How flat is flat enough?

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Active Member
Feb 7, 2023
Greetings to all.
I just posted my nubie intro & I'm in a time crunch, so Ill jump right into my request for advice. I'll start by trying to answer the questions you might ask.

I'm in central Alabama in a house built in 1979. Except for the roof trusses it's a stick build with a 1/2 daylight basement on a slab. The single main level is all plywood subfloor. I've already installed nail-down, prefinished 5/8" bamboo everywhere upstairs except the kitchen & bathrooms. In one room I had a significant sharp hump where I had to remove the subfloor & shave the top of the joist. The rest hasn't been too bad until I found something different in the last room & need some advice.

The last 2 rooms are the MBR & another BR with a common exterior wall over the garage doors. We had to have some helical piers installed in 2020, so I know there has been some settling, but my problem might have been sloppy construction. The floor joists run front to back & the flooring is perpendicular to them throughout the house. From about 18" away from the wall, there's a drop of about 1/4"' all the way across that end of the house. The same drop exists in both MBR & the last room I'm working on now. In the MBR I used staggered layers of felt to build up to a flat surface. In this room, the joist about 18' from the wall seems to sit a little high, causing a hump (parallel to the joist) just before the drop to the wall. From the center of the room to the start of the down slope there's a rise of almost 1/4 bubble on a 4' level. With 25+ years installing draperies & pictures, I may be too picky about level. I know that, up to a point, flat is more important than level with hardwood.

So here's my quandary. Do I sand down the hump & finish out the flatness with trowel-on leveler, or build up the down slope with staggered strips of felt to produce a flat surface in the same plane with the slight rise from the center of the room? Unlike the middle-of-the-room place where I removed the subfloor to deal with the hump, all this subfloor extends under the bottom plate of the exterior wall.

I hope this is clear. Please ask if I need to clarify something. After I read several threads about leveling floors, I'm grateful for this minor problem. My philosophy has been that the only way to be right most of the time is to aim for perfect every time. In light my limited experience I have to do a lot of research.

Sand down the high spots and fill in the low spots with roofing felt. 30# felt is 1/32” thick. 6 layers is 3/16”. Sand a 1/16” and fill the rest with roofing felt. If you are lucky you can cut down the roofing felt into different width strips to fill the dip. 18” wide divided by 6 layers is 3” increments per layer. First layer is 3”, second layer is 6” and so on.

You could trowel fill but why if you don’t have to. Roofing felt won’t crack when your house moves and you can nail through it if you need to.
Thanks for your answer C.J.

I’ll try to clarify what I’m asking. The area I’m dealing with is about 6’ X 12’. The 12’ sides of the space are along an exterior wall and along the center line of the length of the room. That’s the direction of the joists.

Choice 1-If you encountered an area of floor that size, all acceptably flat for hardwood, but with a ¼” consistent slope over 6’, would that be OK for installation? That’s essentially what I’ll have if I build up the area 18” X 12’ against the wall with graduated strips of felt.

Choice 2-If I have to sand the high places to reduce the slope (make it closer to level) and then make the area flat, I’ll be sanding a strip about 2’ X 12’, removing about 1/4” along the centerline of the 2’ width & tapering down in either direction.

The amount the floor will be out of level with Choice 1 is acceptable for my circumstances. I’m hoping someone can tell me if it will be a technical disaster.

Thanks again for any help.

So if I read JPfloor & CJ correctly, for a couple of plywood joints with misaligned surfaces (less than 1 sq ft each) I'll use some leveler. For the 12' X 18" strip against the outside wall, I'll use graduated strips of felt to create the flat surface.

When I was installing draperies & pictures I could see 1/8" drop on a 10' wide window. For that perfect "window-pane" grid in a grouping, 1/32" could drive me nuts. That's why I was worried about 1/4" out of level. Of course, that required me to wear contacts instead of my thick glasses that warp lines.

BTW, my original research about flattening sub-floors indicated using a cement-based product. Trying to remember what I used, I called the tech dept at ARDEX about Henry 549 Feather Finish. They said I shouldn't use it because the nails would glance off. I finally found my leftover & that's what I've used in the whole house with perfect results (Bostich staples & Harbor Freight stapler). Have I just been lucky or does anyone do it that way?

Thanks again for advice. I'm probably troweling the Feather Finished tomorrow morning & do the felt after I paint the room.

I was serious about helping if you're hanging stuff on the wall. I know some cool tricks for problem spots.