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How Do You Build a Floor in an Existing Shed?

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Many people buy an existing home that already has a shed on the property. In fact, it’s a great selling point and adds value to a property. The problem with this is that you don’t know how well your “new” shed was constructed. Chances are you’ll find some things that you would have done differently had you built it yourself or had it built.

Maybe you find that your shed doesn’t even have a floor in it. You might find that it has a floor, but it’s got obvious problems, such as sagging, bouncing, signs of mold or even rotting places. Now what do you do? How hard is it putting a floor in an existing shed?

How Do You Build a Subfloor Over Dirt?

When putting a floor in an existing shed you first have to take everything out of the shed that sits on the floor to allow you room to work. If you have a lot of things up on shelves, you may want to remove them as well so that you don’t knock anything down from them while you are moving around working.

Buy a Shed or Build

Dig out an inch of dirt from the floor. Remove any rocks or roots sticking out. Use a level to make sure your surface ends up completely level and smooth. Then spread a layer of fine gravel over your dirt and rake it smooth.

Measure your shed front to back. You’ll need enough pressure-treated 2 x 4s long enough to run this distance and span your shed base when they are spaced out a foot apart. Lay out these boards on the narrow sides.

For the actual flooring, you’ll need enough 1 x 6 boards to cover your floor space. You’ll nail these floorboards to the 2 x 4s all the way from one end to the other. Now you have a shed floor over the dirt, you have the gravel underneath to aid drainage and you have your flooring raised a bit to keep it away from moisture.

Can You Put a Concrete Floor in an Existing Shed?

You can put in a concrete floor in an existing shed. One possibility is if your shed is small enough to be moved. You can lay a traditional concrete slab nearby and move the shed to it once the concrete has dried sufficiently.

The shed can be bolted to the new slab using special bolts and a masonry drill as long as your shed has sill plates. The bolts are drilled through the sill boards into the concrete to a certain depth, then nuts are attached on top of the wood planks.

Another possibility is a floating slab. This is one that will not be attached to the shed. You’ll need to take out any existing floor, dig down at least four inches, and put in maybe three inches of gravel, then build a wooden frame to hold the concrete as it is poured and dried.

This will definitely be a do-it-yourself job, as most sheds don’t have a large enough doorway to enable pouring from a commercial truck. You’ll probably need to rent a concrete mixer to both mix the concrete and keep it from setting up while you pour it batch by batch and spread it around.

You might use a large wheelbarrow to pour the concrete. The only problem with doing this is that the main reason people put concrete slabs under sheds is so that the shed can be anchored into the concrete, and a floating slab doesn’t allow for this.

If your shed is light enough but you can’t move the whole thing for some reason, you can try renting hydraulic jacks to raise the shed so you can prop it up using concrete blocks and 4 x 4″ beams while you pour a slab underneath it large enough so that the shed will rest on it and you can bolt the shed to the concrete.

Ryans Shed Plans

This is the least desirable option, as there is no way to raise the shed and hold it high enough to make any of the digging, pouring, spreading or smoothing tasks anything but a real pain. Another option is to use concrete pavers which can be lifted and reset as needed. Check out my article Can I Use Paving Slabs for a Shed Base? for more information.

How Do You Replace a Rotted Shed Floor?

If you find that your shed was built with less than good moisture protection and the floor is rotted, you certainly will want to replace it. Besides not wanting possible holes in your floor, moisture from the rotten wood may eventually impact your sill plates or even your joists. Holes can also let in vermin. Remember that any hold large enough to admit a mouse’s head will let in the rest of him as well.

If you only want to replace the rotten parts, cut away a section of the existing floor to see what was used and how thick it is so that you can get comparable flooring to replace the bad parts. A better solution would be to replace the whole floor. There may be places in the flooring that have absorbed water that aren’t readily visible. In addition, mold and mildew can send out filaments that are invisible for the most part that allow it to spread.

Cut away the old flooring with a circular saw, taking care not to hit the joists. Once the floor is off, check the joists for any moisture damage or rot as well. You may have to cut some parts of them off and replace the bad parts. Then you can cut and lay down your new flooring on the joists.

How Do I Protect the Bottom of My Shed Floor?

First of all, use pressure-treated wood, plywood, or OSB board for your flooring. Tell your supplier what you want it for, and he can make sure you get the right grade of pressure treatment. Some people like to install foam insulation between the joists. Using a water-repellant sealant applied to your flooring before installing it will protect each board or panel all the way around.

Another way to protect your shed floor is to look at the terrain outside. Do you see any place where rainwater routinely runs down toward your shed? You may want to look at putting in a French drain to divert this water flow. Water running off your roof eventually digs a sort of channel or ditch that holds water and allows it to seep into the soil under your shed.

Ryans Shed Plans

Digging out the soil around your shed a couple of feet all the way around to a depth of three or four inches will allow you to put in some gravel. This will catch the rainwater and help it evaporate. It will also keep the water from digging a ditch. If water collecting around your shed from roof runoff is a frequent problem, you may want to utilize the French drain system to help collect this water and get it running away from your shed. You may want to consider adding a vapor barrier on the ground before building your new floor. You can get more information in my article Do I Need a Vapor Barrier Under My Shed?

How to Lift a Shed to Replace Floor

What you use will depend on the size of your shed. Of course, take everything out of your shed first, including anything on shelves or stored up in the rafters. If the shed is small you can use 2 x 6 boards as levers and concrete patio blocks or wooden blocks as props. You’ll need at least one helper to place the prop blocks, and possibly two to help you lift each corner.

Lift one corner at a time, allowing a helper to slip the blocks underneath. Then move to the next corner. You may need to go around more than once if you plan to raise your shed up quite a bit. Go slowly and lower the shed back down if your lever board shows any signs of cracking. Somebody can really get hurt if it breaks.

If your shed is larger, say a 10 x 20′, you will need a much more involved process and more help. You’ll need a power jack puller or at least a couple of hydraulic car jacks, which you can probably rent, some 4 x 4″ boards, and some 3-1/2 inch concrete blocks.

Lay your 4 x 4s along one side of the shed with a couple of inches of space in the middle. Dig out from underneath them so you can slide them under the shed. Use a concrete block as a base for the jack or jacks, positioning the jack so that it uses the 4 x 4s on top of the jack. and raise up the side.

If you use one jack, slide two 4 x 4s underneath the shed close to the center, then one horizontally underneath these two. If you use more than one jack, make sure they are both raised up at the same time. Now place your concrete blocks under the floor beams or the 4 x 4s on the corners. Then go to the other side.

You may have to go around more than once to get the shed the height you want. If you raise the shed in steps rather than going for a larger height all at once, it will take longer but it will lessen the chance of shifting or sliding off the blocks while raising it. Check out my article What Are the Strongest Shed Floor Materials to Use? for more information on the strongest shed floor options.


Repairing or putting in a new floor in a shed will really extend the life of the shed and make it safe and sturdy for storing your garden tools and lawn equipment.

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