Joists are the fundamental structure for flooring in modern homes.

    Generally, making a supporting mid-span beam or wall beneath the floor joists is the best technique to reinforce them.

    Continue reading to find out about when you should reinforce your floor joists, causes of bouncy flooring, how to check a bouncy floor, and the best ways to strengthen your floor joists.

    When To Strengthen Your Floor Joists

    The best time to reinforce your floor joists is when you notice problems in your flooring. Unless there is an issue with joist spacing or the wood has deteriorated, most house framings should be able to support a standard-size cast iron tub without problems. 


    What Causes A Weakened Or Bouncy Flooring

    Normally, spongy, bouncy or sagging floors aren’t a cause for concern unless a significant structural issue causes them. As homeowners, we soon become accustomed to a floor with a tiny bounce or spring, and we rarely examine whether there is a problem.

    What’s amusing is that a first-time visitor or home buyer would commonly halt and even shift their weight up and down to check a squishy floor. Below is a list of scenarios of possible causes for your flooring to deteriorate.

    The hardwood floor joists may begin to break over time. A cracked joist may compromise the subfloor’s integrity. It will no longer be as rigid as it once was and may become spongy, particularly if two or three framing components are fractured or damaged close together.

    Cracks can form due to a multitude of factors. For instance, if a large tree fell on the house during a storm, the joists could be damaged.

    Code dictates the structural requirements for flooring, roofs, foundations, and other components. 

    In most cases, an engineer is required to calculate the foundation size, the size of the rafters and how far apart they should be, the size of the floor joists or trusses, and the kind and thickness of the subfloor.

    If an engineer makes a mistake or the builder fails to follow the blueprints, a flooring system may droop, become spongy, or break, which does not occur frequently. 

    When renovations, extensions, or walls have been changed without a set of engineering plans, problems may arise. If You don’t have a permit, the likelihood of incorrect engineering or poor craftsmanship increases, and problems ranging from small to serious may arise.

    If exposed to water for an extended time, the OSB board and plywood will rot. The glues that hold these two items together will fail. Leaks in the plumbing or roof that go undetected or are not repaired on time can impair structural strength, resulting in sponginess. 

    Moisture stains on the floor may indicate a leak or other moisture issues. Check the baseboards and lower portions of the wall for stains as well. 

    When there are black discolorations, mold and mildew are likely to be present, indicating a moisture problem. Musty scents are another indicator of this.

    If the wood subflooring gets wet during construction, it loses its structural strength, especially if left damp for an extended time. Homes constructed during the rainy season that are not adequately roofed and wrapped are more vulnerable to this damage.

    Termites enjoy eating wood, which weakens it to the point where it loses much, if not all, of its structural integrity. As a result of this type of degradation and degeneration, a floor will bend or feel spongy over time, and if not corrected, it will fail.

    How To Check A Bouncy Floor?

    Whether you notice that your floors are too bouncy, investigate them to see if they need to be repaired. But, before making final decisions, you need to know what to look for. Follow the steps in the dropdown tabs below:

    Check your joists for damage and ensure they’re all in good shape. See if the posts that hold the beams and joists are rotted at the base. Ensure that the joist holes are less than a third of the joist’s depth.

    Check for rot with a screwdriver. The joist has to be repaired if the wood is soft, and you can probe it with a screwdriver.

    Examine the joist-supporting beams and posts. On either side of the joist, notching should be limited to the joist’s depth.

    Look for signs of degradation. Examine joist bearings to ensure they’re shimmed and in touch with beams or walls.

    To ensure your joists are the right size, measure them and check their diameters. It’s crucial to check the joist sizes to ensure they’re up to code.

    Best Ways To Strengthen Floor Joist From Beneath

    In the sections below, you will be going over your options for strengthening your floor joists. Many of them can be completed by even the most inexperienced DIY enthusiasts. 

    However, attempting to work around electrical and plumbing may drastically cause issues. So to help you make an informed decision, the dropdown sections below cover options for reinforcing your floor joists:

    The final option for stabilizing sagging floor joists is to build a wall or a support beam. Because beams restrict headroom in places like the basement, this solution is best suited for crawl spaces.

    Use appropriate-sized columns and footings to support your floor joists when using a beam. The columns immediately support the beam, which is supported by the footings.

    Ideally, your footings should be large enough to provide adequate structural support for each column. As a general guideline, we recommend that footings be no less than two square feet in size.

    Floor joist blocking consists of connecting pieces perpendicular to each pair of joists in a straight or staggered line using solid lumber the same dimension as your floor joists. 

    Here are steps for blocking your joists:

    1. Calculate the distance between the spans of two joists. Cut a piece of 2-by-6 or 2-by-8 lumber to the desired width.
    2. Place the lumber block between the two joists. To secure the blocking, use 16d nails on both sides.
    3. This method should be repeated every 24 to 36 inches along the joists. Stagger the blocks in the next cavity between joist sets so that your new nails don’t interfere with the ones in the previous cavity.
    4. Bridging is identical to joist spanning, except that plywood or smaller pieces of timber are used to make an “X” between each joist instead of a solid piece of lumber. 
    5. Cut the plywood into 3 inch wide sections. Their length determines the number of joists you’ll need. You’ll need one pair of bridges in the middle of your joists that are 12′ or shorter. 
    6. Each strip’s ends must also be angled to sit flush against the joist. A speed square or a joist bridging angle table can be useful.
    7. For this task, a nail gun is appropriate. Because some joists are difficult to reach, a pneumatic palm nailer will make the process much easier.

    If you don’t have any, start pounding with 8d nails. Despite their higher cost, structural screws outperform regular building screws. Furring strips are created the same way as before, with the added benefit of not tearing sections with a table saw.

    1. Cut 8-inch pieces of 3/4-inch plywood to the width of your existing joists with a table saw.
    2. With a couple of jacks and a temporary beam, raise your joists to the desired height.
    3. Glue and 10d nails the plywood pieces to the joists.
    4. In the same manner, overlap another layer, alternating seams. For the second layer, you can use 12d nails.

    When sistering floor joists, you must use lumber that is the same size as your existing floor joists. Remove all obstacles from the existing joist, use a jack post to raise the joist to the right height, and apply construction adhesive to the last joist’s face. 

    Apply the new joist and nail it in an x-pattern every 6 inches with 10d nails. Sistering joists strengthen the floor joists while still allowing electrical and plumbing to be drilled through them.

    Steel bridging, some of which are nailless, and steel flitch plates wedged between two joists are just a few of the steel reinforcement options for your floor joists, which is where we’ll talk about the flitch plate method.

    To use a steel flitch plate, get a 12″ or 14″ steel plate the same width and length as your floor joist(s) and bolt it to the joist. Some people use these plates to sandwich a joist or bolt them together to increase strength.


    Keep in mind that a slight bounce is expected, especially if numerous children bounce around or exercise in the room. However, because joists are such an important structural component of your home, it never hurts to undertake a thorough visual inspection.

    Keep your fasteners in mind when blocking, bridging, sistering, or using flitch plates. If your basement is wet, use galvanized fasteners rather than screws unless they are structural. When possible, oversize your fasteners. Otherwise, your efforts may be in vain.

    Read also: Complete Guide to Rafter Spacing

    Editorial Contributors
    avatar for Matt Greenfield

    Matt Greenfield

    Matt Greenfield is an experienced writer specializing in home improvement topics. He has a passion for educating and empowering homeowners to make informed decisions about their properties. Matt's writing focuses on a range of topics, including windows, flooring, HVAC, and construction materials. With a background in construction and home renovation, Matt is well-versed in the latest trends and techniques in the industry. His articles offer practical advice and expert insights that help readers tackle their home improvement projects with confidence. Whether you're a DIY enthusiast or a seasoned professional, Matt's writing is sure to provide valuable guidance and inspiration.

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