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Motordoctor 02-09-2015 06:02 PM

What size screws for underlayment
 
2 Attachment(s)
Hello all, I thought I would ask this question here instead of dogging the poor guy that's been helping me on another forum. He pointed me this way so here I am. I'm redoing a kitchen floor, had to take it down to the subfloor. It had some hideous dark brown wood tiles in a house that has Oak hardwood floors in the rest of the house. Under the wood tiles was black felt, followed by glue, followed by vinyl tiles, followed by some gray gasket looking paper, followed by more felt that I couldn't get off. I figured it would be faster and do a better job just tearing up the old plywood and putting down some new stuff. So I sawed it into manageable chunks and pried it off. Probably a good idea since the subfloor had a couple of water damaged planks that I replaced. These are 7" wide and overlap, not a true tongue and groove. I put down some Poplar 1X8, actual size was 3/4 X 7 1/4. Hopefully that is the right stuff to use, didn't have a lot of choices other than pine, poplar or red oak.

I'm going to put down on top of the 3/4" subfloor, sheets of 1/2" "Plytanium. It's rated CD and exterior. On top of that will be 1/4" Hardibacker and then 18X18 tiles.

So on to the questions: What length screws should I be using on the plywood? I had gotten 1 1/4" deck screws but those will only give 1/2" engagement into the subfloor. Better to get 1.5" deck screws? I would have like to put down some thicker plywood but it will create a bigger transitional problem from the living room into the kitchen. With what I described, the tile will be above the existing oak floor about 7/16". The only other option I saw was to put down 3/8 plywood but I felt that might be a little too flexy. Thanks.

More info
House built in 1950
Deflecto rated L /842
2x8 joists on 16"
140 sq ft kitchen
subfloor 3/4" overlapping planks laid on diagonal

Steve in Denver 02-09-2015 06:58 PM

I'm not a pro, but I'll take a stab at what they are going to say.

- Make sure the existing boards are securely fastened to the joists.
- The CDX plywood isn't suitable - it can have voids that leave some areas unsupported, so you need something with all layers with a C grade or better (and with exposure 1 / exterior glue rating)
- You want to screw your top layer into the subfloor only, not into the joists.
- Your floor will need to be very flat to get good results with the tiles you are planning on using.

I've gotten this detail wrong in the past (and CX or someone will correct me if I'm wrong) but I think the spec is a maximum 1/4" deviation in any 10 feet and 1/8" in any 2 feet.


Have you considered removing the existing subfloor, taking that opportunity to correct any flatness issues with the joists, and installing a 3/4" T&G plywood subfloor?

cx 02-09-2015 07:09 PM

Welcome, Dan. :)

That deflection rating is a bit suspect. You do understand that you're not looking at the dimension of the room or tile installation, but the unsupported span of the joists regardless what's on top of that span?

Steve's got you covered on your plywood selection.

But for the flatness requirement for 18" tiles the tile industry requires no deviation from intended plane of more than 1/8th-inch in ten feet nor 1/16th-inch in two feet measured from the high points in the floor. That is a very, very flat floor and you'll wish it were even flatter when you begin setting those big tiles.

My opinion; worth price charged.

Motordoctor 02-09-2015 08:07 PM

2 Attachment(s)
The deflection is based on 14ft joists, 16" O.C. and the joists are supported @ midpoint by a basement bathroom wall. The wall is framed with 2X4s and is sheet rock with tile, may have backer board, not sure. Sorry I was wrong on the plywood rating, had to search for it. Included a couple of more pictures. I was going to put a dial indicator on the bottom of the center joist and have about 400lbs worth of people stand right above it to measure the deflection. That sound like a reasonable way to measure it?

Motordoctor 02-09-2015 08:33 PM

Quote:

Make sure the existing boards are securely fastened to the joists.
I'm planning on putting a deck screw through each plank into the joists.
-
- You want to screw your top layer into the subfloor only, not into the joists.
- Your floor will need to be very flat to get good results with the tiles you are planning on using.
To check the floor, what would be a reasonable method? A ten foot level or ?


Have you considered removing the existing subfloor, taking that opportunity to correct any flatness issues with the joists, and installing a 3/4" T&G plywood subfloor?

I hope it doesn't come to that but if that's the only way to make this work I would do it. I don't know where to get that plywood at. Here in NYC we are technologically backward, I'm stuck shopping at Home Depots and Lowes.
If I check the floor and it's not to spec, flatness wise, is the self leveling compound a viable option?

Steve in Denver 02-09-2015 08:52 PM

Quote:

Originally Posted by CX
1/8th-inch in ten feet nor 1/16th-inch in two feet measured from the high points in the floor.

I'm not sure I'll ever get that right. I think it has something to do with my apparent inability to actually achieve those results...flat, indeed.

Quote:

Originally Posted by CX
and you'll wish it were even flatter when you begin setting those big tiles

And correct use of the subjunctive to boot. Who is this guy? ;)

cx 02-09-2015 09:40 PM

Steve, the requirement changes when your tile has one side measuring 15 inches. Smaller than that it's 1/4" in ten feet and 1/16th" in one foot. When you get to 15 inches on a side, the tolerance is exactly half that or 1/8th" in ten feet and 1/16th" in two feet.

Achieving the higher standard is, indeed, quite difficult. It's generally accepted in the industry that only with a mud bed or properly installed SLC can it actually be done effectively. I've heard it can be done with poured gypsum, but that's not something usually encountered or considered in residential work.

As for the grammar, if y'all was from Texas y'all would know we most all gots real good grammar hereabouts. :)

Dan, your plywood grade and exposure rating are fine. The nominal 1/2" is the minimum industry requirement for installation over board subflooring, but that presumes your boards to be T&G and to be oriented perpendicular to the joist structure. I generally recommend a thicker material when the boards on on a diagonal because their span is a good bit greater. I have no authority for that recommendation.

The support wall doesn't really qualify. To act as a load bearing wall it must have at least a doubled top plate or have the studs stacked directly below the joists. You could add some studs, with appropriate bracing, and also ensure you have firm contact at the bottom of each joist. You may need some shimming for the later.

Trying to measure joist deflection in situ is not a realistic endeavor in most cases. The design deflection, which is what building code and the tile industry rely upon, is calculated using a fixed load, most commonly 40 pounds per square foot) and known values for joist size, wood species, and grade. There are other factors, too, such as the dead load and material condition that also factor in. You can certainly do as you suggest, but the result won't actually tell you what you need to know.

If you improve your support wall you should have very good joist support for your tile installation.

My opinion; worth price charged.

Motordoctor 02-09-2015 09:41 PM

How many screws should I be putting down in each sheet (4x8)? According to this site, it's about 220 per sheet :scratch:

http://www.patriottimber.com/hardwoo...llation-guide/

cx 02-09-2015 09:48 PM

I recommend you not pay any attention to that site, Dan. Different kinda product for different kinda applications.

Here is a good article from our Liberry that I think shows the very best way to install a second layer of subflooring for a ceramic tile installation. I recommend you adapt as much of their method as you can make fit your application.

My opinion; worth price charged.

Motordoctor 02-10-2015 09:04 AM

Well, I'm not married to the 18" tiles, I think I could take them back since I only opened one box. The fun part is going to be carrying 700lbs (70 lbs/case) worth of tile out of the garage down a snowpacked ice covered driveway to load them into the back of the Jeep.

I bought them as a typical rookie would: "they were on sale". Knew only enough about tile installation to be dangerous. Thought putting down bigger tiles would be less work than 12X12s. Walking through the local HD with the wife, saw the tiles, said "do you think these would look nice in the kitchen?" She liked them and the color so I came back and bought them. HD had them stacked in a display so I was thinking that they were closeouts and I should buy them before they were gone. Rookie mistake #2. Oh well, live and learn. Better to know now then spent hours putting tiles down only to have them start cracking. What started out as thinking that putting down some nice tile shouldn't be too much of a pain and doing the work myself would save lots of money has become "the floortile monster project". Regardless of what direction I take, the old floor had to go, it was hideous and starting to come off so something had to be done.

So on to the questions:

• I can get to the bottom of the floor joists and put a 2X8 support column under each one at the midpoint next to the existing bathroom wall. Should I do it while the floor is at its lightest, with only the subfloor on the joists or do it after I put the plywood down and Hardibacker?
• Should I predrill the screwholes in the plywood? My thought is having the pilot holes will reduce the subfloor deflecting away from the plywood when you drive the screws.
Thanks for your advice.

cx 02-10-2015 09:28 AM

1. I suppose you could, but looks like it'd be a lot easier to add a few studs to your existing wall. And single stand-alone 2x8s won't really provide the support you need. Too much flex if not at least stabilized mid-span. You'd actually do better with just doubled 2x4s in that application and that would be somewhat questionable.

2. That's referred to as screw-jacking and yes, pre-drilling the new layer will help prevent it.

My opinion; worth price charged.

Motordoctor 02-10-2015 10:09 AM

The reason I suggested that method was of course because it would be the easiest. The bathroom wall isn't accessible the full width of the kitchen floor to put anything else. I would have to take out the furnace, take out the return ducting to get to it and probably knock down a wall in the process. But if it won't work, wishing it would doesn't change anything. I guess maybe I should consider some other floor options. Too bad, it would have looked good with the tile in there.

cx 02-10-2015 10:19 AM

Dan, I'm judging only from your posted photo and description. I can't see the rest of the problems. All I can tell you is that you must have a mid-span support under those joists to achieve the necessary rigidity. How you do that is negotiable.

If you have access to a line under the span that is clear enough to install a support under each joist, make that a wall with a double top plate or studs stacked directly under the joists and sheath one side or add adequate blocking. Is that feasible? You need to reduce the unsupported span to ten feet or so.

Or tile over what you've got. Your house, your tile, your decision. :)

cpad007 02-10-2015 10:52 AM

Do things change at all for you if you removed the solid wood plank floor? Does having access from above help you? You might be able to sister new joists to the old ones or do a combo of improving the support and sistering. You'll need at least 2/3 span for sistering to help. I dunno if you have a bunch of plumbing and/or electrical in your way for sistering.

Removing the solid wood planks might get you to the flatness those large tiles need because you can then fix things at the joist level.

Motordoctor 02-10-2015 06:08 PM

5 Attachment(s)
Doing the sister thing would be hard since it's over the bathroom with all the electrical wiring and can light poking up under the subfloor. Here's some more pictures, it's the backside of the bathroom wall, in the laundry room, from left to right.

Motordoctor 02-10-2015 06:10 PM

5 Attachment(s)
More pictures down the tunnel between the joists.....

cpad007 02-10-2015 07:48 PM

Dan,

That's what I call a "Whole Lotta Fun!"

:)

Motordoctor 02-10-2015 08:42 PM

Yeah, I'm thinking that putting tile in the kitchen isn't a good idea. The reason for it was to pretty up the house for sale, not to rebuild the whole house, one room at a time. The underlayment needed replacement, had a couple of rotted sections from water leakage. Took out a couple of the planks too. So that's not a total loss. Other than all the time I wasted trying to scrape off the old tile/glue/felt/schmutz/etc. The final fix was just to saw through the old plywood and pry it off. I spent less time prying up the whole floor than I spent scraping 1/4 of it. Like I said, it's a real shame because I think the tile would have made the kitchen. Now I have to figure out what to put down instead.

I've heard of vinyl tile that looks like ceramic with grout lines, may look into that. I put some snapstone tiles in the basement kitchen, problem with them is the floor (concrete) isn't flat and still has old vinyl tile glue on it that won't budge. I tried every chemical I could but nothing would budge that crap off. Heatgun, scraper, putty knife, etc. I first tried putting down some self adhesive vinyl tiles after putting a primer down, looked good for a couple of days and then the edges started to lift. Pulled those off, and tried putting a good contact cement on the tile and the floor. Same result, stuck good for a couple of days, then the edges started curling up. Lowes was having a sale on the Snapstone tiles, thought that might be a solution but since the floor isn't flat enough, the tiles rock. That project has been on the backburner too until I figure out what to do about that. Since the concrete floor seems to be the problem, and the area is too small to get any serious power equipment in there, I was thinking the only solution was SLC under the snapstones but I don't know if the bond issue is going to be a problem. So far the score is Tiles 2, Me zero.

A couple of old sayings come to mind:
You can't make a silk purse out of a sows ear.
If you don't have time to do it right, when will you have the time to do it over?
And of course this one :good advice is often given but seldom taken.
Being a motorcycle mechanic, I have to make that decision often. Do I spend an extra hour on the gasket surface to get it 100% or would spending 10 minutes to get it to 90% do the job? What it comes down to is this: what's it going to take to replace that gasket and do it right if it leaks. If it's easy, I will take the chance, if it's a PITA, then I take the time to do it right.
Putting 140 sq ft of tile down only to have to redo it a couple of months later isn't worth the risk.

cx 02-10-2015 08:56 PM

Lot of tile pros have learned that sometimes the job you make the most profit on is the one you walk away from. Most of'em learn the hard way, though. :)

Motordoctor 02-16-2015 12:51 AM

Well, I haven't given up on it yet. Just spent the last two days replacing the water heater. Never did one before, so it took me a while. I tried those flex hoses that HD sells since I figured #1 they look cool and #2 lots faster then measuring/cutting/cleaning/soldering copper pipes. Not a good idea. Never could get them to quit leaking at the water heater joint. As soon as you put water pressure to them, they would leak. Tried all I had to throw at it, could slow it down to a slow trickle but not the thing to have. Oh well, live and learn.
So the wife decided that she would really like the tile in the kitchen so I guess I need to make it work. Happy wife=Happy life. So on with the questions.

I'm going to take up the subfloor (diagonal planks from the 40s) and replace it with 23/32" Plywood. I've seen reference to the tongue and groove style but of course I don't see it at HomeDepot. Structurally that better than just plain (meaning not T&G) 3/4" plywood of the proper grade? Georgia Pacific makes the Sturd-I-Floor Plytanium and listed HD as being a vendor but it's not listed on their website. They do list a couple of other local lumber yards so I have to call to see if it's available.

Do you recommend gluing the subfloor to the joists as well as screwing it down?

Since I'm going to have access to the joist with the subfloor off, I can block and sister to my hearts content more or less. Are there some links on the proper way to sister a joist/blocking or should I say how to sister a joist in this instance? Does blocking help with the deflection issue?

So my current plan is now 3/4 AKA 23/32" plywood subfloor laid 90 degrees to joists, 1/2 AKA 19/32" plywood underlayment over that and 1/4" Hardi with a modified thinset over that. Gives me about 1 5/8" total thickness between the joists and the tiles.

Since I'm trying to stack the deck in my favor, I was wondering about a couple of other options.

Is the Ditra more forgiving if the floor is flexing? It says it's an uncoupling membrane but does that make your tile install more forgiving of a marginal floor? Since it would take about $275 worth of the stuff to do the job, is it money well spent in this case? If I put down the Ditra, would I put it over the Hardibacker, or just straight onto the plywood.

Do the modified medium bed mortars that are "flexy" AKA Flexbond/Megalite/Reflex/etc helpful here? Local procurement is limited to HD/Lowes/Bostik. Thanks again for your help.

cx 02-16-2015 11:51 AM

1. You must either have T&G panels or you must install blocking between panels at the between-joist spans. The T&G is usually easier, except when the confines of remodeling make it difficult to install correctly.

2. Yes. As a rule I prefer to glue every part of a subfloor installation. An exception is when plywood is installed over sawn boards or OSB.

3. Form your photos,you're gonna need to remove some of your wiring and probably move some plumbing to do any meaningful sistering. I'm still of the opinion that simply improving the wall you have in place is the easier and more effective method of reducing deflection in your application.

Between-joist blocking does nothing at all to improve design deflection of the joists.

4.
Quote:

Originally Posted by Dan
1/2 AKA 19/32" plywood underlayment over that...

. Nominal 1/2" plywood will be marked as 15/32nds" thickness.

If you have room for 19/32nds" plywood I would not remove the existing subflooring, but would install the nominal 5/8ths" (19/32nds") plywood over the existing diagonal board subflooring, saving a whole lot of labor and some material cost.
Quote:

Originally Posted by Dan
Gives me about 1 5/8" total thickness between the joists and the tiles.

That total is not relevant at all, Dan. The important criteria is what is making up the subflooring thickness.

5. If the floor is flexing, you have a problem regardless the tiling substrate you elect to use. There is no industry standard for uncoupling membranes, so we can't really tell you what they will/won't do. The manufacturers have their own test data and their own subflooring requirements. Meet the requirements and their products should function as advertised.

6. There are no medium-bed mortars. There are thinset mortars (thin-set being a method, not a product type) whose manufacturer's allow their use in thicknesses above the industry standard maximum of 1/4-inch in some applications. The manufacturer's will tell you what to expect from each of their mortars.

My opinion; worth price charged.

Motordoctor 02-16-2015 01:26 PM

Quote:

If you have room for 19/32nds" plywood I would not remove the existing subflooring, but would install the nominal 5/8ths" (19/32nds") plywood over the existing diagonal board subflooring, saving a whole lot of labor and some material cost.
So when I put down the 19/32" plywood over the existing planks, do I treat it as underlayment or subfloor? Meaning do I only screw it to the existing subfloor and not the joists? Thanks again for your advice.

cx 02-16-2015 01:30 PM

It's a second layer of subfloor, no matter what you call it, Dan, and you'll want to fasten it only to the board subfloor below after you've ensured proper fastening of those boards.

My opinion; worth price charged.

Motordoctor 02-17-2015 05:44 PM

I think I may have a solution here. Putting a mid-span beam under the joists in the basement. I talked to the guy at Ellis Manufacturing about it and he suggested using a 4x4 with their screw jacks to shore up the floor. Probably take about an 8~9ft piece of wood to do it (the beam). The screw jacks use a 4x4 as an upright to the bottom of the beam with a Purlin Splicer to attach it to the beam. The distance from the floor to the bottom of the joist is 84". If I can buy them around here, I was thinking that I should put a concrete pier under each jack to spread the load since I have no idea how thick the basement slab is. If not I guess I need to make a couple of them. On a google search they show up @ Home Depot but there's no price/availability.

They also have the option of using 3 2x6" side by side as a beam also. If I really wanted to get fancy I could make a wood I-beam using 4 2x6" screwed and nailed together. As far as the jacks go, would two of them be enough for a 8 ft beam or should I go with three? After looking at them, I could probably make those things way cheaper than buying them but getting metal around here is a real pain in the ass.

Sound reasonable? I took a good hard look at the existing wall in the basement and it's pretty flimsy looking. It's 2x4s with a single side of drywall. There should be enough room for me to put the beam right next to that wall, probably have to compromise a bathroom closet a little but what the heck.

Reinforcing the existing wall would really be a pain, half of it is more or less easy to get at. The other side is behind the furnace/water heater/return duct. I'm just looking for the cheapest/easiest/quickest solution so I can sleep at night not worrying that I really should have done more than I did. When you don't know what you're doing, overdoing it is better than under-doing it, eh? Ask anybody that wastes money on synthetic oil and changes it every thousand miles. I tell them if it lets you sleep better at night, go for it!

cpad007 02-17-2015 06:01 PM

Dan,

Do you have any settling issues? I'm not quite sure why you'd need to use hydraulic jacks at all to add the support wall unless you need to lift several joists a bit. I suppose you could lift things a tiny bit, build your wall, and then let the jacks down to settle on the wall.

I think I would just build a regular old double-top plate 2x4 wall of whatever length you need, 16" oc studs. One thing you need to be sure about is the support from your foundation. You might also toss some 1x material at an angle across it for lateral stability or put up plywood. You might further be clever by using the wall for something useful like a closet or something.

Or are you looking to simply put a beam in with two posts and not really a load-bearing wall?

Motordoctor 02-17-2015 06:41 PM

Quote:

Or are you looking to simply put a beam in with two posts and not really a load-bearing wall?
I just want to sleep at night knowing I did all I could to stop the dreaded cracked tiles/grout. The reason for the screw jacks is probably a stupid one. I don't have faith in myself to cut the uprights to the right length without a bunch of tries. Using the jack, it only has to be within 6" and I'm sure I can do that :) And if the floor needs some adjustment from sagging I can always crank a little up in a particular spot. I'm worried that cutting them a little long and whacking them into place will cause flatness issues on the floor. Although for the price of those jacks I might consider giving it a whirl since 4x4 posts are pretty cheap compared to them.

Houston Remodeler 02-17-2015 06:57 PM

Before we get too far - Have you checked the slab in the basement ?

I someone installed a post in my his first house and after a few weeks the posts went right through the thin slab. :x:

Motordoctor 02-17-2015 10:15 PM

Yeah, I considered that too, going to look for concrete piers like they use for decking. If I can't find those, I guess I will have to make some forms and pour them myself.

Motordoctor 02-19-2015 03:46 PM

4 Attachment(s)
More dumb questions from the peanut gallery. I put some mason's string on the floor spaced 1/4 off the ends to check the flatness. Checked it under the string over each plank. As it sits, the sub-floor varies a maximum of 1/4" on a couple of planks, most of it is 1/8" or less. My questions:



Is that enough to affect the underlayment 5/8" plywood I'm going to screw/glue over the planks? Or will the plywood and the subfloor come to an understanding about how flat to be? Will the screws suck up the low spots to the bottom of the plywood without creating a low spot on the top? Never measured a piece of plywood for flatness/thickness variance so I don't know if I'm being a little too anal here.

There is some "lippage" on the planks in a few spots which I could take a belt sander to and knock it down to match the adjoining plank. Needed or?

If the answer is "get it as flat as possible before you screw it down", what would I use to take up the low spots? Have to be something about 1/8 thick compressed in 7" wide strips.

I'm planning on using Loctite PL Premium construction adhesive under the plywood. Only thing HD had in the large tube size. That OK?

Under the Hardibacker board, recommended thinset? Their data sheet says modified or unmodified. The Bostik rep I talked to say their Reflex thinset would be good but @ $35/bag it's a little pricey. I thought I read somewhere on here that under the CBU it really didn't matter and it was a waste of money putting high priced thinset.

Putting down the CBU, screws or nails? I have access to a nailgun if I need it. Would speed things up I bet.

Thanks again.

cx 02-19-2015 05:23 PM

Dan, if some of those boards are cupped a bit, making high ridges on the outer edges, I'd likely use a 36-grit belt sander on those places and any similar protrusions. Otherwise, you'll pretty much wannna put your plywood over what you've got and screw it down really well. I would not recommend using any kind of construction adhesive between the board and your new plywood.

Once your new subflooring is installed you can check for flatness and determine what more, if anything, needs to be done.

Home Depot sells CustomBlend thinset mortar, the lowest rung on their product ladder, and bedding CBU in floor installations is one of the few applications for which most of us would ever recommend it.

Any salesman who would recommend thirty-dollar-a-bag thinset mortar for bedding CBU is someone I'd recommend crossing off your list of people from whom ever to seek tile installation advice.

The purpose of the mortar under the CBU panels is not to bond it to the subfloor, but to provide a 100 percent footprint for the panels. The mortar holds them up, the mechanical fasteners hold them down.

I would recommend screws for fastening the CBU panels.

My opinion; worth price charged.

Kman 02-19-2015 06:04 PM

Quote:

Originally Posted by Dan
I'm planning on using Loctite PL Premium construction adhesive under the plywood. Only thing HD had in the large tube size. That OK?

I'd recommend you not use any adhesive in your application. Lines of adhesive can create tiny voids which will allow for vertical movement. You don't want that.

Motordoctor 02-19-2015 07:00 PM

Quote:

Home Depot sells CustomBlend thinset mortar, the lowest rung on their product ladder, and bedding CBU in floor installations is one of the few applications for which most of us would ever recommend it.
WOW, that stuff is cheap, only $7/bag.

Do you think I need to fill in the low spots with some patching compound or is that just wasting time? I was looking at this stuff but I don't know if it will take having screws run through it. HENRY® 547 UniPro Universal Patch & Skimcoat.

cx 02-19-2015 08:35 PM

Dan, if you'll visit our FAQ you'll find a brief tutorial showing how to properly attribute quotes you post here on the forums. Very simple once you see it. :)
Quote:

Originally Posted by Dan
WOW, that stuff is cheap, only $7/bag.

And worth ever dime.

When using CBU as your tiling substrate you fasten it to whatever subfloor you have. If your floor is not sufficiently flat after the CBU is installed, you fix it at that level. You can't do any flattening or leveling between the Subfloor and the CBU.

My opinion; worth price charged.

Motordoctor 02-19-2015 08:53 PM

Quote:

Originally Posted by CX
When using CBU as your tiling substrate you fasten it to whatever subfloor you have. If your floor is not sufficiently flat after the CBU is installed, you fix it at that level. You can't do any flattening or leveling between the Subfloor and the CBU.

Excellent. Thanks as always for your help. I got some heavy duty commercial floor jacks to crank the floor up but they are not needed. Just have to put some reinforcement at the midpoint on the joists and I think I'm in business.

Motordoctor 02-20-2015 07:19 PM

Well, I got my answer to the question "does the subfloor lack of flatness transfer to the plywood underlayment." That answer is Yes it does. Put down one sheet and checked it. Not so good. Like a teeter totter that floor. So I either have to grind down the high spots or shim up the low spots.

Have a theory about how to fix it, thought I would put it out there. How about leaving the subfloor/plywood as is, and float the CBU on top of about 1/2" of medium bed thinset. The CBU would rest on the top of the high spots and the thinset would take up the low areas. The only fly in the ointment that I can see is putting screws though hard thinset, since you can't screw it down until it is set up.

The other two options are rent a drum sander from HD and take about 1/4 off the floor or rip all the subfloor up and just put down 3/4 Sturd-I-Floor plywood. It would take about $200 worth of plywood to do it plus ripping all the old stuff off. Probably why I was considering sanding it.

Thanks again for all your help here.

cx 02-20-2015 07:27 PM

See post #33, Dan. Don't think I can make it any more clear.

Up to you what you wanna do, of course.

cpad007 02-20-2015 07:38 PM

I say remove the solid wood planks, fix the joists, and lay down 3/4"...all nice 'n flat and likely level.

Now you have a good foundation to do whatever you want on. :)

Kman 02-20-2015 09:23 PM

Dan, it's important that you find out exactly what is causing the floor to be out of plane. Is it a joist that's high or low, or is the subfloor sagging between the joists? If the plumbing is not in the way, you may be able to check the joists from below with a straight edge or a string. If there's just one or two low joists, you might be able to push them up from below. If the subfloor is sagging in between joists, you could remove a section and replace it.

Motordoctor 02-20-2015 11:14 PM

Quote:

Originally Posted by KMAN
it's important that you find out exactly what is causing the floor to be out of plane. Is it a joist that's high or low, or is the subfloor sagging between the joists?

Looks like I have one joist that's higher than the rest. Of course it would be the joist that I can't get to from the bottom easily (furnace is in the way). So replacing the subfloor probably wouldn't change anything. OY what a pain :wtf:

cpad007 02-22-2015 12:52 PM

If by "replacing the subfloor" you mean removing your solid wood planks, I would think you'd have all the access you need.

Short of shaving down the high joist (not recommended unless you can possibly sister up something next to it for most of its length), you can add to the rest of the joists to bring things to flat and/or level.


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