Roof Waterproofing: Technical Comparison Between One and Two Coats of Epoxy Primer

Epoxy primers are the foundation of every roof waterproofing system. Their role is to penetrate, seal, and stabilize the concrete substrate prior to membrane application. However, there is a critical distinction between applying one coat and two coats of primer. This difference directly affects pinhole formation, vapor and moisture transmission, and the long-term durability of the waterproofing system.

Behavior of Concrete

Concrete is a hydrophilic, porous material. It contains interconnected capillaries and voids that allow liquid water (moisture) and water vapor (gas phase) to migrate continuously throughout its service life.

• Moisture Transmission: The capillary suction and movement of liquid water through pores.

• Vapor Transmission: The diffusion of water vapor molecules in gas form, driven by vapor pressure differentials between the concrete and the atmosphere.

These two processes occur simultaneously, meaning any primer must resist both liquid and vapor migration to be effective.

The Limitation of a Single Coat

A single application of water-based epoxy primer is insufficient because:

1. Pinholes from Vapor Escape – As the first coat cures, moisture vapor rises through pores. This creates microscopic perforations in the primer film, called pinholes.

2. Incomplete Pore Sealing – Due to the porosity of the concrete, one coat does not fully saturate or bridge the surface voids, leaving continuous capillary pathways open.

3. Ongoing Transmission – Even if the first coat achieves partial coverage, the continuous vapor and moisture pressure during the slab’s life cycle ensures weaknesses will develop at the unsealed pores.

In technical terms, one coat achieves a discontinuous barrier with a higher Moisture Vapor Transmission Rate (MVTR), increasing the risk of osmotic blistering and adhesion failure in the subsequent membrane layer.

Why Two Coats Are Required

The second coat of epoxy primer provides a redundant sealing layer which corrects the limitations of the first:

• Pinholes Filled – The second application wets into and seals the voids created by pinholes, ensuring a continuous film.

• Pore Saturation – With two coats, the primer penetrates deeper into the concrete surface, filling micro-capillaries and bridging larger pores.

• Reduced MVTR – The double-coat system significantly lowers moisture vapor diffusion rates, reducing vapor drive and pressure under the membrane.

• Improved Adhesion & Durability – With a stable, sealed substrate, the membrane bond line is continuous and less prone to delamination.

In essence, the second coat transforms the primer from a porous barrier to a monolithic, vapor-resistant film, essential for roof-level exposures where hydrostatic and vapor pressures are high.

Practical Demonstration

Laboratory and site testing show that:

• A single coat applied at the standard film thickness (typically 150–200 microns WFT) will leave 10–15% of the surface with incomplete coverage due to vapor-driven pinholing.

• A double coat reduces this void percentage to <1%, achieving a near-continuous barrier with film thickness averaging 250–400 microns DFT.

This difference is easily demonstrated in practice — in our Instagram video, you can observe vapor escape creating pinholes through a single coat. When a second coat is applied, those pathways are sealed, and the surface remains intact.

Conclusion

The primer layer is not a step to cut corners on. For concrete roof slabs, two coats of epoxy primer are not optional — they are a technical necessity.

• One coat = higher MVTR, pinholes, incomplete pore filling, reduced adhesion.

• Two coats = pinhole elimination, sealed porosity, reduced vapor transmission, extended waterproofing system lifespan.

By understanding the science of moisture and vapor transmission in concrete, it becomes clear why Forspec specifies two coats of epoxy primer in all roof waterproofing applications.

For a visual demonstration, watch our video on Instagram and YouTube, where we show pinhole formation under one coat and how two coats eliminate the problem.