Expert: Ted Barnhill (Principal, Design45 LLC) - 1/22/2008

Question
QUESTION: I want to add some support to my kitchen floor which was tiled approx. 4 months ago.  When someone is walking across the floor and you are standing still, you can feel minor vibrations and I want to add some intermediate support so that our new tile and grout do not start cracking.  I am thinking about placing a 4x4 under some of the joist with jack post for support.  I guess my biggest concern is that it is currently winter in North Carolina so the humidity is very low and I didn't know if I should expect the floor system to want to move upward in the spring/summer due to the joist and 4x4 beam expanding.  I'm afraid if too much of this occurs, then I might start seeing cracks due to the upward pressure caused by the swelling of the wood because of the humidity in warm weather.  My current structural set up is 2x10 joists with (2) layers of 1/2" plywood, (1) layer of cement board and then the tile.  Everything has been in place 27 years except the cement board and tile, which have been in place for approx. 4 months.  The joist are 16" on center and span approx 14'-6".  Would the 4x4 with jack post be an adequate way to add some additional support and take the vibrations out of the floor?  Also, is my concern about movement up in the floor due to swelling in humid times a valid concern or not really an issue.  I definitely do not want to do anything that will cause more harm then good.  My biggest intensions are to keep the tile and grout from cracking.  If I can make the floor more solid that would be great too.  On a side note I did consider joist sistering, however, there is so much ductwork and plumbing in this area that I would have to add lots of small pieces to the joist and I didn't think that would really do much good.  Thank you in advance for your help.

ANSWER: John-

Thanks for the well-formed question.  I agree that you have cause for concern about grout and tile failure with so much floor deflection.  Depending on the species and grade of your joists, they are almost certainly undersized for the spans and loads that you described.  At best they would be expected to see the code-maximum deflection of L/360, or about 1/2" in your case.

If joist sistering is impractical, adding midspan support across all of the joists is logical.  For a long-term solution, you should add a beam that carries across to existing or new footings.  This can be as simple as adjustable steel columns on concrete pads, if this doesn't disrupt the use of the space under your kitchen.

I would not worry about the seasonal changes in lumber as you install the beam.  These are much smaller in scale than your deflection issue.  In fact, if you use new lumber I expect it will have a higher initial moisture content than it will ever regain in future summers.

By the way, 4x4 is very small for a beam, and with widely spaced columns you may see sagging over time.  If headroom is not an issue I suggest multiple 2x10s or larger.

Good luck, and feel free to follow up with additional questions.  -Ted

---------- FOLLOW-UP ----------

QUESTION: Thank you Ted.  My crawl space actually has a a concrete slab poured throughout and I'm not certain how thick it is.  I doubt that there are existing footings under the part of the slab that I would be installing the beam under.  Should the slab be adequate to support the steel jack posts and beam?  I'm not sure if I do have enough room to use multiple 2x10s nailed/screwed and glued together, but if I do, how many 2x10s should I combine?  Also, what kind of spacing should I have the jack post at if I use 2x10s?  If I'm not able to use multiple 2x10s and have to go with a 4x4 or 4x6, what kind of spacing should I use for the jack post then?  With either case, how close to the end of the beam do the jack posts need to be?  When installing the beam, do I just need to make sure it is snug?  I'm guessing that it doesn't need to be too tight b/c it may cause the floor to bend upward.  Is this correct?  (Hopefully this aren't dumb questions, I just haven't installed support beams before and don't won't to cause more harm than good to my floors).

On a side note: after I sent the question this morning I was wondering if the fact that the wood being a little green when purchased would cause it to have more moisture initially than with the changing of seasons.  Out of curiosity, does wood typically expand/contract symmetrically in all directions?  I'm assuming it probably does, but thought I would ask because I'm not certain.  Also, in general how much does it expand/contract due to humidity.  

Thank you again for all of your help.

Answer
John,

Now that I understand your crawlspace situation better, I would propose that you build a short wall under the midspan of your joists.  This will spread the load of your sagging floors over a much larger area, and avoid the problem of new pads and tight space for your beam.  Be sure to use a treated sill plate.  If you align the studs with your joists, a single top plate is fine.  Shim the wall in very tight to your joists.

On wood moisture and dimension changes, it is a complicated issue.  The three directions (tangential, radial, and lengthwise, I think they are called), relative to the tree, have different shrinkage rates.  The tangential is twice the radial, and wood hardly shrinks lengthwise at all.  The moisture content is up to 20% in green lumber.  If your wood gets very dry, a shrinkage of a few percent is the most I would expect.  So, 1/8" or so for a 2x4 or 2x6, worst case.

Good luck!  -Ted