# Method to fill gaps before pouring leveling compound



## VersaBar (Nov 30, 2010)

They make caulk backer rods (made of foam) that you can shove in the gaps and then silicone on top of that. You can also use spray foam for the larger gaps. I would put some duct tape along the larger gaps from underneath if you use the spray foam, just to try and keep it off the basement floor.


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## JazMan (Feb 17, 2007)

Scott,

You can't install anything direct over your plank subfloor, including concrete backer board and obviously SLC. (self leveling compound/cement). 

You must install a plywood underlayment first. The plywood needs to be a good grade, not CDX and the min thickness is 1/2". Go thicker is you can. (ask about how to install the new ply later). You should then install either a concrete backer board or a matting such as Ditra, then your tiles. 

If you choose Ditra, you should install the SLC first, if you decide on concrete backer board, the leveling is done after. Any remaining gaps around the wall or pipes are closed with foam.

Natural stone tiles require a stiffer subfloor system than regular tiles. You must have a double layer subfloor, plus the joists system needs to meet higher standards. Tell us the type and size of the joists, species and grade if possible, spacing from one to the next on center, and the span of the joists.

Jaz


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## scottfs (Dec 1, 2010)

Interesting, the folks at the tile store told me I could pour the stuff directly over the subfloor, so that's what I had planned to do. The original tile floor which dates to 1962 consisted of tar paper over the subfloor planking, wire lath over that, and about an inch and a half of concrete with the tiles directly on the concrete. 

The floor planking is about 3/4" thick and the joists are 2x8's, 16 on center. I figure they are pine but I don't know enough about that kind of thing to say for sure. 

The tile store guys also told me that the self-leveling product they sold me could be poured up to 2" thick, and serve as the total underlayment. (The packaging also claims this) There were 2 reasons I want to use it. First, I was afraid that with natural stone that if the floor wasn't perfectly even they would be more likely to crack and break. Secondly, I want the floor to come out to the same overall height as the old floor, for asthetic purposes and also because I don't want to end up with the toilet flange at the wrong height after I'm done. I figured I could make some marks of where I wanted the height and just pour up to the desired height. If you're telling me I can't do that, I don't know what I ought to do instead.


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## JazMan (Feb 17, 2007)

What you had before was a typical "mud" job, which was/is fine done as you described. 

There's a few errors in what the store told you. 

1. They may not have known your subfloor is an old plank floor. SLC's work well over plywood with proper priming, but not planks since there's way to much flex. (what brand?) 
2. Pouring SLC on your floor will not level your bathroom until the basement is filled too. :whistling2: Which was your question.
3. They may not know or care what the criteria for subfloor deflection is for a successful tile installation. They may be more concerned on making a sale?
4. We find that many of these tile store clerks do not know the specifications to properly install tiles. And if by chance you went to a big box store, their advice is usually off mark most of the time.

I doubt very much if your 2x8 joists will meet L720 deflection standards for natural stone. Standard practice in home building is to meet minimums which is L360. However you didn't say what the span is, and to be specific I would need to also know the species and grade of the joists. Otherwise I can only assume they're Doug Fir or SYP grade #2 which is kinda common. 

Assuming Doug Fir north #2 the span needs to be under 9' 6". If they are Doug Fir south #2 it's 8' 7". SYP (Southern Yellow Pine) is also 9' 6" to meet L720 at 50/20 live/dead load. 



> The tile store guys also told me that the self-leveling product they sold me could be poured up to 2" thick, and serve as the total underlayment. (The packaging also claims this) There were 2 reasons I want to use it. First, I was afraid that with natural stone that if the floor wasn't perfectly even they would be more likely to crack and break. Secondly, I want the floor to come out to the same overall height as the old floor, for asthetic purposes and also because I don't want to end up with the toilet flange at the wrong height after I'm done. I figured I could make some marks of where I wanted the height and just pour up to the desired height. If you're telling me I can't do that, I don't know what I ought to do instead.


No problem with the above once you have a proper subfloor. You can get the height you want with ply and SLC. 

Jaz


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## scottfs (Dec 1, 2010)

I forgot to mention the span of the joists. The span is about 10 feet. (About the length of the bathroom itself) The basement was finished at some point, I'd say about 20 years ago. A bathroom was installed below the one I'm working in and a wall is now present which would cut the span of the joists in question to about 6 feet. Would it be fair to consider the span 6 feet because of the wall? If not, what would you recommend doing?


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## JazMan (Feb 17, 2007)

If the wall below is running perpendicular to the joists, and if the joists are resting on this wall, you will be OK for the joists' deflection part. Now concentrate on the more important deflection from the space between the joists. 

Add ply., 1/2" min. or the thickest you can go. (3/4"?) Then decide on 1/4" concrete board or Ditra and/or the leveling method you want to go with. Since SLC adds no structural strength and is very costly, make up the height with plywood, may even consider 2 layers if needed. Also double check how much the floor is out of level. SLC will self level so the thickness will not be the same. 

Jaz


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## scottfs (Dec 1, 2010)

Thanks for the detailed replies. I can see the top of the wall and the joists from the unfinished side in my utility room and they are resting on top of it with no gap so I guess I'm ok on the joist span. I will have to get some plywood to put down over my subfloor. I have never had to put down plywood before so I guess I'd better go research the best tips on that now!


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