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Dovetail Greene
Dovetail Greene, General Contractor
Category: Home Improvement
Satisfied Customers: 373
Experience:  Licensed Building Contractor & Certified Building Designer
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How do I fix a floor joist that is sagging?

Customer Question

How do I fix a floor joist that is sagging?
Submitted: 3 years ago.
Category: Home Improvement
Expert:  Dovetail Greene replied 3 years ago.


My name's Kel.


All structural elements under load sag.

The question is

is the sag excessive for the span?


Can you tell me how long the joist is from one support to another?

A support might be the foundation and a beam running down the middle of the house.


Is the sag causing furniture to wobble?


Does the floor bounce excessively when someone walks on it?


Is there any heavy pieces like a grand piano over the joist that's sagging?


Are its neighbors sagging the same amount?


How old is your home?

Customer: replied 3 years ago.



Span is 17 feet across. Above the sag is a 8' x 8' shower with about 800 pounds of concrete and limestone. Cabinet installer had to shim the 72" vanity on one end. Vanity sits off the floor on one side about 1/2".


Floor does not bounce in the bathroom. However, it does bounce a little in the adjoining bedroom.


My home was built in 1959. I gutted the entire house and reconfigured a stairway and the kitchen that backs up to the large shower/bathroom.



Expert:  Dovetail Greene replied 3 years ago.

Can you tell me how deep the joist is?

Might measure 7 1/2" or 9 1/2" or 11 1/2"


Seventeen feet seems a might long.

Just to confirm -- The joist is unsupported for a distance of 17'?


For a span that long

most building codes would allow a sag of almost 5/8".

If the vanity is near the middle of the span then having to shim one end 1/2" makes sense.


Since the shower's in place you don't want to jack the joist up -- that would would crack the concrete and limestone.


The simplest repair is to install a 4 x 4 perpendicular to the joists near the center on the joists that're sagging the worst.

Usually make it long enough to support three joists.

Then install a 4 x 4 column to support that.

The column runs from 4 x 4 to concrete floor.

Your concrete floor in the lower level will support the weight.


It's much harder to add joists along side of the sagging ones that run from one end to another. The ends must be shaped so it can be twisted into place right next to the first. Then shims are used on the ends to adjust the height and level.

For a span this long you'd have to add 2 new joists.


Does this make sense?

Have I answered your question?

Customer: replied 3 years ago.

Yes, your answer makes sense. The joists are 8" x 12". Sort of rare for a house built in 1959 but it has been a good thing throughout the home remodel.


I think I will have to bite the bullet, have the electrical wires pulled and gas line and sister up the joists and add the joist hangers. The contractor I spoke to agrees, we cannot jack up anything or we will crack the tile. However, we want to stop the movement.


A post down will not work. The lower level will make a great family room and a post will look awkward and ruin sight lines.


Thanks very much. Your answer lines up with what the professional said that came to look at the job. Having your second opinion helps me know I am going in the right direction.

Expert:  Dovetail Greene replied 3 years ago.

Keep in mind that the real strength in a structural element comes from its depth not its thickness. To keep the sagging from increasing given the weight of the shower your need to add deeper sistered joists. If that won't work for interior design reasons then you'll have to increase thickness. Meaning two new elements.


You might consult with a structural engineer to size the sistering elements.


Could also use an engineered product like LVL rather than sawn lumber.

Dovetail Greene, General Contractor
Category: Home Improvement
Satisfied Customers: 373
Experience: Licensed Building Contractor & Certified Building Designer
Dovetail Greene and other Home Improvement Specialists are ready to help you
Expert:  Dovetail Greene replied 3 years ago.
Hello again Ann. This's Kel checking in to see how the sagging joist project's going. . .
Customer: replied 3 years ago.



Thanks for following up. Engineer recommended sistering three floor joists. This requires pulling wires (electrician) and licensed framer. I am getting bids to do the work. The Engineer put two temporary posts in.


Thanks for your advice. I hope all will be finished in the next 2-3 weeks.



Expert:  Dovetail Greene replied 3 years ago.

Hello again Ann,


Best of luck with your project.

It'll be a happy ending.





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