It's never pleasant to have a foul smell emanating from under your deck. Smells can come from any number of sources, from mold growth to an animal inhabiting the under-deck space. Most smell problems can be fixed with a little cleaning and perhaps some hard work. If the problem seems too difficult to manage on your own, consider calling a backyard cleaning expert to handle the task.
Identify the Smell
Identify the source of the smell coming from under your deck. Sometimes this is obvious, as in the case of skunk odor or other animal scent. But other times the smell may be less easy to pin down. Possible smell sources include decaying organic matter like leaves and branches, discarded food that has fallen under the deck, algae growth and stagnant water. Any of these could lead to mold or fungal growth underneath your deck. If you can access the underside of your deck, take a look with a flashlight and see if you can locate the source of the smell.
Clean Under the Deck
Remove all debris from under the deck. If it is possible for you to get under your deck, remove everything that is beneath it. Use a rake or a shovel to scoop the material into plastic garbage bags. If you cannot access the underside of your deck, you may need to remove a small portion of the flooring in order to clean out all the debris.
Once you have removed all debris from under your deck, prepare a solution of one part chlorine bleach to four parts water. Add 1 tsp. of TSP (trisodium phosphate), sold at home improvement stores, to make the solution produce suds. Scrub this solution over the entire underside of your deck to take care of any mold or other growth.
Wash the entire top surface of your deck and the underside surface of the deck as well. Purchase wood and deck cleaner from a local home maintenance store that is appropriate for the type of deck you have. You can use the bleach solution on decks made out of stone or some artificial materials. Mop the entire surface of the deck and let it dry. Use a sponge to scrub the underside of the deck completely. This should take care of any mold or algae growth.
Seal Off the Deck
If an animal is living under your deck, you may need to purchase an animal trap or contact a local animal control specialist to remove it and any offspring. Once the animal is gone, seal off the entrance points that the animal may have been using to get under your deck. You don't want it or any other creature to return. Repair any under-deck access points with boards, fill animal holes with dirt and cover them with bricks. Be careful to do this only after the animal has been removed; if you seal the animal in it may die beneath your deck and cause a worse smell problem.