The Department of Energy (DOE) defines cool roofs as "roofs that are designed to maintain a lower roof temperature than traditional roofs while the sun is shining." Cool roofs often are touted as having two major benefits when compared with traditional roofs: they contribute less heat load to building interiors, making buildings less expensive to cool, and they help reduce urban heat islands.
But cool roof membranes become less reflective (and less cool) as they age, mostly as a result of climatic exposure. Sometimes, to restore a cool roof membrane's reflectivity, the membrane is cleaned. However, before you prepare to clean a cool roof membrane, there are a few things to consider.
What are cool roof membranes?
There are many forms of cool roof membranes, but the most common is a roof membrane with a high solar reflectance index (SRI). SRI is calculated from solar-reflectance and thermal-emittance values according to ASTM E1980, "Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces." The higher a roof system's SRI value, the cooler the roof system will be in the sun. The highest SRI value is 100 (white), and the lowest SRI value is 0 (black). As in-service roof surfaces become soiled, reflectivity is reduced and SRI values decrease.
Most new cool roof membranes have minimum SRI values of 82. This is known as an initial SRI value. Cool roof membrane products also typically have a minimum three-year-aged SRI value of at least 64. The 2012 International Energy Conservation Code® uses these values in Table C402.2.1.1, Minimum Roof Reflectance and Emittance Options, for low-slope roof systems with slopes less than 2:12 directly above cooled conditioned spaces in climate zones 1, 2 and 3 (southern U.S.). The U.S. Green Building Council's (USGBC's) voluntary LEED® program, Version 4 (LEED v4), also uses an initial SRI value of 82 and a three-year-aged SRI value of 64 as minimum values for "high-reflectance roofs" with slopes of 2:12 or less to qualify for its Heat Island Reduction Credit. The most commonly used cool roof membrane products are thermoplastic single-ply membranes (mostly TPO or PVC) and roof systems with polymer-modified bitumen cap sheets formulated to have surfaces with a minimum initial SRI value of 82 and a minimum three-year-aged SRI value of 64 as calculated per ASTM E1980.
Why clean cool roof membranes?
The benefits of cool roof membranes decline as they become soiled, so you may conclude cleaning cool roof membranes is a good idea. However, the reality of cleaning in-service cool roof membranes is more complex.
In the Economics section of DOE's Guidelines for Selecting Cool Roofs, it states: "… dirtying of roofs reduces solar reflectance. Although annual cleaning can restore up to 90% of initial reflectance, the energy cost savings alone does not warrant the cost. If you do clean your roof, be sure to follow the manufacturer's cleaning recommendations, since improper cleaning (e.g., power washing, harsh chemicals) could damage your roof."
The DOE document cites a Lawrence Berkeley National Laboratory abstract, "Long-term Performance of High-albedo Roof Coatings," as a source for the claim that energy-cost savings do not warrant the cost of roof membrane cleaning. The abstract states: "… We use measured data to estimate the effect of weathering of white roofs on seasonal cooling energy savings and estimate a 20% reduction from first year energy savings for all subsequent years (2-10). Although washing the roofs with soap is effective at restoring original [reflectance], calculations show that it is not cost-effective to hire someone to clean a high-[reflectance] roof only to achieve energy savings. …"
Within the body of the paper, the following statement about cost appears: "Our estimates are based on an annual cooling energy use of 1000 kWh and the average change in albedo achieved through washing. Based on our experience, washing a 2,000 square foot roof would require four person-hours of work at an estimated cost of $25/person-hour. With a cost of $100 per roof … the Cost of Conserved Energy (CCE) worked out to be ~ 70 cents/kWh, far above the average cost of electricity to residential customers."
In real-world scenarios, the cost of washing a 2,000-square-foot roof is significantly higher than $100 because, in addition to the actual washing time, there are costs for mobilization, demobilization, travel to and from the job site, etc. These costs likely would be above and beyond the hourly labor rate, especially for smaller-sized roof areas. There also are areas of the country where typical labor rates are in excess of $25 per hour.
Although the cited abstract addresses roof coatings and not roof membranes, similar studies have found soiling rates for cool roof membranes to be about the same as in the abstract. NRCA's research with in-service roof systems suggests soiling rates for cool roof membranes installed at a 1/4:12 slope (the minimum slope for roof membranes for new construction in model building codes) often results in the reduction of reflectivity in excess of the values quoted in the abstract, so it is safe to assume at least the same number of man-hours would be needed to clean cool roof membranes as noted in the cited paper for minimum-slope situations.
Based on this information, it would seem DOE's claim that the energy-cost savings alone do not warrant the cost of washing cool roof membranes is well-founded for most situations because the labor cost for washing the membrane likely exceeds the energy savings realized by improved reflectivity as a result of cleaning.
This especially would be true for well-insulated roof assemblies, such as required in commonly adopted energy codes such as the International Energy Conservation Code. The more thermal insulation between a roof membrane and an interior cooled space, the less a roof membrane's temperature will affect the interior building temperature. In other words, as thermal roof insulation values go up, the potential for energy savings by washing roof membranes goes down.
Of course, there are other reasons a building owner might consider having a cool roof membrane cleaned. For one, building owners participating in USGBC's LEED v4 program need to comply with LEED v4's Building Operations and Maintenance Heat Island Reduction Credit. The credit's intent as noted in the LEED v4 Reference Guide for Building Operations and Maintenance is "to minimize effects on microclimates and human and wildlife habitats by reducing heat islands. … Dark, nonreflective surfaces used for parking, roads, roofs, walkways, and other hardscapes absorb the sun's warmth and radiate heat, creating heat islands."
There are four options for complying with the Heat Island Reduction Credit; using cool roof membranes is the most common. When cool roof membranes are used, the following performance requirement is noted: "Implement a maintenance program that ensures all high-reflectance roof surfaces are cleaned at least every three years to maintain good reflectance." Although the credit language is vague about how much cleaning is necessary to achieve "good" reflectance, it seems reasonable to assume the effort necessary to maintain cool roof membrane reflectance would be similar to the effort characterized by four man-hours per 2,000 square feet of roof surface annually to restore 90 percent of an original cool roof membrane's reflectivity.
Even if energy savings or urban heat island issues are not a concern for a particular building owner, cleaning cool roof membranes still may be desired for aesthetic reasons because roof areas may be visible from within a building or from other buildings. Whatever the reason building owners decide to clean cool (or any) roof membranes, NRCA supports DOE's guideline for following roof membrane manufacturers' cleaning recommendations.
NRCA surveyed cool roof membrane manufacturers to elicit their cleaning recommendations for cool TPO and PVC membranes and polymer-modified bitumen cap sheets. These product types were selected because they most often are used for low-slope cool roof membranes.
The recommendations from the roof membrane manufacturers that responded are as follows:
Carlisle SynTec/Versico Roofing Systems, Carlisle, Pa.
Products: Sure-Weld TPO, VersiFlex PVC, VersiWeld TPO
Available document: Carlisle Owner's Manual
Summary of cleaning guidelines: Handprints, footprints, general traffic grime, industrial pollutants and
environmental dirt may be cleaned from the membrane's surface by scrubbing with detergent and water then rinsing with
clean water. To maximize reflectivity, white membranes should be cleaned once every two years.
Duro-Last® Roofing Inc., Saginaw, Mich.
Products: All Duro-Last PVC roof membranes
Available document: Information is provided by Duro-Last Roofing on a case-by-case basis.
Summary of cleaning guidelines: Use nonsudsing cleaners such as Spic and Span® or Simple Green® as
"household use" with a soft-bristle push broom and rinse clean.
FiberTite,® Wooster, Ohio
Products: All FiberTite thermoplastic roof membranes
Available document: A letter addressing cleaning options is provided by FiberTite on a case-by-case
basis.
Summary of cleaning guidelines: Clean using a mild soap, such as Dawn.® Using water from a hose and a
soft, flexible bristle brush or broom, clean (do not abrade) the membrane surface. If a stronger cleaning agent is
required, Soft Scrub,® mechanics soap, Zep® industrial cleaner, etc., may be used. Heavy-traffic areas or
areas with "ground-in" dirt may require hands and knees scrubbing. It is possible to use a power washer with a "wide"
tip, but extreme care must be taken to avoid "scarring" the membrane surface. It also is possible to use a floor
scrubber with a nonabrasive pad, but care must be taken to avoid scarring the membrane surface. A power washer or
floor scrubber can damage the membrane surface.
Firestone Building Products Co. LLC/GenFlex Roofing Systems, Indianapolis
Products: GENFLEX™ EZ TPO, Platinum TPO, ReflexEON TPO, UltraPly™ TPO
Available document: Information is provided by Firestone Building Products and GenFlex Roofing Systems on a
case-by-case basis.
Summary of cleaning guidelines: Liquid cleaners tend to leave a film residue; clean with a water-soluble
granular cleaner, such as TMT (Twenty Mule Team) Boraxo powdered hand soap, and water. A polypropylene scouring pad,
such as the type manufactured by 3M,™ is recommended for maximum cleaning. The scouring pad used with granular
detergent allows for enough abrasion to thoroughly clean the sheet without damaging the membrane. Do not use
steel-wire brushes under any circumstances, and thoroughly rinse with clean water.
Products: UltraWhite™ APP Modified Bitumen Cap Sheet, UltraWhite SBS Modified Bitumen Cap Sheet
Available document: Information is provided by Firestone Building Products and GenFlex Roofing Systems on a
case-by-case basis.
Summary of cleaning guidelines: Generally, cleaning these products is not recommended except to remove
accumulated debris. If needed, a soft-bristle broom and clean water can be used. Do not use a pressure washer.
Flex™ Membrane International Corp., Leesport, Pa.
Products: All Flex thermoplastic roof membranes
Available document: Cleaning Procedure for Flex Membrane Roof Systems
Summary of cleaning guidelines: Sponge or spray water onto roof surface. Use a cleaning solution made by
mixing Flex Orange Power cleaner in a ratio of 16 ounces of Flex Orange Power to 1 gallon of potable water. Mop,
spray or sponge the cleaning solution onto the roof membrane and allow to stand for at least 5 minutes. Scrub or
brush the solution thoroughly to loosen dirt or sediment. Rinse the solution from the surface with clean potable
water. A rubber squeegee may be used to direct water to drains or gutters.
GAF,® Wayne, N.J.
Products: EverGuard® PVC and TPO
Available document: GAF Technical Advisory Bulletin No. TAB-C 2014-27
Summary of cleaning guidelines: GAF recommends the following:
Products: RUBEROID® EnergyCap™ APP and SBS
Available document: GAF Technical Advisory Bulletin No. TAB-C 2014-27
Summary of cleaning guidelines: GAF recommends the following:
Johns Manville, Denver
Products: JM PVC, JM TPO
Available document: Information is provided by Johns Manville on a case-by-case basis.
Summary of cleaning guidelines: Clean with a light detergent solution that has a pH value no greater than 8.5.
This may be accomplished with mild cleaners that contain no bleach or chlorine as these may detrimentally affect the
membrane. The cleaning solution exposed to the membrane only should be slightly basic in nature and applied with
minimal disturbance to the sealed laps. Apply the solution with a soft-bristled broom or brush. In no instance should
mechanical scrubbing, such as a floor buffer with scrub pad, be used. Johns Manville does not promote the use of
high-pressure spray to clean the membrane because of the potential risks of damage at seaming locations. If a
high-pressure spray is used, it should be less than 2,500 psi, and the nozzle should be at least 18 inches from the
membrane surface. Care should be taken to ensure the application of solution with the power sprayer never goes against
the laps in the membrane. Extreme caution should be taken if a pressure washer is used. Additionally, after completion
of the membrane's cleaning, all laps should be visually inspected to verify the membrane's continued integrity.
Products: GlasKap® CR, SBS CR membranes, TopGard® 4000 and TopGard 5000
Available document: Information is provided by Johns Manville on a case-by-case basis.
Summary of cleaning guidelines: Cleaning these products is not recommended. Johns Manville stresses the need to
ensure proper drainage.
Polyglass® U.S.A. Inc., Deerfield Beach, Fla.
Products: All Polyglass APP and SBS granulated polymer-modified bitumen cap sheets
Available document: Information is provided by Polyglass on a case-by-case basis.
Summary of cleaning guidelines: For small areas, Goo Gone® cleaner may be used. For larger areas, Simple
Green cleaner may be used. Do not use a pressure washer. A standard hose should be used for rinse water.
Sika Sarnafil, Canton, Mass.
Products: Sarnafil G Membrane (PVC), Sarnafil S Membrane (PVC) and Sikaplan® (PVC)
Available document: A letter addressing cleaning options is provided by Sika Sarnafil on a case-by-case
basis.
Summary of cleaning guidelines: Membrane washing should be done from the field sheet over the overlap (not the
other way around). Washing against the overlap could damage the edge of the material or cause problems with the weld.
Cleaning with a sponge, mop or brush is the least aggressive method and works well on new material and for smaller
areas. Apply water and a nonabrasive cleaner to the area and sponge, mop or brush off. Wire brushes never should be
used. If a rotary buffer is used, use a small buffer with a 12- to 15-inch base and only use a synthetic buffing pad or
polishing pad. A stripping pad will damage the membrane. If a pressure washer is used, a lower psi is preferable. Using
too high a pressure will cut into or through the membrane, causing leaks and possible damage to other roof system
components. A wide-dispersal nozzle also should be used; a narrow-dispersal nozzle can damage the membrane. The
pressure washer nozzle should be kept at a constant 16 to 24 inches above the membrane. Pressure washing only should be
performed if all other options have been unsatisfactory. Lowering the nozzle in an effort to concentrate the spray may
damage the membrane.
Siplast,® Irving, Texas
Products: Paradiene polymer-modified bitumen finish ply sheets: 30 CR FR, 30 FR BW, 40 CR FR
Summary of cleaning guidelines: Cleaning these products is not recommended by Siplast.
Tremco® Inc., Beachwood, Ohio
Products: All Tremco thermoplastic roof membranes
Available document: Information is provided by Tremco on a case-by-case basis.
Summary of cleaning guidelines: Using a 2,500-psi power washer (with or without a rotating head), wash all areas
to loosen and remove dirt. If dirt remains, use a mild, biodegradable detergent and scrub pads to further loosen the
dirt. Rinse with clean water. For areas that may have residual and difficult to remove contaminants on the surface,
wipe areas with acetone and white rags only then rinse. Be sure detergent complies with local regulations and, if
run-off is prohibited, capture and properly dispose of run-off water.
Product: POWERply® FR T24 (SBS)
Document: Information is provided by Tremco on a case-by-case basis.
Summary of cleaning guidelines: Cleaning of granule-surfaced polymer-modified bitumen products is not required.
If an owner elects to have a roof cleaned, first remove accumulated debris by sweeping or air blowing. Then, wash the
roof surface using water at a pressure not to exceed 1,200 psi. A fan spray tip (15- to 25-degree fan width) should be
used and maintained no closer than 16 inches to the membrane to prevent damage. Do not use a point, roto max nozzle or
jet spray as these could adversely affect the membrane. Direct the power wash spray fan across the surface and over the
tops of selvedge laps. Do not direct water spray into the selvedge overlap or flashing details. A soft-bristled brush
or broom may be used to assist with cleaning embedded dirt from the membrane surface. A water-only wash is recommended;
however, a biodegradable detergent suitable for acrylic-coated surfaces may be used to assist with removing embedded
dirt. Follow detergent manufacturer's recommendations for proper selection, dilution, use and disposal. If detergent is
used, ensure all traces are fully removed from the membrane surface and not allowed to dry on the roof surface. If
run-off is prohibited in the jurisdiction where a roof is being cleaned, capture and properly dispose of run-off water.
Inspect the roof membrane upon completion of work, and repair any damage as directed by Tremco.
Additional considerations
It's interesting to note roof membrane manufacturers' cleaning recommendations generally are not available on their websites or in commonly available documents. Also, there is no recognized U.S. standard practice document for cleaning cool roof membranes. Because the cleaning recommendations presented here are general summaries, when considering the cleaning of an in-service roof membrane, NRCA advises consultation with roof membrane manufacturers, especially during roof membrane warranty periods.
Cleaning cool roof membranes presents challenges, and the potential exists to inadvertently cause roof membrane damage during the cleaning process. For anyone considering cool roof membrane cleaning, safety is an additional concern. The cleaning process uses water and detergents that often create slick surfaces. Also, Occupational Safety and Health Administration (OSHA) guidelines for fall protection, personal protective equipment and hazard communication, to name a few, apply to any workers taking part in cleaning activities. The cost of training or hiring OSHA-knowledgeable personnel and the safety equipment and methods used to comply with OSHA rules must be considered when deciding whether cool roof membrane cleaning makes sense in a particular circumstance.
In lieu of cool roof membrane cleaning programs, thoughtful roof system design often can minimize issues associated with soiling. Well-draining low-slope roof systems help reduce or eliminate ponding. This is important because ponded roof areas become especially soiled areas after water evaporation.
Providing more than minimum code-required roof slope, minimizing rooftop equipment, and reducing the distance between roof system high points and drainage locations also can help to minimize areas of soiled roofing surfaces.
Final thoughts
Whatever the situation, cleaning in-service cool roof membranes only should be attempted once roof membrane manufacturers' cleaning guidelines are determined; the costs and risks associated with the cleaning process are understood; and local regulations associated with cleaning, such as roof water run-off requirements, are determined. If a cleaning process occurs, it is good roofing practice to schedule cleaning just before routine bi-annual roof membrane inspections so any damage that may occur during the cleaning process is quickly identified and repaired.
Jason P. Wilen, AIA, CDT, RRO, is an NRCA director of technical services.
For an article related to this topic, see:
"Still Water Runs
Deep," July 2012 issue
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