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After my bathroom was demoed, I discovered that the subfloor is in bad shape. The home was built in the 1960s and the subfloor consists of 3/4" thick tongue and groove planks running diagonal to the joists. I do not know if it was poorly constructed or if this naturally happened over time, but the tongues and grooves of the planks across much of the floor are no longer nesting within each other. As such, the floor does not have much structural integrity.

As you can see in the photo below, a section of the floor broke away easily during demo. There are other sections that flex so much that I feel like I'd break through if I attempted to put my full weight on it.

This is a bathroom that will have a tile floor so the goal is to properly support the tile, a bathtub, etc.. Outside the bathroom is 3/4" hardwood and I'd prefer not having too much of a differential with this.

I have had a few contractors present some different approaches that include cutting out the old floor and completely replacing it or simply adding a thin layer of new plywood on top of the old floor. The thin layer on top seems like the simplest approach but I have concerns with both building on top of a questionable old floor as well as about the extra height (what would be the minimum acceptable thickness for the new layer in this case?).

What is the proper way to repair this floor?

Broken floor

Close up of gaps

  • Are the floor boards rotting? – BlueJay Feb 17 at 21:10
  • They are not rotting in terms of any sort of damage from water. I'd say the individual planks are in decent shape, but there are quite a few spots where they seem a bit soft either from knots or I guess having to deal with extra weight in some spots such as right in front of the toilet. – David Anderson Feb 17 at 21:22
  • It looks like many of them have shrunk, and don't even touch each other anymore. Are those area over any old leaks, or somewhere that was often wet, like right outside a shower or tub? – Xen2050 Feb 18 at 2:57
  • Are you sure those are T&G? They look square-edged to me. Can you lift the boards and clamp up? What do your tiling installers say? (Ours insisted on 1/2" ply on top of our Victorian floorboards.) Do you know why the boards are diagonal rather than square? – Martin Bonner Feb 18 at 10:10
  • @Xen2050 Yeah, they seem to have shrunk dramatically over time. This is the bathroom floor in an area with a tub, toilet, sink, etc. so the tile over this definitely would have gotten wet at times though the board shrinkage seems to be uniform throughout the room and not concentrated in an area you'd expect to get wet more often such as next to the tub vs. under the vanity. – David Anderson Feb 18 at 14:36
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The following options will perform equally, their differences are time, cost, and thickness of the subfloor. Best case scenario you'll still have a 3/4" subfloor, worst case you'll have 1 1/4" (sorted most economical in cost/time to least):

  1. Lay 1/2 or (preferably) 5/8 over the existing floor. It's simple, and should provide enough strength with the existing strips to support tile.

  2. Replace the strips that are weakened (you should be able to find a piece of 3/4 that would fit the space. All strips should cross a minimum of three joists for strength. Follow up by laying 1/2" or 5/8" on top.

  3. Remove and replace. This is a challenging option, those strips probably cross the entire house, what this means is that the walls are resting on the strips. When you remove the existing strips it will cause sagging in adjacent rooms so you'll need to at least block out beneath the walls/between joists to support the flooring on the other side. You'll also have to block out in between joists, and at the edges where the joists are more than a couple of inches away from the bathroom wall.

Once you've blocked out the floor reinforcements take a look at the joist spacing.

Subfloor replacement selection - OSB or Plywood. For tile, plywood is generally recommended by manufacturers over OSB.

Picking sheathing

Not all sheathing is created equal, however if you're picking up sheathing grade from a big box store, they hold a respectable graded sheet which is what I'm going to base the rest on.

What's your joist spacing? What is the nominal size of the joist? If it's 18" and 2x8 (most likely case) or greater, you can probably get away with one 3/4, but I'd recommend 2-5/8" cross installed. If it's 16" and under, you'll be fine with a single 3/4"

If you're going to lay sheathing on the existing floor, you need a minimum (emphasis on minimum) of 3/8". Whether it's T&G or not, anything less than 3/8 is a waste of money as it's not structural.

Install

Either T&G, or square edge is fine, what's important is to space them 1/8" apart (use something that's credit card thickness for spacing). Ideally glued in the seam if it's T&G, and to the joist using an ASTM D3498, or APA AFG-01 conforming adhesive (PL-400/Premium, Lumber Lock, AdvanTech for example). Before applying the glue, make sure all surfaces are clean and dry.

Cutting and Blocking

There aren't any perfect videos, however for the preparation this video has great considerations: https://www.youtube.com/watch?v=CgWtHSsww-4

Notes about blocking, use a minimum of 2x6. If you're going to use screws, I'd suggest structural screws like these

Otherwise 16d common nails will work.

End product should look like this, ready for a well supported subfloor:

enter image description here

Sources

Ditra install guideline (they guarantee replacement if tiles crack so they'll give safe numbers) bottom of page 7 for your setup: https://sccpublic.s3-external-1.amazonaws.com/sys-master/images/h71/h71/8858545389598/DITRA%20Installation%20Handbook.pdf

Page six for the bare minimum required for sheathing based on joist spacing: http://www.tolko.com/system/resources/W1siZiIsIjIwMTcvMDEvMTgvMTBfMjFfMzNfMjM3X0FQQV9FbmdpbmVlcmVkX1dvb2RfRmxvb3JfU3lzdGVtc19Db25zdHJ1Y3Rpb25fR3VpZGUucGRmIl1d/APA-Engineered%20Wood%20Floor%20Systems-Construction%20Guide.pdf

  • What does "block out in between joists, and at the edges" mean, or look like? – Xen2050 Feb 18 at 4:01
  • @Xen2050 I've added details, I was hoping for a good video on it, however they are all missing key points which I've commented on, and added a picture for reference. – Chris Feb 18 at 4:45
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I wouldn't chance going over it either. If it's easily breaking and giving when you walk on it, replace it with new T&G sub flooring. Otherwise, you are going to have a bunch of cracked tiles. You might even have some floor joist issues to deal with and you can't really find that out without removing the old sub flooring.

You can start by cutting as close to the wall as you can with a skill saw. Set the saw depth just low enough to get through the existing sub floor. The circular saw will not get right against the wall because of the guard. You will need to use a reciprocating saw, jigsaw, Bosch Starlock Plus or Starlock Max. Once all of the old flooring is removed. Make sure all nails & screws are removed and the tops of the joists are clean and level. Install the new sub flooring with liquid nails on each joist.

If you run into joist issues you'll need to ask another question.

  • Before you cut the floor around the perimeter of the room, consider if the walls are resting on anything. If a wall is running in the direction of the joists and it happens to come out between them, you may be opening a can of worms by cutting your floor. – BlueJay Feb 18 at 21:38
  • BlueJay, Your comment isn't making any sense. – Jerry_Contrary Feb 20 at 12:53
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    I'm talking about the walls around the perimeter of the room. Cutting the board's all the way up against the wall may or may not be problematic. Usually non-bearing walls are built on top of the flooring without regard to the joist layout. Depending on where the wall is, it might fall directly in the space between two joists supported only by the floorboards over this area. Cutting the floor up to this wall would greatly weaken this support. Like I said this may or may not be a problem. – BlueJay Feb 20 at 16:20
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Cost is the biggest issue. Replacing it completely is the obvious choice. That's expensive. You can easily add 1/4" ply glued and screwed down and it will be totally structurally safe. You may however get squeaky, spongey floors. If you can fit it in your budget redo it with a new flooring material especially if you are going with tile.

It's often less expensive to do it right once.

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