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-   -   2x10 subfloor question (https://www.johnbridge.com/vbulletin/showthread.php?t=129235)

RRappel 05-23-2020 06:59 AM

2x10 subfloor question
I'm redoing the kitchen floor in my 1970s split-level and was hoping to go with electric radiant (either the DITRA-HEAT or the DCM Pro Fast Fit systems) under tile. The kitchen dimensions are roughly 11.5' x 13'. I'm trying to have the kitchen floor be the same height as the dining room adjacent to it and a hallway. Both of these floors are 3/4" oak laid on top of the subfloor. The main problem I have is that the subfloor in both the kitchen and dining room (maybe the whole house) consists of 2 x 10's laid flat and spaced maybe 1/8" apart.

Given my height requirement, I am hoping to avoid a layer of plywood on top of the 2 x 10 planks (or at least minimize the height of this). I contacted a carpenter very familiar with both these radiant heat systems and he says I should be fine with just a layer of 1/4" plywood or luan. If the 1/4" ply won't work, any suggestions on what options I have with electric radiant heat and tile that would meet my height requirements?


Carbidetooth 05-23-2020 08:20 AM

That's really unusual construction. How old is the house and what part of the country?
2x10s on the flat would have lots of cupping, I would think, and that would tend to create gaps between tops of joists and subfloor...not a good starting place.

Off hand, I would disagree with your carp, and would strongly recommend you consider a minimum of 1/2" ply with on top of that after some intensive buttoning down or replacement of the subfloor. Depending on cupping, I might want to run a saw kerf the length of 2x10 to let them pull tight against joists.

All of this in my imagination at this point, having never seen such a subfloor. Photos would be helpful, both context and close-up.

Kman 05-23-2020 08:24 AM

Welcome to the forum, Ron. :)

Assuming the joists below will support a tile installation, the bare minimum you can use would be 1/2" plywood over the subfloor, and that's after you've removed the 3/4" oak flooring.

Any idea what the joist system below is comprised of?

I have to say I've not heard of 2x10's being used for subfloor. Are your joists 48" on center, perhaps?

RRappel 05-23-2020 04:55 PM

1 Attachment(s)
Thanks for the feedback guys.

Location-wise, I'm 20 miles north of Boston. I've been here just over 20 years. I remember when I first saw the house I noticed how solid the floors felt. Since I haven't removed the vinyl floor in the kitchen to do the tile work, the best I can to do for a picture is an area where the kitchen and dining room meet. This shows the existing kitchen floor. I think it's 5/8" ply with some kind of vinyl tile on top, then the sheet of vinyl on top of that.

I don't think the joists are spaced 48". I believe they are also 2x10 (vertical this time :-) but have to do a bit of demolition to check that.

If I went with 1/2" ply, the kitchen floor would be around 1/4" higher than the dining room floor. Is that a reasonable height difference? If so, would I grout the floor and then caulk between the tiles that abut the hardwood?

cx 05-23-2020 11:16 PM

Welcome, Ron. :)

If you don't add that geographic location to your User Profile it'll be lost before we leave this page.

RRappel 05-24-2020 06:12 AM

Thanks very much CX for the welcome and for the information. Updated as suggested.

Carbidetooth 05-24-2020 09:20 AM

Ron, if this is over crawl space, you could get a look at framing from there. But I'm guessing this is upper level of split.

You could also use a strong Neo magnet or stud finder on ceiling below to get idea of joist spacing. The neo magnet looks for drywall fasteners.

Do you know age of house? Building with solid lumber subfloor of any kind would be cost prohibitive now, not to mention the lumber available now is not that great.

RRappel 05-24-2020 05:08 PM

Thanks for the info Peter. The house was built in 1971. And yes, this is the upper level of a split.

Once I pull out the existing kitchen floor, I shouldn't have any problem figuring out the joist spacing. Interesting idea of checking the joist spacing from the ceiling below.

One concern I have is that the kitchen will have a peninsula with seating and the back of the seats face the dining room. I wanted to make sure the dining room and kitchen floor were the same height so if you slide the chair back while you are getting up from the peninsula, you can't slide it too far and have it fall over if the back legs of the chair cross over from the kitchen floor to the dining room floor. I believe the peninsula is far enough in from the dining rooms so this shouldn't be a problem, but having the two floors the same height would be ideal.

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