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damo2000 02-11-2013 10:26 AM

Fixing Floor Sagging with Sistering
Good morning,

I have an issue with some floor sagging and deflection. Can anyone tell me if they have experienced the problems I mention below, and if they have advice on the fixes?

Background: I have an older home, built in 1959. The room/floor I am concerned with is just under 14' wide and 19' long. The joists run the 14' length. The room has hardwood flooring, which was sanded and polyurethaned at least once before.

From the basement, the joists are mostly accessible (air ducts and various cabling is in the way). The joists are close to 16" on-center, but are only 7" tall (i.e. a 2x7?). The distance for the supports for the joists is just under 14' wide.

Problem: Stringing a line across the 14' walls, I determined the floor sags over 1/4", mostly in the middle of the room. More importantly, an adult walking across the floor is enough to make the floor shake the room. A good hard jump in the middle can make the china hutch wobble slightly and make waves in the fishtank. I have tried blocking between the joists, but with little improvement.

Proposed Solution: The deflection is more critical to me than the existing sag. What I want to do is sister the joists with glue and bolts. Then, use blocking with screws and glue. I wish to sister three joists, roughly in the middle of the room (spaced about two apart).

Some issues I have so far.
1) Without serious effort to temporarily remove electrical cables and air ducts, I can only get to about five existing joists, which are roughly in the middle of the room.
2) Without the temporary removal of cables/ducts, the sister joists cannot reach the supports. They can get close (about 12'), so they will have to be attached to the center.
3) The sister joists will be ripped to 7" in height, to match the existing joists. I don't wish to add any additional joist height, as the basement is only 7' as it is. I hope to finish it within the next 18 months or so.


1) Considering the dimensions of the existing joists, has anyone tried to do this solution above? Was it successful?

2) Will the proposed solution work?

3) Is there any benefit in only centering the sister joist, or will this not do enough? Is there any harm in this?

4) Is there any reason ripping the sister joist to 7" will not work?

5) Is there any reason sistering only the most-accessible 5 joists won't work, or is the only way to do all the joists under the room?

6) Curiously, is there a standard or requirement of how long/high the joist needs to be for a room of this size?

7) Is two days to finish this proposed project unrealistic?

8) Is the idea of leaving the 1/4" sag alone reasonable? Obviously, I would perfer perfection, but is it normal to have 1/4" over 13', over 50 years?

Thanks in advance,


damo2000 02-11-2013 10:28 AM

Fixing Floor Deflection with Sistering Joists
Sorry for the misleading title. Please consider this one above.


joecaption 02-11-2013 10:41 AM

One beam made of double up 2 X 8's in the middle of the span sitting on footings at right angles as the joist and block piers will take out all the bounce and allow you to even take out that sag.

JazMan 02-11-2013 04:27 PM


I agree a beam would be easiest and probably best. I'd consider 3-2x8" or lam beam. However, you're gonna lose height and you said you don't want that.

So, looks like you should sister the joists. You really should do them all. Yes, just sistering the center will help, do the mid 2/3 at least. I'd use some beams and a jack to raise and hold the joists, then install the sisters.

Your joists are way over-spanned of course. They're 2x8" which usually (now) measure 1.5x 7.25". They should not have been spanned more than about 12' depending on species and grade. Someone goofed, architect, builder, framers and building department, to name a few.


brucem609 02-11-2013 07:49 PM

I did not see it mentioned should also give the floor a little hydraulic jacking to see if you can level it prior to adding the sisters. This way when you are finishers with the job, you have a more level floor with no deflection.
Don't just put a band aid on the problem, fix the problem first, then make the correction to keep,it that way.
Oh by the way, if it was my house jack it up, and put a 1/8" steel plates on either side of the main joist have the holes,drill in them for you so,they are level, jack,up the joist, insert the steel, and insert bolts. It won't move then

JazMan 02-11-2013 10:36 PM


Originally Posted by brucem609
I did not see it mentioned should also give the floor a little hydraulic jacking to see if you can level it prior to adding the sisters.

Someone already said to do that. Good point!:thumbsup:


Originally Posted by JazMan
I'd use some beams and a jack to raise and hold the joists, then install the sisters.


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