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How To Strengthen Floor Joists From Beneath

Floor joists form the skeletal framework of most floors not directly attached to a concrete subfloor. They provide support for the floors. They give floors the strength to withstand the pressure of human, animal, and other inanimate objects exerted on the floor. 

Oftentimes, floor joists are like a legged wooden slab extending throughout the room. Depending on the home’s design and its size, floor joists can cut across one beam to another or one wall to another, or one beam to a wall.

These conjunctions of an evenly spaced stalk of plywood or board the pavement where choice your floors are installed. Be it the hardwood, tile, or carpet flooring choice. However, as years turn decades and decades become more decades, floor joists may lose their supporting power. They may sag or bounce. And since floor joists are not easily substituted, the need for a floor joist’s strengthen arises. So, today look at how your floor joists can be strengthened from beneath.

Strengthen Floor Joists

To improve your floor joists’ supporting ability, you may add to it either a beam or wall. But before then, test the beam to see if it can support the floor joists. To test the beam, match your floor firmly while positioning yourself near the beam and paying attention to hearing a bounce. Do this again when you are positioned between the external wall and the beam. 

While at the center point, there should be a noticeable bounce sound. However, this should not be when you were positioned near the beam. If the bounce sound is equal in both, then the beam should not be used. This is because the beam is most likely of a smaller size or not equal to the support task. Other ways to strengthen the floor joists include bridging/blocking and increasing the board layer of the joist.

Bridging Or Blocking

Bridging is an economical way of removing the floor bounce and strengthening the floor joists from below. It tightens the floor together and shares the pressure received by the floor evenly across the floor joists. To bridge your floor joists is not extremely difficult, it is something you can do. 

For floor joists that had bridging done on it before but have their floor sagging now, it is possible that the formerly installed bridging is lax. Hence, you can make it firm by nailing.

However, if you are bridging your floor joists for the first time, place a bridge between floor joists at a common gap bearing in mind how long your floor joists are. For more effectiveness, the bridges should be placed linearly between joists, cutting across one wall to the next. This allows the floor joists to bear the pressure of movement uniformly amongst them.

To bridge, take note of the broadness between 2joists and cut plank following a dimension measured. A 2 by 6 plank may be used. Place a cut plank between the joists and stabilize it with a three-and-a-half-inch long nail. Repeat this across floor joists.

Increasing the board layer 

You can also boost the supporting power of your floor joists from below by adding an extra layer of plywood to them. This method is more efficient when a smaller, light strip of the board is used. This is because these plywood strips are more easy and direct to install because of their flexibility. 

Usually, the plywood should be 3/4 inch thick and 8feet long. To add the extra layer of plywood, first sand the lower end of the joists using sandpaper of 80grit. This is to ensure joists and plywood firmly adhere. When satisfied with the texture of the surface, suitable adhesive and screw added a plywood layer to the joists.

Adding An Extra Wall Or Beam

Another way to strengthen your floor joists from underneath is through an extra beam or wall. This method is better used for house basement, crawlspace, or a lower room. This is because the extra beam reduces the headroom. 

When using a beam to strengthen, pay attention to ensure columns and footings are snugly fit. This is because columns strengthen beams and footings columns. Since the goal is to ensure footings are well enough to support columns, it is best if each footing is not below two-square-feet.

However, footings size can also be increased when the gap between columns is broad. But it is best to fit columns that are closely spaced.

Sistering Floor Joists

You could also double or triple your floor joists. This is called sistering. This process is somewhat technical and lots of work needs to be done. Here are the recommended steps to sistering your floor joists.

Step one: To sister joists, first you select a sistering material of the same measurements as your previous joists which will be strengthened. This material can either be an engineered wood or refined lumber.  

Step two: Wear your protective kits. This includes your goggles, earplugs, boots, and helmets to keep you safe.

Step three: Re-align and adjust the sagging floor before proceeding with sistering joists. Do this by making a four-by-four inch post with a hydraulic floor jack. Do this while increasing jack by 1/8inch per day till created post and floor joists meet. This is done so there won’t be cracks during the joist sistering.

Step four: Place suitable adhesive through the length of the joists. This is to firmly fit new joists on the old joists for a well-bonded sistering. This always reduces the risk of funny sounds typical to a loose sister floor joists.

Step five: Go ahead to fix sister joists of the same dimension as your old floor joists by placing it on top of the four-by-four inch post you earlier created.

Step six: Further ensure the joining of the new floor joists to old floor joists by nailing through them. While nailing keep floor joists in place with clamps. Remove clamps when done.

Step seven: Repeat steps to strengthen other loose floor joists. 


Floor joists can be strengthened by bridging, adding more plywood, having an extra beam, or by sistering joists.