You can easily prevent a rotting floor
Having your shed off the ground so air can flow freely underneath your shed is an important part of shed construction and you should think about updating your foundation if it wasn’t built like this because it’s the only way to keep your shed floor dry.
Hopefully your backyard storage shed is built on 4×4 runners and at least 4 inches off the ground. And you may also have patio blocks or another form of foundation under the runners and this is fine too.
Air flow allows moisture from the ground and condensation that naturally occurs on the underside of your flooring to dry preventing any type of water problem associated with blocked air flow.
If you decide to plant a border of flowers or shrubs around your shed, be sure to leave openings on every side for air circulation.
If you insist on framing in the opening around the bottom of your shed (I strongly recommend you don’t), install vents on every side and the more the merrier is appropriate in this case.
If you’re in the planning stage of your shed, think about using a plywood called Dry-Ply on your floor. It’s water sealed and won’t shrink like pressure treated plywood does. If you’re hiring a shed company, ask them to use Dry-Ply.
One other important point about keeping your floor dry is making sure your roof isn’t leaking because one way or another, that water will probably make it to your floor.
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