# Help with sistering/blocking floor joists



## live4ever

We are going to be tiling our second-story kitchen in our 1920's home. The joists are 2x10's, 16" o.c., with a span of 14 feet. These barely meet the deflection requirements for porcelain tile, and certainly fail for natural stone. We're still considering travertine, and even if we don't go that route, it seems clear that reinforcing the joists is a good idea.

Since the joisting system is completely exposed from below, I'd like to sister 2x10s to the originals, as well as add some blocking. None of the joists appear to be sagging so at this point I don't think I need any jacking and so far this still seems like a DIY job. I spoke to a framing professional about it, and it didn't seem like it was worth his time, so as long as it doesn't require some major jacking, I'd like to tackle it myself.

What I'm unsure about (don't have any framing experience) is the best way to attach the lumber. Reading online, I've seen everything from construction adhesive plus nails to decking screws to bolts/washers, etc. Could anyone be so kind as to spell out your preferred way to do this? Thanks in advance!


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## glennjanie

Welcome Live4Ever:
First, if you want to gain strength in the floor, you will need to use 2 X 12s. Doubling the 2 X 10s doesn't do much for the bending or flexing of the joists. The 2 X 12 hanging below the 2 X 10 will leave an unsightly situation, so the joists will need some kind of cover or box around them.
You could add a 2 X 2 on the 2 X 10, fill and finish the crack on the side and use a 1 X 4 on the bottom to make them appear to be one piece. Allow the 1 X 4 to extend past the bottom on both sides with an even margin and it will help to hide the 2 X 2 joint.
I would use Contech PL400 and clamp the beams every 2' for 24 hours.
Glenn


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## live4ever

Thanks for the reply, Glenn!

Hmmm - using a 2x12 certainly sounds like it makes more sense but a lot of what I've read online suggests using identical framing members?

I should have added that the joists are exposed in the garage below so unsightliness is perfectly tolerable - in fact, it's quite unsightly down there already!  That being said, if I'm not too concerned about hiding joints, would you go with the 2x12 or the 2x2 method for the best strength and ease of installation?

What kind of screws/nails would you use in addition to the Contech?

Thanks!


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## travelover

Glen is right that a 2x12 will add a lot more strength if you have the space to install it. When I installed ceramic tile in my 2nd floor kitchen I just blocked the floor and it made a huge difference in floor flex. I used construction adhesive and metal hangers attached with drywall screws to simplify the installation of the blocks. 

If you glue the sistered joists together, the screws will hardly carry any load. Plus the glue is really strong in shear. I'd just use decking/ drywall type screws to draw the two boards together tightly.


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## glennjanie

Hello Live4Ever:
I would use the 2 X 12s for the bending resistance. The members could be drawn up with deck screws but make sure you have full contact with the PL400, spread it with a 1/8" notched trowell.
The thought about the 2 X 2s was just to make the  two joists the same height; they will not add any strength at all.
Glenn


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## live4ever

Thanks Glenn.  I forgot to ask one major question though:  what about the myriad of wires and small pipes running through the joists...?


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## glennjanie

Hello Live4Ever:
Hmmm. That's a good question. You can't cut the wires or pipes and strip them out and you can't notch the new joists to receive them.
If you have the extra headroom in the garage, you could put the new joist directly under each existing joist. You will need to hang them with metal joist hangers on each end, which may require another 2 X 8 on each wall, nailed with 4, 16d nails into each stud to mount the hangers. I say 2 X 8 because that is the longest metal joist hanger available.
The new joists will require blocking to keep them from bending out of shape under the load. The 2 X 12 blocks should be 2" up onto the existing joists to keep them and the new ones together. I would divide the span by 3 and put in two rows of blocking at the 1/3rd points.
Glenn


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## inspectorD

Instead of all that work, just put a beam under the joists halfway, you will not get cracked tile that way either.
Unless you are going with some very large tile...over 12 inces, I would install a mud base with wire lathe and call it a finished job. There are mortars and grouts and thinsets which are made flexible for this type job.
I think you are overbuilding it myself. But that is just my opinion.

Cut the load in half with a beam.


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## travelover

Good point inspectorD. Think of an I beam. In bending, all the stress is on the top and bottom web. The top is in compression, the bottom is in tension, there is no stress in the middle. Wood is weaker in compression than tension, so it is important to reinforce both the top and bottom.

You can beef up the floor by simply sistering a board to the top of the joist and a second one to the bottom, with the pipe and wires in the middle. The pipes and wires should be in the middle of the current joist because that way the beam is not weakened.


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## glennjanie

Inspector D is correct, I don't know where my head was although, I used to have a drill instructor who had a pretty good idea.
Glenn


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## live4ever

Thanks Glenn, inspectorD, and travelover!

I think I read inspectorD's method a little differently than you guys did.  InspectorD - did you mean a beam running perpendicular to the joists?  Doesn't that need to be supported by posts?

or--

did you mean sistering along the lengths of the tops and bottoms of the joists?  With that method I could use 2x4's sistered along the top and bottom of each 2x10 joist leaving room for the wires and pipes in the middle, correct?


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## travelover

live4ever said:


> Thanks Glenn, inspectorD, and travelover!
> 
> I think I read inspectorD's method a little differently than you guys did.  InspectorD - did you mean a beam running perpendicular to the joists?  Doesn't that need to be supported by posts?
> 
> or--
> 
> did you mean sistering along the lengths of the tops and bottoms of the joists?  With that method I could use 2x4's sistered along the top and bottom of each 2x10 joist leaving room for the wires and pipes in the middle, correct?



That is my suggestion. Anything perpendicular would not help unless it is blocking, or as you suggest, supported by a post.


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## hondadrv24

A beam perpendicular in the center of the joists, that is supported with posts, would reduce your joists span to 7'  making your 2x10s much stronger.  That is what INSPECTORD was trying to say.  

What is the dimension of this floor to be supported the other way???  If it is relatively short you may not need more than a post at each end, but if it is too long or you don't have much headroom in the garage then you would need to put a post or two in the middle.  

another thought about the wires running through the boards now is that you could cut them and put in a few junction boxes.  the problem would probably be with your drain pipes though as they usually don't like water running up hill to get out of them.   

Just a couple of thoughts 
Justin

eta: if you could post some pictures from the underside we may have some better suggestions for you


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## Todd-Beaulieu

Obviously, this is a really old thread, but I'll give it a try...

I have a single perpendicular beam on posts as an experiment in one room. I really like it for eliminating the bounce so far, but a neighbor tells me I probably shouldn't be trying to eliminate all bounce. His example was dance floors, but I'm thinking that a home doesn't need give.

Does anyone have any experience with that topic?

Thank you.


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## travelover

I've lived in homes where the cabinets with glassware rattled every time you walked across the floor. Not good.


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## Wuzzat?

The joist bottoms are in tension; somebody might make high tensile strength steel strips 1.5" wide by however many feet long that you need, to be bolted to the bottom edge of the joists, once you jack up the span center.

Just two screws with sufficient shear strength at each end of the steel strip may be enough.

I don't know what the mat'ls cost vs. labor vs. inconvenience tradeoff is for this method, though.

http://www.freepatentsonline.com/5497595.html
http://www.freepatentsonline.com/5497595.pdf


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## Wuzzat?

live4ever said:


> The joists are 2x10's, 16" o.c., with a span of 14 feet.
> I'd like to sister 2x10s to the originals



Doubling the joist width means doubling the moment of inertia of its area which means you'll get half the deflection you had before, but I guess the tiles increase the loading per sq. ft. so you'll get slightly more than half the deflection.   

How much does the tiling increase the pounds/sq. ft. dead load?


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## kenncornn

http://www.youtube.com/watch?v=R_OF9KgWhzI


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