Subfloor

[Eggman73]

I'm getting ready to put down large porcelain tiles. My current subfloor is 5/8 OSB. I'm currently flattening the subfloor and am planning on adding another layer of 3/4 OSB on top of it, followed by 1/4" hardiebacker. Is this acceptable? Do I need to use construction adhesive between the OSB panels? Do I need to use thinset between OSB and hardiebacker?

 

[Tile Tom]

Just make sure it's plenty flat, run each layer opposite of the previous one & offset your seams.
Being as the first layer is 5/8" I would definitely take the time to make sure the layer of 3/4 seams fall on joists. Then use 3" deck screws to secure it to the joists along with some 1" deck screws in between to pinch the 2 layers together.

 

[C.J.]

Just make sure it's plenty flat, run each layer opposite of the previous one

As in 1/4 turning the subfloor?

I used to believe that 1/4 turning the second layer of plywood would add strength to the floor system kinda like plywood itself gains its strength from its cross ply construction. In reality plywood and OSB have a strength axis that runs the long direction of the sheet. This is why subfloor is installed perpendicular to the joists. The subfloor itself resists deflection between the joists. 1/4 turning subfloor would result in dips between the joists. For this reason the second layer of subfloor should be run the same direction as the first layer, perpendicular to the joists. That way you are taking advantage of the strength axis in the plywood, which is the whole purpose of adding a second layer. Definitely offset the side seams by at least 1’

Something else to consider is to look at your joists themselves. 5/8” subfloor is bare bones minimum code which leads me to wonder what your joists are like. I’ve seen plenty of older houses that don’t have sufficient joists to meet L360 (for standard tile installation). If you’re installing large format tiles you will need to meet L720 (or is that just for stone?) which prolly ain’t gonna happen if you have an older house. Just something to ponder and if your tiles should crack down the road you’ll know why.

 

[Eggman73]

Thank you. I just finished reinforcing the joists to the point where there is practically no bounce at all. Will definitely use your advise...once I have it sufficiently flat. With OSB being uniformly flat, do I need to use thinset between it and the hardiebacker?

As in 1/4 turning the subfloor?

I used to believe that 1/4 turning the second layer of plywood would add strength to the floor system kinda like plywood itself gains its strength from its cross ply construction. In reality plywood and OSB have a strength axis that runs the long direction of the sheet. This is why subfloor is installed perpendicular to the joists. The subfloor itself resists deflection between the joists. 1/4 turning subfloor would result in dips between the joists. For this reason the second layer of subfloor should be run the same direction as the first layer, perpendicular to the joists. That way you are taking advantage of the strength axis in the plywood, which is the whole purpose of adding a second layer. Definitely offset the side seams by at least 1’

Something else to consider is to look at your joists themselves. 5/8” subfloor is bare bones minimum code which leads me to wonder what your joists are like. I’ve seen plenty of older houses that don’t have sufficient joists to meet L360 (for standard tile installation). If you’re installing large format tiles you will need to meet L720 (or is that just for stone?) which prolly ain’t gonna happen if you have an older house. Just something to ponder and if your tiles should crack down the road you’ll know why.

 

[Eggman73]

As in 1/4 turning the subfloor?

I used to believe that 1/4 turning the second layer of plywood would add strength to the floor system kinda like plywood itself gains its strength from its cross ply construction. In reality plywood and OSB have a strength axis that runs the long direction of the sheet. This is why subfloor is installed perpendicular to the joists. The subfloor itself resists deflection between the joists. 1/4 turning subfloor would result in dips between the joists. For this reason the second layer of subfloor should be run the same direction as the first layer, perpendicular to the joists. That way you are taking advantage of the strength axis in the plywood, which is the whole purpose of adding a second layer. Definitely offset the side seams by at least 1’

Something else to consider is to look at your joists themselves. 5/8” subfloor is bare bones minimum code which leads me to wonder what your joists are like. I’ve seen plenty of older houses that don’t have sufficient joists to meet L360 (for standard tile installation). If you’re installing large format tiles you will need to meet L720 (or is that just for stone?) which prolly ain’t gonna happen if you have an older house. Just something to ponder and if your tiles should crack down the road you’ll know why.

Thank you. I'm pretty sure I've reinforced the joists to the point where I've exceeded L720. Time will tell

 

[Tile Tom]

As in 1/4 turning the subfloor?

I used to believe that 1/4 turning the second layer of plywood would add strength to the floor system kinda like plywood itself gains its strength from its cross ply construction. In reality plywood and OSB have a strength axis that runs the long direction of the sheet. This is why subfloor is installed perpendicular to the joists. The subfloor itself resists deflection between the joists. 1/4 turning subfloor would result in dips between the joists. For this reason the second layer of subfloor should be run the same direction as the first layer, perpendicular to the joists. That way you are taking advantage of the strength axis in the plywood, which is the whole purpose of adding a second layer. Definitely offset the side seams by at least 1’

Something else to consider is to look at your joists themselves. 5/8” subfloor is bare bones minimum code which leads me to wonder what your joists are like. I’ve seen plenty of older houses that don’t have sufficient joists to meet L360 (for standard tile installation). If you’re installing large format tiles you will need to meet L720 (or is that just for stone?) which prolly ain’t gonna happen if you have an older house. Just something to ponder and if your tiles should crack down the road you’ll know why.

You learned me something today Chris. I have always 1/4 turned any new layers of subfloor. I've always lined up my sheets to not fall on previous seams and for the seams to fall on joists. No science or facts behind it just doing as I was taught/told to do....

 

[C.J.]

You learned me something today Chris. I have always 1/4 turned any new layers of subfloor. I've always lined up my sheets to not fall on previous seams and for the seams to fall on joists. No science or facts behind it just doing as I was taught/told to do....

I also used to believe that 1/4 turning the sheets was the way to go. There was a thread on this exact topic on TFP when it was still around and I learned me something from that thread. That and gutting a house and redoing the entire floor system (joists and subfloor) have taught me a lot.

The joists themselves limit deflection parallel to the joists. The subfloor limits deflection perpendicular to (between) the joists. Subfloor itself is also supposed to be T&G to eliminate movement between the sheets of subfloor itself between the joists. All of these things working together make up the flooring system.

 

[Mark Brown]

The best place to get any and all information regarding underlayment/sub-floor as the term becomes interchangeable from time to time in our world is this

That is the most current and best advice anyone can ever get. Pages 44-67 I think. I included the whole thing because it is full of a lot of useful information when someone has the time to look over it. I know I sure learned a lot.

I mostly learned that a lot of the things people learned are incorrect.

 

[C.J.]

Good call @Mark Brown. Knowing is half the battle. I think there is a lot that we have all been wrong about at some point in our career.

 

[Mark Brown]

I was wrong to ever get in this trade....

 

[C.J.]

I was wrong to ever get in this trade....

I used to think that in ‘08 but not any more. I’ve been doin damn good the past 5 years and just absolutely killin it the last 3. Sure would like to see some of my old high school teachers that said I’d never amount to shit without a college degree cus I think we all know teachers ain’t gettin much respect these days. They do have insurance though.