocking, but I've come to the conclusion that the only thing that is really going to get it steady (remember we are talking 2x6 at 24"oc spanning almost 16') is a post/beam setup - which will honestly probably be easier than trying to sister in joists with having to take out a fair bit of electrical and all.
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Old 06-03-2010, 03:32 PM   #7
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Another underbuilt floor issue


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Originally Posted by acme54321 View Post
That wouldn't do much, if anything. The vertical member of an Ibeam is what gives it stiffness. Just sistering another 2x6 to the floor joist would be more effective, but putting posts in the center is going to give the best results without losing too much height.

I think if you did the math, you'd find you are wrong about saying another 2x6 next to a joist would be stonger than a 2x4 under it.
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Old 06-03-2010, 04:14 PM   #8
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Another underbuilt floor issue


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Originally Posted by forresth View Post
I think if you did the math, you'd find you are wrong about saying another 2x6 next to a joist would be stonger than a 2x4 under it.
Damn! You're right.

My way has more headroom though
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Old 06-03-2010, 07:10 PM   #9
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Another underbuilt floor issue


There is currently some sort of half-assed 2x4 running perpendicular and then some short pieces nailed to the bottom, but I'm guessing that really isn't doing much compared to having full span 2x4 running along the underside. I was going to tear all that down once I'm done the bridging, as I don't believe it is doing much to help (hope I'm not wrong, but it doesn't look like it is).

Hell, maybe I'll try the 2x4 under the joists before going beam... I guess it depends on how much the blocking helps, if it's close, I might try the 2x4 and hope i don't need the beam.
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Old 06-03-2010, 08:29 PM   #10
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Another underbuilt floor issue


Consider using a lam beam and hangering the joists off of that---

The beam would be mounted level with the top of the floor joists--a section of each joist would have to be removed --the width of the beam. Two temporary support walls will be needed to hold up the floor while the beam is installed.

Decks are supported this way when the deck is to low to the ground for a beam to go beneath it.

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Old 06-05-2010, 10:47 PM   #11
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Another underbuilt floor issue


Just a small update, I haven't gotten too far yet, but having laid a solid subfloor I'm somewhat more optimistic that after finishing with the blocking I may not need the post/beam, which would be nice. Just the subfloor has solidified things pretty nicely.

Thanks for everyone's advice so far...back at it tomorrow.
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Old 06-06-2010, 05:08 PM   #12
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Another underbuilt floor issue


For existing span there should have been 9" x 2" joists at 16" centres, which would suggest that someone has previously taken out a supporting wall!
Existing joists are suitable for a span of aprox 10' which would fit in with your fix.

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Old 06-19-2010, 11:35 PM   #13
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Another underbuilt floor issue


Another update - installed subfloor and blocking and wow, really fixed it right up. There is really almost no bounce at all to the floor anymore. I'm leaving it at that - it seems good and safe and everything to me. I have a friend who is a (good) contractor coming in to help me (well, I'll help him by handing him stuff) remove a load bearing wall, so he'll tell me if I should do anything else.

Glad that seemed to work so well though.
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Old 06-20-2010, 11:24 PM   #14
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Another underbuilt floor issue


Just for grins, think about installing extra long sheets of plywood to the underside of those joists, glued and screwed. It will give an extra bit of strength - I'm talking about sourcing 12' or 16' long sheets, BTW!

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