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Old 10-02-2006, 12:07 PM   #1
ghumphri
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1/2 Inch Plywood Subfloor - Now What?

Hi All

Beginning demolition of my kitchen re-do. I will be installing porcelain tile over either Ditra (my install) or SLC (contractor pour) depending on timing of other related projects and the responses I get here.

The structure is a 1962 era hip-roof ranch on a basement. 2x10 Spruce, 16" o/c, with the longest unsupported span measuring 13.5'. I have a little over 400 sq ft to tile (kitchen, 1/2 bath, foyer).

The subfloor is only 1/2" plywood. Can you please advise how I should modify the floor to properly support porcelain tile using the following methods? (my speculation added for comedic purposes only)

- Ditra. Lay 5/8" plywood over existing 1/2" subfloor. Ring shank nails through the subfloor only -- not the joists.

- SLC. Contractor recommended adding layer of 1/2" plywood over the 1/2" subfloor (same nailing method as above). He would then pour 3/8" - 1/2" of SLC on the surface. Ready to tile in 12 hours. Seems a little thin to me, but contractor insists SLC is robust at this thickness. Estimate $2.50 sq ft.

Thanks for looking!

Glenn

Last edited by ghumphri; 10-02-2006 at 12:24 PM.
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Old 10-02-2006, 01:13 PM   #2
Dave Taylor
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Dunno Glenn.....

I reviewed your previous posts which mention this kitchen floor and.... I'm a bit confused. Are you sure your joists are Doug Fir..... for sure.... and is their nominal height 9.25" or 10"

Is this floor bouncy (AT ALL) now?

Does this floor need SLC for any leveling or smoothing reasons?

I get a bit leery with a 13.5" unsupported span.... but I sure am leery of SLC on this span at 3/8" - 1/2" thick.... if the only idea is to add structural strength??

The Least I would consider using would be 5/8" ply properly laid (3/4" sounds better)......... then add CBU or Ditra.

I would also consider adding some solid blocking at (at least) mid span, maybe more.

My thoughts.
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Old 10-02-2006, 04:55 PM   #3
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Hi Glenn,

What Dave said.

The floor structure has to measure up before any cement products are added. Another layer of plywood and the Ditra would be my choice, but of course I'm biased.
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Old 10-02-2006, 05:03 PM   #4
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IF you floor is out of whack (not flat or level), then the slc makes sense. You need the subfloor strong enough prior to adding it, as it isn't particularly strong (good in compression, not in tension). If you go with the slc vs say Ditra (you could use both), make sure the primer is used and the reinforcing lath. So, first thing I'd do is see how much if any it is sloped, dipped, etc. If it's not bad, then Ditra, if it's bad, slc, then decide on the Ditra or not.
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Old 10-03-2006, 05:54 AM   #5
ghumphri
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Thanks for the replies.

The 2x10 (9.25 nominal) joists are spruce - not Doug Fir (as noted by the ink stamps on the lumber). How does spruce compare to Doug Fir for deflection?

The floor is slightly spongy in one area in the middle of the kitchen. This is when I jump on it (200 lbs). The blocking is a great idea. I have the room in the basement for access.

The SLC was recommended as a tile backer (ilo of Ditra or SBU) by a local contractor (all he does is pump SLC for tile floors). Accufloor of Michigan is the business.

I have one area that is sloped, but with 3 layers of vinyl underneath, I have no idea what I'm going to find! Suspect the luan (sp?) layers are the cause.
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Old 10-03-2006, 02:32 PM   #6
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Spruce is generally a little softer than doug fir.

I don't like the idea of using slc unless it's necessary. I don't think it should be used in place of a tile backer or membrane.
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Old 10-03-2006, 02:45 PM   #7
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I with Mr. B Glenn.....

Sorry about not responding earlier... I musta been nappin'.

That "slightly spongy" in the center of your kitchen floor is gonna be death on SLC.... and it does not bode well for ceramic either.

Gotta get that floor stiffened up first before anything..... whatever it takes.

My take
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Old 10-04-2006, 02:03 PM   #8
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What's the best method to "block" the joists up to reduce deflection?

Again, 2x10 placed 16" o/c. Thanks!

Glenn
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Old 10-04-2006, 03:18 PM   #9
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Hi Glen,

Well the best thing you could do is build a bearing wall or add a beam under the floor in the basement. Put it in the middle of your span and you will have much better deflection numbers. You said you had room in the basement so is adding a bearing wall or a few posts with a beam an option for you?
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Old 10-04-2006, 06:42 PM   #10
ghumphri
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Thanks for the reply. Unfortunately, a bearing wall in this location would be extremely challenging due to access to my laundry room and the heat/return trunks located above.

How about bridging-in pieces of 2x10 perpendicular to the center of the span? I would not be able to do this to each joist because of plumbing and electrical, but I could manage to get most of them.

Glenn
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Old 10-04-2006, 06:54 PM   #11
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Blocking doesn't provide a cure for deflection, it helps share some of the load between joists and keeps them from twisting with heavy loads...sorry.
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Old 10-04-2006, 06:59 PM   #12
ghumphri
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Thanks, jadnashua. Makes sense.

What type/grade of plywood should I install over the existing 1/2"? I'll most likely go 3/4" .

Glenn
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Old 10-04-2006, 07:38 PM   #13
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No amount of plywood will solve the problem of the joists not being strong enough...but, assuming you fixed those, then 5/8" probably would be okay, but more is better.
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Old 10-04-2006, 09:21 PM   #14
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Hi All,

I agree with much of what has been said. I think Glenn is on the right track.

In 1962, and generally until the early '70's, subfloors up here were built using a layer of 1/2" AND another 5/8" plywood, usually in that order. Sometimes the 5/8" was installed over the joists instead of the 1/2". If Glenn has in fact measured correctly, someone along the line might have removed a layer during some renovation? I recommend adding at least another 5/8", and Ditra.....of course.

The guy who wants to pump some SLC for you is making me mad! Yikes, I too think it should be thicker, but it doesn't sound like you need it in the first place. I think charging about 1K and being done well before lunchtime may have something to do with his recommendation?

If the subfloor feels a little spungy in one area, this might be due to a loose sheet of ply? I would re-screw or re-screwnail the entire subfloor and check again. If still to bouncy, do one of the above mentioned, or perhaps you can 'sister' the joists, at least in the middle 3rd of the span?

The deflection values will vary depending on the criteria used to make the calculations. I know the 'Deflecto' meter errors on the safe side, which is good. According to my span chart, your joists system will meet L360 at upto a span of 15'5" if the criteria is the standard 40 lb. live load and 10lb. dead load and 1st grade Spruce-Pine-Fir. I prefer 50/15 load, which comes out to a max span of 13'6" for 1st grade, and 14'11" for Select Structural Grade. I always check the chart AND also walk or jump around and if I have any doubts I go with my gut feelings and the heck with what it says on paper!

As for VersaBond, it's kinda funny to me, but thru the years it has often been referred to as 'lighly modified', as if that distinction is a good thing, a positive. What it really means is that is is a "CHEAP" modified thinset mortar. Having said that, I also feel that it is adaquete for most basic purposes, and is a good value.

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Old 10-04-2006, 09:27 PM   #15
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Structure

I agree with some of what I am reading. This is an issue I go round and round with some folks on a regular basis.

But before I begin, let me just say that 1/2" plywood is never enough . If you put 1/2" plywood over steel i beams 16" OC it will be totally UNACCEPTABLE, and what you are describing is totally to be expected, so I wouldn't despair just yet.

ANY cement product is prone to cracking if the underlayment below is deflecting too much. And in my opinion, that includes cement backer, as well as hardi backer(over time).

Step 1 would be satisfying the minimum requirement of the thinset. 1 1/8" of plywood is the minimum. USE 3/4. Its a couple more dollars a sheet and has much more strength. But the connection is something that can make all the difference.

What I am about to describe is in all actuality, the construction of 1 1/4" plywood, and is the result of my trial and error on 30 or so subfloors.

Before applying the sheet, clean the 1/2" panels thoroughly, and place additional screws down in between the nails that are there already. Mark on the base of the walls where the joists are to make screwing the 3/4 easy later. Next buy a case of floor underlayment glue in the large caulk gun size. Most people just squirt this stuff randomly between sheets, which actually hurts the integrity of the floor. Dump a whole tube on the floor and using a 1/8" v notched trowel spread it thoroughly on the existing floor, where each new piece of 3/4 is supposed to go. Place the 3/4 perpendicular to the joists, offset each row by at least 24" and dont let any seams line up with those of the 1/2" below. Keep 1/8" space between the panels, and screw them down INTO THE JOISTS. The evenly trowelled underlayment glue will connect the pieces together, and if you place your toes on both sides of the screw you are driving you will be able to really suck down the plywood, but only if you are driving the screw into a joist. This will not work into a piece of 1/2" plywood. Take your time doing this, and then come back the next day and walk around on it. You will probably laugh a little at this point.

If you have any deflection at this point it will be what I call wide area deflection. (jumping up and down seems to make a whole 8' by 8' area move up and down slightly) This is quite subjective, but by doing step 1 first, you have wasted no time.

If you have tremendous movement at this point, and a wall is out of the question, the best way to reduce it is to sister a second 2x10 next to every other 2x10 that you can, or just place a few 2x4 members under some of the joists in the center area of the floor. You don't need a whole wall, 1 2x4 every 4 feet under the floor that I mention above will be more than sufficient. Place them where you can, cut them so you have to tap them in with a hammer, nail them at the top and then put a bead of liquid nails around the base. (I realize these methods may seem or really be impossible at this point, but if you can't put a wall underneath the center, this is the next best thing.)

If you want to minimize a reasonable amount of deflection, blocking IS effective, and any span over 10' should be blocked if deflection is an issue. Blocking is required in certain situations by code because it is effective. By removing twist in the joists, you strengthen the floor system as a whole. Just measure and cut 2x10 blocks and nail them in allong a chalk line snapped in the middle of the ceiling. Tighter the better. And its easier to alternate from the left side of the chalk line to the right side of the chalk line as you go (easier to nail, and easier to fit in tightly).

I wish someone told me this a few years ago, hope it helps.

edit - after hitting submit i noticed jaz's post, and I agree with all written, except I would only use versabond on concrete. Flexbond or fullflex as a minimum. Superflex is the best, but all have the ability to flex to a greater degree than the "slightly modified" stuff. Why take a chance. Spend the extra 10-20 a bag.
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Last edited by Grasshoppa; 10-04-2006 at 09:35 PM. Reason: more
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