5 Reasons Your Floor Joists May Cause Uneven Flooring

joists under wood floor boards
  • Intermediate

Sloping floors are more often seen in older homes but can be a common occurrence even in newer homes as recent as the late 1990s, but it is not normally considered a major flaw requiring extensive repair.

It’s usually a result of the natural deflection of the floor joists or even the main girder onto which one end of the joists are attached, the full weight of the structure, and its content causing them to sag in the middle.

Sloped or tilted floors that are straight, sloping in one direction down to the opposite wall could however be an indication of possible problems with the foundation or bearing walls and should be investigated further.

If this proves to be sound and solid, it will not necessarily need to be corrected unless the tilting is marginally marked and serious.

If the tilting is not excessive, it will not present any problem when laying down rigid floorings such as laminate planks, hardwood planks, or larger ceramic or stone tiles.

Uneven floors are not all about “dips”, however, as they can sometimes be elevated or distorted upward and create “heaves” in the floor.

But either way, while some types of flooring such as vinyl are more forgiving when it comes to floor variations in the flatness of the subfloor, rigid flooring materials will require more prep work.

The Possible Causes of Sloping Floors

It’s always better to know what you’re dealing with when you run into irregularities with the structure of your home, and you should investigate serious problems in order to make sound decisions and take the proper corrective measures.

The following lists most of the problems that can affect the floors and how they should be corrected.

1. Settling

A house that settles is a natural process in which the soil beneath the foundation begins to shift making the house sink slowly and at irregular rates throughout the foundation.

The uneven sinking of the structure over time can damage the foundation forming cracks in the walls and floor opening an opportunity for water infiltration and damage.

This is particularly common in older homes, but can also occur in newer construction jobs done by sloppy contractors.

2. Foundation Damage

Faulty foundations resulting from poor or plain sloppy construction practices will also result in inadequate load-bearing walls, failing to provide proper bracing and support under the overlaid floors and the rest of the structure.

3. Material Failures

Old or substandard materials from sloppy contractors will come to fail or weaken over time, but may possibly be corrected with sister joisting or other bracing and supporting practices.

4. Lumber Movement

Lumber dimensional activity or movements can cause the joists’ planks to bow, twist, cup, warp, or crack especially if they weren’t properly kiln-dried, as dimensional stability is only attained when the lumber is dried below 20%MC (Moisture Content).

5. Moisture Damage

Water and moisture always seem to find their way to infiltrate through foundations, roofs, seals, and water barriers, to seep into floor structures and cause their damage through rot, mildew, fungus, dry rot, eventually damaging floor joists, causing them to sag and create unevenness in one’s floor, as the floor loses its integrity.

The same results can also be caused by leaks within the house from leaky sinks, bathtubs, showers, toilet bowl gaskets, etc.

uneven curved floor

Defining the Heaves and the Dips

People wrongfully tend to quantify the seriousness of the sloping floor by the angle at which the floor is sloped. If it slopes 1/4-inch per 20-feet it is much less severe than if it slopes 5/8-inches within 8-feet.

This, however, does not accurately describe the degree to which the damage has been progressing and the amount of deterioration the foundation and the joists have already suffered.

Before attempting to level the floor, you have to find out what’s causing the irregularity and rule out severe structural issues such as those mentioned previously.

In the meantime, you can check where all the dips and the elevations and one easy way to achieve this is by using a few marbles on the floor and watching them roll away from the heaves and into the dips of the floor.

The amount by which the dips and the heaves vary from the sound floor level can be checked and measured using an 8-foot straight edge or level, placed on edge at different spots around the floor to see where it lays flat tightly against the floor along the full length, and where it shows gaps in-between.

Where the straight edge “rocks” back and forth, you’ll know that you have a high spot.

floor joists exposed under boards

Evaluating the Severity of the Problem

An accurate inspection should be done in the basement or crawlspace where the structure is totally visible to see and touch and get a better assessment of the layout with clear structural warnings such as cracks, fungus, rot, dry rot, discolorations, etc.

Some of those signs could also unveil structural or water damage that also need to be addressed.

Reinforcing and bracing from the basement can help renew the structural integrity of the building when leveling the floors. Just remember that it’s important to do any of those repairs right the first time to prevent having to repeat the same task in the future.

Minor defects in floors can often be solved without extensive carpentry work. They can often be corrected with self-leveling compound products for wood sub-floors and concrete floors.

It can even be applied over ceramic tile floors eliminating the need to remove the old ceramic tiles. Just make sure that the product is compatible with your floor.

floor joists

Is It Bad Enough to Call in the Pros?

In some instances, depending on your skill levels, it might be better to hire a professional to determine the extent of the required fix for the problem and get it done before any floor leveling takes place.

Some of the more demanding conditions would be:

  1. Rotted, cracked, or broken joists
  2. Damage from termites
  3. Cracked foundation.

Recognize a Delaminated Subfloor

When the subfloor is submitted to constant moisture such as a drip from leaky plumbing, the water is steadily absorbed by the substrate which is made of wood fibers or plies of laminate bonded together with adhesives.

The wood absorbs moisture causing it to swell and break the bond, undermining the substrate’s integrity.

You have to carefully check the joists, however, as they could have started rotting, dry rot being a common problem in such cases.

Once this is confirmed, the cause of the moisture condition has to be corrected before replacing the damaged subfloor panels with new ones.

Correcting Dips in the Floor with Shims

If the uneven floor is due to floor joists that have warped or twisted over time or were incorrectly installed, you will need to install shims over the joists to fill in the gaps and bring up the substrate level.

The shimming is done by first removing the flooring from the joists and attaching wedge-shaped pieces of wood on top of the low areas of the joists to make them level.

The shims can be attached to the joist by gluing and screwing them on. Any high spots can then be sanded down before the substrate is reinstalled providing a perfectly level surface.

Adding Sister-Joists

Many older structures were also built with floor joists systems using undersized lumber or with the joists spaced too far apart, thus creating bouncy and saggy floors.

Such structural defects require a special process to increase rigidity and strength of the floor, called sistering the joists, where a new joist is attached to the existing one, adding more strength to the floor.

The ends of the new piece are placed to sit securely on the same sill plates that support the original joist before bolting the two of them together. You should note that such repairs could require the relocation of plumbing and electrical wires.

Another situation where sister-joists are used is when you find a joist that has cracked or got damaged in some way to alter its integrity within the structure. The technique for adding the sister-joist is still the same.

floor joist

Cutting Down a “Heaving” Joist

Occasionally, a floor joist may warp upwards, elevating the subfloor and making it higher than the sound floor around it. This will quickly become apparent as you use the marble to test the floor, as it rolls away from that spot.

Fortunately, this is an easy fix once the substrate is removed from the heaved area of the floor and exposes the joist.

You then simply have to stretch a chalk line across the full length of the joist, hooking it to both ends at the top edge up against the substrate, and snap it to make a line the full length of the joist.

The section of the joist above the mark can then be trimmed off using a portable power plane, a reciprocating saw, or other tools to bring it straight and level.

Once this is done, you can reinstall the subfloor. The floor is then ready for you to apply any type of flooring to it.