Sistering Floor Joists

[Dan Mages]

Is there any structural problem with sistering a 2x10 with a 2x8?

I need to sister some floor joists to reduce deflection before I install stone in the front hallway. The current floor joists are in great shape with no structural problems. The joists are 2x10" I cannot sister with 2x10s without having to rip out lots of stuff and creating more work than it is worth. I can get a 2x8 in there with no problems. Are there any issues?

Dan

[Mike Null]

I have no expertise here but I would think 3/4" plywood might be an alternative worth checking.

[David Linnabary]

I have no expertise here but I would think 3/4" plywood might be an alternative worth checking.

Why not do both, add the 2x8 with a plywood layer in between, built it like a header, throw a little construction adhesive between the layers.

David

[David Helm]

You can sister with the 2X8 but be sure your sisters extend from bearing point to bearing point. If you don't all you are doing is adding weight with zero addition of strength.

[Greg Cuetara]

Not sure that I quite agree with David. If you sister the middle part of the beam you will be adding strength to increase the moment capacity of the beam but will be doing nothing to increase the shear strength at the ends of the beam. Both the moment capacity and shear strength needs to be checked if you are dealing with the original structural capacity of the structure. If you are only looking at increasing the stiffness then almost anything you do will increase the stiffness. I would definately glue and screw whatever you do end up sistering to the original joists. Obviously if you match the depth it will give you the best bang for increasing stiffness but if you added a 2x4 it would help...how much is a matter of math. with all that being said if you can block the ends to provide adequate shear capacity also that would again help and provide more stiffness.

I assume like you said there are no structural problems and you are only trying to reduce the deflection so that you do not get cracking in your tile or joints.

If there are structural problems call in a pro to help you out and assess your specific situation.

Greg

[Steve Peterson]

There are web sites that would give better answers (sagulator, etc.), but the simple math is that a 2x8 has about 60% of the strength of a 2x10. The strength is proportional to the square of the height, and 7.25^2 is 61% of 9.25^2.

Can you put a 2x8 on both sides of the existing joists? That should double the existing strength. Glued and screwed would also help by preventing knots from acting as a weak spot on individual members.

Steve

[Von Bickley]

What about a perpendicular dropped sill under the existing floor joists.

[Jerome Stanek]

is there bridging between the joists

[Dan Mages]

There are web sites that would give better answers (sagulator, etc.), but the simple math is that a 2x8 has about 60% of the strength of a 2x10. The strength is proportional to the square of the height, and 7.25^2 is 61% of 9.25^2.

Can you put a 2x8 on both sides of the existing joists? That should double the existing strength. Glued and screwed would also help by preventing knots from acting as a weak spot on individual members.

Steve

Both sides of the joist would be pure overkill. I will also have issues with ductwork and piping. I can get one side easily, both sides will be too much work.

What about a perpendicular dropped sill under the existing floor joists.

That is an option, but would require posts placed on concrete footers dug into the slab in the middle of the storage area of the basement... yep, more work.

is there bridging between the joists

Yes, there is bridging between the joists.

I have a bunch of 1/2" plywood scraps in the garage. Would sandwiching the joist with plywood make a difference?

Should the sisters rest on the mud plate and the center beam, flush with the bottom, or flush with the floor?

Dan

[Tom M King]

Use Weyerhauser Framer series 2x8's, and latex fortifier in the thinset and grout. If the span is over 8', I'd get a steel flitch plate or two in there.