Build Your Shed On A Concrete Slab

The ultimate shed floor... — The ultimate shed floor.

At first glance you might think a concrete slab would be the ultimate floor and foundation system for your new shed..

But a slab has two main drawbacks you should consider before you make a final decision.

The cost: A concrete slab could easily cost 3 to 10 times the amount of a wood floor. Depending on if you do it yourself or hire someone.
Your shed is no longer portable.

No Longer Portable

Not being portable is important for 2 reasons.

You can’t move your shed if you need to.
And permanent structures are often treated differently by the law.

Permanent structures often need building permits regardless of the size. This means you will need to build it to code and have it inspected, both which will cost you additional time and money. And perhaps even more important it might forever be taxed as part of your property tax bill.

But on the plus side is you will have a floor that can carry any kind of weight, will never rot, and your shed will be resistant to almost any source of movement be it water, wind or frost heave.

Physical Dimensions

Build your slab the size of the shed floor. In other words if you are building an 8×12 shed then build an 8×12 slab. This way the siding will be able to hang over the side of the slab an inch to prevent water from seeping under the bottom plate and into your shed.

If you want to put an apron in front make sure it slopes away from the shed or is a little lower than the floor.

The top surface of your slab should be about 5 inches off the ground. This will allow your siding to overhang about 1 inch, then provide a 4 inch gap between the ground and the untreated wood siding.

This 4 inch gap is your best defense against termites and moisture getting to the shed. And often times this is in the building code.

Make your slab 4 inch thick and it will hold all the weight you can put on it. Any thicker is a waste of concrete and money.

Ramp

If you want a ramp it’s best to pour it the same time as the rest of the slab. You can frame it in and make it part of the slab. This way you will never have to worry about a wood ramp rotting and having to be replaced.

Anchors

You can install “J” bolts in the concrete when it’s wet or you can install expandable head anchor bolts after the walls are in place. If you are building to code check which option they recommend or allow.

Put an bolt 12 inches in from each side of both corners, then a bolt every 48 inches. And put a bolt near the edge of each side of the door.

Expandable Bolts

To install expandable bolts put your 4 walls in place first.

Then use a hammer drill and masonry bit the diameter of your bolts. Drill a hole through the bottom plate and into the slab sufficient for your bolts.

Put the nut and washer on the bolt and drive it into the hole, then tighten it up with a wrench. This will expand the bottom of the bolt and turn it into a good anchor.

Weather Proof

Use pressure treated 2×4’s for the bottom plates and put a foam seal between it and the slab.

If you’re using “J” bolts in the slab then put the foam on the bolts before you raise the walls.

If you’re using expandable bolts then staple the foam to the bottom plate before you raise the wall. This will keep it from moving around.

3 Inches Of Extra Wall Height

My plans allow for a 4 inch overhang so that you can nail the siding directly to the floor for extra strength.

But the siding only needs to overhang a slab about 1 inch.

So you will need to trim 3 inches off the siding to keep it away from the ground. But a better way to use those extra 3 inches would be to build your shed 3 inches taller and gain some storage height.

Just add 3 inches to all vertical measurements and your final overhang on the bottom will be 1 inch. Just what you want. Plus your shed will be 3 inches taller.