Do I need to reinforce the floor before tiles

I get a different answer every time I ask this question, so here goes. My mudroom floor (over unfinished basement) is made of standard 2x8 on 16" centers. Not sure of the plywood subsfloor, so I'm guessing it's standard 3/4" plywood. The house is 8 years old. The span is 10 feet. I want to tile only a 6' by 14' section. I want to use slate, which weighs in at 55 pounds per 10 sq. Ft, so that's (5.5) pounds per sq. ft. I find it hard to believe that a standard floor as described above wont support this. My question to the pros out there - do I need to reinforce the floor to handle the weight of the tiles?
Thanks

When I run the numbers I get L/480 deflection for your floor which is good for tile and porcelain but not for slate. If you have access to the floor from below it shouln't be too tough to beef up the floor. For slate I'd want to add a second layer of plywood as well.

Rich

Terry,

The weight of the flooring is not the point, having a subfloor system that is stiff enough for the tiles is what you have to have. The minimum stiffness requirement (deflection) for natural stone tile is L720. (That means 1" in 720" or any fraction thereof). If I also knew the species and grade of your joists I could better tell you what the chart says the deflection of your joists should equal.

We can come up with different results with the same joists depending on what criteria we want. That is, do we want the joists to be rated for 10lb. Dead weight and 40 lb. Live weight per sq. ft.? (normal minimum for standard flooring) Or, because we are installing ceramic or stone flooring, do we want the joists to be stiffer? The answer is obvious. I recommend using 20lb. Dead and 50lb. Live for ceramic and thin set stone floors. Remember this is only in regards to the joists, a satisfactory joist deflection # does not have anything to do with the subfloor.

We also need the same deflection requirement (L720 for stone and L360 for ceramic) for the subfloor. We know that a double layer of plywood is needed when installing natural stone tiles over a subfloor that is 5/8" or 3/4" and where joists are 16" O. C. Measure the thickness and make sure it's plywood or OSB and not particleboard.

Let us know the species and grade of the joists. I think your joists deflection # is better than what Rich said. I think you are pretty close. Be sure to measure from the begnining (face to face) of the supporting beam or outer wall, not center to center. Unlike Rich's recommendation that he would advise the second underlayment, I say you need it.

You didn't mention what type of CBU or membrane you intend to install over the plywood before the tiles.

Jaz

Thanks for the answers. I made a mistake in my earlier post. The existing floor joists are 2x10. (or 9 1/4 when I measure). They are supported on one end by a poured concrete wall and the other end by a steel I-beam (10 foot span from wall to I-beam). Regarding the grade & species of the existing floor joists. I don't know & I dont know how to determine. It looks like the standard stuff all stores sell, if that helps.
Since I have easy access to the floor from the basement, I thought a series of "sister-joints" would be easy to install - but only if needed.
As for the subfloor, I planned on adding 1/2" plywood topped with 1/4" inch Hardiboard. I did not plan on a membrane.
Do my esisting floor joists seem adequate?
Does my planned subfloor seem adequate?

Terry,

Now you're talkin! The joists are more than stiff enough as is. Once you re-fasten the subfloor, then add the underlayment, you should have a real good subfloor. If you want to sister the joists even better, no need to go the whole length. Be sure to install the underlayment correctly including the proper directiom, nails/screws and gaps between sheets and walls.

Don't forget the thinset under the Hardi too.

Jaz

OK - I measured again from the face of sil on the cement wall to the face of the sil on the I-beam and the span is only 9 feet, which of course makes my floor stiffer than I originally thought. I found a span chart for L/360 and 10-40. I can't find a span chart for L/720. Seems like one should exist somewhere on the internet? Anyone out there have a link?

Also - from my reading, I think I should use Ditri and skip the Hardiboard. I was surprised to discover that Hardiboard adds no strength to the subfloor. Any comments?

So I now think my joists are stiff enough as is. I think the subfloor should be 3/4" plywood and Ditri board. Any comments?

Thanks for your input.

Hardie adds no strength to the floor, what do have now?, you need 2 layers of wood 1/2" or better and the top layer plywood, then you can used Ditra or a CBU, whatever you like.

The joists are fine at that span.

Hi, I have the same question as to whether I should
Reinforce the subfloor before installing a tile floor.

My specs. are:
2x8 @16" O. C. (maybe doug or hem fir)
Subfloor consistes of wood strips 1x4; no plywood here (its a 40 year old house).
Original 3/4" flooring has been removed.
In addition, we found a few termite holes in the subfloor. House had been treated long ago and has not had any further termite problem since.

Now, the plan is to install porcelin tiles over a mud job. Final average height addition = 2 inches.
I know that cement @ 2" depth would add 24#/sq. ft.
Mud is less, 3/8" porcelin tile is more, so its probably safe to use the original 24#/sq. ft.

I, personally, would like to rip out the entire subfloor, replace it with 3/4" plwood, sister every other joist and add solid blocking; however, this is my mom's house and my brothers think I am making her waste money.

So, what do ou guys think?

Forgot one spec: joist span s are 12'0"

I can you that floor won't qualify for a ceramic tile installation with the 12' joist spans. The "mud job" may improve things slightly but I wouldn't go near it.

I agree, your joists system might just barely pass minimum standards for standard flooring, but not for ceramic or porcelain tiles. How much short it is I don't know with only the info you supplied.

You may not have to remove the 1x4 subfloor, but you need to either sister every joist or build a supporting wall under to reduce the span.

Your idea of ripping the sub and installing 3/4 sub is better, but may not be necessary if you do all the other steps right.

Jaz