Kitchen floor installation - reality check

[Guest]

1. Is the 3/4" plywood structuraly enough to support the backer board and tile without cracking the tile or grout? Are the drywall screw sufficient for the plywood subfloor? I will be using cement board screws for the backer board. I could also possibly squeeze in another 3/8" of subfloor.

2. I have not yet installed the cabinets, but was considering putting down the tile first, then the cabinets on top. I want to this because it will make layout easier. What are the pros/cons of doing this? Would you recommend it?

[Guest]

Re: Kitchen floor installation - reality check

Jim

Instead of using drywall screws, use screws rated for subfloor installation (they are much stronger than drywall screws).

Farther down in the Forum listings you will find numerous postings re: your question of tile or cabinets first. I would always prefer to tile first and set the cabinets second.

The manufacturers of tile backer boards all say that your underlayment and joists meet their requirements, assuming the deflection meets L/360 requirements. In a situation like this, if I have any question about the "bounce" in the subfloor, I install a second layer or plywood-glued and screwed- and then install 1/4 inch backer board.

Plywood adds structural integrity, tile backer boards do not.

Rob Z

[Guest]

Re: Kitchen floor installation - reality check

Jim:

Sounds like you are off to a good start. Like Rob said, drywall screws are not a good choice for the subfloor, they are good for, well, drywall. But all is not lost! Use some screws rated for subfloor or deck installation or 8d ring shank nails.

The criteria for the subfloor is meeting the deflection criteria of L/360; the length of your floor joist in inches divided by 360. This is a minimum. This value is based on the theoretical live and dead loads experienced by the floor. The easy way to check it is to look in a span table. Maximum joist spans(the distance between support points) are listed by deflection and loading criteria.

Hardibacker: I have found it difficult to install screws with hardibacker and use hot dipped galavanized roofing nails. Remember, you need to comb out a layer of thinset mortar between the subfloor and the hardibacker. If you do use screws, use the hi-lo rock on screws intended for backerboard applications.

Definitely install your tile prior to the cabinets. Both layout and installation will be easier.

Dave

[Guest]

Re: Kitchen floor installation - reality check

Thanks for the advice. I will supplement the drywall screws with subfloor rated screws.

I am a bit concerned about my floor. The span is 14' 7", so not very big. However, I can visually see some deflection... not necessarily when I walk over it but if someone were to stand in the middle and jump on it a few times... I notice the joists deflect a bit.

Maybe this is nitpicking, I am not sure. I checked a span table, however I don't know what type of lumber the house was built with, nor the structural 'grade' (1,2, and 3 were the options).The span table calculator I found required these so I don't know if the results I got were accurate.

The house was built in 1955, ranch with full basement, I don't if that helps in determining the grade or lumber used. I have tiled my foyer, for which the span is alot less, but it only has 2x4 joists. I notice the same deflection in it when I do the 'jump' test, but the grout nor the tiles have cracked.

In any case, I don't think adding more subflooring would with deflection, but only with flex in between the joists. My joists are 16" O.C. and I have not noticed any flex in the floor between them. I have considered sistering up a few more joists to help with deflection, but I don't know if this is necessary.

[Guest]

Re: Kitchen floor installation - reality check

jim

why don't you run your situation past Paul on the Building Science Forum? or, ask him to read what we have discussed so far and see if he can comment here?

rob z

[Guest]

Re: Kitchen floor installation - reality check

Besides knowing that you have 2 x 10's 16" oc it is important to consider the span. Schulter makes a product called Ditra the is used for problematic situations, they just warrantied a job for me that had 2 x 10's at a 15' 1" span.

[Guest]

Re: Kitchen floor installation - reality check

Jim:

The maximum span for a 2x10 floor joist using the code required 40psf live load is 16'-5". It is my opinion that while this is OK for strenght, it yields a bouncy floor (deflection) and not one I would want to install tile on. Deflections will be greatest at either the center or at the location of a point load, a piano lets say. You are two feet shorter than this but report being able to see the deflection in your joist. Can you tell how much? Is it less than or equal to the allowable L/360? While the grade of lumber can have an impact on your deflection due to an increase in the modulus of elasticity, I doubt you have anything better than No. 2 grade lumber used for your joists.

If you can make the floor bounce by jumping on it then it would be my recommendation to strengthen the floor system prior to installing tile.

Dave

[Guest]

Re: Kitchen floor installation - reality check

I agree that I need to strengthen up the floor. The 'bounce' I've detected is in the middle of the span, right where you would expect. How can I measure the defleciton?

I am under the impression the only way to strengthen the floor would be to sister additional joists to the existing joist. I don't see how adding additional layers of subfloor would have any effect, but this would be effective, I think that it would be easier than sistering an additional joist.

[Guest]

Re: Kitchen floor installation - reality check

Jim:

Do you have access to the bottom of the joists? If so, stretch a string(tight)from end to end on the bottom edge of the joist. Measure at the center to get your deflection. For a 14'-7" span your allowable is 0.4861 inches or about a half inch.

The easiest way to strengthen your floor would be to add a support beam at the mid span of your joists. If this is not an option you could try adding solid blocking between the joists at approximately the third points. Be sure to screw the subfloor into this blocking as well. Another option is to glue and screw a layer of 1/2" plywood to the underside of your joists creating a diaphram.

I am guessing that this is an older home and the problem is the deflection is already there, and has been there. A condition called creep is where defelction takes up a permanent "set" for lack of a better word in the lumber. In other words if you add a beam you cant take the deflection out by jacking it up, you can just support what is there and keep it from creeping any further.

If you are unsure what your best option is have an experienced builder or structural engineer come look at it for advice.

Good Luck,

Dave

[Guest]

Re: Kitchen floor installation - reality check

Jim

Check out the postings in the Building Science Forum re joist deflection problems (posted Aug 25-26). The original post for some reason is not listed anymore.

I traded emails with Professor Woeste-one thing he told me was that the additional layer of plywood adds more mass to the floor and thus resists movement more than a lighter floor.

My posting mentioned what Dave said regarding adding a layer of plywood on the underside of the joist. Unfortunately, I haven't found the original article where I read about the subject.

I have had good luck in the past adding a second layer of plywood-glued and screwed-especially if it's 5/8 T&G. Also, adding the blocking between joists helps by transferring loads to neighboring joists. Just these two things allone can make a great deal of difference.

Rob Z

[Guest]

Re: Kitchen floor installation - reality check

I ended up adding another layer of 1/2 underlayment, I just couldnt get anything more than that without having a HUGE differential in heights between the old and new floors. The 1/2 was glued and screwed, 6" O.C. I then added 1/4" Hardibacker, with adheasive and HiLo cement board screws.

I sistered up a few additional 2x10 to the existing joists, I could not get them all the way to the beams and foundation walls, but was told even going part way will add stiffness. Finally I added blocking between the joists.

I will tell you if it works in a few weeks.

[Guest]

Re: Kitchen floor installation - reality check

How did it work out? I have almost the exact same situation in my new kitchen, 2x10 joists spanning 14 feet with 3/4" tongue and groove plywood. With what I perceive to be alot of deflection. I read somewhere that using regular wood glue instead of liquid nails is better because it laminates the 2 plywoods into one. I will eventually be installing 1/2" granite tile. But please tell me how it works out.

[Guest]

Re: Kitchen floor installation - reality check

I used subfloor adhesive to laminate the 1/2" to the 3/4". I then screwed the 1/2" every 6" on center. I did not necessarily screw it into the joists, but just in to the 3/4".

I didn't end up sistering up additional joists. I just put down 1/4" Hardibacker using thinset mixed with adhesive and HiLo screws. I also used some nails because some of the HiLo's wouldn't sink in flush and they strip easily.

It's been a month since the job was done, it turned out great, floor seems very solid.