We’ve All Been Forgetting to Clean This Part of the Deck—and It Could Cost You

• Cleaning your deck every spring, including the underside, is important to helping it last longer.
• If you don’t clean the underside, mold and mildew can build up and compromise the deck’s integrity.
• You can clean the underside of the deck with a DIY cleaning solution, or a pressure washer.

Cleaning your deck is essential to keep it in good condition, but if you neglect the underside, you may find the deck begins to weaken and may become rotted out from below. 

Cleaning the underside of the deck, as well as the gaps between the planks, isn’t difficult and doesn’t need to be time consuming. Ideally, you should clean the underside of the deck at the same time as cleaning the top of the deck, to ensure the entire structure is clean and free of mold, mildew, and other harmful debris.

Find out what you should clean the underside of the deck, what happens if you don’t, and how to do it.

Why You Need to Clean the Underside of the Deck

Typically, you will clean the deck more frequently than any other type of maintenance, so at least once a year you should scrub, rinse, apply cleaner, and clean both the top and underside of the deck.

“Cleaning the underside of your deck is necessary to prevent issues and to make the most of your outdoor living space,” says Dave Kile, owner of IBP, LLC. “Depending on the type of deck you have—wood or composite—you can DIY your own deck cleaning solution.”

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What Happens If You Don’t Clean the Underside of the Deck?

Generally, the deck should be thoroughly cleaned at least once a year, though it’s also a good idea to sweep the deck every so often to get rid of dust, leaves, or other debris that may have fallen onto it.

If you only clean the top of the deck and forget to clean the underside, it can lead to a variety of problems, that can weaken the structure and make the deck less appealing for regular use.

“Over time, mold, mildew, algae, grime, and other elements create buildup under your deck and on your deck boards,” Kile says. “If left uncleaned, this buildup can trap moisture, leading to wood rot, structural damage, and even pest infestations.”

How to Clean the Underside of the Deck

Clear the Area

Before you start cleaning the deck, remove any items you may have sitting on the deck or stored underneath the deck.

Similarly, if there are any nearby plants that you want to protect, cover them with a plastic drop cloth.

Remove Loose Dirt and Debris With a Broom or Blower

Get rid of any loose debris, like dust, dirt, leaves, or even cobwebs by using a broom or a leaf blower.

“While doing this, you should look for loose fasteners, damaged wood, or other potential items that need repair and take care of those first,” Kile says.

Rinse the Underside of the Deck and Mix Cleaning Solution

Spraying the underside of the deck with the garden hose will help to remove any remaining debris that may have been missed with the broom or blower. This step also helps to prepare the surface for the cleaning solution.

“Use two gallons of warm water, two cups of powdered oxygen bleach, and a quarter cup of dish soap (ammonia-free),” Kile says, specific to wood decking.

For composite decking, he recommends one cup of white vinegar, a gallon of warm water, and one quarter cup of baking soda.

Apply the Cleaning Solution

Once the underside of the deck is rinsed and the cleaning solution is ready, you can start to apply the solution to the underside of the deck. Though it may be a bit more time-consuming, the best results are typically achieved by using a scrub brush, soft sponge, or a light sprayer to apply the cleaning solution.

“If you’re using a homemade cleaning solution, avoid applying it with a power washer or pressure washer since undissolved granules can cause a clog,” Kile says. “As you apply the solution, use a generous amount and brush it into a lather.”

Scrub Stubborn Stuck-On Grime

After applying the cleaning solution, you may notice a few areas that seem especially dirty or that may have bits of stuck-on grime. Using more cleaning solution and scrubbing these areas with a stiff-bristled brush can help to remove any stubborn dirt, mild staining, and grime.

Clean the Underside of the Deck

Give the cleaning solution about 15 minutes to work before you bring out the garden hose again. Use a spray nozzle to help direct the flow and increase the pressure of the water.

When you are ready, spray the underside of the deck to clean away the dirt and grime that has built up over the year. If you find that any spots that still require additional attention, repeat the previous step until you are satisfied with the results.

Replace Items After the Deck Dries

The deck will be dripping wet immediately after cleaning it, so you will need to wait until both the top and underside of the deck are fully dry before you can replace any items you removed.

This will usually take a few hours, depending on the temperature, humidity, and current weather conditions. Once the deck is dry, move these items back into their original place and put away any cleaning tools or products.