Home>Discussions>INTERIORS>Molding & Carpentry>Replacing plank subfloor with plywood
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A. Spruce
Re: Replacing plank subfloor with plywood
Galileo007 wrote:

This is exactly what I am dealing with this week. Our house has diagonal, 7" wide, 5/8" thick planks on top of joists that span between cinderblock walls. I had to pull a few of them up and of course they spintered to pieces being 60 years old. I am not aware of anyone selling 5/8" boards anymore... It's a 5x8' bathroom and my plan is to cut the floor out from wall to wall. I will install a 2x4 ledger on the edges of the room sistered to the joists to support the edges of a new floor, since the planks run underneath the wall and a new subfloor will stop at the wall and won't reach the joist. I will then install 5/8" OSB with 3/8" plywood over that to even out the floor, then I will put 1/4" backer and tile over that for a full 1 1/4" under the tiles.

Am I running into trouble with that plan? Any pitfalls you can foresee?

Do yourself a favor and forget about using OSB. Plywood is much stronger and stable, plus, when it gets wet it doesn't lose all of it's structural stability. OSB is basically particle board, it swells like a sponge when wet, then it crumbles apart.

You don't need to double up the subfloor, particularly since you are setting your own ledgers around the room, you will be able to accommodate plywood thickness and finished floor height. You will need to make sure your deflection is within tolerance if you want your tile to survive. Head on over the the John Bridge tile forum.

Galileo007
Re: Replacing plank subfloor with plywood
HoustonRemodeler wrote:

Install NO plywood thinner than 1/2"

Install NO plywood with a face grade lower than CC (its easier to find BC in most places)

The second layer of plywood needs to be installed in a very specific manner as described here.

All plywood should be installed with the face grain perpendicular to the joists.

Ok, my subfloor layer will be 5/8" OSB, so I think I'm good there.

The underlayment plywood that goes on top (the second layer) has to be about grade BC? It will also be 3/8" BC then. I'm assuming that's ok. The stuff I got at Lowe's is 3/8" with a face that isn't that great, but I didn' know that layer mattered that much, or does it?

That paper was informative... I will stagger the joints accordingly. My original question was about supporting the OSB subfloor at the edges of the room since I will have to cut the planking off there. I think a ledger sistered to the existing joist to provide a lip will work since it's the edge of the room anyway.

Galileo007
Re: Replacing plank subfloor with plywood

Ok, from multiple posters I see that BC plywood is better than OSB. Since I'm getting rid of all the planking in that room, I don't have to worry about finding 5/8" BC plywood, I can go with the 22/32 or whatever.

I'm not trying to double up subfloor. I'm just trying to increase the thickness to 1 1/4" as 1 1/8" is required for tile and I'm surpassing that with 1/4" hardie backer (5/8" + 3/8" + 1/4" = 1 1/4")...

Appreciate all the help. The deflect-o later said I'm ok, although I'm not sure what type of wood my joists are. The home was built in 1952, and a 2x8 was 2x8 back then, plus my joists have the diagonal cross braces between them.

One last thing... should subfloor be butted up next to each other with no gap while the second layer has a 1/8" gap? I'm assuming a gap on the outer edges, so I'm talking about the joints between panels.

Mastercarpentry
Re: Replacing plank subfloor with plywood

We're pretty knowledgeable here but we defer to the John Bridge Tile Forum because those guys are OUR experts when we have questions, so I would suggest that you get your answers there. Mention HoustonRemodeler and you'll get the fast informative responses you're looking for.

Me? I'd do it much differently than you want to. If you want to know why head over to John Bridge.

Phil

riverbluebell
Re: Replacing plank subfloor with plywood

I am hoping someone can give me some input on this question. I am a homeowner and know nothing about this topic. I had someone come in to do a bathroom remodel for my disabled son. They sawed the plank subfloor out and replaced it with a piece of plywood which lays on the joists. My question is about the planks (about 7 inches wide) that were cut, and are now "seamed" (screwed together) with this piece of plywood. I am worried about the "seam" between the cut end of these planks and the edge of the plywood. The plywood seems well-supported, but the remaining length of the plank runs under the wall and into the bedroom, and is clearly weakened by this "seam." This seems very unstable. Am I missing something?

Mastercarpentry wrote:

We're pretty knowledgeable here but we defer to the John Bridge Tile Forum because those guys are OUR experts when we have questions, so I would suggest that you get your answers there. Mention HoustonRemodeler and you'll get the fast informative responses you're looking for.

Me? I'd do it much differently than you want to. If you want to know why head over to John Bridge.

Phil

A. Spruce
Re: Replacing plank subfloor with plywood
riverbluebell wrote:

I am hoping someone can give me some input on this question. I am a homeowner and know nothing about this topic. I had someone come in to do a bathroom remodel for my disabled son. They sawed the plank subfloor out and replaced it with a piece of plywood which lays on the joists. My question is about the planks (about 7 inches wide) that were cut, and are now "seamed" (screwed together) with this piece of plywood. I am worried about the "seam" between the cut end of these planks and the edge of the plywood. The plywood seems well-supported, but the remaining length of the plank runs under the wall and into the bedroom, and is clearly weakened by this "seam." This seems very unstable. Am I missing something?

If I am reading you correctly, you are saying that the planks were cut between joist, and instead of installing framing to support the patched in planks, they screwed a strip of plywood to the remaining plank end and the patched in plank? If this is the case, then you do have cause for concern, that is not a suitable repair for a floor, much less the portion of wall that is now unsupported due to the cutting of the planks under it. Would I be correct to assume that both ends of the removed planks were done in this manner?

IMHO, new framing should have been grafted in to support both sides of the cut planks. When I've made repairs of this nature, I installed new joists with hangers and/or blocking to fully support both sides of a patch. This is particularly important if you are laying tile.

riverbluebell
Re: Replacing plank subfloor with plywood

Thank you so much for replying. Yes, you described what they did, exactly. They were supposed to be installing a linear drain for a curbless shower and I was shocked to see that they had unnecessarily made this giant patch in the subfloor, especially without properly supporting it. What kind of spe******t can fix this kind of thing?

A. Spruce wrote:

If I am reading you correctly, you are saying that the planks were cut between joist, and instead of installing framing to support the patched in planks, they screwed a strip of plywood to the remaining plank end and the patched in plank? If this is the case, then you do have cause for concern, that is not a suitable repair for a floor, much less the portion of wall that is now unsupported due to the cutting of the planks under it. Would I be correct to assume that both ends of the removed planks were done in this manner?

IMHO, new framing should have been grafted in to support both sides of the cut planks. When I've made repairs of this nature, I installed new joists with hangers and/or blocking to fully support both sides of a patch. This is particularly important if you are laying tile.

A. Spruce
Re: Replacing plank subfloor with plywood
riverbluebell wrote:

Thank you so much for replying. Yes, you described what they did, exactly. They were supposed to be installing a linear drain for a curbless shower and I was shocked to see that they had unnecessarily made this giant patch in the subfloor, especially without properly supporting it. What kind of spe******t can fix this kind of thing?

I am unfamiliar with linear drains for a curbless shower, however, I am quite certain that it should not involve weakening the structural strength of the surrounding floor to do it. There really is no "special-ist" for such things, all you need is someone who isn't a hack and who knows how to do things right. This furthers my long standing side rant about how useless "codes" are when they are not written for the tradesmen that are required to follow them. They are written in legalese, a foreign language to most people in general. /rant

Suffice it to say, your project should be permitted, and should require inspection. Make sure you are there when the inspector shows up to point out the flaws as you see them. The inspector will either tell you that it's fine, or that it has to be brought up to code, either way, your concerns will be addressed and on record, so for the bathroom to pass final inspection, the inspector will have to see and pass everything from start to finish.

Side note, the language filters flagged "special-ist" because it saw "cial-is" within the word, which has been a spam problem, hence the ***** in your post. :cool:

HoustonRemodeler
Re: Replacing plank subfloor with plywood

You can get the answers to your problem by starting a thread (with pictures) on the John Bridge Tile Forum. The Pro's there can help with the structural issues as well as the waterproofing requirements.

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