taking up subfloor

[John Dirks Jr]

With 1" combined plywood and Wonderboard, then tile on that we'll be about 1/4" higher in the kitchen than in the dining room. We can live with that. I was not trying to make it perfectly level between the two but rather to keep the transition minimal.

I'm seriously considering leaving the original 3/8" ply and adding another 3/8" ply on top of it as Brandon suggests. Staggering it of course and putting 1/4" Wonderboard on top of that. What type of bonding material would be best between the two layers of 3/8" plywood?

Wouldn't 2 layers of bonded and screwed together 3/8" plys be as strong as a single 3/4" sheet?

[AHI in AR]

With 1" combined plywood and Wonderboard, then tile on that we'll be about 1/4" higher in the kitchen than in the dining room. We can live with that. I was not trying to make it perfectly level between the two but rather to keep the transition minimal.

I'm seriously considering leaving the original 3/8" ply and adding another 3/8" ply on top of it as Brandon suggests. Staggering it of course and putting 1/4" Wonderboard on top of that. What type of bonding material would be best between the two layers of 3/8" plywood?

Wouldn't 2 layers of bonded and screwed together 3/8" plys be as strong as a single 3/4" sheet?

My thinking would say no.

At the risk of complicating matters more, if you have access to the area below, how about simply adding additional floor joists to cut your on center distance in half? If I understand correctly, you're dealing with a relatively small space anyway, so the expense shouldn't be that great. Doing that, you wouldn't have to cut through your original 3/8 inch subfloor and compromise the strength you do achieve from that membrane.

[Jim Katen]

. . . Wouldn't 2 layers of bonded and screwed together 3/8" plys be as strong as a single 3/4" sheet?

It seems like a foolish idea. I don't see any savings in time and only a modest savings in materials at the expense of risking failure of the entire job.

If I were really dead set on trying to manufacture my own 3/4 ply, I'd do some bench tests first.

[ericwlewis]

. . . Wouldn't 2 layers of bonded and screwed together 3/8" plys be as strong as a single 3/4" sheet?

It seems like a foolish idea. I don't see any savings in time and only a modest savings in materials at the expense of risking failure of the entire job.

If I were really dead set on trying to manufacture my own 3/4 ply, I'd do some bench tests first.

Imagine the immense pressure that is applied when they laminate plywood, not to mention the full coverage of adhesive and heat applied. I know we're not talking OSB here but, have you ever bought multiple sheets from a big box store and when you get about 10 sheets into the stack, they're warm?? Still curing I suppose.

Experience has taught me, don't skimp, you'll be sorry.

[John Dirks Jr]

OK, I'm convinced. I'll strip to the joists and lay a single 3/4" plywood toped with 1/4" Wonderboard.

These floors are strong so I'm not woried about loosing structural integrity by removing the original 3/8 ply. The original build has blocking as well.

This pic is not under the kitchen but at an unfinished part of the basement. It's an example of what the kitchen floor structure is.

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Here is the kitchen in general. We already painted those cabinets once. We'll be painting them again.

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[gpdewitt]

Couple of tips from an old tile setter:

Since you're going to remove the sub down to the joists, make sure to block with at least a 2x wherever your plywood seams run perpendicular to the joists. Think where the H clips go between sheets of roof sheathing. Even with 3/4 ply, there is enough flex there to crack your tile/grout later.

Check your joists both ways for flatness, in case you need to shim. Subfloor needs to be flat within 1/4" in 8' for tile to look good.

Have you considered Ditra? I put 400 SF of it in my house a couple years ago, at about $1.75 per SF, and with normal porcelain tile it matched my hardwood easily. It's also less work to put down and I believe more durable. Here's a link:

http://www.schluter.com

Jim Katens' post is spot on and that method will work fine too.

[ericwlewis]

Have you considered Ditra? I put 400 SF of it in my house a couple years ago, at about $1.75 per SF, and with normal porcelain tile it matched my hardwood easily. It's also less work to put down and I believe more durable. Here's a link:

http://www.schluter.com

Holmes uses that stuff everytime he sets tile. I would like to try it some time. Thanks for the link.

[Chad Fabry]

I've used Ditra a half dozen times. I especially like it in showers but it's also an excellent isolation membrane.

[kurt]

OK, now we're talking........

I love Schluter Systems. Ditra is a miracle, as are all the Schluter products.

The fleece backing on the stuff provides a rock solid mechanical bond to the substrate/subfloor. All the little squares in the Ditra anchor the mortar, while on the underside, the little channels created by the squares allow vapor to dissipate out of the assembly.

It acts as an uncoupling layer, minimizing movement that causes cracks. The point loading capability of the material pretty much removes any possibility of cracks.

I can't say enough good stuff about it. Schluter also has a number of transition thresholds that would answer the height differential issue.

And the blocking that DeWitt mentioned.......do it.

[John Kogel]

Hello John. that looks like Douglas fir, to answer your earlier question. By now, it is hard as steel. Save that 3/8 plywood for other projects, it's good stuff. If it is nailed as well as I think it is, you may have to cut the nails.

I've used that orange Ditra mat, bought from Home Despot. They sold me 2 sacks of high grade thinset, one for the subfloor and the mat and a different formula for the tiles. It became a costly project, but no complaints, good product.

[John Dirks Jr]

And the fun has commenced.

The original 3/8 plywood is quite spongy in some areas, especially at seams that are not over joists. Also, it runs perpendicular to the joists which I'm told is a weaker way than if layed parallel. However, I've decided to leave it down and overlay it with 5/8 plywood and 1/4 wonderboard.

Questions.

1) What kind of 5/8 plywood should I use and should I get T&G if available?

2) Should I run the new layer of 5/8 perpendicular or parallel to the joists?

3) What kind of adhesive should I use to bond the two layers of plywood together?

4) What kind of screws should I use to tie the plywood layed together and should the screws hit the joists as well as the field? What kind of spacing?

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[Chad Fabry]

1) What kind of 5/8 plywood should I use and should I get T&G if available?

Tongue and groove is best- osb or plywood 21/32"

2) Should I run the new layer of 5/8 perpendicular or parallel to the joists?

Absolutely perpendicular

3) What kind of adhesive should I use to bond the two layers of plywood together?

I like PL polyurethane, glue above the joists and the field.

4) What kind of screws should I use to tie the plywood layed together and should the screws hit the joists as well as the field? What kind of spacing?

1-1/2" or 2 inch torx drive deck screws 6" oc will work awesome. Absolutely hit the joist -3/8 plywood has next to zero fastener holding power.

[gtblum]

4) What kind of screws should I use to tie the plywood layed together and should the screws hit the joists as well as the field? What kind of spacing?

Nail it. You're already gluing it. Why waste money on screws? As far as spacing goes, 4" on the edge and 8" in the field just like any other floor.

The nails will hit the joists when they do. Who cares? You're gluing it. It's a floor, not a Steinway piano. Are ya done yet?

[John Dirks Jr]

You guys are great for helping me through this and I thank you. No matter what way I decide to do it, all the suggestions are worth pondering and help the mind get around all the things worth considering.

Choke me if you want to but after further consideration, if I lay 5/8 over the existing 3/8, the new edges will not be landing in bearing at some critical edges and junctions. With the 3/8 as flimsy as it is, that scares me. The wall that is parallel to the joists and the parallel line at the cabinet is what worries me the most. Joists would not be at those critical edges. I think the 5/8 would have potential to move too much. It's easy to see the trouble spots now that Im down to the subfloor and can see exactly where the joists are.

I think I should to tear to the joists, block out every edge and spot where a junction will be and lay a single layer of 3/4 topped with 1/4 wondernboard.

Havn't got any new underlayment/subfloor material yet. What are my best choices in 3/4 material? Should I glue the subfloor to the joists as well as screwing it?

[John Dirks Jr]

At the critical edges that are parallel to the joists, I think I'll have to first add perpendicular blocking, then fill it in with blocks that run parallel to the joists. Then it will be a solid bearing point all the way around the edges. Especially in front of the cabinets since thats where most people stand and move about.

Now that the plan is back to 3/4 directly on joists, should I still run the subfloor material perpendicular to the joists?

[gpdewitt]

I would use exterior grade ply run perpendicular to the joists. On the two face plies and center ply grain runs long ways, so more grain running perpendicular to joists that way. Glue and screw, deck screws, you don't want any movement at all ever.

Follow wonderboard manufacturers instructions as to fasteners, subfloor prep, screw type, and I beleive modified thinset under the wonderboard. They also may want you to space out the subfloor and/or wonderboard, and tape the joints with alkali resistant joint tape and thinset. Can be done while you're setting your tile. Dry test your layout in both directions and every transition and corner to adjust for largest possible cuts.

Have fun, and post result pics.

[gtblum]

John, if the 3/8 runs under the cabinets, leave it. When you glue and nail the 5/8 over it, it will be stiffer than a convict on a conjugal visit. When you screw the fiber rock / Hardie board or whatever down, it will stiffen up that much more.

You're over thinking this little floor into the ground. Just make sure you stagger the top layer over the first and to it's own seams. It's really that simple.

If you tear it down to the joists, you're going to cost yourself a whole lot of time, money in extra material and more than likely, more headaches trying to figure out how to stiffen everything up between the cabinets.

What happens when the day comes to put new cabinets in? Are you going to have to tear the whole floor up again? Get er done, Brother!

[John Dirks Jr]

I have little confidence in this 3/8. Its delaminating areas and very weak. I know that blocking out for 3/4 will work, even if its extra time and money. Maybe if you were here to see what I see, you'de loose confidence in the 3/8 like I have.

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[John Dirks Jr]

Furthermore, one of the 3/8 pieces, the one right in the high traffic area of the doorway to the dining room was scabbed together. The top layer of laminate is not even one piece. You can see some paper tape remaining at the junction. Glue and nails or screws are not going to do anything on top of this piece of crap. What's it going to hold on to. A delaminated piece of 3/8? I doubt it.

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[gtblum]

Nope. That's not bugging me a bit.

By the time you get a couple of hundred nails, some glue, and screws down through it, it'll be fine. Go for it.

[Tom Raymond]

Staple some expanded wire mesh down and coat the whole thing with modified thinset. When it's dry coat it again using a 1/4 x 1/4 notch trowel, and when that's dry once more with a smooth trowel. Cover that with a latex isolation membrane and tile it.

Really, don't do that. That's how it would have been done in 19-freaking-81. I just wanted to see if your head would explode[:-dev3]

[kurt]

Mine almost did.......that's how they still do it on some jobs around here.

[John Dirks Jr]

I took out the 3/8 and blocked the perimeter. Next I'll map out and cut the 23/32 plywood I picked up. I'll lay that out to mark the joists then remove it and block in all the junction points.

I went with 2x6 for blocking and in order to leave room for existing wires and that fact that I didnt want to interfere with the furring strips for the ceiling tiles for the room below. some blocks are screwed and some nailed depending on how easy the access was at each area. The blocks that are screwed parallel to joists are also glued.

My back and knees are sore and I got cuts on my fingers. I've done some construction work before but I now have a new found respect for those that do this stuff day after day all the years long.

Hack job maybe but it will be solid when done.

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[Brandon Whitmore]

No matter what you do, don't pull up the subfloor.......oh wait, too late for that.

Now you gotta pull a permit. []

[John Kogel]

Hello, John. Now that you're committed to the project, your phone should start ringing off the wall. []

You done good to scrap that plywood. I was picturing something worth saving, not that crap. The fact that it was laid wrong made it a waste of effort to work with.

Always lay the plywood across the joists and always stagger the ends. Always land seams on a joist or blocking. Glue and screw is the standard nowadays. Glue eliminates squeaks. Use a long straightedge to check the floor joists for flat. Use the power plane on the high spots.

When I went to lay my engineered hardwood floor, I discovered a huge bump in the subfloor, under the cheezy laminate. They'd shimmed and padded both ends to mask the hump in the middle. I built a plastic tent over the hump and took it down with the Stanley hand planer, a whole 5 gallon pail of wood shavings. But there's no more hump.