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2nd floor subfloor slopes from the outer walls towards the middle. Varies about 0.5in to 1inch depending on the wall and distance to the most depressed area of the floor.

The subfloor is 5/8" plywood and the joists (2x8, 11ft span) run 2 different directions. Since the plywood is not thick enough and joists are NOT all running the same direction, I am planning to add 1/4" plywood sheathing so I can run hardwood all the same direction.

Now all this Wouldn't be that much of a problem, but because of the change in direction of the joists, the slope in the primary bedroom is noticeable to the eye as well as you can feel a heavy dip when you walk through.

Before I add the 1/4" sheathing, I want to make the floor as flat as possible, but I need a material like Roofing Shingles that isn't as heavy BUT still versatile enough to change thickness easily to lay on the old subfloor. My concern is the shingles are going to add too much weight and cause more sagging since the joists are only 2x8 plus all the extra material added.

TIA

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    Hopefully you've been made aware that that much sag in that short of a run is very worrying. Not only does it warrant further investigation and possibly a little outside expertise, but a solution like @FreeMan suggested that would add strength would be very wise.
    – pbarranis
    May 8 at 9:36
  • I quite agree. Since you have a criss-crossing pattern in floor joists, it sounds like something is either built improperly or has failed. May 8 at 22:11

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I'm not sure how the joists run 2 different directions, is there a load-bearing wall somewhere in the middle of the room?

In any case, instead of filling the dip, consider actually fixing the floor.

  • Pull up the existing subfloor material.
  • Sister the existing joists with new ones ensuring that the tops of the new ones are flat and level.
  • Install new subfloor on top of the new joists.

An additional advantage is that, depending on what's below, you may be able to increase the size of the joist to help prevent sag in the future.

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  • Yes, there is a load bearing beam that allows the joists to change directions. Because the joists change direction, the subfloor flatness is affected. In regards to the dip, the 1.5" dip is over a 15 feet distance. The slope of the house is relatively the same on the rest of the 2nd floor. May 9 at 3:17

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