When discussing commercial roofing systems, coatings, and waterproofing materials, you may come across the term “mil.” Many property owners hear roofing contractors mention coating thickness in mils but are unsure what the measurement means or why it matters. Understanding the meaning of mil in roofing helps building owners make informed decisions about roof repairs, restorations, maintenance programs, and long-term protection.
A mil is a unit of measurement used to describe the thickness of roofing materials, coatings, membranes, and protective layers. In roofing, one mil equals one-thousandth of an inch (0.001 inches). It does not refer to millimeters or a measurement of strength. For example, a roofing coating applied at 20 mils has a dry film thickness of 0.020 inches.
Why Are Mils Important in Roofing?
Roofing systems depend on proper material thickness to provide durability, waterproofing, and resistance against environmental conditions. Whether applying a silicone roof coating, acrylic coating, polyurethane coating, or liquid-applied membrane, manufacturers specify the required thickness in mils to achieve the expected performance.
The correct mil thickness helps ensure:
- Water resistance: A properly applied coating creates a continuous protective barrier that prevents moisture intrusion.
- Longer roof lifespan: Adequate coating thickness protects roofing materials from UV exposure, weather damage, and deterioration.
- Warranty compliance: Many roofing manufacturers require specific mil thickness levels to qualify for product warranties.
- Consistent performance: Proper thickness ensures the coating performs as designed throughout its expected service life.
Applying too little material can reduce protection, while applying excessive material may increase costs without providing additional benefits.
Dry Film Thickness vs. Wet Film Thickness
Roofing professionals commonly discuss two types of mil measurements:
Wet Film Thickness (WFT)
Wet film thickness refers to the thickness of a coating immediately after it is applied while the material is still liquid. Contractors use special tools called wet film gauges to measure the coating application during installation.
Dry Film Thickness (DFT)
Dry film thickness refers to the final thickness after the coating has cured and the solvents or water have evaporated. Roofing manufacturers typically specify required coating thickness based on dry film thickness because it represents the actual protective layer remaining on the roof.
For example, a manufacturer may require a silicone roof coating system to achieve 20–30 dry mils depending on the application, warranty requirements, and roof conditions.
Common Mil Thicknesses Used in Roofing
The required mil thickness varies depending on the roofing material and application. Some common examples include:
- Roof coatings: Typically applied in multiple layers ranging from approximately 20 to 40+ dry mils depending on the system.
- Waterproofing membranes: May require thicker applications to create a durable moisture barrier.
- Protective coatings: Thickness requirements depend on exposure levels, traffic, and environmental conditions.
- Single-ply roofing membranes: Materials such as TPO and PVC membranes are often measured in mils, with thicknesses commonly ranging from about 45 mils to over 80 mils.
A thicker membrane or coating does not automatically mean it is the best choice. The correct thickness depends on the roof type, climate, building usage, manufacturer specifications, and installation quality.
How Contractors Measure Roofing Mil Thickness
Professional roofing contractors use specialized equipment to verify coating thickness during and after installation. These measurements help confirm that the roofing system meets project specifications and manufacturer requirements.
Proper measurement is especially important for commercial roof restoration projects because insufficient coating thickness can lead to premature failure, leaks, and potential warranty issues.
Does a Higher Mil Thickness Mean a Better Roof?
A higher mil thickness can provide additional protection, but it is only one factor that determines roofing performance. A successful roofing system also depends on:
- Quality of installation
- Proper surface preparation
- Compatible roofing materials
- Correct application methods
- Regular maintenance and inspections
For example, a thick coating applied over a poorly prepared roof surface may fail sooner than a properly installed system with the correct mil thickness.
Choosing the Right Roofing System for Your Property
Understanding roofing terminology like “mil” helps property owners communicate effectively with contractors and evaluate roofing proposals. When comparing roof coating systems or restoration options, always look beyond the thickness measurement and consider the complete roofing solution.
A professional commercial roofing contractor can evaluate your existing roof condition, recommend the appropriate coating system, and ensure the correct mil thickness is applied according to manufacturer guidelines.
Shieldline Roofing’s Expert Opinion
Membrane thickness is only one factor in a roof’s performance. While a higher mil rating generally improves durability, we also consider the membrane material, installation quality, and the building’s expected foot traffic before recommending the right roofing system.
Our Key Insights
In roofing, one mil equals one-thousandth of an inch (0.001″). Commercial single-ply membranes are commonly available in 45-, 60-, and 80-mil thicknesses, with thicker membranes generally offering greater puncture resistance, longer service life, and improved durability in high-traffic or harsh-weather environments.
Final Thoughts
In roofing, “mil” refers to the thickness of a roofing material or coating measured in thousandths of an inch. It is a critical measurement used to determine whether a roof coating, membrane, or waterproofing system can provide adequate protection. Proper mil thickness, combined with expert installation and routine maintenance, helps extend roof life, prevent leaks, and protect your commercial property investment.
For commercial roof repairs, restoration, and coating solutions, working with an experienced roofing professional ensures your roofing system is installed correctly and performs reliably for years to come. Learn More
