Can I frame a small knee wall on a concrete slab to reinforce floor joists?

[jeffesonm]

I'm renovating a small bathroom and want to drop the floor down a few inches so the tile is flush with the hardwood flooring in the hallway. This will require removing 2" or so from the top of the floor joists. Joists are 2x8 16" o.c. spanning 11'. I could sister 2x6s along side the cut joists but according to the span tables I've checked, 2x6's alone won't cut it and it's not real clear how to calculate the contribution of the cut joists to the overall structure. FYI this is a ranch so only one floor to worry about, and full access to crawlspace below (and attic above.)

Pic of basement, bathroom area back left where all the plumbing is. Looks like piers under the main beam are sitting right on the slab, but maybe there are footings under them?



Can I build a small knee wall in the crawlspace to reduce the span in half? Then maybe sister 2x6s on the cut part for good measure? Would I need to cut the slab and make footings, or can I just tapcon a pressure treated 2x4 to the slab and frame up the knee wall?

 

[jeffesonm]

Any suggestions on this plan?

 

[vinny11950]

Hi, Jeff, I would strongly say DO NOT cut your floor joists! Bad idea because you are weakening main structural lumber that was designed to bear loads at that size. Additionally, I am not even sure it would pass inspection if you wanted to sell the house later on and would have to be fixed, plans submitted by engineer to town and all that.

I have a ranch with 2 x 8 joists and they barely do what they are supposed to do. I couldn't imagine cutting them down some.

Anyway, there probably are footings on those support columns, something like a big blob of poured concrete that is deeper than the rest of the concrete floor. So yes, if you put a knee wall to support the mid span, you would have to cut a big hole in the concrete floor, dig some more and then build your footing. You can Google it and it will give you the necessary measurements. Still, not sure if it would pass inspection, so you would have to have it approved.

I am guessing you are putting new, fat tile and it is going to be really high from the hallway flooring? I had that issue in my bathroom reno, so I went with 3/4 plywood, ditra, and tile. I am was still high but the transition was not too much.

 

[semipro]

I'd suggest that after you cut the existing joists you try sistering additional joists first and check the floor bounce. Its entirely possible that proper sistering of dimensional lumber may provide enough support.
It may also be possible to use steel rather than wood to provide extra support. Angle steel attached to the top or bottom of the existing joists may provide more support than the additional lumber.
If neither of these options provides enough support you can then add the support under the floor.

 

[sportbikerider78]

How thick is the hardwood? That is likely 1/2 - 3/4 and on a 3/4 deck base. Why can't the tile be stripped down to the decking 3/4 then 1/4 hardiebacker followed by a tile that isn't more than 1/3? That could get you very close and you can 'cheat' the tile that touches the wood by putting it on a slight angle at the transition.

If it was me, I'd do a nice wood transition piece. I have a similar situation with hardwood flooring and tile. My transition is a big 1/2" and i'm having a carpenter come in and make a nice sloped transition. It is what it is. I chose to leave the additional 3/4 decking under the old tile because it would have been a nightmare to pull up. Plus, they had cracking tiles and I wanted additional support.

 

[jeffesonm]

Thanks all. I am doing a single slope shower floor to a linear drain, so need to account for additional thickness of 1/4" per foot slope. I will probably get some select 2x6s and sister all the joists in the area, and cut the 2x8s down to be flush with the 2x6s. If it's still bouncy I'll do the knee wall.

 

[begreen]

The alternative is to put in a shower with a curb at floor level. This is how we did it. First we tore out flooring down to the joists. Then put 1.25" plywood down. That was 13 yrs ago. Still not a crack in sight.

 

[iron]

quick math for you:

strength comparison (using section modulus) --- 2) 2x6 = 15.1 in^3; 1) 2x8 = 14.1 in^3. therefore, double 2x6 is slightly stronger than a single 2x8

stiffness comparison (using moment of inertia) --- 2) 2x6 = 41.6 in^4; 1) 2x8 = 52.7 in^4. therefore, single 2x8 is stiffer (i.e. less bouncy) than a double 2x6.

by being less stiffer, you floor will deflect more under load. in this case, you could expect 26% more deflection.

perhaps you could sister with either two more 2x6s or a single 3x6. whatever you do with the sistering, make sure you glue and screw to the subfloor or it will not contribute to the strength/stiffness and you would like.

 

[begreen]

Flexing is something you do not want to see under tile, especially in a shower.

 

[jeffesonm]

Absolutely, I do not want my new nicely tiled shower floor to crack.

Using the calculator on the John Bridge tile site, my 2x8s 16" oc spanning 11' are just barely enough to support tile at L/377 deflection. Walking across the floor in the adjoining dining room causes the glassware in the china cabinet to jingle, so I feel like even without cutting joists they could use some reinforcement.

Some mid span support seems like a good way to shore things up, I just really don't want to cut through the basement slab and pour footings.

 

[iron]

2x6's @ 4" OC should do the trick.

 

[jeffesonm]

Well I decided to do it right... fortunately to have a buddy with a demo saw he let me borrow and cut a 12" wide by 6' long chunk out of the crawlspace slab. Dug down 6", threw in some rebar and mixed up 6 bags on concrete. Stuck a few anchors in there and will build a 2x6 knee wall supporting the floor joists mid span. Plan to sister the joists over the bathroom with 2x6s from the outside wall to my new knee wall and then cut the existing 2x8s down to be flush with the 2x6s.

 

[vinny11950]

Looking good. Pays to do it right in the long run. Do it once and forget about it.