–Karin B., Toronto, CN

A:  It’s one thing to have a floor with occasional depressions and gaps; it’s another thing when the entire floor slopes in one direction or the other.

You have to ask yourself:  Is this a flooring problem or a structural problem?  Because when the slope is pronounced–indeed, when we use the word slope at all–it’s no longer a flooring problem.  It’s a problem with joists, structure, foundation.  All that big, hairy, scary stuff.  So what to do?

Flooring Slopes to the Center – Sagging Floor

If you’ve got a floor that slopes inward–from the perimeter to the center of the room–you’ve got sagging joists.  Your fix goes beyond using leveling compound.  You’ll need to strengthen the floor joists and even add beams and piers underneath–can you handle it?  Depends on how hardy you are; most homeowners will call in a contractor at this point.

Or if you can determine that the joists are structurally sound (i.e., not quickly rotting away), you can “sister” the joists, which does two things:

1. Strengthens the joist.
2. Provides a new, level surface for your subfloor–you’re essentially circumventing the whole “slope” factor and running level boards next to the sloped ones.

Good thing:  it works.  Bad thing:  it entails ripping up the entire floor–finish flooring and subfloor alike.

One End Slopes to the Other End – Foundation Subsiding

It’s a larger problem when one end of the floor is higher than the other end.  This means a subsiding foundation, and bigger construction work.  You will need to call in a contractor–not necessarily a foundation company, but just a competant contractor–and jack up the lower end of the house, insert beams, and lower the house.  Simple, huh?

If the floor isn’t too far out of level, you might be able to insert tapered wood shims atop the joists and re-install the floor.  By “not too far out of level,” I mean something like half an inch vertical per ten feet, roughly.  Again, you’re ripping up the entire floor (bad thing) and once your fix is completed, you have other problems to deal with, such as:

• Installing new baseboards and trim to accommodate this out-of-square room.
• A noticeably different look to the room.  For instance, windows on the “corrected” side with be closer to the baseboards than on the other side.  The human eye can pick up these differences.

I like to talk a lot about the benefits of layering and how layering can save your butt (in other words, if you mess up this layer, there will be another, covering layer atop to cover up your mess).  So, you really have to make this one count, because you’re at the next-to-last layer.

High Spots and Bumps in Subfloor

If you need to bring down your subfloor, you can try either a hand plane or a power sander.  If you have a drum sander on hand, this can level out any bumps in the subfloor, but you need to have a steady hand to control this.

Actually, if you go overboard and gouge out some of the subfloor, it’s not the end of the world.  The gouges can be filled in…or if small enough, the finish floor may be able to bridge them, as long as you’re installing solid wood or engineered wood flooring.

If your subfloor is of the solid wood variety (i.e., not plywood or OSB), you can single out a floorboard or two that are creating the bump, and which cannot be easily sanded or planed down, then you can pop them out and replace them.

Have a crawlspace or basement?  Then you have access to the bottom of the floorboards.

It really doesn’t matter how ugly a job this is–you’re going to cover it up anyway.

Got plywood or OSB subfloor that is warped or bumpy and can’t be sanded down?  Then either screw the thing out or pry it out and replace it.

Low Spots and Depressions in Subfloor

Some flooring, such as solid wood or engineered wood, can bridge small depressions.  Any depression that is more than 3 or 4 inches wide is, in my opinion, too wide.  It’s not the depth of the depression that matters; it’s the width.

Floor leveling compound is made for just such a thing.  Spread it on, trowel it smooth.  Guides often tell you to sand it smooth–but what’s the point?  As long as it’s level, you can forgo the sanding.

When you have depression that are three or four feet in diameter or more, it’s the joists that are the problem.  You need to get access to the bad joist(s) and replace or repair.  Sometimes a joist will be bowed simply as a function of aging.  This type of joist can receive wedges on top to bring the level up.  While it’s probably fruitless to try to straighten out these warped joists, you can most certainly sister them and prevent them from warping any more.