Things You'll Need
Garden hose with nozzle
Liquid dish detergent
Medium-hard nylon brush
Cedar shake siding can improve the looks of most homes, but over time some cedar shake can become dark and old-looking. When this happens it is time to spruce up your siding and put the old vibrancy back into it. There are two primary reasons for cedar shake to become dull and old-looking: mildew, or what is called tannin. Removing these two stains and restoring the original light and bright look to your cedar shakes involves two very different processes. So the first thing you need to do is to determine the root cause of your problem.
Determine the Cause of Your Problem
Test a small part of your siding to determine exactly what type of staining problem you are dealing with--tannin or mildew. Start by cleaning a small area. Mix a commercial wood cleaner in a regular garden sprayer according to the manufacturer's directions and spray the cleaner onto a small area of your cedar shakes starting at the top and working your way down. Allow the cleaner to soak into the wood for 15 to 20 minutes.
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Gently scrub the test area with a medium-hard nylon bristle brush to work the wood cleaner in.
Wash the cleaner off using a garden hose with a spray nozzle. A power washer can be used, but do not spray on the highest power setting, as this could damage the shingles. The shingles in your test area are now clean and ready to be tested to determine the cause of their discoloration.
Test For Mildew
Mix three quarts of water with one quart of household bleach and 1/2 cup of ammonia-free dish washing liquid in the garden sprayer.
Spray the test section of the cedar siding with the bleach and water mixture and allow it to soak into the test area for 15 minutes. There is no need to scrub the area.
Rinse the bleach solution off with a garden hose. If the shakes appear clean then the problem is mildew and the water/bleach/liquid detergent mixture should be sprayed on all of the siding and allowed to sit for 15 minutes before being washed off.
Test for Tannin Staining
Fill your garden sprayer with a wood brightener, available at most hardware stores and home improvement centers, following the specific manufacturer's instructions. Spray the brightener on the test area of your siding. The wood brightener is designed to remove tannin stains from your shingles--tannin is a chemical naturally present in cedar which can migrate to the surface of the wood and cause staining.
Wear gloves and eye goggles and gently scrub the brightener into your cedar shingles using a medium hard nylon bristle brush.
Wash off the wood brightener after you have scrubbed it into your shingles using a garden hose and a power nozzle. If the wood is cleaner but still not as bright as you'd like, clean a second time with wood brightener.
If the bleach and water solution brighten your wood then the wood brightener will not be necessary as the staining is not caused by tannin.
Do not get the bleach and water mixture on your clothing as it will bleach out any colors.