restructuring subfloor [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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03-24-2015, 09:30 PM
I need recommendations for restructuring my 1940 subfloor. The more I think about it the more options I think of.

Current joists are 2x6 with no markings, ok condition, and ~16" on center. At the top of the picture the joists sit on top of the sill and block foundation wall. At the bottom of the picture the 3 long joists are notched and sitting on a ledger on a 2x6 x 3 beam. The 3 short joists were cut short at some point to add stairs to the basement. The long joist span is 10'3". The short joist span is 7'6". The two non loading bearing walls on the left and right fall between the joists. The load bearing wall at the bottom is not directly over the beam, but bearing on the joists in question. The stairs are being removed.

Adjoining bedroom (and portion of bathroom) is 3/4" hardwood layed directly on the joists. The non load bearing walls (left/right) are sitting on hardwood. The load bearing walls top and bottom sit on the joists. The hardwood will remain in the bedroom. I will remove it from the bathroom.

My first thought is to sister 2 2x6 to each current joist and add 2 2x6 under each wall. I could hang these on the beam or build a support wall below next to it. The basement is unfinished with 6' clearance.

I would like to use DITRA Heat under ceramic. To minimize transition height, I would add cleats and lay 3/4" plywood between the joists. Then minimum necessary above that. It is a consideration to add a support wall somewhere under the tub in order to lower the joist structure across the rest of the room ;although, I do like to walk around without banging my head in the basement. I could add mid span support up to 3' from the block wall and at the existing stairwell location of 7'6", but nowhere in between as this is the path into the basement from the bulkhead entrance.



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03-29-2015, 09:20 PM
One question at a time...

Are there any unforeseen issues with sistering a 2x6 with two additional 2x6 to make a triple 2x6 spanning 10'3" ?

03-29-2015, 10:37 PM
Welcome, Shawn. :)

Sorry you were overlooked earlier, but making another post to force your thread to the top of the queue is the right thing to do in such circumstances.

I think a lot of the problem might have been that it's not easy to understand what you've got there. I, for example, have no idea at all what you have for a joist structure in the area where you indicate there usta be a stairway, but now there's not.

Biggest downside to trying to double sister those joists is running out of working room between them. And your plan to drop your new subfloor between the joists just exacerbates that. You're gonna end up with plywood cuts less than a foot wide and four feet long and trying to fasten your ledgers/cleats in a space of only about eight inches. Can be done, of course, but I'd rank it at least bitchy, or maybe a bit worse.

You could actually achieve your required L/360 deflection by sistering in a single 2x6 of good species and grade to each existing joist. That makes things a little easier. If you can do that, I'd personally recommend you sacrifice two inches of headroom in the basement and do your sistering with 2x8 material instead. Or build your support wall as you suggested and don't do any sistering at all.

If you did the support wall, you could also leave the existing subfloor and add a layer of half-inch plywood, your Ditra Heat, and tile. You'd have a transition to make, but tile-to-hardwood is one of the easiest of those to do.

That's about as far as I can go with the information we have.

My opinion; worth price charged.

03-30-2015, 08:18 PM
The joist at the stairs were not properly supported when the stairs were added. When I remove the stairs the joist will end in mid air and must either sit on a new wall or be sistered to reach the full 10'3". Playing with deflecto it seems like a 7'6" 2x6 sistered to a 10'3" 2x6 spanning 10'3" would be marginal?

03-30-2015, 08:31 PM
Shawn, the absolute minimum sister that will provide any meaningful reduction in floor joist deflection is the center two thirds of the joist span. Sistering a 7' 6" piece to one end of a 10' 3" joist span doesn't get you where you need to be.

My opinion; worth price charged.

03-31-2015, 09:29 AM
A properly built 2x4 load-bearing wall is the best bang for your bucks and you can put it right where those joist are floating in mid-air...if you can afford the space in your basement. Maybe it can become a closet? :tup2:

04-04-2015, 05:31 PM
If I support the cutoff 2x6's at 7' ( and the 10'ers) do I need to sister to the 7' piece to extend to 10' or is it ok for the two joist spans to be physically not attached?

I would be willing to give up the headroom to do 2x8 and also drop them an inch so as the make the threshold less extreme. Is there any sensible way to accomplish this at the exterior block wall that is currently 2x6 on the sill?

04-04-2015, 06:07 PM
Support how?

04-04-2015, 07:23 PM
Why not use the sisters as the cleats to recess your subfloor?

04-05-2015, 08:00 PM
Forgetting the difference in cost and labor which if either would perform better?

1) sister 2x8 to each joist and place 3/4" ply on top. 2x8 would extend from central beam to as close to sill as possible. Transition ~7/8" with DITRA Heat.

2) sister 2 2x6 to each joist spaced down 3/4" and place 3/4" ply between and 1/2" on top. 2x6 would extend from central beam to as close to sill as possible. Transition ~5/8" with DITRA Heat.

In either case, should I get as close to the sill/block as possible or notch so the the sisters sit on the sill?

04-05-2015, 08:23 PM
Shawn, there would be no reason to use 2x6's as the cleats if you're already sistering with 2x8's. You could use 2x4's as the cleats.

My suggestion, and I don't see why it wouldn't work, would be to sister the joists with 2x8's but to set them 3/4" below the existing joist so that they also serve as the cleats, killing two birds with one stone.

But to answer your other question, it certainly doesn't hurt to extend the joist as close to the support wall as possible, but the minimum would be to have them attached to the center 2/3 of the span. There would be no benefit to having the sisters extend over the support wall, provided the original joists are well supported and in good shape with no cracks or damage.

04-05-2015, 08:35 PM
I was thinking (1) OR (2). Sister a single 2x8 with no cleats and single layer of ply OR sister 2 2x6 as cleats with ply between and on top.

04-06-2015, 12:08 AM
The single 2x8 would give you about the same deflection rating than the 2 2x6's. What would really help aside from that would be having two layers of plywood. If you go with option #2, that would reduce your between joist space to about 11 1/2", which is much better, and you would still need a second layer of plywood if you put the first layer between the original joists.

So option #2 would be more work and give you the same joist deflection, but be better for your between-joist deflection.

09-29-2015, 04:25 PM
When laying subfloor on sisters joists, should the panels meet on the same 2x or should they each sit on their own 2x? Should I be concerned with the new 2x shrinking causing a step in the subfloor?

09-29-2015, 05:46 PM
Welcome back, Shawn. :)

Is this the same project from this thread? ( If so, we try to keep each project on one thread. I can merge the two for you.

If the sisters are properly attached with glue and screws, they shouldn't be going anywhere. You could have one panel end on a joist, and the next panel start on the sister. Of course, this is dependent on the sister running the entire length of the span. If you've run it just part of the span, that's a different story.

If you're joists are 16", 19.2" or 24" on center, the plywood should be starting and ending on a joist if you center the edges.

09-30-2015, 09:39 AM
Same project.

The sisters are full length, glued and screwed. Glad to hear that the easy way is ok.