Strengthen Joists

You wouldn’t believe the types of questions I get about how to strengthen joists.

So, this means we’re talking about basements, crawlspaces, and attics mainly–accessible places where you can get to the joists without ripping apart half of your house.

Now, none of my advice about how to strengthen joists applies to houses that have severe problems.  Things like:

  • Deep cracks in the joists.
  • Sags lower than a couple of inches.

Wood Shims
Wood Shims

Method 1 – Wood Shims

Using the wood shim method is a stop-gap fix.  Better to sister with a new joist, if possible.

Underneath, you hammer in wood shims between the floor joist and the bottom of the floorboards or subfloor.  “Whoa!” you say.  That’s not strengthening the floor joists; that’s something else!

Indeed.  This is a quick-and-dirty method.  What this does is help force a gap between the joist and floor, raising the floor somewhat.  This works for slight sags.

Method 2 – Sister with New Joist

So, this is a bit harder, but it is more effective.

  • Buy a floor joist the same length as the sagging or weak joist.
  • Lay new floor joist alongside the old one.  You will not be removing the old joist.
  • Make sure new joist is resting next to the same contact points as the old joist.
  • Option A:  Raise up old joist with a 40 ton house jack until it is level or close to level; OR,
  • Option B:  Do not raise up old joist.  Keep the sag in place, and merely strengthen instead of level out the joist.
  • Attach the new floor joist to the old joist with 2 1/2″ screws, driven every 8 inches.

How to Get Rid of Vinyl Flooring Bubbles

Sheet vinyl flooring can develop bubbles of trapped air.  Since these bubbles are covered by a seamless sheet of vinyl on the top, and by the underlayment or subfloor on the bottom, they will never go away.  How do you get rid of them?

One of the reasons you bought sheet vinyl flooring in the first place is its seamless nature.  With tile vinyl floor, you have seams everywhere.  These seams are bad because they can let moisture seep through down to the underlayment.  It should be noted that most seams hold up well–but there is always the possibility of failure.

By contrast sheet vinyl has almost no seams.  Good for daily living, bad for trapping in air bubbles.

Bubbles under sheet vinyl flooring occur during installation, not after.

Keep in mind that the following method only works for relatively small bubbles (up to 6″ diameter).  Larger bubbles require you to cut out a section of the flooring and replace.

Renting a 100 pound roller is a good idea is you want the job done right.  Our improvised “ironing” method mentioned in this article doesn’t work nearly as well.  You can rent a roller for almost nothing at any local rental yard.

Vinyl Flooring Roller

To get rid of these trapped pockets of air under your vinyl, use a canvas needle (a sewing needle used for sewing canvas) or even the tip of an X-acto knife or utility knife, and puncture the bubble.

Most people will not happen to have a floor installer’s 100 pound roller on hand, but if you do, go ahead and use it to flatten the bubble.  Lacking a roller, you can kneel on a short piece of 2×4 wrapped in a towel, and “iron” it out.

Complete by sealing up with a vinyl flooring seam sealer found at most hardware stores.

Fixing Concrete Slab Prior to Installing Flooring

If you’re going to install flooring directly on concrete, then that concrete has got to be perfect.  You have no more flooring layers available to smooth out things before your finish flooring goes down.  So, you need to deal with cracks, depressions, and high spots.

And keep in mind that we’re not even talking about the surface itself, which needs to be absolutely clean, oil-free, stain-free, dirt-free, and sealant-free.  That’s a whole different matter…

Cracks or Depressions in Concrete Slab

One good thing about repairing cracks in the concrete is that none of this is visible.  It’s really difficult to repair concrete cracks and not see the repair areas.  Here, all you want to do is get the thing level and smooth.

  1. Use your ever-ready masonry chisel to chip away at the loose edges of the crack.  Anything loose must come out.
  2. Then, chip away at the bottom of the crack to form a (rough) inverted “V” shape.  This will help form a space so that the epoxy goes in…forms a plug…and then won’t come out.
  3. Clean it all out with a pressure washer or hose.  Shop-Vac the crack thoroughly.
  4. Fill the crack with epoxy patch designed for concrete.
  5. Trowel smooth.

High Spots in Concrete Slab

Concrete Floor Grinder

High areas in the concrete also need to be brought down.  Otherwise, you’ll be bowing out your flooring.  Theoretically, this can be done.  And that’s one of the benefits of laying ceramic tile:  it conforms to the shape of the flooring below.  After all, you’ve seen ceramic tile laid on all kinds of curved surfaces such as swimming pools; it can certainly lay down on gently undulating concrete slab.

But do you really want this?  Given an option between smooth and flat concrete, you’ll take the “flat” option.

About the only solution is to use a concrete floor grinder.  These are serious machines that rent out for $80 to $100 per day.  High cost, but you probably will not need the floor grinder for more than a day or two.

Fixing Squeaky Floor Without Ripping Up Carpet

If you’ve got a carpeted floor that squeaks, you may think that it’s impossible to silence the noise without ripping up all the carpet.  Not so.  In fact there is a neat little trick that helps you silence almost every squeak in your carpeted floor, without even pulling back a single corner of the carpet.

Where Floor Squeaks Come From

First, it’s important to understand why you’ve got a squeaky floor, and there could be either (or both) of two reasons:

  1. Your flooring or subflooring rests on top of wood joists.  Whenever wood comes into contact with wood, there is the potential for noise.  So, if you’ve got a concrete slab floor, you most likely will not have squeaks.  It only happens in houses with raised foundations.
  2. The existing flooring nails are moving in and out of their nail holes every time someone walks on them.  Metal against wood equals squeaking.
  3. Both.

That said, you’ve already identified with 90% certainty where the noises are coming from. That’s good.

Longstrip Flooring

Fix #1 – Insert Wood Shims

If you’re feeling ambitious, you can get under the floor to the crawlspace or basement with a flashlight and look for any noticeable gaps between the tops of the joists and the bottom of the subfloor.  Have someone walk on the floor above when you’re below, and you might even see the gap closing–there’s your squeak.

In this case, smear a little construction glue on a wood shim and tap it gently into place.  This should stop the squeak.

Fix #2 – Pound in Finish Nails

But why do that when you can do something easier?  Now, this fix only works with carpeted flooring.

  1. Position a 6d finish nail so that it is between piles, as much as possible.
  2. Hammer the nail straight through the carpet into the offending joist.
  3. Use a nail set to keep pounding the finish nail as far down as possible.  This will get the nail head out of sight, out of mind.  You will never again see it.

How to Fix Sloping, Out of Level Floor?

Q:  My floor slopes and is out of level.  One end is noticeably higher than the other end.  Peas roll off the plate!  How can I fix it?  Can I use levelling compound or is something more “serious” required…and I do not look forward to your answer because I’m afraid what it will be.

–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

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.

Which Sandpaper Grit to Use for Sanding Wood Floors?

When sanding wooden floors, your temptation may be to start by heavily abrading them.  This temptation is intensified when the floors are grooved, pitted, stained, worn, and scratched.  What’s the best grit to use?

Best Answer

For either soft or hardwoods, start around 50 grit and gradually step down to 120 grit.


Sandpaper grit designations might be the opposite from what you think.  A higher number means a finer, softer sandpaper grit.  A lower number means coarser and more abrasive.


Hardwood Floors

  • First Pass:  40 grit.
  • Second Pass:  60 grit.
  • Third Pass:  100 grit.
  • Optional Last Pass:  120 grit.

Soft Wood Floors

  • First Pass:  50 grit.
  • Second Pass:  60 grit.
  • Third Pass:  100 grit.
  • Optional Last Pass:  120 grit.

Can You Use a Coarser Grit for Problem Floors?

Yes, but you need to look at your floor’s history a bit.  If it has been drum-sanded eight times already, you’re probably walking on rice paper now.  Not only would I advise against sanding, I would advise you to lay down all-new floors!

If this is the first or second drum-sanding, then feel free to lay down hard on those floors with a coarser grit–provided the floor has imperfections that warrant this.  Just abide by all the sanding rules to avoid destroying your floors.  Some stains and imperfections just need a good, hard floor-sanding, and that’s all.