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.
Good article on air spaces. I have a deck that is 10′ off the ground. If I installed a water collection system so the space under the deck could be used as an all season/weather patio, would that reduce air flow to a point where there could be an issue?
It very well could. Not only will it limit air flow and ventilation but if anything gets caught in the collection system and there is a backup of water, that could create an issue in the product absorption of additional moisture.
Using best maintenance practices will help reduce this chance by making sure there is no objects that can cause a back up of water and maintaining the deck surface clean of debris. Having said this however, boards could still move.
Thank you for your question,
I am building a 28’x16′ deck that is 7′ off the ground and want to use the area underneath for an all weather patio. Can tongue and groove with silicon caulk be used to create impervious deck cover?
The deck will have slight sloped for drainage.
If you give us a call at 1-877-232-3915 one of our sales representatives will be more than happy to assist you with any installation questions you have on tongue and groove decking. Or feel free to send an email to email@example.com and we’ll get you in touch with someone.
Have only one inch minimal clearance between concrete pad and ipe boards. I will be placing it over a wood 2×1 that slope up to 2×2 so water under the wood can run out on one end. There will be spacing between boards and openings on the one end were water runs vor ventilation purposes. I will be treating concrete with a epoxy coating for waterproofing reasons. What do you think of this. Will it buckle the ipe boards? Do you recommend this material for this job? Please help!!!
Hi Tanya Oliver,
What you describe is something we don’t recommend doing with our decking products. You may have success if you epoxy both sides and fasten down the boards very well and every few inches, but it’s always uncertain because the wood won’t be getting adequate airflow and the epoxy is a product we don’t recommend either. For the kind of application you describe you can use our Deck Tiles. These square wooden tiles are perfect for covering up concrete pads, balconies, or any other flat area.
Thank you for your question Tanya,
We are using the IPE RAINSCREEN on our outdoor kitchen. The wood will be mounted vertically with only the furring strip between the IPE and the cement board. Do we need to seal or put oil on all 4 sides prior to installation? Will we have adequate ventilation?
Oiling all 4 sides prior to installation is always recommended, but in reality, the face should be sufficient.
I don’t have enough information to answer the ventilation question. If it is under shade, you should be fine. If it is exposed to the sun, you will want to make sure there is some opening for air flow at the top and bottom. In either exposed or shaded application, ensure your furring strip is ¾” in thickness and holding your Rainscreen ¾” off the cement board.
FYI, where we do not have installation instructions for this, it is typically recommended to install your horizontal furring strip at somewhat of an angle still to allow for any water that does get through to run off.
Hope this helps!
After some advice. We have had a deck done that’s comes out from the house, is right up to the retaining wall under the fencing on two sides and the other side is boxed in. The deck at its lowest point is 400mm and at the highest point 500mm. We have left 5mm gaps between the merbau timber that has been done in the 140mm thickness. After reading up on ventilation I am worried that 3 of the 4 sides are completely blocked in with no airflow what so ever and the fourth side only Gets the airflow between the 5mm gaps between the decking on the side of the deck. Will this cause problems? We’ve had the deck for 6months (through cold wet months) and we don’t have any problems yet. Any advice on how we could increase airflow if needed? Thanks your help is appreciated.
The problem is that we don’t specialize in Merbau. If you’d like to open more air flow, creating vents on the boxed sides will help out. However, you may want to consult with a structural engineer to ensure this is safe.
I’m building a ground level, floating, flush beam deck in my back yard. It’s on double 2×6 beams and 2×6 joists. My concern is that it is situated between the house and a long retaining wall with a concrete slab at one end and raised turf at the other. That makes it completely closed in (with a gap) on all 4 sides. There’s anywhere from 1” to 5” of clearance between the bottom of the joists and the ground.
I installed a massive French drain and drainage channels under ground level to remove all standing water. There’s also landscaping fabric and a 1” to 2” layer of drainage aggregate over that.
I was banking on the searing heat we have hear in Woodland Hills CA for most of the year to force out the moisture but… Given that any below deck vents would cut into support joists, I’m wondering if surface vents could be a solution (if I experience any future cupping)? What are your thoughts? Thanks in advance. Mark.
Any ventilation you add will help, as will following all the advice in this article (thick, narrow boards, pre-oiling all sides). However, I can’t make any guarantees when it comes to ground-level decks.
I am considering putting wood decking over a 10×40 feet concrete patio. The concrete pad is about 17 inches above ground on one end and 24- inches above ground on the other as the yard slightly slopes. About 27foot section is covered and the rest is open. At the most I can have 3.5-4 in vent space if I place the sleepers ( 2x4s ) on their side. Is that sufficient for airflow do you think? Also should I be concerned that in heavy rain the water could make its way into the enclosed portion from under the deck from the area not protected by a roof? Thank you for any advice you have.
That’s quite a bit less than the 18″ of vent space we usually recommend. On top of that, we aren’t fans of installing decking over concrete, since that adds additional moisture concerns. However, we also acknowledge that you have to work with what you’ve got.
So the first thing we’d suggest is to use deck tiles instead of deck boards. The tiles are built to handle poor ventilation, and can be installed directly over concrete, no sleepers required.
If tiles really aren’t your thing, and you’re heart’s set on traditional deck boards, then you should use thicker, narrower boards like 5/4×4. The extra thickness and narrowness will help the boards resist moisture-related warping and cupping.