Once you as a commercial roof contractor have correctly identified the composition of the substrate, which we covered in our previous blog titled “Commercial Roof Substrates Overview,” and have determined it to be sound and dry, the next step is repairing flaws and preparing the surface for coating.
Above all, the application surface should be clean, dry, stable, and free of grease, wax, dirt, loose roofing material, contaminants, moisture, and other conditions that could interfere with adhesion.
When preparing a substrate, the tools and equipment required will depend on the existing roof and the coating system being installed. A general list of roofing equipment can be found here: Roofing Tools and Equipment List.
Always follow the current coating manufacturer’s requirements for substrate preparation, primers, cleaning methods, application conditions, and compatibility.
Proper Aging of the Roof
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Before repairing or coating a built-up asphalt roof, the condition and age of the asphalt should be evaluated. Newly installed asphalt materials may require an appropriate curing or weathering period before some coating products can be applied.
The surface should be free of oil, solvent residue, moisture, loose material, and other contaminants that could interfere with adhesion. Required curing periods vary by asphalt product, coating system, weather conditions, and manufacturer specifications.
Preparation of Built-Up Roofs
Built-up roofs may develop conditions such as alligatoring, splitting, blistering, delamination, gravel displacement, exposed felts, and other signs of deterioration.
Before coating, the roof should be inspected to determine whether these conditions can be repaired or whether the existing roof has deteriorated beyond what a coating system can reasonably restore. Wet insulation, structural deterioration, drainage problems, and unstable roofing materials should be addressed before restoration proceeds.
Alligatoring
An alligatored roof has a weathered surface with interconnected cracking that resembles alligator skin. This can indicate an aged or brittle roofing surface.
Severe alligatoring should not simply be covered with coating. The affected areas need to be evaluated to determine whether damaged materials should be removed and repaired or whether more extensive replacement is necessary.
Splitting
Splits can result from several conditions, including roof-deck movement, membrane deterioration, thermal movement, or other stresses.
If structural movement is suspected, the cause should be investigated before coating. Installing a coating over a roof that continues to move excessively may lead to premature failure.
If splitting is isolated, damaged roofing should be repaired and the affected area checked for moisture. Wet insulation or other deteriorated materials should be replaced as necessary.
Blistering
Blistering may indicate trapped air or moisture within the roof assembly. Blistered areas should be evaluated before coating, and any wet insulation, deteriorated roofing, or unstable materials should be addressed.
The repair method should be compatible with the existing roof assembly and the coating system being installed.
Gravel Turnover and Irregular Surfaces
Graveled built-up roofs can present challenges for direct coating because loose aggregate, uneven surfaces, excessive asphalt, and irregular profiles can make cleaning, adhesion, and consistent coating thickness more difficult.
The roof should be evaluated to determine whether the existing surface can be properly prepared or whether removal of loose aggregate and installation of a suitable restoration substrate is more appropriate.
Felt Exposure
Exposed roofing felts can indicate that the protective asphalt layer has deteriorated. Areas with exposed, dry, brittle, or unstable materials should be evaluated and repaired before coating.
Delamination
Delamination can refer to separation between layers of a built-up roof or separation within another substrate such as plywood.
Loose or unstable materials generally should not be coated over. Affected areas should be repaired or replaced with compatible roofing materials before restoration proceeds.
Removal/Replacement Guidelines
Some general principles to keep in mind when replacing faulty or deteriorated roof sections include:
- Use materials that are compatible with the existing roof and the proposed coating system.
- Create a stable, reasonably uniform surface for coating application.
- Repair large gaps, voids, and deteriorated areas with materials designed for the specific condition rather than relying on surface coating alone.
- Remove dust, loose material, debris, and contaminants created during the repair process.
- Make sure repaired areas and the surrounding roof surface are sufficiently dry before coating.
Preparation of Severely Weathered Built-Up Roofs
A severely weathered or alligatored built-up roof may not be suitable for direct coating. Depending on the roof condition, options can include removing deteriorated materials, repairing or replacing unstable sections, installing an appropriate recover substrate, or replacing the roof system.
The correct approach depends on the existing assembly, structural condition, moisture, drainage, applicable code requirements, and the requirements of the restoration system being considered.
Graveled Built-Up Roofs
Direct coating of a graveled built-up roof can be difficult because the irregular surface may make cleaning, adhesion, and uniform coating application challenging. Loose aggregate can also interfere with the formation of a continuous coating membrane.
Any underlying splits, wet insulation, unstable asphalt, or other deterioration should be identified and corrected before deciding whether coating is appropriate.
New Substrates Over a Graveled Built-Up Roof
In some restoration projects, a new compatible substrate may be installed over an existing built-up roof after the roof has been evaluated and prepared.
A properly designed recover assembly can provide a more stable surface for the new roofing or coating system. However, adding insulation, cover boards, plywood, foam, or other materials should be based on structural capacity, moisture conditions, fastening requirements, drainage, applicable codes, and the specifications of the selected roofing system.
Preparing Boardstock or Plywood
Boardstock, cover boards, or plywood may be used in some roof assemblies to provide a stable substrate. Preparation requirements vary by material and coating system.
Joints, seams, fasteners, surface moisture, dust, and debris should be addressed according to the roofing system manufacturer’s requirements. These materials should be sufficiently dry before coatings or other roofing products are applied.
Preparing a Metal Roof
Metal roof preparation generally begins with a thorough inspection of the panels, seams, fasteners, flashing, penetrations, corrosion, coatings, and other surface conditions.
Typical preparation may include:
- Removing loose rust, oxidation, dirt, chalking, and other contaminants
- Replacing or securing loose or damaged fasteners and washers
- Repairing damaged seams, penetrations, flashings, and panel conditions
- Treating corrosion using compatible preparation methods and primers where required
- Removing oils or other contaminants using methods approved for the existing metal and selected coating system
- Making sure the surface is clean and dry before coating
Preparation methods for galvanized metal, previously coated metal, bare steel, aluminum, and other substrates vary. Current manufacturer instructions should be followed rather than assuming one cleaning, etching, or priming procedure is appropriate for every metal roof.
Preparing a Concrete Roof
Concrete can serve as a roofing substrate, but its condition and moisture content need to be evaluated carefully before coating.
Efflorescence, spalling, cracks, laitance, contamination, and trapped moisture can all interfere with coating adhesion or indicate underlying problems.
Loose or deteriorated concrete should be removed and repaired with suitable materials. Larger cracks may indicate structural movement or deterioration and should be evaluated before proceeding.
Cleaning, surface profiling, moisture testing, primers, crack repairs, and curing requirements should follow the specifications of the coating system and repair materials being used.
Cleaning the Roof
Assuming that necessary repairs have been made, the next step is cleaning the roof.
Different roof systems may require different cleaning methods. Loose material, dirt, dust, biological growth, oils, chalking, and other contaminants should be removed without damaging the roof or introducing unnecessary moisture into the assembly.
Dry-cleaning methods such as sweeping, vacuuming, scraping, or power blowing may be appropriate in some situations. Pressure washing or other wet-cleaning methods may also be used when compatible with the roof system.
Before using water, open cracks, holes, failed seams, and other paths for water entry should be repaired or protected so that cleaning does not introduce moisture into the roof assembly or insulation.
After wet cleaning, the roof should be allowed to dry thoroughly before coating.
It is crucial that cleaning and preparation be thorough. The long-term performance of a roof coating system depends heavily on substrate condition, adhesion, proper detailing, application, and maintenance.
Preparing a Previously Coated Roof for Recoating
A previously coated roof may be eligible for recoating if the existing coating and underlying roof remain sound and compatible with the proposed new system.
Before recoating, the roof should be inspected for adhesion loss, cracking, blistering, ponding, damaged materials, wet insulation, failed repairs, contamination, and other deterioration.
The existing surface should then be cleaned and prepared using methods and products approved for the coating system being applied. Adhesion testing, primers, repair materials, and additional preparation may be required depending on the existing coating and roof condition.