How to level/flaten the attic floor?

Hey guys, i've found a bunch of different info about how to level a floor. Even something about roofing felt.
We've got some 1/4" plywood and 1 - 1/2" plywood. The homedepot guy sold me some leveling cement, but then people said it would crack or be too heavy?

1" thk concrete in that area might weigh a half ton.

Felt paper is not going to level your floor, it could prevent cement from running through floor.. but I wouldn't be pouring that much cement in an attic.

How much is floor unlevel from end to end or side to side? Which way is it unlevel? Which direction do the floor boards in pic run ? Which way in relation to floor boards is it unlevel? Does the floor slope, or are there differences in height from 1/4" to 1 1/2 plywood?
Looks like a closet doorway in pic 1 ? Where are passage door ways? Will level
floor across door ways slope from side to side or be a transition step up or down?

You may need closly spaced furring strips across or paralell with joists over planks.

Hi that's the closet door the exit is directly across from it. It is bowing from the middle mostly on the right as seen on the photo. There are other boards underneath. The other room was also leveled with plywood. The right side is about 3/4" down. I think start there with 1/2 plywood. I guess glue/nail it down. Then use the cement leveler to fill the steps? Or try shim it i guess would creek too much?

Plywood has a grain.
Put 1.5" wide x 4' lg shims, of varying height of 3/4" and less, necessary to achieve level, each 16" across the plywood grain, 7 shims total for each plywood sheet.
Cutting these shims may prove challenging. A single shim may be 3/4" high at one end and zero at the other.

1000 lbs spread over 100 sq. ft is only 10 PSF of dead load. The same thickness of wood would give you maybe 300 lbs.

It sounds weird to put concrete in an attic but tiled bathroom floors have much the same configuration, a very dense, brittle material-then brittle or not underlayment-then wood.

First find out why your floors are sagging and or not level.

Andy.

What about roofing felt or shingles? Too heavy?
http://www.oneprojectcloser.com/how...oor-using-asphalt-shingles-construction-felt/

The size and spacing of the joists under floor is what determines how much weight it can support. If by "bowing" you mean that floor is sagging, rather than a slope, it would be wise to check joists to see what you got.

Apart from the labor involved in removing existing flooring, it is easier to level a floor from joist level.

It is easier to put in furring strips across the slope than trying to cut long tapering ones. Glue and screw them thru existing floor into joists below.
Also check for any creaking from floor you have now. Screw loose flooring to joists.


If slope runs along joists; one end higher than other, or ends higher or lower than middle;
If the "right side" is 3/4" inch low I would put a 1 1/2" X 3/4" strip the full length or width of that side. You could use plywood or rip it off a 2x. Come 16" closer, to high side and measure height for next strip, rip it from 2X, and glue & screw it. Continue on untill you reach high side.

If slope is across joists put strips on top of joists, you may need some in between joists .

Glu and screw underlayment on strips to joists below. You mite want a light weight filler between strips as a sound deadener. This does not have to be as watery or heavy as self leveling cement as you can screed it level with a board dragged across strips. there are "concrete" mixes with light weight aggregates.

The bow is not that bad, i know what caused it.
I think i can leave it as is, the room is small already. Maybe put carpet or laminate tiles over it. Any ideas about filling the cracks first?
What kind of under pad should i use, cork?

I was thinking just put 1/2" ply wood to the right and level it off with the cement (just a little.) then put either carpet or floating tiles.
Would that work?

"filling the cracks" what cracks would that be? There are various floor patch products.
I am not a big fan of SLC over planks. They expand and shrink cracking SLC. Even with felt or other barrier thin sections may still crack. You need to cover entire floor with 5/8 or 3/4 tongue and groove plywood. Level that ply as much as you can with the graduated strips on 16" centers..
You know what caused sag, what is the cause and have you corrected the problem? Is the floor springy or bouncy at all?

The under pad should be whatever the maker of flooring you choose recommends, over what ever your final sub-floor surface turns out to be.it won't be cork.