Note: This is cleaning/treatment/restoration only. We do not offer roof repair services.
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Proper Care and Maintenance of Wood Shingle and Shake Roofs

How To Deal with Green Moss on Cedar Shake roof?


I get e-mails and phone calls weekly about how someone can clean their cedar roof throughout the U.S. and Canada. Most requests are to buy my solution. I do not sell it.

Cedar Roof Overview

The following is an overview for all you cedar roof homeowners. If you have done your research or are about to, you will find the following will hold true as a common theme for cedar roof care.


Wood shingles and shakes can provide superior performance in areas that experience high winds or damage from hail. Although they do have certain limitations (susceptibility to rotting, splitting, and warping), proper maintenance and care can minimize these limitations. See also:

Most wood shingle and shake roofs are made from western red cedar (Thuja plicata) because of their exceptional properties, including defect-free straight grain, dimensional stability, low weight (low density), impenetrability to fluids, and probably most important – decay resistance, derived from natural substances found within the wood. In this discussion, when I mention shingles and shakes, western red cedar is the species I mean.

Other wood species, notably redwood (Sequoia sempervirens) and cypress (Taxodium distichum), have similar properties and are sometimes used; but decreasing supplies significantly restrict production and distribution.

Cedar Shakes Roof Care and Maintenance

When wood is exposed to the effects of sunlight and precipitation, its surface begins to change. This change is partly physical and partly chemical, and it’s cumulatively referred to as weathering.

The first noticeable change is the color. The initial red-brown color tends to fade, and a graying process begins. This change to a silvery gray is the result of ultraviolet radiation from the sun stripping the surface layer (less than 0.01 inch deep) of certain cell-wall materials.

This first change in color is rather rapid, occurring within the first year of exposure (under more severe conditions, within several months). Gradually, the silver gray will change to a darker, more graphite gray, then to a brown/black color indicating the colonization of the surface by microfungi that complete the initial phase of the weathering process. White and Brown Rot Fungi.

By nature, wood rapidly attracts water to its surface. When this happens, the wood swells. As it dries and the moisture content decreases, it shrinks. This repeated wet to dry cycling causes the development of compression and tension stresses; these, in turn, cause microscopic cracks to develop.

Over time, these cracks grow larger and larger, caused by additional wettings or by freezing and thawing. The cracks also trap water and allow wood-rotting organisms to penetrate deeper into the wood. As the water penetrates deeper, the wood becomes harder to dry. With increased moisture, the wood-rotting organisms prolong their destructive activity and damage more wood material than would occur during normal weathering.

As the roof continues to age, the surface and sharp edges from manufacturing are eroded by abrasive particles carried by wind and water and by the action of sunlight. Wood-destroying organisms also continue to slowly degrade the surface and the thicker butt edge. The roof loses its new appearance. Debris from trees accumulates in the valleys and between the shingles.

After each rain, some areas stay wetter longer. Soon, other plants are beginning to grow, primarily lichens and mosses. More debris collects, and the roof appears to be natural and rustic. There are splits developing, and in certain areas cupping and curling begin to become more apparent. Then the leaks begin. Sometimes, this takes 30 years sometimes, much less.

Good housekeeping for a wood roof requires removing all leaf litter, pine needles, and debris that accumulate between the shingles and shakes and in the valleys of the actual roof structure. It’s best to do this before the wet season begins.

The debris that accumulates has two deteriorating effects on the roof. First and foremost, debris tends to retard the shedding of water. Secondly and more destructively debris retains moisture that allows wood-destroying fungi to grow.

Wood-rotting fungi require water as part of the life cycle. If you can reduce the amount of water so that the minimum level is never reached, the organisms can’t grow.

To promote drying, it’s very important to remove overhanging branches that excessively shade sections and retard the drying of the roof surface. Never allow tree branches to touch and rub against the surface of the roof. The mechanical action of this rubbing can literally wear sweeping grooves into the surface of the shingles or can loosen the fasteners that hold them.

Finally, in very woody areas, branches may retard air flow so that surfaces dry very slowly. In all cases, you’ll need to make sound judgments about pruning, trimming, or removing problem-causing trees. Excessive growth of lichens and mosses is a sure indicator that your roof is not drying properly and that enough moisture is available to support wood-rotting organisms.

Removing the debris accumulation is relatively simple. In most cases, anyone who’s willing to climb on the roof can wash off most of the material with a garden hose.

It’s easy to remove litter from around chimneys and in valleys; you also need to clean all the keyways (those areas between the individual shingles and shakes).

Professional services sometimes use high-pressure washers (available through rental firms). These are not recommended because cedar is a soft, low-density wood. Excessive or imprudent use of high-pressure systems can detach shingles or erode many years of wear in moments even though the roof will look like new.

Chemical Treatments For Cedar Roofs

Bruce Cleaning

There are several types of treatment used in the cedar shake roof cleaning industry.

  1. You can easily prepare cleaning solutions to help remove dirt, mild stains, and light growths of molds and mildew.
  2. For more persistent stains, bleaching formulations will be recommended.
  3. In some cases of excessive moss growth, you can apply chemical solutions to kill the mosses and any fungi that may be present.
  4. You may sometimes apply preservative chemical solutions that will retard the growth of molds, fungi, and mosses.
  5. Additives such as pigments and ultraviolet absorbers can slow deterioration caused by sunlight.

Considered as a whole, the chemicals listed below are chosen for their fungicidal or herbicidal effects. Note that these chemicals will perform differently under various environmental conditions. As a result, the frequency of application required for long-term protection depends on the chemical you use, the amount of precipitation, average temperatures, and the roof exposure conditions.

The chemicals recommended are available in products found in hardware stores, building supply centers, or lumber yards.

Cleaning Solutions For Cedar Shake Roofs

Because I have developed my own cleaning solution, which not only includes the proper cleaning ingredients i.e. algaecide, mildewcide, and fungicide, etc. but also has preservative ingredients within the solution and for business purposes I will not detail in this overview.

For the purpose of this overview, I will discuss what is the commonly recommended cleaning solution based upon research. You’ve removed the debris from your roof, and you can see light staining or irregular discoloration. Cleaning the roof with a mild detergent solution may provide an improvement.

Bruce Cleaning

Bleaching Formulations

For more persistent stains, you can increase the sodium hypochlorite (chlorine bleach) concentration, even to the point of using it full strength from the bottle for spot applications.

Formulate stronger solutions by using granular chlorine (calcium hypochlorite), mixed at a rate of 2 oz. per gal water. This chemical is used for algae control in swimming pools and is available through pool supply houses.

For safety, wear eye protection, rubber gloves, and aprons. It’s best to apply these bleaches with brushes for small spots; spray large areas (use plastic or stainless steel sprayers).

Don’t let bleaching solutions stay on wood more than hour before rinsing. These solutions provide more uniform results when you use them in lower concentrations; repeat the application, rather than use a single high concentration when you start.

It’s best to use these solutions out of direct sunlight. Apply the chemical only to the stain. Avoid drips, runs, and splashes of bleach or excessive bleaching on the roof surface these can mar the roof’s appearance. It’s not necessary to scrub bleached areas, but rinse the wood thoroughly.

Preservative Treatments For Cedar Roofs

Chemical solutions may also be applied to red cedar shingles and shakes that will retard the growth of molds, mosses, and (more importantly) wood-rotting fungi. These chemicals are most commonly referred to as wood preservatives, and they’re an effective means of extending the life of your roof.

When you apply preservative chemicals to your roof, the roof condition at the time of treatment will determine the actual amount of increased longevity. If a roof has aged for many years, the treatment won’t be as effective simply because the years of exposure will have taken their toll. In all instances, except when the roof has badly deteriorated, you can expect preservative chemicals to extend the life of your roof.

When you apply preservative chemicals to shakes and shingles, you need to remember several important considerations.

First, the coating should be as uniform as possible. Avoid drips and runs. It’s better to apply several light coats and obtain good absorption than to try and force it in all at once. The important point is that you’re trying to reach a certain level of preservative retention. If the retention level is too low, your treatment will be ineffective.

Second, since much of the decay occurs in the butt region of the shingle or shake, it’s a good idea to be conscientious when you apply the solutions, to cover the exposed butts as well as the exposed shingle surface.

The three methods available to provide proper application are brushing, using a thick-napped roller, or spraying. Because of the irregular surface, you’ll probably get a more uniform coating with a multi-gallon, pump-type sprayer set to produce a coarse spray.

Spraying also requires a calm day (wind less than 4 mph) to prevent overspray from drifting over plants or into your neighbor’s yard. Brushes are also effective because they allow for a thorough treatment of the keyways and ends.

The chemicals listed below will most likely be part of formulations in over-the-counter products or part of the formulations applied by roof servicing contractors. These chemicals are also available in concentrated form through specially licensed supply companies. This is because of the toxicity, possible problems in preparation, and the limited demand from home owners. In every case, regardless of the concentration, be extremely careful when you apply, handle, expose, and store it. When you purchase over-the-counter products for long-term moss control, be sure that one of the wood-preserving chemicals listed below is on the label.

Currently, the preservatives in the following sections are available as either aqueous solutions or oiled-based products.

Copper Naphthenate is an effective preservative well-suited for applying to roofs. Solutions should consist of at least 1 percent copper (2 percent is better); apply them at a rate of 1 gal/100 sq. ft. for shakes, and 1 gal/150 sq. ft. for shingles. Literature indicates that using this concentration will provide up to 5 years of protection in the Pacific Northwest. Note that copper naphthenate has a greenish color that may prove unsightly. To overcome this, choose a solution that’s pigmented to simulate the red-brown cedar tones or use another type of preservative.

Zinc Naphthenate While copper is much more effective than zinc in retarding the growth of biological organisms, people use zinc solutions to retain the more silver or gray appearance on the roof’s surface. It’s recommended that you use either more frequent applications or higher concentrations of active metal content, for example, 2- to 3-percent solutions for 3 years of protection (or 4 percent for 5 years) per 100 to 150 sq. ft. as above.

MTC and TCMTB MTC are (methyl bis [thiocyanate]). TCMTB is (2-[thio cyanomethyl thio] benzo-thiazole). These generally are used in combination as an effective roof treatment. They can be used in clear coatings or in pigmented solutions, and either as waterborne solutions or with solvent/oil mixtures.

Copper 8 Quinolinolate is a safer alternative to those listed above, but some tests indicate its effectiveness is short-lived not much more than 12 to 18 months.

Polyphase Ployphase is 3-iodo 2-propynyl butyl carbonate. In solutions where concentration levels are greater than 0.58 to 2 percent active, you might get 2 years of protection. The durability and long-term performance of this product are still unknown, but it’s limited because of light and water sensitivity.

TBTO TBTO is tri-butyl tin oxide. Some literature indicates its use is questionable in treating existing roofs because of mold and mildew resistance and chemical degradation. It’s often used together with trichloromethyl thio phthalimide (Folpet), which has weak fungicidal properties.

Zinc and Copper Strips For those who prefer not to use toxic chemicals, there appear to be safe and relatively inexpensive treatments to control moss. Galvanized metals have a surface coating of zinc. For example, if you fasten a 2-inch-wide strip to either side of the ridges and run it the entire length of the roof, rain will leach zinc from the metal’s surface and wash it down the roof.

As noted earlier, zinc can effectively kill or retard the growth of mosses and fungi. Its effect has long been noted in personal correspondence between wood scientists; this effect is prominent below chimney flashings and vents.

However, I couldn’t find any scientific study to verify how long it will remain effective or how much metal is required. Its effect below chimney flashings can be seen for at least 15 feet and appears to last for the life of the flashings.

If your roof is so large that the material at the ridge doesn’t retard moss growth at the bottom, place more strips across the surface. After a year, you should be able to judge how well the strips are controlling the growth of moss; add more if they’re needed.

The greater effectiveness of copper over zinc has been noted with copper and zinc naphthenate. So copper strips are also likely to control moss, and possibly retard wood-decaying organisms, because of the greater effectiveness of the copper. This fact has been observed below copper flashings.

In one case, on a roof considered a favorable site for moss growth, two strands of 10-12 gauge copper wire had been placed, one near the ridge and one some 15 ft. below (at the halfway point on the slope). No moss was observed growing. As with the zinc strips, I couldn’t find any supporting scientific studies to confirm these methods but those willing to try may obtain favorable results.

Cedar Shake Roof Oil Treatments

Oils are sometimes advertised to replenish the natural oils in the wood. In red cedar, these oils account for less than 3% of the total mass. While they do contribute to dimensional stability and decay resistance, applying replenishing oils to weathered shingles may be of questionable value, especially if they contain no preservative or water-repellent chemicals.

You might get some temporary benefit, however, with regard to controlling cupping and warping. But some information indicates that applying oils may have some limiting factors. The main limitation is that unless you include an effective mildewcide or preservative, oil like linseed may provide an additional food source for mold, mildew, and fungi.

Furthermore, petroleum-based products are likely to be oxidized (broken down) by sunlight, to have only a short-lived effect, and to increase the flammability of your roof.

Stain, Varnish or Seal for Cedar Shake Roofs

Whatever you do, do NOT stain, varnish or seal your cedar roof. Cedar needs to breathe.

New Growth Cedar vs. Old Growth Cedar

There has been debate about the newer cedar not being as good as the older cedar?

The way I see it. Cedar is not man made, nor a hybrid, but a natural wood.

Any material in life will be as good as you care for it. Your cedar roof is no different.

Let me ask? Why do you go to the dentist to clean your teeth? Because they are organic they need to be cared for to reach their natural lifespan. A cedar roof is no different; it is organic and needs to be cared for to reach its natural lifespan!

NOTE: The materials or information cited above came from research from many different sources, in part, and from my personal experience. This writing is for informational and educational purposes.

Though any person can DIY, I would recommend that they hire a professional.

Bruce Sullivan is a cedar roof cleaner from Des Moines, Iowa area. He has also offered his service throuhgout the Greater Midwest.

Credit goes to research done by:

S.S. Niemiec, former Research Assistant, College of Forestry, OSU

T.D. Brown, Professor emeritus, Wood Science, OSU

Cedar Shake Roof Cleaning in Des Moines Iowa
Cedar Shake Roof before & after cleaning

Meet Bruce

50 Years of Experience &
Over 2,000 Shake Roofs Cleaned

Bruce Sullivan has over 50 years of experience in maintaining, cleaning, and restoring cedar shake roofs. Making him one of the top authorities on cedar shake roofs in the US.

Watch the Cedar Shakes Cleaning Process in Action in this video

Play Video about Roof Cleaning How to Remove Moss, Lichen, Fungi from a Cedar Roof Sullivan Roof


What can I say? Bruce and Keith provided an exceptional service, and they are really nice guys too. Found out at the last minute they were in Minneapolis, and they fit me in for a cedar shake roof cleaning the next evening after their regular job was done. Even cleaned up my gutters, small fence, and small deck at no charge. The place looks outstanding! Highly recommended.

Mike Richards

I highly recommend this business. Due to the work performed we are going to get another 10-14 years out of our cedar shake roof. We were told by some roofing companies that the roof should be replaced in 2 years. I lucked out and found Sullivan Roof Cleaning while researching companies that perform this service, and was pleased to learn that the company does work outside of Iowa. If you need your cedar shakes cleaned, go with Sullivan Roof Cleaning.

Eric Oelrich, 
St Cloud, MN

Bruce did a great job cleaning the whole exterior of my house. The roof has new life! It looks practically brand new. All the mold and moss on the roof, on the gutters, under the eaves, completely gone. He exceeded all our expectations. When he tells you what he can do for your house, you can believe and trust him. Great guy, great service.

Stuart Malone
Kansas City, MO


Cedar shakes are organic. Though a resilient material. Being organic they need to be maintained. Over time, like any organic material that are subjected to weather elements, they reach a stage of deterioration. Like UV exposure, moisture and seasonal changes. Which results in moss, mold, mildew, lichen and brown rot fungi. These infestations need to be removed to further preserve the integrity of the cedar shake.
The proper way to clean a cedar shake roof is from a ladder. By spraying a special formulated solution onto the shakes and then rinsed with a garden hose pressure. Best done by a professional cedar shake roof cleaner. Not DIY recommended.
No. Pressure washer will require walking on the shakes, which will harm them, and even at its lowest setting will also damage the shakes.
On average, cedar shakes should be cleaned every 10 ten years. Subjective to the grade of the shakes and the amount of surrounding tree cover.
Avoiding walking on a cedar shake roof, other than for repairs.
Never seal a cedar shake roof. Cedar shakes need to breathe. They expand and contract in rain conditions. Seal will shorten the lifespan, to the point of premature replacement.
Staining has no real preserve value and can also harm the integrity of shakes, as stated, they need to be able to breathe. Also, if the shakes are not properly cleaned staining can seal the deteriorating infestation within the shakes.
Depending on the grade of the shakes and installation, and if maintained properly, 40 to 60 years.


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