Flattening and strengthening Floors in Kitchen, Bath & Laundry Room

[Always Learning]

Hi Everybody! It’s my first time posting! I have read lots of information on the site and I need some advice. I’m going to give as much information as possible, so hopefully this doesn’t get too long! The house is constructed with 2x10 joists. The kitchen and bathroom both have a span of 12’ so we’re good to go for ceramic tile! According to the deflect-o-lator on this site we have a deflection of L/558. The joists change direction under the bathroom floor but the span is still 12’ in each direction. The laundry room is unique because it’s constructed with sleepers laying atop a slab. The span is 12’8” although I’m not sure if it matters since the joists are sitting on a slab. The house was constructed all at the same time and the original subfloor is 1/2” plywood throughout, labeled C-D APA 32/16 Interior PS1-74 Exterior Glue. When we first purchased the house, we had somebody doing some work for us and after removing the old sheet vinyl flooring and luan from the kitchen and bathroom, he added a second layer of 1/2” plywood. We have a couple of problems that need to be addressed.
1. There is a large hump in the kitchen floor, as well as a second, smaller hump in the kitchen. The framing is visible from the basement and appears to be in very good condition. My only explanation for these humps is that the house was framed with green lumber (labeled NLGA Rule No 3 S-GRN HEM-FIR-N) and the humps formed as it dried. The large hump is about 1/2” high over a 5’ span, so it’s pretty substantial. Our plan to remove these humps is to cut out the sub floor, shave down the humps and then replace the subfloor in that section only. We’re going to do that tomorrow. The bathroom floor seems fairly flat, and I’m not quite sure about the laundry room yet. After I remove the old layers of flooring, I’ll be able to visualize it better, but right now there *may* be a small hump near the doorway that extends into the hallway, which may pose a problem. I’m wondering what you think of our plan to remove the humps in the kitchen? The only other idea I had was to shim the whole floor, but that would essentially raise the whole floor by 1/2” before we even address problem number two.
2. I don’t feel that the second layer of 1/2” subfloor that was added to the kitchen and bathroom has added enough stiffness to the floor, so I have decided to completely remove it from the kitchen and start over. I’ll finish doing that today and after we remove the humps, our plan is to add a layer of 3/4” plywood to stiffen the floor. Any opinions on that plan? I haven’t given any thought to how I will adhere the tile to the floor, whether is be an uncoupling membrane, another layer of plywood, etc. but I’m thinking long term, that I may not want to put thinset directly on the 3/4” plywood. And the reason I say that is because it’s an investment (time and money) and if the tile looks dated after 20-30 years, it would be better if it could be replaced without damaging the 3/4” plywood. Any thoughts or ideas on that? Unfortunately, I cannot remove the second layer of 1/2” plywood from the bathroom because the plumber has completed his rough plumbing work including hooking up the cast iron tub which is sitting on the plywood, as well as the toilet flange. My only idea for the bathroom is to use small tiles and cross my fingers. The laundry room was constructed with particle board over the original 1/2” subfloor, so after I get all the particle board removed, the plan is to put down new 3/4” plywood over the old subfloor just as we’re doing in the kitchen.
3. We have 2 step downs from the kitchen, one into the family room and one to the basement stairs. We also have 2 transitions from the kitchen to hardwood floors, one to the dining room and one to the hallway. Can anybody tell me how these are normally addresses or share some links, etc so I can figure it out? I’m assuming that we will run the new 3/4” plywood right up to the edge of the transitions. Then maybe I need stair nose for the step downs? And something else for the hardwood floor transitions? We also have 2 doorways to address, which both will transition to hardwood floors, one in the bathroom and one in the laundry room. I’m assuming that saddles should be used here? Should the plywood run under the saddles? It doesn’t in the bathroom, so maybe I will need to add some plywood before installing the saddle?
I really appreciate everybody’s insights on this. Thanks for your help!

[HooKooDoo Ku]

I'm just a DIY, and I have no experience tiling a wooden floor.
But I first used this site about 10 years ago to tile a bathroom on slab and learned a lot.

But I think I recall the "ideal" way to tile a wooden floor was to follow the following steps (hopefully one of the experts will chime in a correct any mistakes I make).

1. You usually want a 3/4" subfloor with the "strength" direction (usually the 8' dimension of 4x8 sheets) perpendicular to the floor joists.

2. Add a 2nd layer of 1/2" plywood. The "strength" direction should still be perpendicular to the floor joists, but the 2nd layer should be shifted a half sheet in each direction so that none of the joins line up. This second layer should only be attached to the 1st layer. Don't drive any screws for the 2nd layer all the way down to floor joists.

3. Add a 3rd layer of CBU (such as 1/4" Hardie Backer). This layer isn't designed to add any stiffness to the floor, so there is no need to use 1/2" CBU. You want to first lay down a layer of thinset and set the CBU in the thinset and screw it down to the 2nd layer. The purpose of the thinset is to ensure ALL of this CBU is supported by the floor with no unsupported pockets.

What I don't know is if you only want "cut" out sections of subfloor. Doing so, you loose the benefit of what should be tongue-n-grove edges that keeps sheets from moving relative to each other... but then the offset 2nd layer may help take care of that for you.

When it comes to stairs, one issue you might face is that building codes. While I'm not sure about numbers, I think building codes say the rise from one step to the next can not be more than 7-3/4", but it also says the difference in the rise in one step to the next can not be more than 3/8". I think the idea is that you don't to get into a rhythm of lifting your foot a certain amount only to suddenly have a riser be taller than you've gotten used to stepping and it trip you up. By the time you add all the layers above AND tile, your final step from the stair case to the tiled floor might violate these building codes.

[ss3964spd]

Welcome, Adrienne, you've got a lot going on there.

The 2nd layer of 1/2" isn't doing you any good. Actually, the 1st layer of 1/2" isn't, either; 1/2" ply doesn't have tongues and grooves so the seams where they abut are not supported unless there is blocking directly below them, and 1/2" is unsuitable for any tile installation. Remove all the 2nd layer, everywhere. Remove enough of the 1st layer to address the hump(s) at the joist level. Replace that 1st layer, then cover the entire floor with 3/4" T&G, B/C EXP1/Exposure 1 ply. Screw the 3/4" ply directly to the joists (the screws probably won't get enough of a bite in the 1/2" ply anyway).

For the bathroom, I'd still remove the 2nd layer of 1/2". Use a multitool to cut it along the tub apron. Sacrifice the toilet flange - which really shouldn't have been installed yet anyway; "should" be installed after the finished floor is in.

Install either 1/4" tile backer on the floors or, to save about an 1/8" of an inch, install something like Ditra on the 3/4" ply then the tile on that.

Joseph has a good point regarding the stairs.

[cx]

Welcome, Aerienne.

1. The tile industry standard for flatness of the substrate for tiles with no dimension longer than 15 inches is no deviation from intended plane of more than 1/4" in ten feet nor 1/16th" in one foot. How you get there is up to you.

Your nominal half-inch plywood is not acceptable as a first layer of subflooring at all. In your case where you have the CD grade, it's even worse and it has not T&G edges and I doubt seriously that you'll find any blocking under the between-joist gaps. You really want to remove all that subflooring and start over.

Planing down the joist tops to bring them into plane can be effective, but you've got to be careful how much you remove, especially near the center of the span. I don't think you'll be able to judge whether that is an effective approach for you until you've removed the subflooring.

2. You don't indicate the joist spacing you have. If it's 16" on center or less, a single layer of nominal 3/4" exterior glue plywood (or OSB) with no face of grade lower than C is adequate as a single layer subfloor for all manufacturer's of tile installation substrates. Joseph's recommendation of an additional layer of nominal 1/2" plywood is a good one if you have the vertical space for it.

Originally Posted by Adrienne

...I may not want to put thinset directly on the 3/4” plywood.

I trust you're talking about thinset mortar for bonding your tiles. You must not do that. While there is an industry recognized method for bonding tiles to plywood, you must install the second layer of plywood to comply with that method. We (TYW) generally recommend against use of that method for our DIY visitors.

3. I can't address the transitions to other floor coverings without knowing what the subfloor is under each and what the final flooring height difference might be in each case. Ceramic tile to hardwood flooring is among the easiest transitions to accommodate.

Raising the height of the finished flooring only at the top landing of that stairway is a serious consideration, as Joseph pointed out. How do you intend to deal with that?

My opinion; worth price charged.

[Always Learning]

I want to thank everybody for replying to my post. I can’t thank you enough for this resource. Really, I mean it! Thank you! I’m sorry I haven’t replied sooner. I’ve been working so hard over the last 5 days. I’ve removed all of the second layer of 1/2” plywood in the kitchen and then I had some help come over (family) and we removed a section of the original 1/2” subfloor over the most egregious hump. There was an unbelievable amount of nails used. There were a ton of screw nails that didn’t want to come out. Then there were a whole bunch of nails with small heads that were difficult to grab. And to add icing to the cake, there were a bunch of Sheetrock screws used by the first guy that we had hired, and many of those broke and we couldn’t get them out. We spend most of the day working on that. Today we will try to plane the top of the joists flat and reinstall some new 1/2” plywood. Which brings me to my question. I have received two different suggestions. One is to install 3/4” plywood on top of the old 1/2” subfloor after flattening joists. Basically, the old 1/2” is useless and will act as a shim. The other suggestion is to cut out all the old 1/2” subfloor and install new 3/4” subfloor directly on top of the joists. Because of how many nails are holding the plywood down, and the fact that I am getting help and not doing this alone, I think I will get pushback if I try to suggest that we remove all the old 1/2” subfloor. That would be my preference, but it’s not completely up to me. Any thoughts on this? Other than losing 1/2” of floor height? I promise I’ll address all other issues in my next post, such as the stairs (Thanks Dan!). Help is on their way here right now. Thanks everybody!!!

[cx]

That's a hellofa hump, Adrienne. I'd wanna know why it's there. Such as the rest of the floor having settled due to some structural deficiency and only the center is properly supported? I'm a little more cautious because of the poor grade of your joists in the photos.

That's a lot of wood to remove from a floor joist. And given your description of the nailing of the existing subflooring, the person from whom you rent your planer is not likely to be happy with all the notches you'll put in the blades.

Originally Posted by Adrienne

There were a ton of screw nails that didn’t want to come out.

Yep, that's why deformed-shank nails are commonly indicated for subfloor installation.

I'd still recommend you remove all the existing subfloor. Yeah, it's a great PITA, but it'll make for a better installation of the new subfloor. The nails that prove too difficult to remove can simply be driven flush or below the top of the joist rather than pulled out. Still bitchy to remove the subfloor, though.

The recommendation for subfloor installation usually involves adhesive along with mechanical fasteners to achieve design rigidity. When you have that extra "shim" of half-inch plywood in there, you're building in some available movement between layers. Is it enough to be a problem? I dunno. I'd just rather not have it. Up to you.

My opinion; worth price charged.

[ss3964spd]

Adrienne, just be sure you understand what is involved with removing all of the 1/2" plywood. You'd need to cut around the entire perimeter of the space right next to the sole plates of the walls, and then install blocking between the joists so the new layer of ply has something for the edges to rest on. If there are any interior walls that are not resting directly on a joist, or spanning across joists, you'll have to install blocking under those walls so they are properly supported.

If you have to plane off a lot of material from the joist tops to get the hump out it may weaken them enough to the point that they deflect too much for ceramic tile. From here, though, it appears you could sister some new joists onto those you need to trim, thereby restoring the strength they need. In your case you wouldn't need to use 2X10's for the sisters, 2X8's would do.

[Carbidetooth]

Adrienne, I gotta agree with CX on his point about figuring out what has happened to cause such a hump. Something has moved and figuring out what and why will prove to be valuable info. Maybe it's done moving, but often structural deficiencies get worse, mitigating your efforts to correct the symptom and not cure the problem.

Pulling ring shank nails is no fun, but a nail puller of some sort will help. Here's some pics of one that has removed many. The key is a smallish claw which I drive diagonally under the nail head with a hammer and subsequently lever up the nail. Pull what you can, remove the sheet and drive remaining ones into joist unless you're going to plane it.

Structure forensics is a part of remodeling I kinda like, but often leads to additional work if one wants it right.

[ss3964spd]

Inspecting for structure deficiencies is certainly a good idea while so much is torn apart, but I have admit that I'm struggling to figure out what kind of deficiency at either end of those joists could cause a hump to develop over time in the (roughly) middle of the joists without a support beam under the joists where the hump is.

Assuming the hump wasn't there, at least not to that extent, when the house was built the only thing I can imagine is the joists developed a crown, likely shortly after they were installed.

[cx]

Your point is well taken, Dan, but I'd still wanna know.

Peter, I've never seen a Cat's Paw with a wide, flat chisel-looking thingee on the other end.

[Carbidetooth]

Dan, the first thing I'd check is whether it's a single joist or a group of them.
A single joist could crown like that, I suppose, but if it's more than one, that seems really unlikely. I'd be looking at the foundation walls next, somehow exerting pressure on joist ends, but that seems like a reach too. Dunno, but am curious.

CX, you'd love that tool for removing casing and baseboards, best I've found. I also like the hammering platform directly in line with the claw. I have others that curve there.