Need to raise the subfloor, then install finish floor. Am I on track?

last month

last modified: last month

Hi all,

I have a large "bonus room" in what used to be the attic. A previous owner installed kneewalls and gussied up the space a bit, and I want to take it farther. It currently has a rough ~attic floor (softwood planks).

One particular problem is that a large section of the floor is raised about 2". This was to accommodate an arched ceiling in the large living room just below. The result is that some of the floor is 2" lower than much of the rest of the floor. I would like to raise the lower level to be even with the higher level, and then finish the whole floor with a suitable finish layer.

I am seeking input on my plan to accomplish this. On the lower sections, I am intending to screw in 2"x4" sleepers (probably 16" o.c.), then cap that off with 1/2" plywood. The lower floor is a bit variable, so I will need to trim and/or shim the 2x4's so the plywood is flush with the raised floor section everywhere. Is this a sound plan?

I believe 1/2" plywood is marginal for this application, but sagging calculations and some preliminary testing suggest it is okay. Not bouncy. Worst comes to worst, I can space the sleepers more closely.

Let me post some pictures of that part of the project, and talk about the finish floor requirements in a later post.

Thanks!

Featured Answer

PRO

PPF.

last month

What are you using for a finish floor?

Someone will ask if I don't -- is the floor structure (joists) capable of supporting the use of the space as a room?

Attic space floors are usually just the ceiling structure from the floor below and are not intended to support loads associated with living space.

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Comments (6)

angie_diy
Original Author
last month
last modified: last month
Good question! You had me scared for a minute. However, the house (as originally built in the 1920s) had living quarters on this floor. There are 2 bedrooms and 1 full bath (with a cast-iron clawfoot tub) on that floor. Those rooms take up ~1/3 of that floor, and the rest was attic space. Again, this is the way the house was built; those rooms were not added on later.

I cannot directly measure the joists, but the distance between the ceiling of the 2nd floor below and the finish floor of the 3rd floor is 11". So, subtracting out the thickness of the plaster and lath ceiling and the subfloor and finish floors, I would guess that the joists are 2x10s.

As for the finish floor, I am thinking of using luxury vinyl plank, but I am totally open to suggestions. There is another whole can of worms to talk about, though. I am planning to build a sauna in part of the room. This will be a dry sauna, but some water will definitely be tossed around and trod in from the shower. I believe water-resistant LVP would be a good choice, but would love to hear other's opinions.
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PRO
G & S Floor Service
last month
1/2" plywood will not be stiff enough. You need at least 3/4" Rip your sleepers to 1 1/4". Space it at 12" OC. You should be able to get two sleepers out of one board.
angie_diy thanked G & S Floor Service
angie_diy
Original Author
last month
Okay, thank you for that advice. I will use 3/4" material for the raised area, and sleepers of an appropriate thickness.

My next concern is about possible underlayment needs. Let's assume for the sake of discussion that I will use LVP. Does anyone have advice about appropriate underlayment? Given that I am trying to match the height of the raised section, and that the texture of the existing softwood plank subfloor is variable and not that smooth, I am wondering if I would be well-advised to put in an underlayment of sheet goods, like Luan plywood or 1/8" hardboard. Or do you think there is an appropriate roll-out underlayment (like cork?) that can provide some leveling and smoothing functionality? What would you do if it were your home?

Thanks as always!
cat_ky
last month
I would use 1/4 inch underlayment type plywood on the entire floor, before I put down any flooring.
angie_diy thanked cat_ky
millworkman
last month
I would use the underlayment required by the LVP manufacturer as their product warranty will only be in effect with the underlayment they require.
angie_diy thanked millworkman