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GeraldMadison 05-09-2019 08:15 PM

Deflection Calculations
I have a floor that spans 11'2" from an exterior block wall to a double 2x12 beam on cinderblock piers spanning 6' apart. I added a 2x10 syp joist to the side of every joist that is beneath the bathroom floor and replaced most of the 3/4" plywood subflooring with advantec and added 1/2" BC spaced 1/8 at all edges and screws 4" at edges and every 6" in the field. My question is what is the deflection of this floor? Deflecto doesn't consider double joist and adding the width to give a single joist width would not be correct I'm sure.

Tool Guy - Kg 05-09-2019 08:37 PM

Welcome to the forum, Gerald! :wave:

I could start giving you some answers making assumptions, but let’s not assume.

What size are your original joists?
Are your original joists the same species of SYP or.....?
What is the O.C. spacing of the original joists?


PC7060 05-10-2019 05:07 AM

What Bubbu said... let us know the original size and spacing and we will be a able to give you a reasonable idea of the deflection.

Sounds like you’ve done all the right things so far. Are you planning to put a natural stone on this floor?

Kman 05-10-2019 07:14 AM


Originally Posted by Davy
Are you planning to put a natural stone on this floor?

Sounds like he's planning on parking a truck on it. :D

Gerald, the Deflecto may actually do those calculations for you, but like the others said, we don't have the size and spacing of the original joists. If they're also 2x10's, the you would just change the joist width to 3", which is what you have with the doubled up joist now.

That's assuming you sistered it properly, i.e. the sister has to be at least 2/3 the span, it should be glued and nailed/screwed to the original joist, and the original joist is in good shape.

wwhitney 05-10-2019 09:34 AM

FWIW, there's a simple formula for determining deflection when you have sistered two different types of joists. Namely, if joist type 1 alone would have deflection L/A, and joist type 2 alone would have deflection L/B, then together they have deflection L/(A+B).

So if your original joist and their sisters have slightly different dimensions, or are different species/quality, you can run the deflectolator twice and add the denominators.

Cheers, Wayne

GeraldMadison 05-10-2019 11:39 AM

Wow! So many replies and so short of time. The joists are all the same maybe an eighth over 9 in tall. we sistered to every joist from one end to the other with no breaks. All are home runs. We used 3 in screws fastened top and bottom, every 10 in. The joist were originally spaced 16 on Center and we doubled every joist. Most on the same side some on the other due to the fact that there were other things in the way, i.e. Plumbing Etc.. The 3/4 plywood we left in place, we blocked with two-by-fours flat and screwed the subfloor to blocking where there were no t&g. We replaced what we took out with 3/4 inch advantech. We then installed half-inch BC plywood and used inch and 3/4 screws and fastened as mentioned above. I don't know as far as what species the southern yellow pine that we used or what was there to start with but all was in great shape with the exception of one area near the shower where maybe the top quarter inch or so was compromised buy water damage. I started to put 3 in in the deflection calculator but as I understand it a 3 inch solid Timber would be different in its ability to handle a load than two inch and a half timbers fastened together. If that is an incorrect statement then so be it I will just plug in the numbers and see what I get. Thanks to all who replied to my post! I will check back for any additional input.

PC7060 05-10-2019 04:27 PM

I ran the numbers on the tools; looks good.


Originally Posted by Deflecto
Thank you for using the John Bridge Forums Deflect-O-Lator :-)

For joists that are SYP or Douglas Fir, in good condition, 9 inches tall, 3 inches wide, 16 inches on center, and 12 feet long between supports, the deflection calculated is 0.138 inches.

This translates to a deflection of L / 1043.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile or Natural stone, Congratulations!

Those results aline with my calculations too.

PC7060 05-10-2019 04:29 PM


Wow! So many replies and so short of time.
It’s a good excuse for not doing my labor intensive work! :D

GeraldMadison 05-12-2019 08:05 PM

Thanks for the input! I would like to know what, if any, difference there is in solid timbers v/s double or triple joist of the same girth. I just don't see in my pea brain how they could be the same. :scratch::scratch::scratch::scratch::deadhorse:

PC7060 05-12-2019 08:12 PM

Multiple section beams are treated as a single integrated structure provided the separate elements are attached using approved methods. Could be nails for joists or structural screw for LVL sections; just have to follow the approved schedule.

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