Another subfloor question

[JohnMickish]

I think I have a dilema in my foyer floor. Here is the history.

House is a 1974 rambler. We are doing a large remodel of the foyer/kitchen area of the house. The kitchen floor will be hardwood and the foyer will be tile, like it was originally.

The tile was in good shape and only had one broken tile, in the middle of the floor. Pulling up the foyer tile floor, it was discovered that the subfloor was 1/2" ply, with feltpaper and screen nailed down, then 1 1/4" cement, then morter then tile.

Here is my question. The 1/2" subfloor has a bow in it right in the main traffic areas. My plan was to put another layer of 1/2" ply then backer board then tile. Do you guys that are in the know think that the combination of ply and backer board will give enough strength to prevent the new tiles from cracking due to the bow? I need a total of 1 3/8" build up to match the kitchen floor. We are using 3/8" ceramic tile, 12X24 inch.

Am I better off just cutting out the original sub and put in a new one?

[cavalierowner]

The first thing I would be looking at is that bow and what caused it. What is the span of the joists, the joist size and spacing? If the joists are not right, then you could always have issues with cracking.

The original mudbed installation is a first rate installation, especially given the era it was installed. It's still listed in the Tile Council of North America book because it works and has withstood the test of time. That's the system I would suggest. I see you're an electrician. Any of your current jobs have a good tile sub (Grazzinni, Advance, etc.) you could talk to about how to do mudbed tile systems, assuming this is a DIY?

[Surface Tension]

Normally that would be enough although 3/4" and 1/2 being better. What do you have for floor joists and centers.

Is the bow a hump or a sag? If its hump something settled and a lollycolumn, beam, or partition wall would be under the hump.

If the bow a sag between floor joists or are the joists sagging. If the joists are sagging from being undersized there is going to be some deflection. In that case I'd think x bracing, blocking, or even adding floor joists to stiffen the floor.

If the joists and centers are sufficient enough to give you a solid floor and the sag is between joists adding 1/2 plywood over that would have an air gap.

If screws could pull that plywood together then you could use a leveler before putting the backer board down.

So it sort of matters why there is a bow but I'd try and fix that first.

Edited, I was typing, got pulled away while you replied.

I agree that was a first rate install.

I wonder if you got a crack in the mudbed or being this is an entry and over the decades had water slowly leaking through and the sub floor sagged.

[NoWiser]

I'd tear everything up and check the joists to see if there are any problems there. If the joists look OK, I'd put down a GOOD subfloor, like 3/4" Advantech, pour another base with levelquick and then tile.

[frazwood]

I agree with the above post. My guess, though, is that your home is structurally fine (2x10" joists, 16" OC, spanning 12 feet or less) since it originally had tile and it only cracked once in more than 30 years (how was the grout? that cracks more easily).

Another thing to consider is the floor transitions.... you might be able to have your tile floor be the same height as your hardwood floor, if you design it correctly.

I have previously used a couple of layers of plywood (as described above) with a layer of 1/4" cement board (thin-set mortared and screwed into the plywood) followed by tile on top of that. The cement board is absolutely a great surface for adherence of thin-set mortar (buy a good, high quality modified thin-set).

I've also heard (but never tried) that DITRA is a great product. It's a thin, separation membrane that is waterproof. You use thin set to apply it to the plywood (i.e., it effectively replaces the cement board layer) and then use thin-set to apply your tile to the DITRA. Because it's waterproof, though, you need to use a non-modified thin-set mortar. (note: it looks like DITRA is more expensive than cement board, but in the end they are about the same price... the screws that you use for cement board are kind of pricey and you use a lot of them).

The nice thing about DITRA is that it comes in two thicknesses... this might be useful for getting the correct floor heights.

[Nhamm]

First things first check the joist spacing. I have pulled up quite a few floors in that area and there have been some ugly sub floors put in that I have seen.

Ditra product is good but the reps that I have spoke to about it really push for it to be used on cement floors. Cement board is better suited for plywood subs.

I would do whatever you have to do to solidify that area. Even if the transitions are off a tad you can work with that somehow and much better than a cracked floor in 5 yrs bc you wanted them flush.

[JohnMickish]

I should have put this in the first post but didn't think of it untill reading some of the posts.

The normal flow of traffic for the past 40 years just happened to be running between two of the floor joists, not across the joists. It's a straight path from the garage door to the livingroom hallway.

The floor joists are on 16" centers and are in good shape. The house is great structurally.

The sag in the floor isn't much, but you can notice it if you are looking for it.

What about floating a leveling agent under the backer? I just don't want my new tile to crack.

[cavalierowner]

OK, we will assume that the span is OK for the joist size. Can you get at the area that's sagging between the joists? If so, I would piece some 2x4 blocking between the joists @ 24" o.c. just to be sure the subfloor doesn't move any more. I am really a big advocate of the mudset system like is there now for a supported floor. It really is the best system and has the added benefit of resolving the sag that's there now. Whatever you do, be sure the floor is absolutely flat before you start setting that large tile or you are all but guaranteed to have lippage from one tile to the next.

[nofishfisherman]

OK, we will assume that the span is OK for the joist size. Can you get at the area that's sagging between the joists? If so, I would piece some 2x4 blocking between the joists @ 24" o.c. just to be sure the subfloor doesn't move any more.

I was thinking the same thing. If the traffic pattern in the past has been right down the middle of 2 joists I'm sure the same traffic pattern will continute in the future and will potentially start to sag again. Adding some blocking between the joists should prevent any future sagging. Once you know it won't sag any more you can level it however you feel is best.

[JohnMickish]

That sounds like a solid plan. Thanks guys.