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How to Strengthen Floor Joists From Beneath

How to Strengthen Floor Joists From Beneath

Whether you bought a home with an existing shed or bought or built your own, at some point you may start wondering if the flooring is strong enough. You may also wonder how sturdy your pre-built shed joists are.

After all, sheds always tend to have more and more to hold as time goes on, not less. When you get that new lawn tractor, you may even think you see a bit of sag. Is there anything you can do to strengthen floor joists from beneath?

There are several ways to strengthen floor joists from beneath. If you don’t have enough space to work underneath your shed (and you probably won’t), this means that you’ll have to jack up your shed temporarily while you work. In addition, you’ll want to jack up and level any sagging parts.

Jacking Up the Shed

Of course, you’ll want to take everything out of the shed. Slide the tip of a hydraulic jack under a supporting beam at a corner and jack it up a bit higher than you want, then place your jack stand underneath. Put another jack stand at the other corner on the same side. Repeat on the other side.

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You’ll want to make sure you have a level, sturdy base underneath your jack stands. A flat concrete block would be good for this. For a larger shed, you may also want to place one in the middle of your long sides. A helper or two would be ideal.

How to Strengthen Floor Joists from Beneath

Plywood. One way is to cut plywood to the size of your joists and nail them along both sides of your existing joists to sort of artificially widen them. The plywood will provide added support to the floor.

Blocking. This involves cutting planks the same size as your joists and attaching them perpendicularly between the joists. Two or three 10d or 12d nails on either side will do. Make sure the blocks fit snugly and that the bottom of the block is even with the bottom of the joist. Alternating placement may allow you an easier time when nailing but putting the blocks in a straight line will result in better support.

Steel plates. These are ¼” or ?” plates made to run the length of your joists. They usually come with pre-drilled holes and are fastened to the joists using carriage bolts. Check out my article How Do I Strengthen My Shed Floor? for more ideas on strengthening your shed floor.

How to Sister Floor Joists

This involves adding an identical joist to the existing ones and attaching it with screws or nails. This is a good way to help joists that were too weak to begin with or joists that have damage. Some authorities recommend this fix rather than replacing a damaged floor joist.

How to Brace Floor Joists

This involves forming an X-shaped cross brace between joists. You can use 1×2 or 2×2 lumber, but it must be perfectly dried. New lumber may shrink and lose its effectiveness. Actually, plywood cut in 3-inch wide strips works better and is used in newer homes and sheds.

The trick will be figuring out the proper length and angling the cut ends to fit. Using 8d nails or structural screws is recommended. Standard construction screws are not a good idea. The number of braces you use will depend on how much help your floor needs.

How Do You Replace Rotten Floor Joists?

If your shed is old, perhaps built by a previous owner, you may have floor joists that are rotting, especially if the builder didn’t take steps to keep moisture away from them when the shed was built.

If there is only one damaged spot and the rest is sturdy, you may just want to saw and remove the bad part and replace it. If you want to replace the whole joist, you’ll need a reciprocating saw. Cut the joist into 3- or 4-foot sections, then use a pry bar or nail bar to remove it from the flooring or subflooring. You may need to cut nails at either end, depending on how the joist is attached there.

Cut a new joist of the same type of lumber as the old one. Set it into place, replacing the joist hangers if necessary. Attach it to the footplate by toenailing upward. Attach it to the flooring or subflooring. Need some more help with your floor, check out my article How Do You Build a Floor in an Existing Shed?

How Much Weight Can a Shed Floor Support?

The average shed floor can support 20 lbs. per square foot. This means that a 10×12 shed can hold up to 2,400 lbs. This, however, varies with the type of flooring, size of joists, and spacing of the joists.

Naturally, a shed built with 2 x 12 joists on 12-inch spacing can hold more than one with 2 x 8 joists on 24-in centers. A floor with ¾ inch plywood holds more than one with ½ inch flooring. Even your foundation plays a part. A shed with a good, solid foundation will resist sagging better than one set on joists directly on the ground, as ground tends to shift and erode with time. Get more details on how much weight your shed floor can hold in my article How Much Weight Can 3/4 Plywood Hold?

What Size Floor Joists Do I Need?

One simple formula is to note the span the joist will make and round it up to the nearest foot. Divide that by two, then add two to that answer. This will give you the depth your joist lumber should be.

For example. If your joists will span a distance of 15.5 feet, round that up to 16, then divide that by 2, resulting in 8. Add two to get 10. You’ll need 2x10s for the joists. Using 2x12s will give you some room and peace of mind when you get more heavy things to put into your shed. In my article What Shed Floor Joist Spacing Should I Use? I go into detail joist sizes and spacing for a strong floor.

Do I Need to Use Pressure Treated Floor Joists?

You should definitely invest in pressure-treated lumber for floor joists. No matter what kind of foundation your shed has, the joists are the first parts of your shed to come into contact with any ground moisture evaporation and the parts most likely to be exposed to moisture over time. If you skimp on this part, you’re gambling with the structural integrity of the whole shed.


If you’re considering buying a home with a shed, make sure your home inspector takes a good look at your shed, including the joists, if possible. Once you have the home, take a look yourself. You can help yourself a great deal if you find problems and fix them before they become worse. Even if you buy or build your own, inspecting your shed’s underside periodically is a good idea. The goal is to help your shed continue to do its job.