A lot of people ask how much space they need under their deck, or if extra ground clearance is even necessary. At AdvantageLumber.com, we recommend at least 18 inches of open air under the joists. Anything less is risky. But why?
Most people don’t know that wood breathes. It absorbs and expels moisture; it moves! In a controlled environment like the interior of a home, this movement is minimal. Furniture and cabinets last a very long time without any gap spacing. Outdoors however, wood is exposed to the onslaught of all four seasons. This means that natural wood will expand and contract as it’s soaked with rain and dried up by the sun.
This is why proper ventilation is essential. In order for wood to be able to move evenly, it must have good airflow across all four sides. If airflow is restricted on one side, the board(s) will expand/contract unevenly, which is what causes buckling, warping, and twisting.
We always recommend at least 18 inches of open space beneath the joists because this allows air to pass unhindered and unaltered below the deck. If you attach a deck too close to concrete pads or soil, the temperature of these objects will change the temperature of the air passing underneath your boards, causing a temperature difference between the top and bottom of your deck. Proper airflow will also keep the bottom of your boards dry. Without it, the top will be dryer than the bottom, which will cause cupping.
Building a deck any lower than 18 inches is a risk we don’t recommend taking; however, if you insist on building lower, here’s some advice. Use thicker, narrower, and shorter material. A 5/4×4 board has more dimensional stability and less chance of cupping than a 1×6 board. Oiling both sides prior to installation will help mitigate cupping/warping by adding a small layer of protection against the elements. Also, any concrete under your deck should be sealed with some type of epoxy coating before you start building.
Before you take on your next deck project, always remember to give your boards a good amount of breathing room. This will ensure a better performance and a longer lifespan for your deck no matter what Mother Nature throws at it.