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Old 01-22-2018, 04:26 PM   #1
dceddia
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Subfloor for kitchen tile

I've done a good bit of reading here and found a lot of good info, and now I've got some specific questions about my own situation.

We're looking to tile our kitchen which is around 180 sqft total, but the tiled area would be less because of cabinets. The current plan is to take up the linoleum/vinyl, clean up the floor, install Ditra (possibly with heat), and put down 6-12" tiles.

Our floor specs:

Joists:
- 2x8 joists, 16" OC (may qualify as "kind of cracky or knotty", not sure, see pics)
- unsupported length is between 11' and 12' (see question below)

Subfloor:
- 1x8 (actually 3/4" thick) planks
- 5/8" plywood (glued & screwed? I don't think so -- can't see any glue around the planks, and there are ribbed nails sticking through all over...)
- 1/4" underlayment

The planks run perpendicular to the joists. The 5/8" plywood runs perpendicular to the joists for about half the floor, and then seems to switch to parallel for the other half There's black goo oozing out between the 5/8" plywood and the 1/4" underlayment (it's sticky - seems like tar of some kind). The 1/4" underlayment is then glued to the linoleum (not sure if it's actual linoleum, or some other kind of sheet vinyl).

Here are a few pictures under the floor. It's accessible from the basement, but there's a gas pipe, some electrical, and some water pipes running across it.

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Here is the side profile of the subfloor layers:

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Deflecto ranges from L/177 (12', knotty) to L/377 (11', SYP/douglas fir).

So my questions are:

1. How should I measure the unsupported span of the joists? Does that include the piece of board that's laying on top of the support or not? Is the unsupported span "A" or "B" in this picture:

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2. Can you tell what kind of joist wood we have? Would you consider it "kind of cracky or knotty"?

3. Are the joists sufficient?

3a. If not, can we beef up the subfloor and leave the joists alone? Or do we need to reinforce the joists somehow?

4. Is the subfloor sufficient? I've read that 5/8" plywood is the minimum, but since ours is sitting atop those spaced-out planks I'm not sure how that affects the floor.

5. The money question: can we put tile on this?

Thank you!
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Old 01-22-2018, 06:50 PM   #2
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Newb DIY're here... so no expert, but I'll throw my thoughts in:

1. B - Based on the Deflecto I think you're either L/289 or L/222


2. Either way I think it's classified as "unknown", so you're still under L/360 - sorry

3. I don't think so

3a. From what I researched, you can NOT make up for joist deficiency with subfloor... but I've read some post here (or elsewhere) about gluing/nailing a 2x4 along the bottom of joists to beef them up significantly... maybe you could get away with that instead of having to do full sister joists... I'll let the experts weigh in on this.

4. I don't know... and I'm interested in what the experts have to say... I haven't gotten to re-doing my kitchen yet, but I have a very similar situation with 1-by T&G planks... you would think they would offer some level of support, so that in addition to the 5/8 Ply may be enough?

5. Based on 3/3a... you need to beef up the joists

Well, that's my $0.02.... but again, wait for the experts to weigh in.
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Old 01-22-2018, 07:14 PM   #3
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Thanks, Chris. :)

1. Definitely "B", as Chris said.
2. At first glance, it looks like fir. But there's soooooo much dust/patina on the wood to make my diagnoses pretty close to a squinty-eyed guess.
3. No...you're close though.
3a. No. Beefing up the subfloor does nothing to improve your structural deflection. Either: shorten the span (consider building a basement closet in the basement that would run a structural member across the joists, effectively shortening them to 10' or less), sister at lest the center 2/3rds of each joist with at least a 2x6 (2x8 would be preferential as sturdier floors are always appreciated), or apply a 2x along the bottom as Chris mentioned (but do realize you'll have to temporarily jack the joists up while your adhesive adhering the two is curing. And old joists with dirty surfaces are poor choices to adhere to. And you gotta use an adhesive that doesn't allow "creep", like most construction adhesives). Reducing the span of the joists or sistering the joists are two options that are usually easier.
4. Likely. But do investigate how well it was installed. Any wiggly vertical movement would be super detrimental to a stiff, brittle tile floor. Tell us what you know about its installation. The plywood that is running parallel to the joists doesn't qualify as sufficient. Some pros would replace those sections. And other pros would proceed as is. I stick to the rules when I'm dispensing advice in this DIY forum.
5. There are multiple paths that will lead you to a successful tile job here. But the joists and plywood need some work before the Ditra goes down.

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Old 01-22-2018, 11:22 PM   #4
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Ok, thanks - so joist improvements seem like the first order of business.

We have considered installing a sauna in the basement in that area, which is sorta like a closet... But in the interest of only taking on one huge project at a time (multi-part kitchen renovation) - could you say more about the basement closet idea? Would a long back wall of a closet effectively shorten the unsupported length of the joists it sat under? Does it need to be anything different/stronger than a standard wall construction to make that work?

Re #4: I don't know much about the subfloor construction, aside from what I can see looking at where it terminates at the door, and underneath. The house was built in 1959, and the kitchen was redone some time in the mid-80s (well... the cabinets were installed mid-80s, so I'm just assuming the floor might've changed then too). There are multiple spots in the floor that creak underfoot, which probably means dreaded vertical movement...

I don't think the plywood is tongue and groove, based on the seams I can see between the planks underneath. The edges of those seams don't appear tapered, and at some of them I can see through to the 1/4" underlayment.

Those 3/4" planks running perpendicular to the beams -- are they basically ignored as far as tile goes? Is it all about the 5/8" plywood and above?
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Old 02-07-2018, 09:54 PM   #5
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Sistering joists with plywood?

I've been doing some more thinking about how to reinforce my subfloor. After my first thread here determined my floor is not quite adequate (between L/222 and L/289), I looked at how I'd sister the joists.

There's a bunch of electrical, water lines, and a gas line running perpendicular to the joists so it would be tough/expensive to get full-length (12') 2x8's up there.

I came upon this article (finehomebuilding.com/1997/07/01/stiffening-a-floor) that suggests using two pieces of 1/2" or 3/4" plywood sandwiched together with the seams staggered, something like this:

---- 12ft joist ----
8ft + 4ft ^ glue + screw
4ft + 8ft ^ glue + screw

I don't know if that diagram makes any sense But does this seem like a reasonable idea? Will it provide enough structure? It seems like 2 layers of staggered plywood would be better than a single 8ft 2x8 sistered to the middle, but I'm no structural engineer.
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Old 02-08-2018, 12:46 AM   #6
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When it comes to sistering solid sawn joists, anything that you could do with plywood would be better done with solid sawn lumber of equal dimensions. The plies of a plywood sister with the grain running vertically would provide no bending strength. So your 1/2" plywood sister would be roughly as strong as a 1/4" solid sawn sister.

If you can get a centered 8' long full dimension sister on a 12' span, that would definitely be stronger than anything you've proposed. If you specify the number of joists to be sistered and the exact locations of the obstructions, we might be able to help you with a better design.

Cheers, Wayne
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Old 02-08-2018, 07:06 AM   #7
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Is building a wall below the joists to cut down the span an option for you?
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Old 02-08-2018, 08:22 AM   #8
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Is the electrical and plumbing running under the joists or thru the joists?

If it is running THRU the joists, then you could attach a 2x4x12' laid flat to the underside of the existing joists. It would sort of help turn the joist into something of an I-beam.

If it is running under the joists, is there room to slide in 9' long or longer sisters? A sister doesn't have to run "wall-to-wall" to be effective. I don't recall the rule of thumb myself, but if you sister I think you want to sister at least the center 2/3, but 9' would be 3/4. Also, a sister doesn't have to be as tall. There is no reason why you couldn't add 2x8 sisters to a 2x10 joist. But of course the taller would be stiffer.
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Old 02-08-2018, 05:05 PM   #9
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Dave, let's keep all questions related to this project on this thread so the history is in one place, and questions and answers aren't duplicated.
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Old 02-09-2018, 08:40 AM   #10
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Mud it

Good God The amount of unneeded thinking and hand wringing over creaks and deflection and pipes and electicity and joist spans. And all because the oldest and most effective and time tested solution has been discarded. This is what should be done on a floor. Range it with long straight edges and a 4 foot level. Use pieces of tile to see where low and high spots are. Decide the height of the finished tile floor your comfortable with and take up the layers of floor till you come to the height that will when 3/4 inch plus the tile thickness plus 1/4 inch for mortar give you that finished height. Nail down tar paper and self furring galvanized metal lath make up a 4/1 mix of coarse sand and Portland into a dry pack and mud the entire floor. keep it all level and ,trust me, you will never have to worry yourself about joists and creaks and whatever. The 3/4 inch is the height of the tar paper and wire and mud that will go over the hi spot on the floor..Other areas lower will get more mud to keep level thru out. Unfortunately, and as sad as it is to say, The hardest part of all this is finding a tile MECHANIC who can do this.
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Old 02-10-2018, 12:30 PM   #11
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Sorry to say that industry standards and the TCNA handbook disagree with our friend Bookie. First, insufficient joist strength is not made up for with a mud bed. Especially with a mud bed of insufficient thickness itself.
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Old 02-10-2018, 02:36 PM   #12
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I thought mud over paper had to be a minimum thickness of 1 1/4 inch
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Old 02-13-2018, 08:03 PM   #13
dceddia
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Oh sweet, thanks for merging the threads Kevin

Some answers below...

Quote:
Is the electrical and plumbing running under the joists or thru the joists?
Everything runs under the joists.

Quote:
If you specify the number of joists to be sistered and the exact locations of the obstructions, we might be able to help you with a better design.
We've got 6 joists to be sistered. Here's a diagram:

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If that's not big enough to read, it goes:

43" - gas pipe - 52" - copper pipe - 5.5" - copper pipe - 37"

From what I can tell, if we can remove the gas pipe temporarily, we can get an 8' 2x8 up there, position it in the middle, and cover the middle 2/3rds of the unsupported span (a tiny bit more even, 68%). We could do the same with the copper water lines, but those aren't just 2 straight lines (there are T's coming out of it at a few points) so it would be a bit more work to put back together (but we wouldn't have to mess with gas, either).


Quote:
Is building a wall below the joists to cut down the span an option for you?
We probably could build a wall, but the space is currently an open storage area and we'd prefer not to divide it.

Our current plan is to sister the middle 2/3rds with 2x8x8' boards, moving pipes as necessary. I'm gonna see if I can slide any joists in from the left side, over the supporting beam. Seems unlikely but worth a shot.

One more question: there is some existing cross-bracing, the X-shaped boards nailed in along the joist span. We'll need to take those out to fit sisters in -- should we replace them? And with what?
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Old 02-13-2018, 11:06 PM   #14
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You could remove the current plank subflooring to drop in full length sisters from above.

You could rebuild the existing wall, making sure it is load bearing, and possibly scoot it a little closer to center. You don't need to reduce the span by much to have a big effect on deflection.

You could remove the gas pipe and some/all of the subfloor above the 37" segment to the right of the copper pipes. That should let you get full length sisters into place.

Cheers, Wayne
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Old 03-25-2018, 09:39 PM   #15
dceddia
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We've made some progress over the last few weeks!

It turned out we could get 2/3rds-length sisters up and over the pipes, so we installed those.

Then we started removing the existing flooring. The linoleum and the 1/4" underlayment have been removed. About half the floor is covered by 5/8" plywood, and the other half is hardwood.

Since we can't tile over hardwood, and the plywood isn't T&G and isn't consistently laid perpendicular to the joists, we're going to remove all of it down to the plank subfloor.

So the next question: what to do with the planks?

They're 1x8" (actual 3/4" x 7", 7 1/4"ish) not in great shape - many are cracked, and I don't know if it matters, but it looks like they were previously used as a concrete form of some kind.

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As I see it, my options look like this:
  1. Leave the planks; install new 5/8" plywood over them
  2. Remove the planks, and put a single 1 1/8" plywood layer on top of the joists.
  3. Remove the planks, and put down 2 layers of plywood (1/2" and then 5/8"?)

#1 seems easiest, but it raises the total floor height by at least 1/4" (old was 1/4" underlay + linoleum, new will be 1/4" ditra heat + 1/4" tile + thinset).

#2 seems second-easiest, but I'm not sure if it's sufficient to have a single layer of plywood over joists, or if it's better to have subfloor + underlayment.

Ideally we'd keep the new floor the same height as the old floor and save some time on cutting doors/door trim/cabinet above fridge, which I think we can do if the subfloor is 1 1/8" in total - which is why I was thinking of doing 1/2" + 5/8" ply instead of 3/4" + 5/8". Not sure if that's advisable though.

What's do you think is the best option?

Thanks in advance!

Also, I suppose the thread title could be changed to something more general at this point since we're well past the "considering" stage, haha. Maybe "Subfloor for kitchen tile install" or something.
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