Metal roof waterproofing: Causes, possibilities & material

Metal roof waterproofing is the most cost-effective of all metal roof repair options. Metal roof waterproofing allows business owners and building managers to extend the life of a commercial metal roof by up to twenty-five years.

With over 1000 different metal roof sealants to choose from, it can be difficult to find the best solution for your situation. This article is intended to clear up the confusion and give business owners and building managers all the information they need to make an informed decision about metal roof waterproofing.


Metal roofs can be recoated for a variety of reasons. Initially they have a factory finish coating, which can be galvanized or Galvalume finished steel panels or a fluoropolymer based paint such as Kynar 500. However, when the metal panels reach the end of their life expectancy, their coating is compromised.

“Regardless of the original coating, the protective coating is affected by long-term exposure to the elements,” says Bob Arnold, vice president and general manager of Topps Products Inc. in Stilwell, Kan. “Rust forms, and over time it compromises the structural integrity of the roof and negatively impacts the aesthetics of the building. All metal roofs expand and contract with temperature changes. This allows the seams to open and moisture to penetrate the building envelope. With the right repair agents and recoating products, a metal roof can be restored to like-new condition using the original substrate as a foundation.”


The following is an overview of the advantages that speak for a new coating compared to a complete roof renewal.

  • Cost: Recoating saves money compared to roof replacement.
  • Speed: less work than a roof replacement.
  • Safety: recoating minimizes accidents, interruptions and downtime.
  • Drought: During a coating renovation, the building contents are not exposed to the leaks that occur during a roof replacement.
  • Cleanliness: less dust and debris than roof replacement, which is important in factories and processing plants.
  • Savings: Re-coating offers an immediate tax advantage over roof replacement.

Important: Re-coating a metal roof should only be considered if the existing roof is sound and performing exceptionally well in other respects, such as weathering resistance, base metal corrosion and wind resistance. Some items that should be of concern when recoating a roof are:


Here is an overview of the main waterproofing work at a glance.


This is the traditional method of extending the life of a black roof. Reflective asphalt coatings, often used as a coating for a metal roof, provide additional protection by creating a renewable weather barrier with added protection at transitions and flashings and reflecting the sun’s rays. Asphalt coatings can be solvent-based or water-based.

Polymer (elastomeric) coatings

Polymer-modified asphalt coatings offer improved elongation and recovery properties compared to conventional asphalt coatings. Expansion and contraction are important because metal plates expand and contract with temperature changes.

Pigmented latex coatings

Pigmented latex coatings are the most commonly used materials for coating metal plates because they are more resistant to chemicals, oils and solvents. Like polymer coatings, they also offer stretchability and recovery.

Polyurethane coatings

Polyureate coatings are used when industrial chemicals or oils are present.
Solvent-based elastomeric coatings exhibit good elongation and recovery and provide good metal adhesion, but are less resistant to solvents and oils than acrylic or polyurethane coatings. These types of metal roof coatings are generally not used for restaurants where there is significant buildup of oils and solvents.


Here are the instructions for applying a polyurea spray film. This is sometimes best suited for metal roof waterproofing due to its physical and chemical properties.

Substrate preparation

The first step is substrate preparation. In this, the old coating or waterproofing is removed. This is usually done with a sandblasting unit or, if sufficient, with a high-pressure water jet unit. It is important to remove old defective material to create the proper substrate for a new coating.


If the substrate has been prepared cleanly, it must be primed. The primer ensures the correct adhesive pull of the sealant and must be matched to the material accordingly. We use only quality products from the German chemical company BASF. The application is usually done by roller. The primer should cure for at least 6 hours. In damp or cold weather, ideally even longer.

Waterproofing (polyurea spray film)

Once the primer is applied and fully cured, you can start sealing the metal roof. As mentioned at the beginning, we recommend polyurea for this purpose. The spray film is applied by means of high pressure and hardens completely within a few seconds. A clean and even application is important here to ensure a homogeneous seal.

Final sealing

Then comes the final sealing. This provides additional protection from UV and other weather conditions. Again, this is applied with the roller. Afterwards, the final sealant, also called top coat, must be allowed to cure again for at least 24. In addition, the topcoat can also change the color and surface of the roof, if you have special requirements for appearance and slip class.


Roof coating and waterproofing replacement, especially for metal roofs, offers several advantages over tearing off the old roof and building a new one. Renovations in the form of new coatings are in most cases much more cost-effective, and ecologically more sustainable. Especially if you choose the right coating system.

It is important that the base roof is not damaged and is still completely intact.

The most sustainable option is to remove the old coating or film and re-seal with the innovative product polyurea. The spray film convinces with high resilience and durability. For a non-binding initial consultation and quotation, please feel free to contact us.


Submit project request

Initial technical consultations and information as well as quotations are free of charge. We request that all necessary information be provided to allow for accurate bidding.

25 years of innovation in the waterproofing and coating sector with polyurea.

Scroll to Top