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Sagging Floor Joists: How to Fix Them

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V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com

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Sagging Floor Joists

Floor joists are narrow beams that support the floor.

They are part of the structural system that transfers the floor’s weight to the foundation below.

Floor joists sag can be due to termites, rot, sinking foundations, or improperly designed supports. The sag itself can hamper your everyday life and your house’s value. Quickly rectifying the cause of sagging joists and correcting the sag in your floor is crucial to keeping your home safe.

What Causes Floor Joists to Sag?

Saggy floors generally result from weak joists or lack of proper support.

Typical Floor Construction
Typical Floor Construction

Weak Joists

Water damage, wood rot, termites, carpenter ants, and the like can attack the wood of your joists and weaken them.

Improperly notching joists while installing utility lines can also significantly weaken them.

Such damage results in sagging over time as the joists get weaker. 

Lack of Proper Support

Joists shift as the foundations or soil beneath them settle, and the floor may get loaded beyond what the joists can safely carry.

Insufficiently supported joists start sagging mid-span or sloping from one end of the joist to the other. 

Solving these issues requires addressing the cause first and then correcting the sag itself.

When Should Sagging Floors Concern You?

A sagging floor can begin to affect your everyday life long before it’s in danger of caving in.

The following is a list of indicators that call for having your floors examined.

Quality of Life Concerns

Some degree of sagging is common, especially in old homes.

Slightly sagging floors can make tables, chairs, bookcases, and other interior furnishings unstable.

Moreover, sudden dips in the floor can act as a tripping hazard, particularly for children and those with mobility impairments.

Not all such sags will endanger your house’s structural integrity.

Correcting such sags, if feasible, can improve your quality of life.

Structural Concerns

More prominent sags are cause for serious concern.

Heavily sagging or bouncy floors indicate that your floor needs more support.

If neglected for too long, such floors can collapse entirely.

However, building codes allow for small amounts of floor sag.

Your floor joists can safely deflect up to 1/360 times their span length.

Steel joists are less likely to sag than wooden floor joists.

If you can easily notice your floor sagging, the odds are it is already sagging far beyond this limit.

Get your floor joists looked at without delay.

Also, keep a lookout for other structural defect warnings accompanying your saggy floor.

These signs include doors or windows not shutting properly and cracks in your walls, ceilings, or foundations.

Sloping floors, where one edge is higher than the other (rather than the middle of the floor having a depression), can signal a sinking foundation. 

Additionally, if you can inspect your floor joists via a crawl space or basement, look for signs of damage here.

Pay attention to problems like cracks in your joists, termite infestations, moldy wood, water leaks, or wood rot.

Joists are notched or cut to allow HVAC ducts and other utilities to pass through.

Measure these cuts to make sure they’re not present in the middle one-third of the joist’s span or touching the bottom of the joist.

Imperfect notching impairs joist strength.

Furthermore, notches at the edge of the joist should be less than 1/4 times the joist depth, while notches in the center are less than 1/6 times the depth.

Holes should also be smaller than 1/3 times the joist depth and at least 2 inches away from the joist’s top and bottom edges.

Addressing these structural concerns promptly is crucial because floor sagging tends to worsen over time.

How to Fix Sagging Floor Joists

Here’s the step-by-step on fixing a sagging floor joist.

Step 1:Assess the Depth of Sag in the Joist

The first step is to locate precisely where and how much the floor sags.

You can measure the distance between the ceiling and the floor at different points.

Alternatively, use a carpenter’s level.

Also, go inside a crawl space or basement ceiling where there are exposed joists to inspect them.

You can fasten a string a few inches from the top of the joist at both ends.

If the joist is level, the string will remain at the same distance from the top at all points.

Additionally, look for any other issues in the joists here, such as joists breaking or rotting, improper notching, termite infestations, or water damage.

Step 2: Address the Root Cause

Addressing the cause of the sag is vital.

If the joists are affected by the foundation settling, plumbing leaks, or termites eating away at the wood, your first action should be handling these issues.

Dry out the area first if moisture is the source of wood rot.

To prevent this issue from reoccurring, ensure the space under your joists has proper drainage and, if required, encapsulation.

Step 3: Repair the Joists and Correct the Sag

Next comes repairing the joists and leveling the floor.

Specific situations call for different approaches.

Possible methods to consider are listed below.

Jacking up the Floor

If the sag is quite pronounced, you might need to jack up your floor before dealing with the joists.

Set up a jack below the damaged section.

Consider adding a temporary beam if you need to hoist multiple neighboring joists.

Jack up the floor slowly to protect the floor and walls above.

Raise the floor by about 1/8th of an inch every day.

Sistering Floor Joists

Sistering Floor Joists
Sistering Floor Joists

Securing new boards to damaged joists strengthens the joist system.

This method works when the joists are only slightly damaged or overloaded.

Cut a healthy lumber board into the same size as the damaged joist.

Bind the two together using a strong adhesive along with nails or screws.

Sistering both sides of the joist may be necessary if the lumber is very weak.

Alternatively, bolt the damaged joist to one or two flitch plates to improve the joist’s strength and dampen sagging.

Do not opt for this if you expect to drill through the joist later.

Install Post and Beam Support

Jack Post
Jack Post

When the joists lack sufficient support from below, add new support columns and beams.

For extra posts below your joists, you can choose from steel push piers, brick piers, permanent jacks, and concrete footings.

You can place additional beams perpendicular to the joists between new piers.

Replace the Joist

Provide temporary vertical support to the floor and then pry out a damaged or rotten joist.

Attach a new joist of the same dimensions to the floor using construction adhesive in place of the old joist.

Secure the new joist to the floor using a joist hanger.

Blocking and Bridging

Floor Joist Blocking and Bridging
Floor Joist Blocking and Bridging

Blocking involves adding solid pieces of wood that fit snugly between the joists to reinforce them.

In bridging, strips of wood or metal connect two adjacent joists in an X-shaped manner.

These measures strengthen the joists and make them stiffer.

Pour Self-Leveling Compounds and Shims

You can pour a self-leveling underlayment or add shims in the sagging area for shallow dips in the floor.

The underlayment spreads all over the depression in the floor and creates a flat surface on top as it hardens.

Install new flooring above this.

Shims are thin strips of wood hammered between the joist and the floor to fill any gaps.

However, these methods do not address structural issues but correct the sag in a floor.

How Much Does it Cost to Fix Sagging Floor Joists?

Floor joist repair expenses vary with the extent of damage and how accessible the joists are.

At the lower end of the scale is sistering the joists, which costs between $100 and $300 per joist.

Joist repair can get pricier if access is through a cramped crawl space or involves removing layers of subflooring and insulation.

Repairing inaccessible joists can take up to $1000 or more per joist.

Replacing joists is more expensive, at times costing up to $2,000 per joist.

The rule of thumb is that if repairing the joists costs over 50 percent of what it costs to replace them, replacement is the wiser choice.

Aside from dealing with the joist, typical additional costs include redoing the subflooring, insulation, and flooring on top of the joists.

Using a jack to lift the floor off a damaged joist is complicated and sometimes necessary during joist repair or replacement.

Jacks cost around $400-$600.

Termite removal, if any, can range from $250 to $2,000.

When there is water damage, you also need to plug the source of the leak, remove any mold, and dry out the wet areas.

Dealing with a water-damaged sagging floor can set you back between $2,000-$6,000.

Getting a structural engineer to inspect your floor to assess the cause of damage can be very beneficial.

Inspections generally cost between $300 and $500. 

Furthermore, joist work usually requires permits that can cost you $50 and upwards.

Will Homeowners Insurance Cover Sagging Floors?

Probably not. Most home insurance policies focus on sudden and accidental issues, which don’t include sagging floors.  

You may get coverage if an unexpected hazard such as fire, vandalism, burst pipes, storms, and other “covered perils” damages the floor.

Your insurance may also cover damage from flooding or earthquakes, depending on your policy.

But most sagging floors are considered wear and tear issues that are gradual or preventable through timely action.

Proper maintenance and acting quickly when you notice any issues are the best safeguards from major repair expenses that you’re likely to have.

Can a Sagging Floor Collapse?

Yes, sagging floors can collapse.

However, collapse generally takes a long time to occur.

Address the root cause of the sag immediately to avoid the building up of pressure on the floor’s joists or subfloor.

The sag will keep getting more pronounced, and eventually, the floor can break.

If you can perceive a sag in your floor by looking at it or walking across it, get it checked out.

Frequently Asked Questions (FAQ)

Can You Replace Floor Joists Without Removing the Floor?

You can avoid removing the floor if the joists can be reached from underneath.

The exposed joists in a crawl space or an unfinished basement are accessible this way.

How Can You Tell if a Floor Joist is Rotten?

Check your floor joists for signs of rot if you notice your floor sagging or high moisture levels in the walls or near the foundation.

Also, rot visibly damages the wood, changing its color and causing cracks.

Moreover, it makes the timber either spongy or dry and powdery in appearance, and a musty smell accompanies it.

Does Rotting Wood Indicate the Presence of Mold?

Not necessarily.

Both wood rot and mold are the result of fungi growth and develop in similar conditions – high moisture levels and warm temperatures.

However, rot does not indicate mold is undoubtedly present, but one often accompanies the other.

V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com

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