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live4ever 02-24-2009 09:46 AM

Help with sistering/blocking floor joists
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!

glennjanie 02-24-2009 10:43 AM

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.

live4ever 02-24-2009 11:24 AM

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?


travelover 02-24-2009 11:52 AM

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.

glennjanie 02-24-2009 03:46 PM

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.

live4ever 02-24-2009 07:20 PM

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

glennjanie 02-26-2009 08:00 PM

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.

inspectorD 02-27-2009 05:38 AM

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.:D

Cut the load in half with a beam.:)

travelover 02-27-2009 06:18 AM

1 Attachment(s)
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.

glennjanie 02-27-2009 10:21 AM

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.

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