How to Get Rid of Odors Under an Outdoor Deck

Finding the Source

1. Determine 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, urine 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: If they have fallen between the cracks, ashes or butts 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 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.

2. Remove the Cause

   Once you've determined the cause of the odor, 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.

Removing the Odor

Treatment options depend on the source of the odor. Follow the directions below for the cause of the odor under your deck.

1. For Dead Animal Odor

   If you find a dead animal below your deck boards or between the joists, it can produce a very unpleasant odor. Remove the animal first, then use one of the following methods to get rid of dead the animal smell outside:

   • Vinegar: Undiluted vinegar can get rid of the smell of dead animals when poured on the area. This may take multiple gallons of cleaning vinegar depending on the size of the animal. Saturate the soil, then repeat if the smell persists after it dries.
   • Baking soda: Add a few tablespoons of baking soda to a spray bottle filled with water, then spray the area a few times each day. If you can't locate the dead animal, baking soda is the best option to remove the smell.
   • Ground coffee: Coffee grounds can absorb the smell of dead animals. Coat the soil underneath your deck with a few bags of fresh coffee, then if the smell continues, reapply more coffee until it resolves.
     

2. For Skunk Odor

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

   • 1 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 the odor dissipates.

3. For Dog Urine

   This smell can get quite pungent, especially if it builds up. Try these 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.

4. 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

5. For Mold and Mildew

   Usually, you will find mold and mildew in a cool, damp area on, not under, your deck. Chances are that 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 effective baking soda and sweeping method.