Steel shed and a (larger) concrete pad. Is a subfloor needed?


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Old 02-22-23, 01:28 PM
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Steel shed and a (larger) concrete pad. Is a subfloor needed?

Hi all,
I recently bought a 15x20 steel shed and I have a concrete pad in my backyard that's slightly larger that I was going to put it on. I've been recommended to put a 15x20 subfloor that's flush with the shed dimensions, I guess to keep water from pooling near the shed base but I wonder if it's necessary. To put a wood foundation under a steel shed that will likely outlast the wood seems counterproductive. Is it possible to just cut the concrete foundation down to the exact dimensions once the shed is up (or before I put it up)? It's only maybe a foot larger on each side. Then I can just seal the shed base and the concrete with silicon and any rainwater can just run straight down into the dirt.
Am I missing something? I'd really rather avoid introducing wood into this equation if at all possible.
Thank you.
 
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Old 02-22-23, 02:10 PM
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You wouldn't put a wood floor over your garage floor so you don't need a floor over your shed slab.

If you want to be really protective, you could lay a course of block or even PT wood around the perimeter of the the walls and attach the shed through the block/wood into the slab using expansion bolts.

The two garages that I have were built that way, with block, not so much to get it above the floor for water but to get it above the grass line for trimming.
 
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Old 02-23-23, 05:05 AM
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If the concrete slab is larger than the shed you might want a raised floor in the shed but it's not required. It will be almost impossible to keep water out of the shed. Rain will land on the concrete not covered by the shed and some will likely seep under the shed walls. Don't think you can seal around the edge to keep the water out. It will fail. If you want a dry floor for storage you'll need to elevate the floor inside the shed.

You can pour a raised slab sized for the shed on top of the existing slab. Expensive but it will last the longest and will be the thinnest at about 4" (you really shouldn't pour concrete thinner without special precautions). Another option is to cover the floor inside the shed with bricks or concrete pavers or stepping stones. It will be easy except for the weight but the floor will be difficult to clean and there will always be nooks and crannies for insects & snakes. You can frame out and build a pressure treated wood floor. Because there will be no ventilation below the floor to keep it dry it may have a shortened life.
 
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Old 02-23-23, 09:59 AM
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Thanks for the responses. The shortened life for a wood floor would be my main concern, shortened means less than 20 years? If it does need to be changed, I would basically have to take the shed apart somehow to replace it and that sounds like it'd be a massive headache.
 
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Old 02-23-23, 11:48 AM
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How long depends on the wood you buy and your local conditions. I hate to say the modern "safe" treated lumber has a much shorter life than the old ACQ good stuff. If you want to build a wood floor that lasts look at the treatment level of the lumber.

Most 2" thick treated lumber is lightly treated for above ground use only. In a wet environment it doesn't last long. I've got some that was used in a small deck similar to yours (on the ground, no ventilation) and it had noticeable damage in 7 years and at 13 years was totally useless. You will get longer life if it stays dry but if you regularly get water underneath, without ventilation it will rot sooner than you want.

Treated timbers, stuff over 4" thick like 4x4, 4x6, 6x6 are treated for ground contact and will last much longer. I am now doing another experiment where I did the framing with 4x6 then topped with synthetic decking. So far, 13 years, it is holding up great though it was a lot more expensive.

If you live in a coastal area lumber yards (not big box home centers) will carry wood treated to a very high level for making docks in salt water. That stuff will last a really long time even in the 2" thick form. I can't recall if I've seen 5/4 decking boards treated to the higher levels. If you live inland check with your lumber yard to see how much it is to special order the higher treated wood.
 
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Old 02-23-23, 12:13 PM
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Cement blocks mortared to the slab, never have to worry about anything rotting away.

Water does not pass through and they are cheap.


 
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Old 02-23-23, 12:54 PM
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What does "slightly larger" mean?
 
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Old 02-24-23, 09:06 AM
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Slightly larger meaning like a few feet wider, say 16x24 instead of 15x20
 
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Old 02-24-23, 09:29 AM
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If its just a pad, cut it to the right size before you build the shed. Raising it up will create a problem with all your door openings unless they can be modified. If they can, then the solid block around the bottom is a good idea. You don't want it in the dirt.
 
 

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