Deflect fail - 24" oc joists [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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03-23-2016, 11:05 AM
This is my first tile job, putting down ceramic tile in kitchen and bath. The deflecto-meter says:

"For joists that are Unknown wood, but in good condition, 7.25 inches tall, 1.5 inches wide, 24 inches on center, and 11.5 feet long between supports, the deflection calculated is 0.804 inches.

This translates to a deflection of L / 172.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood."

Over the joists is 3/4" 1x8 planking. Over that is 3/4" plywood, topped by sheet vinyl. Are my tile dreams crushed?

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Tool Guy - Kg
03-23-2016, 11:31 AM
Welcome to the forum, Tim. :wave:

Wow, if those really are your measurements, you would need to strengthen the floor joists before proceeding. I'm guessing this part of your house is several decades old?

Before we go too far, let's ask this: Is this an area where you would be able to work on the floor joists, or is it kind of impossible to get at?

I ask, because if you doubled up your floor joists, you'd be up to L/343. And if you used SYP joists, you could get that up to L/396. Or, if it's possible to cut down the span by building a support under the joists, you could substantially stiffen the floor.


03-23-2016, 12:13 PM
Thanks for the reply, Tool Guy. The joists are accessible via an unfinished basement. However, I'm doing this job to help sell the house. I'm not sure the extra work stiffening the floor is justified since I won't be here to enjoy the fruits of my labor. Looks like my best option is to go back with vinyl, even if that is a less desirable surface for selling the house.

03-23-2016, 01:33 PM
Upon a little further investigation, the top layer of 3/4" plywood is tongue-in-groove. Would the Ditra spec'd for 24" joists spacing get me back in the tile game?

Steve in Denver
03-23-2016, 03:04 PM
I think some of us are having a hard time believing those measurements- is it possible that one (or more) of the measurements is wrong?

In terms of the T&G flooring helping, the short answer is no, not for the purposes of joist deflection.

03-23-2016, 03:49 PM
The Ditra installation manual (and any reputable company's) all require the floor to be stiff enough (joists) AND the subflooring as absolutely need both. It's fairly easy to add more subflooring to reduce the in-between deflection, but only shortening, sistering, or adding more joists can improve the strength along the joists. Adding more subflooring does very little to improve along-the-joist strength, only in-between them.

FWIW, that flooring system should never have passed an inspection, if it is as you stated.

03-23-2016, 03:54 PM
1x8 is incorrect,sorry. Those planks are 3/4" thick, 6" (or 5 5/8") wide. The plywood on top is 23/32". Joists span 11.5', and are 1.5" thick, 7.5" wide, 24" oc. It is an older house, built in early 1950's.

03-23-2016, 04:29 PM
I know little about this subject, so you'll have to forgive me. What is substandard about those dimensions, from a Ditra perspective? Ditra has a spec for use with 24" oc joists and a subfloor consisting of 3/4" plank under 15/32" plywood, which is less subfloor thickness than what I have. Is it the length of the joists from beam to beam that is substandard, or the dimensions of the joists themselves?

03-23-2016, 05:09 PM
Problem isn't related directly to ditra, you'd have the same problem with any type of decoupling layer. Basically your floor needs to be very stiff to prevent tiles and grout from cracking and separating. A floor behaves a little like a trampoline when you walk on it, the stiffer the floor the less the deflection.

You have a soft trampoline. ;)

Is there wiring or plumbing in the way of sistering new joists next to the old ones?

03-23-2016, 05:43 PM
Unfortunately, there is a maze of wiring, plumbing and ductwork that would make sistering serious work.

Strange thing is, the floors all feel really solid. One tile contractor was so impressed by its solidity, he wanted to skip any underlayment except for something called Ardex (?), which is some kind of poured-in-place deal.

Houston Remodeler
03-23-2016, 05:59 PM

FWIW my house was built exactly the same way, with kinda sorta 24 on center joists. Every time someone walks across a room we have to adjust the pictures on the walls.

Although it was built in 1927. To install tile we added a beam at 5.5 feet and some more joists to make the granite happy

03-23-2016, 06:32 PM
Housing standards call for a residence to have stronger floors than yours does. ANY installation of tile requires that minimum (or, preferably, a bit more). Schluter calls for the framing to be structurally sound, and it is the industry standards that the deflection must be L/360 or better (stiffer) to install ceramic tile (it must be twice as stiff to install natural stone tile). Yours does not meet that minimum. And, over 24" OC, it requires TWO layers of subflooring to meet the deflection requirements between the joists.

ANY tile installation would need at least the minimum requirements, it is NOT a Schluter specific requirement. Many installation methods and materials do not support 24" OC, but Schluter has tested and verified theirs works, but only if the whole structure meets at least the minimum.

Tool Guy - Kg
03-23-2016, 07:48 PM
You might want to consider DuraCeramic ( This is a hybrid product that looks like a 16" square tiles. The core is made from crushed limestone (so it's slightly flexible) and an extra durable vinyl layer forms the top. They are installed over plywood using adhesive and can be spaced apart for the look of grout joints. You can get the look of tile without having to strengthen the joists. :idea:

03-23-2016, 08:39 PM
How long would a beam in the basement need to be? How long would posts at each end need to be? Got a concrete floor down there or dirt?

Might be cheaper than you think to solve this. :)

03-25-2016, 05:30 PM
Thanks to you all for saving me from an expensive mistake. I've decided to stay away from tile.

03-25-2016, 06:50 PM
Good call. Glad we could help. :)