Hey all,

I was directed here from the FAQ page. How can I tell if my floor meets the L360 or L720 deflection req. Its an older home...the joists are 2X8, 16" spacing and 11 feet unsupported span (I think).

What would I have to do to meet the L720 req( I bought travertine), if that's not possible, how about L360?.

Thanks in advance.

Your floor is just over the L/360 so its probably ok for ceramic tile.
To be able to install travertine, the joist span would have to be shortened somehow to 9'-9" or less.
The other option is to sister all the joists in the tiled area.

Two layers of 3/4" plywood or OSB over the floor joists along with Schluter Ditra may help but I wouldn't guarantee it.

Sistering the joists would really help remove deflection from the joists, but I'd also go with the extra plywood to remove deflection from between the joists.
If you have to remove blocking from between the joists to accomodate, be sure to replace with solid blocking when you're finished.

You could also bridge a support mid-way under the existing joists if that is an option.

Thanks so much for your replies. I am a complete novice here, so I'll try to address each comment.

>> Your floor is just over the L/360 so its probably ok for
>> ceramic tile.
So, the floor is okay for L360, correct. If I decided to go ahead and return the travertine and find a ceramic tile, all I would need is 1 layer of 3/4 plywood or OSB along with Ditra and I should be fine, right? What about Hardibacker? I already purchased that.

For L720, that's where the extra work comes in.

>> The other option is to sister all the joists in the tiled area
I am assuming "sistering a joist" means adding...well, nailing another 2X8 to each joist?

I also want to clarify what "unsupported span" means. This is the length of the joist that is not supported by wall or beam, correct? I ask this because my bathroom is over my garage, so obviously there is no supporting wall, however there is a red steel beam. The beam actually sits right at the end of the bath. If my measurements are a bit off, then the beam starts at the doorway of the bath. Is my understanding accurate?


>> You could also bridge a support mid-way under the existing
>> joists if that is an option.
I think it is an option because my bath is over my garage, so the garage ceiling is basically the bath floor. Also I would assume that when I pull up the vinyl bath floor and the sub-floor, I should be able to see to the joists. However, could you expand on "bridge a support mid-way under the existing joists". Do you mean (1) add a support beam to cut the unsupported span to 6 feet OR (2) place 2X4 (or something) in between the existing joists?

Again, I am a complete novice here so I hope I didn't sound too confusing. After I figured how get my floor to the L360/L720 standard, the real issues lies with finding a contractor that is going to do this. I had to fire my current one as he insisted on not pulling up my 2-layers of vinyl flooring and just nailed the Hardibacker on top of my existing floor. Is there a way to find a contractor that is familiar with Ditra?

Thanks for the help.

Quick question... As I was browsing through the site, I read that a specific plywood is recommended with Ditra? If yes, which one is it?

Thanks.

As far as calculations go, you are at about L/340 which just makes it for tile. Thats without adding any additional plywood.
Can you tell us what the thickness of the subfloor is?


Some people use this term to describe the section of a floor josit that spans between two supports.(IE: foundation walls,exterior walls and beams)

That term is not used when your talking about code specifications. It's actually referred to as supported span when doing size calculations.

What Harry means by "bridge a support midway", refers to constructing a wall or placing a beam in the middle of the span under the floor area in question.

You may have miscalculated the actual span of your floor joists. Can you tell me what the distance is from wall to wall and the dimension from the walls to the beam in your garage?



No easy way other than asking each tile setter or contractor you call whether they use Ditra floor underlayment.

Maybe this is a single-car garage?

Hello RC,



Thanks for your help. My house is a raised-ranch, so 4 of my rooms (3 rooms and the bath) sit over the garage. I went and measured the dimensions of my garage and its 22X25(width X depth), which makes sense as the width of the rooms along the front of the garage are 12" and 10" respectively. The red steel beam spans the entire width of the garage, so its 22". It has a post on one end and the other end does sit on a ...ummm.. brick-like wall post..basically its a full brick wall/post that stops right under the beam. I am guessing it was structured that way on purpose. ( I hope this makes sense)

I was wrong about the dimensions, the beam starts about at 12"4' (about the middle of the depth of the garage), but that is the end of the bathroom, so I think my unsupported span is 12" (my bath is a 12X5). If you imagine that you are standing in front of my garage, the bath is in the rear corner on the right.
~