How to Get Rid of Odors Under an Outdoor Deck

How to Tackle the Stench Under, On, or Around Your Deck

Wooden deck of family home.
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It's not uncommon for homeowners to one day find an extremely unpleasant, perhaps even nauseating odor, coming beneath a deck. Before going extreme and tearing up the decking or calling professionals, be reassured that you probably can identify and eliminate the odor yourself. All it takes is some investigation, research, and cleaning solutions you might already have on hand.

Tools and Supplies You Will Need

Not all these supplies will be required for every odor; the items you need will depend on the nature of the odor and its cause.

Determining the Cause of the Smell

The most difficult part of eliminating smells under or on a deck can be identifying the source. How you deal with the odor will depend on its cause. These are the most common causes for unpleasant odors around your deck:

  • An animal—dead or alive: Various creatures bravely visit yards and patios in search of food. These include squirrels, rabbits, raccoons, possums, skunks, woodchucks, moles, rats, mice, and chipmunks. Such creatures may leave excrement behind, and if even a tiny animal dies beneath your deck, the smell can be extremely powerful as the animal decays. These common smells caused by animals will be hidden out of plain sight.
  • Mold or mildew: These fungal infestations created by shade and damp conditions can create a significant odor. Often, signs of mold or mildew will be present above the deck, but the serious fungal buildup may also be below the deck on framing members.
  • Dog or cat urine, especially if it accumulates over time, can create a powerful odor. You will likely know if your pets are relieving themselves on or under your deck, as you will likely see them entering or exiting the space from time to time.
  • Pet poop: No matter how good you are at cleaning up after the animals, it happens from time to time. Here, too, you will likely know if your pets are engaged in this behavior on or under your deck.
  • Cigarette ashes or butts that have fallen between the cracks can create a telltale ashy, smoky smell. It often gets worse in humid weather.
  • Food that has fallen between boards: Think back to your last barbecue or when a household member was eating a chili dog—maybe something spilled and found its way under the deck. Rotting food—even a small amount—can create a very unpleasant odor.

Identifying the cause of a bad smell can involve some detective work, and it often requires some hands-and-knees crawling under the deck while wearing long sleeves and pants and carrying a flashlight. If you don't have access to the space under the deck—such as if it is a ground-level deck without sufficient space to get beneath it—you may need to remove deck boards to identify the cause of the smell.

Removing the Cause

Once you've determined the cause of the odor, the next step is to remove the offending material. For example, it's possible that a squirrel or rat entered the crawl space beneath the deck, and for some reason died while down there. Its lifeless body decomposed and you began smelling that unavoidable, horrid odor as time went by.

Once you find the decaying material—whether it is an animal corpse, pet excrement, or bits of picnic food that fell beneath the deck—you'll have to remove it. Hold your breath and put whatever material is causing the smell into a plastic bag and tie it shut. If you are squeamish about touching this decaying material, a garden trowel or a long-handled grabber tool is a good alternative.

Warning

Wear gloves if you're handling decaying material directly.

Treating Animal-Based Odors

Treatment options depend on the source of the odor.

For skunk odor: If a skunk sprayed under or on your deck, either use a non-toxic deodorizer or a homemade solution like this one: 

  • One quart of 3 percent hydrogen peroxide
  • 1/4 cup baking soda
  • 1 teaspoon of liquid dish soap 

Mix ingredients together, then dip a sponge or brush in the solution and scrub the deck. Rinse with water and the odor should be gone. If not, repeat until odor dissipates.

For dog urine: This smell can get quite pungent, especially if it builds up. A few solutions:

  • Baking soda: Sprinkle it over the areas, then sweep with a broom, making sure it fills in grooves of wood and in between deck boards. This also works on concrete and other areas where your dog might urinate. A little bit of baking soda won't harm nearby plants.
  • Vinegar mixed with water: Spray or dip a rag or brush in a bucket of the solution and scrub the deck. Reapply until the smell is gone. 
  • Apply a commercial odor neutralizer or eliminator: Note that these can get expensive if your dog is a repeat offender.

For cat urine or spray: Some substances that can remove this must pungent of pet odors include:

  • Over-the-counter pet odor eliminator or neutralizer
  • A vinegar solution (see above)
  • Coffee grounds
  • Cleaning with a power washer every week or so
  • Cayenne pepper
  • Baking soda, which can be swept with a broom into cracks and grooves of the deck to neutralize odors

Dealing With Mold and Mildew

Usually, you will find mold and mildew in a cool, damp area on, not under, your deck. Chances are, the deck has been a bit neglected and needs to be treated and sealed. You can either do this yourself or hire a professional.

If going the DIY route, clean the deck with a water and bleach solution to kill the offensive mold and mildew. After that, sand it and apply oil-based wood sealer (if the deck is surfaced with wood). This needs to be done only every five years or so. A hand-held power sander will make the job easier.

Practicing Routine Cleaning

If the odor is a mystery, not too strong, or if you just want to freshen up things, a general deck cleaning using a commercial deck-cleaning solution and power washer may eliminate the smell, and it also will keep things smelling fresh going forward. The solution you use can vary, depending on how strong you want to go. For weekly spot treatments, try a spray solution of vinegar and water or use the baking soda and sweeping method—it actually works.