This article Reprinted from Western Roofing magazine, March/April 2007, Volume 30, number 2

 

Moss Control

WSRCA Moss Control on Steep-Sloped Roofs Document

by the WSRCA Technical Committee and Jim Carlson, WSRCA technical advisor

 

 

The growth of moss, algae, and lichens on roofs is a problem that has been around for hundreds of years. These types of organic growth can be found on many roof surfaces, including four of the most popular steep-slope roof coverings: asphalt composition shingles, tile, wood shingles, and shakes. Not only are these simple forms of plant life an unsightly nuisance that detract from the appearance of a roof, they hinder drainage of rain and snow melt, which can contribute to lateral water migration and leaks. Build-up of moss can degrade a roof covering, and lead to leaks and even premature roof failure.

                  Moss produces spores that are released into the air, and once airborne they fallout (i.e., alight) on surrounding roof surfaces. The moss spores will begin to grow on the roof surface if moisture, oxygen, and a food source are present. Nearby trees and shrubbery, especially those that overhang the roof, can provide an organic food source, as well as shade that helps retain moisture on the roof surface. Older wood and asphalt shingle roofs that have lost natural oils and resins from exposure to the sun and weather tend to hold more moisture than their newer material counterparts.

                  A lot of people believe that moss only grows on the north side of a roof, but it may grow on any side of the roof, even on what is considered the sunny side, when the conditions are favorable. Many homeowners decide to just remove the build-up of moss growth, which is where additional trouble can begin. Addressing the visible symptom of moss growth, rather than the real problem, can lead to unnecessary expense and often shortens roof life.

                  There are many potential remedies for getting rid of moss and other organic growth on roofs, but the solution can often be found by carefully cleaning the roof and then altering the pH of rainwater runoff. Exposed, unfinished sheet metal strips applied horizontally across the roof change the pH of the rainwater runoff as it flows over the metal strips, thus inhibiting the growth of moss. Before the removal process is undertaken, careful thought and consideration, with the help of a quality-orientated roofing contractor, is important.

                  First of all, is the roof in good enough condition to withstand the removal process? Generally, if a wood shake or shingle, or asphalt shingle roof is over 15 years old, is embrittled and weather weakened, it may be better not to attempt to remove the moss unless well trained, knowledgeable roof technicians are retained. Foot traffic and the use of high-pressure water blast equipment on an embrittled roof can cause severe damage, create leaks, and hasten premature roof failure.

                  Therefore, even though many homeowners may consider moss removal to be a handyman or even a do-it-yourself project, this is one project that should be left to roof professionals. Moss covered roofs can be very slippery, especially when wet, and the sloped and sometimes uneven surfaces of a roof can be dangerous to walk and work on. An experienced, quality-orientated roofing contractor can assess the condition of the roof to ensure that the roof is in a condition that will allow moss removal in order to prolong the roof's service life. Even if the roofing contractor does not do moss removal, they most likely can put you in contact with a reputable company that does do that type of work, and then can return to install materials such as sheet metal strips that will help inhibit moss growth in the future.

                  Money invested, hiring a professional roofing contractor, will be well worth it in the long term. As should be the case with anyone that works on your home, request proof of insurance and a list of references from satisfied costumers with roofs similar to yours.

                  Methods used to remove moss growth on a roof include: careful brooming, and controlled whisking of the roof surface to dislodge and remove moss accumulations, then controlled low-pressure rinsing and/or washing of the surface to remove residual particulate. Washing and/or rinsing solutions can sometimes include various water-diluted compounds such as zinc sulfate or copper sulfate. Proper environmental precautions should be exercised when using these compounds. These compounds and others may be used to inhibit moss growth in some locals, but generally have a short period of effectiveness. Knowledgeable roof professionals that will exercise appropriate precautions are best suited to apply these compounds, as they can be very corrosive to metal flashings, gutters, and downspouts, and can kill surrounding plant life and stain landscaping. Cleaning chemicals should only be used where environmentally feasible and only where allowed by law. A less intrusive and longer lasting option of moss removal is to do the following:

•               Assess the condition of the roof. Have an experienced roof professional carefully inspect the roof to assess its condition, and identify any damaged locations. Deficiencies may include missing or damaged shingles or tile, curling or cupping of the shingles, cracking or splitting, granule loss on asphalt shingles, and inappropriate flashing details.

•               Perform the needed roof maintenance and flashing repair. Replace cracked, torn, or degraded roofing units, maintain valleys, check and repair all flashings, etc. as necessary.

•               Broom off loose moss and debris. It is best to have this work done during a period of dry weather when the moss has dried out, as the drier it is, the easier it is to remove. Using a stiff-bristled push broom, and starting at the top of the roof (e.g., at the ridge), the roof should be broomed directly down slope, toward the eave, never broom sideways or upward against the bottom of the shingles or tile. Stubborn accumulations of moss may be lightly scraped with a whisk broom, but care must always be exercised to avoid damaging the roof covering.

•               Low-pressure rinse and/or wash and rinse. If using this process, caution must be exercised when including this step in the moss removal process. This step must also be started at the top of the roof and worked down slope to the eave. Always work from above the shingles. Hold the tip of the washer wand at about a 20° to 40° angle above the plane of the roof to avoid direct pressure on the shingle or roofing surface. On asphalt and wood shingles, as well as shakes and tile, gently work to clean the residual moss and debris from the spaces between neighboring roofing units and the cut-outs in asphalt shingles. A low pressure should be utilized. Some people advocate using higher pressures, but higher pressures can be very harmful, especially to older roofs. Careful downward washing, using low pressure is best for roof preservation. Note: Rinsing/washing at excessive pressures may affect or void the roof covering warranty.

•               Re-inspect the roof surface. This step may seem redundant, but brooming, pressure washing, and walking on the roof can cause damage. Carefully check the edges of all shingles, shakes, or tiles, ensuring that they have not been damaged or broken. Valleys can also be a very vulnerable location, since roofing materials do not always conform to the underlying roof deck at angle changes, creating a void under potentially embrittled shingles or fatigued metal. Foot pressure can easily puncture the unsupported roofing and/or flashing materials, causing a roof leak. A check of the ceilings and/or underside of exposed roof sheathing inside the house during the initial rinsing is also a good idea to ensure that water is not entering the roof cavity or interior. If any evidence of water is found, stop rinsing and check the roof area above to make sure all damage has been located and repaired.

•               Install copper, galvanized steel or zinc strips. Installing sheet metal strips of copper, galvanized steel or zinc along or near the ridge of the roof has proven very effective in inhibiting future moss growth on the roof covering from 8’ - 15’ down the roof. For roofs with a longer truss or rafter length, an additional metal strip may be required across the halfway point, and at other locations depending upon roof size and individual roof area layout.

                  Allowing rainwater to flow over copper, galvanized steel, or zinc strips changes the pH of the runoff and inhibits growth of the moss. It is recommended that these strips be a minimum of 6” in width and of sufficient thickness (16 oz. to 20 oz. copper, or 26 gauge to 24 gauge zinc or heavy galvanized steel) to prevent wind flutter, blowup or blow-off by strong winds. It is advisable to have a turned-up hem at the top edge, to serve as a capillary break under the ridge cap or shingle tabs, and a turned under hem at the bottom exposed edge to stiffen the metal strips.

                  Note: Over time, zinc strips have proven quite effective in retaining its properties for inhibiting the growth of moss. As copper patinas, a protective "glaze" forms on the aging copper surface, somewhat reducing its ability to alter the pH of rainwater runoff. Galvanized steel also experiences a reduction in effectiveness as its galvanized zinc coating erodes away with age and weathering. Installation of these metal strips may require removal and replacement of the ridge cap, ridge vent, and some shingles, thus the reason it is so important to enlist the services of a professional roofing contractor.

                  If the rafter length is 15’ or less one continuous strip of metal at or along the ridge of the roof should be sufficient to control the moss. The ridge cap shingles, and/or ridge vent may be carefully lifted or removed, so that they can be reinstalled after the metal moss-control strips are in place. The 6” to 8” wide strip of metal should be positioned so that at least 3” of the metal will be covered by the ridge cap and achieve adequate weather coverage of the fasteners, with a portion of the remaining metal exposed to the weather once the ridge cap is reinstalled.

                  The lengths of metal should be laid end to end, leaving an approximately 1/8” to 1/4” gap between the strips to allow for expansion. The metal should be fastened at approximately 12” o.c. with low profile, round-head, ring-shank nails or screws made of compatible metal. The fasteners should be placed a sufficient distance below the top edge of the metal, to guard against blow off, but still provide a minimum 2” coverage of the fastener heads by the overlying shingles, shakes, or tile. Roofing nails or screw fasteners must be of sufficient length to penetrate into dimensional decking a minimum of 3/4”, and puncture through the underside of the plywood roof sheathing a minimum of 1/2”. Sufficient allowances should be made for roofing materials' thicknesses, especially at locations where additional thickness of roofing materials are encountered. Also, care should be taken not to penetrate through tongue-and-groove profile roof decks where the underside of the roof deck is the finished ceiling (e.g., cathedral ceiling).

                  Dissimilar metals should not come into contact with one another, so if copper strips are being installed, use copper nails/fasteners, and cover the heads of steel or galvanized shingle nails that will come in come in contact with the copper strips with a film of asphalt roof cement or a layer of asphalt-saturated roofing felt (e.g., No. 15 felt).

                  If an additional strip(s) of metal is required on a roof with rafter length of over 15’, installation of the moss-control metal may require removal and replacement of at least one course of roofing in lieu of the ridge cap, though installation of the metal strips will be basically the same. When replacing asphalt shingles, it is recommended to apply a dime-to-quarter size dab of asphalt roof cement or polyurethane sealant under each shingle tab to help prevent wind lift or blowup of the shingles that overlay the metal strip. The reinstalled shingles will be separated from the underlying shingle's self-sealing strip by the newly installed metal strips, so the asphalt roof cement will help as a hold down aid or intermittent seal(s).

                  Help keep the roof free of new or recurring moss growth by keeping the roof surface free of vegetation and debris, and by trimming overhanging trees and foliage back away from the roof surface.

 

In Conclusion

                  No matter what your personal beliefs or philosophies are, it pays to be conservative when it comes to dealing with the roof on your home. Replacing the roof can be very costly, as can repairing damage, or replacing items below a leaking roof. So if moss is a problem on the roof of your home, contact a roof professional to assess the moss problem, and just as important, assess the condition of your roof. Then let the experienced professionals do the work. This is not a do-it-yourself project. •••