How to Strengthen Floor Joists from Below

How to Strengthen Floor Joists from Below

How to Strengthen Floor Joists from Below

Floor joist strengthening becomes necessary when the floor of your house starts wobbling or bouncing.

In older homes, joists start cracking or sagging due to years of degradation.

6 Ways to Strengthen Floor Joists 

To strengthen floor joists, block the joists by nailing a piece of lumber between them. Another method is to sister the joists with a piece of lumber. Plywood and steel plates are also effective in increasing the strength of run-out joists.

Another solution is to provide a mid-span beam or wall underneath bouncy joists to support them.

After years of use, the quality of floor joists tends to deteriorate. They start to lose their strength, which consequently leads to sagging or cracking of floors.

If the spans of floor joists are too large, they may start to bounce or wobble with time.

As efficient as floor joists are, they are not that easy to replace.

However, there are alternative approaches that can be adopted to strengthen floor joists and increase their useability.

A few of the most widely used methods are listed below.

1. Using Beams or Walls as Support for Floor Joists

 Using beams or walls as a support to your floor joists is an excellent way to strengthen them.

This option opts for cases where space is not limited and adding columns does not reduce the effectiveness of the design.

If obstructions and loss of headspace are no problem, beams and columns provide sufficient strength to joists.

They are even applicable to basements. The aspects to be taken care of are the configuration of beams and columns.

When adding beams, it is important to include proper footing to support the columns.

Normally, small footing sizes are required for wooden beams, but in the case of steel beams, a larger footing size may prove to be more optimal depending on the load of the beam.

The rigidity of the beam will be in direct relation to the spacing between each column and footing.

The closer the columns and footings are, the more strength the beam will offer as a support to the floor joists.

A foundation may be created by cutting a hole in the ground beneath the basement and filling it up with concrete.

Then a beam is fixed under the midspan of the floor using hydraulic jacks, and a column is installed between the beam and the foundation.

Instead of wooden beams, custom-built-up beams can also be utilized in the strengthening of joists.

Built-up beams offer more strength and can be incorporated if the floor loads yield a very large value. Similarly, a wall may be erected to support the joists from underneath.

Floor Joist Strengthening with Beam Support
Floor Joist Strengthening with Beam Support

You might be interested in this- Joists vs Beams vs Girders

2. Joist Sistering to Strengthen Floor Joists

If one joist does not suffice, try adding another joist alongside an existing one.

This process is known as sistering of joists and is proven to be quite effective in increasing the strength and stability of floor joists.

The size of the sister joist may be kept the same as of the original joist, but on the other hand, using a taller joist provides more resistance.

Provided that the taller joist will need to be notched to meet the fixed level of the original joist.

The increased depth proves more strength and reduces the wobbling of joists by a great deal.

Engineered wood joists can be used to sister the initial joists instead of standard lumber joists.

Sister joists can be applied to both sides of the original when more stiffness is required.

If the existing joists are sagging or bowing downwards, they are first jacked up, and then a sister joist is attached to straighten them out.

The adhesive used between both joists further helps reduces the squeaking sounds that may have developed due to the deterioration of the joists.

The top of the sister joist is aligned with the original. The bottom section, if longer, is cut from the middle and then rested upon the sill plate.

10d to 16d nails are then used to join the joists together.

3. Using Plywood Strips

When floor space is restricted and sister joists do not seem like an approachable choice, plywood strips serve as an excellent replacement. Two strips of flexible plywood are applied to one side of the wobbling floor joist.

Plywood strips are not only shorter, but lighter and more flexible, which makes their installation process quite easy. Especially in spaces where full-sized joists cannot be added.

¾" strips of plywood are cut and matched to the dimensions of an existing floor joist. The connection is made solid using a strong construction adhesive.

The overlap is further strengthened using 8d nails or 2" screws.

One major recurring inconvenience is the bounce that develops on the floor over time. In such cases, replacing old floors may not be a solid option.

Alternatively, it is recommended to use plywood strips to reduce the bounce on existing floors.

Normally, planks are used to construct the floors above joists. When considering remodeling or replacement of an old floor, or even when adding a new one, plywood can be considered.

Plywood subfloor can be nailed into existing floor joists, creating a sheathing over the existing deck. The flexibility of plywood consequently considerably reduces the bounce of the floor.

Plywood also offers a solution to strengthening the joists of second floors of houses, where the joists are not as easily accessible through the first-floor ceiling.

The transitioning of material and the installation process itself are made quite more convenient when opting for plywood.

In addition to the top side of the floor joists, the plywood sheathing can also be installed at the bottom of the joists.

Plywood floor sheathing is applied by spreading a large amount of construction adhesives on the old run-down planks.

The new sheets are laid perpendicular to the floor joists. Screws or Nails, having sufficient length are penetrated through the sheathing and into the joists.

The same method can be applied to install the sheathing on the bottom side if the level of the joists is the same throughout the deck.

If the level varies by more than ½" the sheathing cannot be applied using this method.

Strengthening Joists using Plywood plates
Strengthening Joists using Plywood plates

4. Using Metal Wraps

Wrapping steel straps around the joists helps in distributing the loads more efficiently along the ends of the joists.

The way to do this most effectively is to start wrapping from the top of one side of the joist, then towards the bottom of the midspan of the joist, and then again towards the top of the other end of the joist.

The load applied towards the center of the joist in this configuration is distributed along with the nails that run through the joists.

Metal straps are quite easy to install. The joists are first raised using jacks and temporary beam support is set up.  Raising the joists helps in ensuring that the metal wraps tightly when the jacks are removed.

Start from the top of one end by nailing metal connector nails along every hole for the first 2 ft. after that nail holes after every 6". Nail the opposite side in the same way.

 The strap should be folded crisply along the bottom at the midspan. Additional straps can be installed if required for additional support. Once the straps are installed, remove the temporary beam support.

Metal wrapping to strengthen floor joists
Metal wrapping to strengthen floor joists

5. Using Steel Plates

Steel can be used to reinforce different types of floor joists. One such technique is called the steel flitch plate method.

In this method, steel plates are sandwiched between two adjacent floor joists and bolted at the ends.

To gain maximum effectiveness, ensure that the plates run along the full length of the joists. In this way, the plates are supported at the ends.

Using steel plates limits the space for drilling. Consequently, it can hinder the process of passing ducts and wires.

Although steel provides more strength, the overall benefits of using a simple wooden sister joist may be more practical.

Floor Joist Strengthening by Steel Flitching
Floor Joist Strengthening by Steel Flitching

6. By Blocking of Floor Joists

Solid blocking is another technique used to improve the performance of engineered I joists and trusses.

Using wooden blocks between adjacent joists helps in transferring weight along the joists in a more unified manner.

Floor joist blocking is only effective if it is tight.

Metal and wooden blocks are prone to loosening as time passes, due to expansion and contraction.

Shrinkage and seepage may also pose a threat to the system.

The best time to induce the blocking method is during early spring when the moisture content in the joists is at its lowest.

This is because the technique works best when the joists are dry.

The blocks are cut in proportions that are a bit larger than the space offered by the joists. This ensures that the blocks will be tightly packed.

They are then installed perpendicular to joists.

If ductwork is in the way, use two separate smaller blocks.

Drive nails through the joist and blocks.

Additional support can be provided by inducing more blocks in an equally spaced manner.

Floor Joist Strengthening using Blocking
Floor Joist Strengthening using Blocking

7. Other Options

Increasing the area of the band joist in the frame can also be a good option to reduce the bounce of a floor.

This can be done by installing double band joists across the longitudinal sides of the perimeter

What is the best way to Strengthen joists?

The best way to strengthen floor joists is to provide support at the midspans using beams or walls. But space restrictions can make this impractical in some situations.

Steel reinforcement and metal wrapping are also quite effective but are costly as compared to the other options. Blocking may also yield sufficient strength but is prone to loosening.

Sistering of joists is easily executed, provides sufficient strength, and is cost-effective as well. Plywood offers the same benefits with the added advantage of being even smaller in size.

All options offer some advantage over the other. Identifying the best method will depend upon the site conditions and restrictions.

If space is plenty, beam support would be the ideal way to go.

Why Do Floors Sag?

 In residential houses, the floors may start to sag over time due to insufficient support and load transfer mechanism.

When not enough beams are provided to support the weight of the above floors, the existing beams must work harder and as such, they start to weaken over time.

Another reason why floors start to sag may lie in the rotting of floor joists.

If the crawl spaces are not appropriately sealed, rotting may occur because of moisture and humidity acting on the joists.

To avoid damage, you can use low-pressure fans and dehumidifiers to dry the joists.

Since wood is a common material used in constructing a joist system, mold may develop and eat away at the strength of the system. Termite action may also cause the joists to rot.

The soil existing below the foundation may have a limited bearing capacity, causing the load to cause a punching effect. Thus, poor soil may also cause the floors to sag by initializing the settlement of foundations and columns.

For large-span construction, opt for floor trusses instead of floor joists to prevent sagging.

What are The Signs of Damaged Floor Joists?

  • Moisture causes the wood to rot.
  • Mold starts to develop because of moisture.
  • Sagging of floors starts to occur.
  • Cracks may form in the joists.
  • Sloping and unevenness of floors may develop.

How much does it cost to reinforce floor joists?

The costs to replace floor joists is quite expensive, costing up to 6000$ to 1000$. The price may increase up to a total of 20000$ if jacks are required.

 Alternatively, using strengthening techniques to solidify existing joists is much cheaper. Consider sistering the joists, where it only costs about $100 to $300 in total to sister a floor joist system.

Frequently Asked Questions (FAQ)

Does drilling holes make joists weaker?

Drilling holes in joists will weaken the strength of floor joists. The extent to which this happens depends on the size of the drilled hole. Therefore, the allowed limit of a hole is a diameter of 1/3 times the depth of a joist. Holes must also be centered (at least 2" away from the top and bottom).

How do you stiffen bouncy floor joists?

Bouncy floors can be stiffened by several methods as mentioned above. The application of plywood panels is an excellent method to reduce the bounce present on floors.

Can you replace the floor joist without removing the floor?

If sufficient crawl space exists, then it is possible to replace floor joists without removing the entire floor. However, this is an ideal situation and it may not always prove to be efficient to replace floor joists.

About 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

DIY Profile
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

Connect With Me


The comments are closed.