Q: Last June we installed a new redwood deck. After waiting two weeks, my husband treated it with Cabot Australian tree oil. We now have areas that are almost black and others as pristine as the day the deck was installed. I have wiped off the black areas and have found the rag to be covered in a black residue. An apricot tree hangs over part of the deck but the color between the tree’s droppings and the color on the deck is inconsistent. What is our problem and how can it be solved?

A: You’ve posed the first of what are usually many of our annual deck maintenance questions. Without question there’s a fungus among us. You have a mildew problem.

The clues are that the apricot tree shades the deck and the black residue covers the rag when you wipe the discolored area.

The deck is protected from sunlight by the shade of the tree. Especially in winter, areas getting little sun are prone to be damp, making them perfect breeding grounds for mildew.

Kevin had a similar problem in Alameda, Calif., with a deck that was on the north side of his house. It got some sun in the summer, but in the wet winter months the black slime came out. At one point he even started to grow a nice crop of moss.

Fortunately, the solution is simple. Kill the mildew with a mildewcide, pressure-wash the deck and reseal it.

By far, the best way to clean a wooden deck is to blow the slime and moss away with a pressure washer. Pressure washers may be purchased at all the large home centers. Costs range from a couple of hundred dollars to $1,500 and up. For general household use, we’d recommend that you use a pressure washer capable of generating at least 1,500 pounds per square inch (psi) of pressure. Pressure washers are also available to rent at your local rental center. It should cost around $50 for a four-hour rental.

When using a pressure washer, be sure keep the wand about 12 inches above the surface of the wood and keep it moving. If you don’t, it’s very easy to blast softer wood away and leave a rippled effect on the deck.

Commercial deck cleaning solutions designed for use with pressure washers are sold where pressure washers are sold or rented. These solutions contain a mildewcide, as well as other cleaning agents.

Another simple solution is to use a 50/50 mixture of bleach and water to kill the fungus. Any store brand will do. Test a small out of the way part of the deck by applying the solution. The mildew should disappear without changing the color of the deck. Rinse the solution off with a garden hose. If the result is acceptable, repeat the process on the remainder of the mildew. Then pressure-wash the deck to remove accumulated dirt.

It will probably be necessary to reseal the deck. Be warned here: As long as your tree shades your deck this will probably be an annual maintenance project. From our point of view though, it’s not too much work for a shaded place on a warm summer’s day.

Removing moss from bricks

Q: This year for some reason, we have a lot of moss on our brick stairs and landing. I have searched for something to eradicate the moss. Labels on the packages say: “Could be harmful to brick!” Any ideas on what to use on this moss? Or do I just wait until everything dries out and brush it off?

A: We suggest you rent a pressure-washer and blow the moss off the brick. We don’t think you’ll need any cleaning solution, just the high-pressure stream of water from the wand.

Start at an inconspicuous place so you get the hang of the machine. Hold the wand of the pressure washer a minimum of 10-12 inches away from the surface of the patio to prevent damage to the mortar joints. Clean a small area at a lower pressure and a wide fan spray pattern. If you are not satisfied with the results, gradually increase the pressure and/or decrease the fan spray pattern until the surface cleans easily.

Do not use any more pressure or any narrower fan pattern than is required to do the job. Be careful of the mortar joints. They are old and can be easily dislodged by a high-pressure stream of water.

After you do the initial cleaning, you’ll be able to get by without pressure-washing for maintenance.

Purchase a nozzle for you garden hose that will produce a narrow stream of water.

Depending on the water pressure at your home, you’ll be able to clean up the stairs on a regular basis without going to the expense of either buying or renting a pressure washer.


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