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-   -   3/4 inch dip in center of sub floor, what to do? (https://www.johnbridge.com/vbulletin/showthread.php?t=98376)

iagaoe 01-05-2012 09:49 AM

3/4 inch dip in center of sub floor, what to do?
This is my first house that is above grade, all other houses i've owned have been slab.

My kitchen currently has laminate glued to plywood sub floor. The wood is in good shape, but we noticed a dip in the middle of the floor. This dip at its max is about 3/4 of an inch. It spans probally 3-4 feet with the max dip in the center.

We will be tiling this floor, and will be using schluter ditra over top of the plywood. My question is is it ok to float alittle extra mud under the ditra and let it dry and then tile over top of that? Should i use leveling coupond before i mud for the ditra? can i leave the dip, because our island will actually be straddling the dip sideways? or lastly i have a guy say he could jack up the sub floor and reinforce it from under the house?

This is a house we'll be living in so i want to make sure i do the right thing. Any tips would help.

Thanks for any replys!

bbcamp 01-05-2012 09:56 AM

Chad, that is a rather significant dip. Do you know what caused it?

Assuming your floor joists are adequate (check them with our Deflecto tool), I would consider removing the subfloor in the affected area, and sistering the joists to bring them up to plane with the rest. Reinstall the subfloor, then install the Ditra and your tile.

If your joist are sagging because they are undersized, you need to install a beam and reduce the span.

iagaoe 01-05-2012 10:01 AM

So in other words....don't work around the problem by trying to float extra mortar, but insted address the problem directly?

I kind of know thats the answer i'm going to get but wasn't sure if a self leveler or mud bed would be suffiencient?

He mentioned he thinks its sagging between the joists due to normal foot traffic and house settling?

But what do i know...?

bbcamp 01-05-2012 10:05 AM

Some sagging is inevitable, but yours sounds excessive.

I would prefer to fix the cause of a problem than apply a band-aid fix. I'm not saying that a patching compound is a band-aid or that you won't use it, but because your dip is sooo deep, I'd think little time researching the root cause and fixing that would be time well spent.

Bodie Powers 01-05-2012 10:55 AM

It seems unusual that settling would cause an abrupt dip over a 3-4 foot area. I'm wondering if a single joist become bent due to green lumber curing around an abnormally large edge knot, causing a sudden dip in that spot. I'd be curious to see what you find under the floor.

Regardless of the cause, do what Bob said.....fix it at the source.

iagaoe 01-05-2012 11:37 AM

Ok so if i attack it from the source, jack up the floor, level and brace it, can i lay the ditra straight ontop of the plywood that is there? Or would it be better to rip the ply and put cement board down?

The plywood has laminate, will i need to scrape all the laminat off?


bbcamp 01-05-2012 12:02 PM

You can adhere Ditra to plywood. The plywood has to be clean enough for thinset to stick to it.

iagaoe 01-05-2012 01:09 PM


Found out that there is no beam, that is whats causing the sagging.

We're gonna pour another footer, jack the floor and support it. Hopefully that will correct the dip?

Once that is done, do you think i should rip the plywood floor and put cement board down? I'm getting mixed information, some people are telling me to, others are saying not to so that i keep the integerity of the floor(it is pretty solid)?


bbcamp 01-05-2012 01:15 PM

You said laminate. Are you talking about some sort of sheet material or what? Pictures would help.

iagaoe 01-05-2012 01:22 PM

No, the house is above grade so there is the sub floor, 3/4 inch plywood, then laminate glued to it.

What would be better choice once the dip is corrected. Should i rip the plywood off of the subfloor and install cement board and skip the ditra, or should i clean up the plywood(remove the laminate) and use ditra?

Just curious which would provide the better longevity and also provide the best strenght?


bbcamp 01-05-2012 01:25 PM

I understood that this was above grade. I am confused about the type of floor covering you are calling laminate. Is this like Formica? Or is it like lineoleum?

iagaoe 01-05-2012 01:32 PM

Doh, lanolium....geez, i'm sorry!

Just that plastic glued to the floor...ha

bbcamp 01-05-2012 01:38 PM

Scrape it up. If the subfloor looks good, use Ditra. If you ding it up too bad, use backerboard or install 3/8" plywood and then Ditra. (The second option, plywood and Ditra, would give you a great subfloor, so if you have the height, consider it.)

Had it been Formica-type laminate, I would have said to install backerboard right over it. Vinyl flooring is too spongy for that.

iagaoe 01-05-2012 02:05 PM

thats the thing i don't have the height to add much more.

The plywood now is level with the wood floors in the dining area. So regardless i'll have a lit and need a transition but if i add more plywood i think it would be too much.

I think the options i was going with was putting ditra on top of existing or ripping existing to replace with cement board.

Thanks for all your time!

iagaoe 01-05-2012 02:08 PM

ohh and i'm not too familiar with formica type?

This stuff is old....house was 55-60's....how do i know what type it is...i would love to just lay right over top of that so i don't have to scrape, but if i can't i'll get to work.


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