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Why is Ventilation Under Your Deck So Important and How to Achieve It

ventilation under deckingMany homeowners often ask us about the amount of space that is necessary under their deck.  Ask any expert in decking, and they will surely recommend you to have a minimum of 18 inches of open-air below the joists and that anything lower than that can prove to be risky.

The Need for Deck Ventilation

Why is deck ventilation important, anyway?

A lot of people are not aware that just like them, wood also breathes. Wood can expel and absorb moisture. In fact, wood can also move! But this movement tends to be minimal within a controlled environment, such as a home’s interior. Cabinets and furniture can last for a very long time with no gap spacing at all.

However, things are different outdoors where wood can get exposed to the harsh onslaught of different seasons and weather elements. It means that natural hardwood is going to contract and expand as the sun dries it up, and rain soaks it wet.

This is why proper deck ventilation becomes essential. There should be a good flow of air across all the four sides so that the wood can move evenly. If there is a restriction of airflow even on just one side, the board or boards will contract or expand unevenly. This will then lead to twisting, warping, and buckling.

How to Ensure Proper Deck Ventilation

As mentioned earlier, it is recommended to have a minimum open space of 18 inches under the joists as it will allow the air to pass unaltered and unhindered under the deck.

If the deck is attached too closely to soil or concrete pads, their temperature is going to alter the temperature of air that passes under the boards. This can then cause a difference in temperature between the bottom and top portions of the deck.

Proper flow of air can also ensure that the bottom of the boards stay dry. Without this, the top is going to be dryer compared to the bottom, and this can then lead to cupping.

It is never recommended to take the risk of building your deck below 18 inches. But, if you want to simple advice lower, there is simple advice to keep in mind. Make sure you use shorter, thicker, and narrower material. 5/4×4 board has lesser risks of cupping and better dimensional stability than 1×6 board.

You can also oil both sides before installation, as this can help mitigate warping or cupping by adding some layer of protection from the elements. You also need to use an epoxy coating to seal any concrete below your deck before you begin building it.

Before you proceed with your deck project, don’t forget to give the boards a substantial amount of proper breathing room. It will help guarantee a longer lifespan and better performance of your deck and make it withstand everything Mother Nature may throw its way.  Check this page for decking accessories.

14 thoughts on “Why is Ventilation Under Your Deck So Important and How to Achieve It”

      • It’s certainly not as as important with composites since most won’t absorb moisture. The big worry would be the pressure treated sleepers(nailers)rotting quickly if no ventilation and with poor drainage. Some composites don’t warranty decks over sleepers so look into the brand you plan on using.

  1. Does underdecking impact air flow? I just added under decking and am now worried that it will rot out the bottom of my deck?

    Any advice?

    • If moisture gets trapped between the deck and underdeck you will have problems. Cutting in some vents in the underdeck might help if you think you will have trapped moisture.

    • If there was any moisture in the rails then he was probably right to leave the bottom unpainted. Painting wood with moisture inside will lead to peeling, bleeding or the wood rotting from the inside. You should be fine to paint the underneath in the summer when you are certain the wood has dried internally. Semi solid stains are better than paint as they breath and let moisture transfer.

  2. This is a great article and you are banged on with your points about ventilation importance how to achieve. Be clear about what you really need and want to accomplish at this point- you are right with this point. When you look for same type of great content, like here then check out this https://roofquip.co.nz/ also.

  3. I enclosed my deck and made it into a room and carpeted the floor, underneath it’s about 3 foot crawl space and I insulated underneath. I just recently applied some 10 mil plastic to keep bugs out and to give insulation some protection was that a good idea ?

  4. We’ve just had some wooden decking boards laid today just above soil level, they have been hosed down and tomorrow my partner is talking about covering the whole thing with tarpaulin for winter, will be 6 months under cover. We have a difference of opinion, I think the natural wood being wet also won’t be able to breath and have visions of the decking being a complete mess when the tarpaulin is lifted 6 months from now. My partner is of the opinion there isn’t a problem. What’s your expert opinion?
    Thank in advance

    • I would be afraid of covering a deck for the winter. Personally, i have never heard of folks covering their decks in the winter and i’ve seen plenty in Colorado. If your deck is made with a good durable decking product, then it should hold up well without covering.

  5. Outdoor Deckwood in Jaipur: Decking/Deckwood can be built to any design and specification which makes it ideal for gardens and areas near the water. Decking helps in creating a perfect space for outdoor dining and relaxing.

  6. Black bears have pulled off the heavy duty
    outside deck vents
    (twice now and reinforced the last time).
    The vents were on opposite ends of the
    porch . 4 sides of vents mortared into the brick & with extra long screws.
    I am thinking to brick up the vent spaces & re- create vents on each end of the porch floor about same area as before.
    The porch deck is about 5 x 20 .
    The under the deck surface is dirt floor.
    the space under the deck to dirt floor is about 3 feet crawl space.

  7. I just installed composite decking on my upper deck and i have an uncover system under it and i have a lower deck and the under cover system keep the rain off and i enjoy setting there. Will my composite decking have enough air flow since i have the under cover system. The spacing between board in 1/8 in.


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