Stalled out with SubFloor Level - Ceramic Tile Advice Forums

winty03 · 03-19-2010, 07:06 AM

Good Morning Everyone,

Just tore out our kitchen's flooring to replace the vinyl laminate with ceramic tile. We have stalled out with installed because of a few questions regarding the subfloor. What the floor consists of at this point is(original installed)....floor joists with 3/4 inch plywood on top. We have shimmed the joists from below, where needed, to fix some of the major dips or humps we noticed during demo. We have screwed each sheet of flooring every 6" across the room (dimensions 8" x 15"). There are small drops, (ex. where 2 sheets of plywood meet) and or where the room transitions into another, there’s a slight slope. Should we be concerned with those small dips and or the slope of the floor? We have not done the SLC before...should we get into that to fix small dips? The slope is gradual and would mess with 2 walls and threshold, so I don't believe we want to mess with raising the slope. Lastly should we sand off all the old glue down to the wood, to achieve good contact with glue and hardibacker? Any advice would be appreciated! Thanks

Picture of current floor

bbcamp · 03-19-2010, 07:15 AM

Hi, Chris!

Did you use glue with those shims? We usually don't recommend shimming the joists because shims can split or fall out if not glued in place.

Slight dips, those less than 1/8", can be filled with thinset as you tile.

Slope is not a problem for the tile. If you lived with it , it probably wasn't a concern for you, either. What really matters is getting a flat floor.

Scrape the glue off, no need to sand it down to plywood. The thinset under the Hardibacker will take care of minor imperfections.

After you get the Hardi down, use a straight board laid on edge to check for out of planeness. Let Us know what you find, and we'll help you deal with it.

John Bridge · 03-19-2010, 08:15 AM

Hi Chris, Welcome aboard.

Don't worry about bonding the backer board to the floor. The idea of the thin set is to fill any possible voids under the board. The screws do the fastening. So use the cheapest thin set you can buy for under the backer.

Use good thin set for the tile, though.

winty03 · 03-19-2010, 08:18 AM

So you think we should remove shims and glue them in? What should we do with the shims.....other than that I think we are ok..O about the thinset.....or the glue from the subform to the hardibacker........is that contractors general glue? Or do you use the same glue as gluing down tile? Thanks again!

bbcamp · 03-19-2010, 08:36 AM

If you haven't glued in the shims, take them out and screw the plywood down into firm contact with the joists. We'll take care of flattening your floor after you get the backerboard installed.

You use thinset to embed the backerboard. It's just there to fill any voids. The screws do all the holding. As John said, you can use very cheap thinset for this.

winty03 · 03-19-2010, 08:45 AM

Reading other literature they said to shim...crappy!! Well theres not that many shims...maybe like 10..but its mainly in one area....so we will remove shims and screw subfloor tight, then we thin set and screw down the hard backer right off the bat....then we get to leveling?!!? How do we go about leveling after the hard backer is laid? I know removing the shims going to give us our problems again..."high spot" we had wooden chairs that were being shelled out because of uneven floor and rocking....how do we go about fixing that on top of the hardi backer?

bbcamp · 03-19-2010, 08:52 AM

It depends on how much out of plane you are. That's why you measure after the backerboards are down. We'll figure out what's best for your floor.

As far as shimming goes, many pros do it (I don't know of any of our regulars, though). We don't recommend it for the reasons I gave earlier. You will hear many different ways to do things from different sources. It's up to you to decide which source you want to listen to.

winty03 · 03-19-2010, 09:19 AM

Ya we have got many variations.....just first time we have delt with a bigger floor with some uneveness. The smaller floor we tiled was rather level..... how do you go about and or measure after the backer board....I am thinking our high spot..we are 1/4" low one joist over...just use level on 10ft 2x4 and measure the dips?

bbcamp · 03-19-2010, 09:52 AM

Chris, you don't need level, you need flat. Use a straight board, 10 feet long or what will fit in your room, and lay it on edge of the floor (don't need a level for this). Measure between the bottom of the board and the floor. Make note where that exceeds 1/4" and where it is less than 1/8". Move the board around the room and in different directions until you have a topographic map of your floor.

winty03 · 03-19-2010, 10:03 AM

So is our goal to use thinset..between subfloor and hardibacker...and then again on top of the hardibacker to get the floor as level as possible before glue and tile?

winty03 · 03-19-2010, 07:39 PM

Ok, we removed all shims and screwed floor down tight .....we did some measuring with a 10ft 2x4 and we have one spot thats high.....everything else seems to be about the same height except that one joist. From that joist to the wall its about 1 1/4" difference (6ft span)..excluding that high spot the floor is with in 1/4 to 1/2" whats our options or next step?

UPDATE: We are planning on just adding 1/4 inch plywood to the entire floor except for the high spot and use either SLC or thin set to smooth from top of high spot to edge of 1/4 plywood. Looks like it will put us flat, with a slight slope. Not excited about the increased height of the floor, but will get us past the uneven floor.

bbcamp · 03-22-2010, 01:53 PM

I'm confused. Are you saying that the high spot is 1-1/2" higher than the rest of the floor, and that's over a 6 foot span? Or did you mean 1/4" to 1/2" higher?

1/4" plywood's no good under tile, so forget that. If you have only one joist that's causing a problem, it's probably easier to remove the subfloor over it and plane it down. That's assuming the 1/4" to 1/2" dimension above.

Your floor has an over-all slope that you don't want to correct, so that lets SLC out.

winty03 · 03-22-2010, 05:42 PM

From the high spot.....which is right behind the dishwasher in the picture to the fall wall is a difference of 1 and 1/2 inches....that the high spot...and the lowest part of the slope difference...we opted out of the 1/4 and just going to go with 3/8 inch plywood over the floor to bring the floor up to the high spot and going to leave the gradual slope towards the wall. so over all we will be 3/8" higher on the entire floor except the high spot which we will thin set smooth and leave the gradual slope alone. After the 3/8" subfloor add on we are flat and gradually sloping towards the far wall...we are flat...but not level.

Hammy · 03-22-2010, 05:52 PM

Chris, post us a picture of the straight edge showing the 1.5 " high / low spot in the floor. Hammy

winty03 · 03-23-2010, 06:22 AM

Pictures to come this morning....

another view

*UPDATE* Last night we cut out the top 3/4 inch of our "high" joist and then bolted on sister joists to the side of the orginal joist...our floor is now flat with gradual slope. Should be good to go from here.

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