Metal Architecture Home

Is Your Roof Ready to be Recoated?

Mcn  Prod Feature  Sept15 3 Low Rez

Materials, tools and procedures to prep for recoating


Photo courtesy of Mule-Hide Products Co. Inc.

Correct metal roof surface preparation is a critical step before roof coating. Poor surface preparation will reveal itself later in the form of call backs and even potentially costly mistakes when roof leaks cause damage.


Inspection and Evaluation

To ensure watertight integrity and coating success, contractors should first complete a thorough inspection of the entire roof. Determine whether or not its metal panels are structurally sound. Look for deficiencies and damaged areas that need to be addressed before the coating system is installed.

"These include loose or missing fasteners, previously installed coatings that have begun to peel or chalk, poorly attached vents, open seams or side laps, loose or cracked perimeter edge flashing, broken or improperly flashed soil pipes, stains from ponded water or the accumulation of dirt and debris, panels with excessive rusting or holes, and panels that have become seriously deflected," says Kate Baumann, director of marketing and procurement, Mule-Hide Products Co. Inc., Beloit, Wis.


Repair, Replace, Retighten

Once they are found, these deficiencies and others such as laps, cracks, blisters, holes, splits and perforations require repair. "Sometimes this can be done with patching cement or tapes, but sometimes this requires a whole new piece of metal," says Robert Anderson, building envelope solutions manager, Firestone Building Products Co. LLC, Indianapolis.

Flexible flashing materials that have deteriorated must be removed and replaced. Missing or stripped fasteners, screws and washers must be replaced. Loose fasteners and loose panels need to be retightened and secured. "If a fastener is stripped and/or won't tighten, change it out for an oversized one," says Doug Loomis, vice president, Energy Coating Systems Inc., Conroe, Texas. "Additional fasteners can better secure the seams."


Photo: Firestone Building Products Co.

Adhesion Test

An adhesion test, completed according to the coatings manufacturer's instructions, gives contractors confidence that the chosen coatings will properly bond to the existing metal panels and coatings. "Acrylic coatings do not adhere well to some metal finishes, such as Kynar 500, and to some coatings, such as silicone," says Baumann. "Clean a small area of the roof and perform the adhesion test to ensure the coating will properly adhere to the roof surface. The adhesion test should be performed using the product that would have direct contact with the roof panels⎯typically the metal roof primer. If the adhesion test is satisfactory, the contractor can then prepare the entire roof surface."


Clean It Off

A clean, dry metal roof is an absolute necessity prior to a successful recoating. Without it, adhesion will not occur. "It is critical that the surface preparation effectively removes debris that will prevent adhesion of the coating and also prepares the roof to be coated by ensuring that a proper surface roughness is achieved to allow the coating to stick to the substrate," says Michael Endredi, project engineer, The Garland Co. Inc., Cleveland.

The roof should free of any dirt, dust, oils, heavy rust, residual coating, debris and fresh roof cement. "If anything but silicone coatings will be used for recoating, silicone caulks and sealants also must be removed," says Baumann.

Always follow the coatings manufacturer's guidelines and industry standards for cleaning. "Use a method and product approved by the coating manufacturer, keeping in mind that coatings are really roof systems," says Anderson. "It's important that you have all of the approved system components that you need to have a long-lasting roof."

Baumann says a wire brush and a good cleaning detergent may be required for spot cleaning, and it may be necessary to add a fungicide to remove heavy accumulations of fungi and algae.

Photo courtesy of Energy Coating Systems Inc.

Brushing, vacuuming or power blowing can clean a metal roof, but pressure washing the entire roof surface is typically recommended. "Use a minimum of 2,000 psi with full-strength cleaner at a rate of 0.25 gallons per 100 square feet, followed by a water rinse," says Baumann. "Care must be taken to ensure that the existing roof surface is not damaged during pressure washing.

Using too high a pressure can cause irreparable damage to seams, joints, flashings and membranes, which could allow water to be forced into the roof system, requiring the complete removal of wet areas. In all cases, contractors should prepare the building owner for the possibility that some water may enter the building during the power-washing process."

"We start with 4,000-psi pressure washers to remove dirt, old paint, tar, silicone and numerous other products applied over the years," says Loomis. "Sometimes a hammer and chisel are used. If we're still left with a chalky residue that won't come off, or are going over a previously coated surface, we then apply an adhesion product for insurance."

A roof's size, shape, slope and contours all lead to some methods of preparation being more feasible than others. "If, for example, a roof is too large, hand abrading may become an unfeasible method to achieve the required surface roughness due to time and labor constraints," says Endredi. "If the condition of the existing substrate allows, chemical etching within a power washer may be able to achieve the same required surface roughness while adhering to time and labor requirements. On the other hand, the contours of the roof might be such that much of the roof must still be hand or mechanically abraded in areas where chemical etchers could not effectively reach."


Primed for Recoating

Many metal roofs need to be treated with a rust-inhibitive primer prior to recoating. Primers prepare the roof surface for the coating and improve the overall adhesion of the coating to the roof substrate. "The primer is what connects the coating system to the roof and if it fails it does not matter how well your top coat sticks to the primer because the whole paint system will detach from the roof," cautions Endredi. "Factory-applied roof coatings are typically PVDF resin based, which makes them inherently slick materials that are difficult to adhere to."

Photo courtesy of Topps Products Inc.

Corroded or oxidized areas must be treated with a rust inhibitor. "Hopefully this will just be needed over a few rusty fasteners, but there are times when an entire roof should receive a coating of a rust inhibitor," Anderson says. Follow with a thorough water rinse.

Some rust-inhibitive primers, such as zincchromate-based primers, may be incompatible with some coatings. Primers come in a variety of binder and solvent combinations: cut-back, emulsion and polymer based. Consult the manufacturer's requirements for the specific coating and substrate.

When preparing to recoat your roof, don't forget to forecast the weather. Anderson says most coatings have a temperature minimum (typically around 50 F) for proper application, and many also have a restriction that they shouldn't be applied if rain is expected within 24 hours. Also, don't forget to properly mix your coating. Coatings are available in a wide variety of containers from 1 gallon to 55 gallons; most need to be stirred or mixed prior to application.


Sidebar: Estimating the Recoating

Proper estimating is the first step in the successful completion of a job. Here are some guidelines to help contractors estimate the product quantities needed when recoating a metal roof system:

• When estimating the amount of finish coating required, add 20 percent to the total roof area to compensate for the panel profile.

• For HVAC penetrations, ridge vents, etc. estimate the amount of flashing grade coating needed at 50 to 75 feet per gallon.

• Determine the number of fasteners requiring actual replacement.

• Estimate approximately one gallon of flashing grade per 300-400 field fasteners to be coated.

• Estimate additional flashing grade at 100 feet per gallon to coat seams and end lap joints.

Kate Baumann, director of marketing and procurement, Mule-Hide Products Co. Inc., Beloit, Wis.

Photos courtesy of The Garland Co. Inc.