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Unread 01-16-2008, 11:18 AM   #1
T-Bone430
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Subfloor Question

I am redoing my kitchen and after I demo'd the existing floor, I realized that I only have 3/8" plywood over my joists. Obviously I need to build up the subfloor, but I am not sure what the best way to do it is.

At the adjacent floors I have 3/4" hardwood floors and I also have a exterior door at the tiled area. The sill for this door is also about 3/4" above the existing 3/8" plywood.

Please let me know what you think.
Thanks in advance
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Unread 01-16-2008, 12:26 PM   #2
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T-Bone, are you sure your sub-floor is only 3/8" thick? I've never heard of sub-flooring being that thin.

As long as you are looking into building up the sub-floor, might as well check out the joist structure. What size are they, how far apart are they spaced, and with is the unsupported span. All that data needs to be entered into the Deflecto tool found on the blue bar towards the top of this screen. Tell us what you get. Also, let us know if you need any help with that.

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Unread 01-16-2008, 12:33 PM   #3
Dave Taylor
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Howdy T-Bone...........

and welcome to the forum.... gotta' real name we may use?

I don't recall ever seeing 3/8" plywood subfloor (the wood layer directly over joists). Are you sure its only 3/8" and (if so) how (why) did it ever get used as a residential subfloor?

What are you planning on re-doing your kitchen floor with?

Tell us about your kitchen floor.... its length and width.

Tell us about the joists under your kitchen floor...... their type of wood and their condition.

Do some exact measuring and tell us the joist width, joist height, joist distance on center and.... the longest unsupported joist span holding that kitchen floor up, T-Bone.

With this info the good folks here-a-bouts at TYW forums will be able to assist you.

Thanks T-Bone.

* * * * * * * * * * * * * * * * *
Hmmm..... that Mike Mike is quick.... must be them jelly donuts.
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Unread 01-16-2008, 05:29 PM   #4
T-Bone430
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My name is Kevin.
As far as my joists:
Joists: 2x8's spaced at 16" o.c.
Span: 10'-0"

Let me know what you think I should do. Thank you
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Unread 01-16-2008, 05:31 PM   #5
T-Bone430
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I am planning on using ceramic Tile for the floor.
As far as the condition of the Joists, they appear to be in very good shape.
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Unread 01-16-2008, 06:07 PM   #6
jadnashua
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Your deflection ratings are okay for ceramic, but the subflooring is totally inadequate for tile. If you don't want to or can't reset the exterior door, you might have to do some major remodeling. Do you have access to the joists from below? There's ways to beef it up from below. If not, I'd tear out the 3/8" (I also can't believe it is really 3/8") and put down 3/4" T&G, then a membrane like Ditra from www.schluter.com. If you can handle resetting the door, then you could put more ply on top of what's there, but it will be nearly impossible to get it adhered properly - screws just won't hold in 3/8" ply and you don't really want the second layer adhered to the joists - normally the second layer is only attached to the first, not the joists.
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Unread 01-16-2008, 06:46 PM   #7
T-Bone430
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I stand corrected, I have about 1-1/2" above the 3/8" plywood to the bottom of the door, so I do have a little room to play.

I just finished my basement and put in a drop ceiling so I can get to the joist below.

I really do not think that taking out the 3/8" plywood would be a solution because the bearing walls rest on this ply. Structural I am pretty sure that just replacing the ply at the span areas will not do much.

Could I laminate 1/2" plywood to the 3/8" wood, so that it would then be about 5/8" thick, then apply Ditra or use Hardiboard.
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Unread 01-16-2008, 08:30 PM   #8
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Well Kevin.....

you definitely have a non-standard floor with that 3/8" plywood subfloor..... and its gonna' take a non-standard fix before a ceramic covering even has a chance to survive..... a non-standard fix I don't give out often. It can be a contraversial fix here-a-bouts and does carry some risk of failure.

Add more ply over that 3/8" subfloor.... 5/8" at a minimum AC or BC or CC sanded and plugged, exterior (type 1) glued.... applied basically as per Here

Now for some exceptions:
Glue your new ply underlayment to the 3/8" sub using a full spreadable glue like Titebond II.... applied evenly.

Fasten this new underlayment into all joists using galvanized deck screws.

Add solid blocking between joists where you are able.... and fasten the new underlayment into this solid blocking too.

Use Ditra as a final uncoupling surface to tile over.... and be sure to follow all manufacturer's instructions carefully when installing it.
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Unread 01-16-2008, 08:35 PM   #9
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Welcome, Kevin.

Please go to the UserCP above, find Edit Signature and enter your first name there so it will appear with each post and we won't hafta search for it.

Let me be the forth to express substantial skepticism that you actually have a 3/8ths-inch subfloor. Been wrong before, but.......

But if, in fact, that's what you're starting with, and you're not willing to remove the 3/8ths ply, I would recommend you pretend it's not there and lay 3/4" T&G plywood over it and fasten that firmly to the joists. That will give you a single-ply subfloor that is slightly above minimum specs. for most substrate manufacturers.

And I would strongly suggest you add some blocking under those partition walls that are sitting on nothing but that 3/8ths plywood.

And for this part:
Quote:
Originally Posted by Kevin
Could I laminate 1/2" plywood to the 3/8" wood, so that it would then be about 5/8" thick,
If you can do that, I can get you full time work dealing with transition problems our other visitors have alla time.

Seriously, there is really no good way to do that lamination for the reason Jim (jadnashua) pointed out. It's not impossible, but it's very, very improbable.

My opinion; worth price charged.

Looks like ol' Dave done snuck in there while I was typin'. But I'm stickin' by my advice, even though it means disagreeing with him twict in one day.
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Unread 01-16-2008, 08:56 PM   #10
Jaz
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Kevin,

I too don't believe your subfloor is only 3/8" thick. If it is, you need to find both the builder and your town's building inspector person.

Please though, can you give better answers? I mean you haven't described the situation very well so far. This forum does not charge by the number of words you type here.

How old is this house? Where is it? What did you remove and how thick was it altogether? I think you had a 2 layer subfloor system that was used here up to about the mid '70's. When was te house built? etc. etc.

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Unread 01-16-2008, 09:03 PM   #11
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Good evening Jaz!

Hopefully the OP meant to say 3/4", but he did repeat it was 3/8".

double, triple check the thickness again.
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Unread 01-16-2008, 09:21 PM   #12
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Hi Mark,

I noticed when you found this forum a few weeks ago.

Unless someone really messed up, I think Kevin has 1/2" plywood, and the second sheet of 5/8" was removed in the demo. We'll wait until he remeasures. We have had people swear they had 1/2" only to find it was really 3/4".

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Unread 01-17-2008, 05:40 AM   #13
T-Bone430
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OK gentlemen, we will start this over. I removed three layers of linoleum that was installed by a previous owner. Under the first (installed) layers I did remove tar paper and a 1/2" piece of plywood (so you may be correct on the (2) layer subfloor system). Under this 1/2" piece of plywood is a piece of 3/8" (yes 3/8") piece of plywood. This is now my starting point.

However the 1/2" piece of plywood is only found in the kitchen area. The rest of the house appears to have tounge and groove 3/4" hardwood floor that sits on the 3/8" subfloor that I discribed above.

As for the house, it was built in 1962 in Maryland.
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Unread 01-17-2008, 06:32 AM   #14
ddmoit
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Hey Kevin,

I think the subfloor experts here are going to advise you to remove that first layer of 3/8" plywood. It's hard to construct an subfloor suitable for tile starting with 3/8". The main reason is that the second layer needs to be screwed into the first (as opposed to the joists). There's just no good way to get a screw to bite into 3/8".
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