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One of my rooms on 1st level has about 5 inches lower level of floor compared to other rooms adjacent to it. I plan to remove existing laminate finish and OSB laid on wood subfloor support beams.

As a next step, I am thinking about laying new joists onto existing subfloor support beams and then when I put back OSB sheathing I have new height with laminate finish equal to the floor height of adjacent rooms. The room is above basement and subfloor is totally accessible underneath. Joists are on 16" centers.

Please share advice if this approach is ok.

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Maybe this is a rather long comment but we have a lot of sunken rooms from the 60s-80s in my area and have done this a couple times.

You will not remove the subfloor. First it is just a waste and second the subfloor is binding loadbearing joists in the basement.

You will want to run the new joists perpendicular to the loadbearing ones below. If you are going up 5" I would use 2x6s and cut them down to height. Do not come up with formulating 2x4s and whatever to "stack". Stacking doesn't work out nice if you want flat. I would personally put these 12" apart. The other thing is I would really really try to get these at 18' so you can run one the length of room. I know you can't get these at big box but lots of lumber yards around me have 2x6s at 20' (decks and small additions).

As for matching up the subfloors this might be trickier than you think. The builder and crew probably didn't care that the entry way and the dining room were the exact same height. This could be off a half inch. I personally would remove the flooring way ahead of time to give myself a proper plan. For instance maybe the floors were raised with subflooring and maybe you only need to go up 4" - not saying that is the case but having options is good.

Last note on the subfloor. Because the other rooms are smaller I would plan to have my new subfloor binding the rooms. This could mean removing all of the subfloor from the higher rooms or just the sheets touching. I have seen guys not do this and you feel that little dip (even if you self level) or that little sway... No reason to have issues after all of that work.

And just because I have to give my opinion... I don't think this particular sunken room is really that bad at all. You are not gaining more use of a dining room and it is not hurting near entry way. I get we rather it level but I don't see this house being hurt by this - maybe looks worse because of outdated flooring and trim? If we were flipping this house we wouldn't level that room. We would make that opening to dining room as big as possible and finish the basement. Stairs on the edge of the basement... that is ideal for a 2 bedroom/1 bath huge rec room type of basement. Yes you have the ideal house we try to buy.

Note: I purposely left out other things involved with raising a floor. But the incomplete list is - moving ducts, moving electric and the fact that you don't have a bottom plate to attach trim. Most of the time we just cut out the bottom two feet of drywall... The new joists and installing them is like 20-30% of the job.

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    @isherwood - overlapping the subfloor between the rooms. That way if the small differences in height will be worked out over a sheet. It also makes you deal with leveling via shims instead of using some floor leveler or some cheap fix. We frame these like decks and level them like a big kitchen island.
    – DMoore
    Commented Apr 4 at 21:45
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    If the new floor joists are laid directly on the subfloor, that would make each joist cavity a sealed space. If there was ever a leak of water into those spaces it would be hard to ventilate to dry it. Elevating the joists 1/2" above the subfloor with periodic spacing shims would create one continuous space to facilitate drying in case of water ingress. Does anyone ever do this? We (two 80-year-olds) have a sunken living room 5" below the rest of the house. It is treacherous. We have most of the step down blocked with furniture and vertical grab bars at the two remaining open spaces. Commented Apr 5 at 1:39
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    @JimStewart - it is a valid concern and did cross my mind when I answered the question. The fact that you are shaving off 4x6s and they span 17', even with shims I am not chancing them bowing or warping. I don't like the 1/2" gap. To me there are just too many logistical issues plus having a 17' span on shims... If leaks were a worry it would be easy to pop holes in the existing subfloor. You would also almost suredly have holes in existing joists to run cabling/electric and stuff like that.
    – DMoore
    Commented Apr 5 at 4:51
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    @StayOnTarget I would toenail them but the key is a large amount of blocking. I would have probably 3 rows of blocking over the 17 feet... probably could get by with 2.
    – DMoore
    Commented Apr 5 at 21:44
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    Also the new joists do not need to connect to the old. The new floor should be able to stand on its own. Nailing it in isn't structural it is just so it doesn't move. In fact I would suggest letting it float as the movement in the structural joists will probably be greater than this new addition. With proper blocking and perfect cuts you could just lay the lumber in place (not saying I would do that
    – DMoore
    Commented Apr 5 at 21:51
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We live in a 270 house tract development in Dallas TX built 1970 where many of the houses (incl ours) have a sunken living room. This is on a hill trending down from east to west; streets are E-W. One plan similar to ours but larger has two sunken areas. We have lived here 46 years and I have seen a number of houses getting the sunken rooms raised with joists. Houses do sometimes get water in them and it is a concern to me that these joisted areas have provisions to dry in case of water ingress.

In our development the sunken rooms are close to grade level if they are on the uphill side of the house and in some cases pooled rainwater has flowed into the sunken rooms. Others have had catastrophic plumbing leaks when no one was home (tank water heater rupture, washing machine hose rupture, toilet supply line failure). Water ends up pooling in the sunken room. With this experience I favor joisting so that this space can dry out in the event of getting wetted.

Two houses in this development were flooded with sewage and the sunken room filled with sewage. These two houses were on separate adjacent streets at the bottom of a hill and the city sanitary sewer on both streets (years apart) developed a blockage. Sewage from houses uphill filled the city line in the street until it came out of the shower and toilet of the lower of the two houses on the corner. The individual house lines have no check valves in the sanitary sewer lines. A simple preventative to that is to put a spring loaded relief cap on the sewer cleanout outside at ground level, but AFIK none of the downhill corner houses have them at present.

These are all slab on grade and originally 1 story so my concern is perhaps not directly relatable to the current question, but I see it as somewhat relevant. Our plan is only sunken 5" but some are significantly more. Ours was originally ringed with a 3' tall railing with two openings which served as warnings of step downs, but we removed this decades ago in a remodel. My wife (mobility impaired) would like the living room raised but so far she can use a grab bar.

One measure short of raising the floor would be to use a contrasting flooring material in the lower room. This would at least alert people to the grade change.

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