Direct Answer
In roofing, a mil is a unit of measurement used to describe the thickness of roofing membranes, protective coatings, and other roofing materials. One mil equals one-thousandth of an inch, or 0.001 inches. It is not the same as a millimeter.
For example, a 60-mil commercial roofing membrane has a nominal thickness of 0.060 inches, while an 80-mil membrane has a nominal thickness of 0.080 inches. In general, a thicker membrane can offer greater resistance to punctures and physical wear, but thickness alone does not determine the quality, durability, or expected lifespan of a roofing system. Material composition, installation quality, roof design, and maintenance also matter.
Understanding mil thickness helps commercial property owners compare roofing proposals, evaluate membrane specifications, assess coating applications, and determine whether a proposed roofing system meets the project’s requirements.
Who This Applies To
- Commercial property owners
- Property and facility managers
- Commercial real estate investors
- Developers and general contractors
- Condominium and HOA boards
- Building owners comparing TPO, PVC, and other roofing systems
- Owners considering roof coatings or restoration
Not for: Mil thickness alone cannot establish whether a roof is suitable for a particular building or whether a coating or membrane meets a manufacturer’s warranty requirements. Always verify the complete system specifications with the manufacturer and qualified roofing professional.
1. What Does One Mil Mean?
One mil equals 0.001 inches, or approximately 25.4 micrometers. Roofing manufacturers use this measurement to specify the nominal thickness of certain membranes and the wet or dry film thickness of coatings.
| Measurement | Thickness in Inches | Approximate Thickness in Millimeters |
|---|---|---|
| 10 mil | 0.010 inches | 0.254 mm |
| 20 mil | 0.020 inches | 0.508 mm |
| 45 mil | 0.045 inches | 1.143 mm |
| 60 mil | 0.060 inches | 1.524 mm |
| 80 mil | 0.080 inches | 2.032 mm |
These conversions illustrate the difference between mils and millimeters. A 60-mil membrane is approximately 1.52 millimeters thick, not 60 millimeters thick.
The National Roofing Contractors Association (NRCA) roofing glossary defines a mil as 0.001 inches and identifies it as a measurement commonly used for roofing membrane thickness.
2. What Does Mil Mean for Commercial Roofing Membranes?
Commercial single-ply roofing membranes, including thermoplastic polyolefin (TPO), polyvinyl chloride (PVC), and ethylene propylene diene monomer (EPDM), are available in different thicknesses depending on the product and manufacturer.
For example, commercial TPO membranes may be specified at 45, 60, or 80 mils. These numbers describe the membrane’s nominal thickness rather than its weight or overall quality rating.
| Membrane Thickness | General Consideration | What Owners Should Evaluate |
|---|---|---|
| 45 mil | A thinner membrane option available for certain applications | Project specifications, expected exposure, and manufacturer requirements |
| 60 mil | A commonly specified commercial membrane thickness | Building use, durability requirements, installation method, and warranty terms |
| 80 mil | A thicker membrane option | Potential benefits for demanding applications, cost, and complete system performance |
These examples are not universal recommendations. Available thicknesses, approved assemblies, and minimum requirements vary by membrane type and manufacturer.
For more information, see the NRCA roofing guidance on membrane systems.
3. Why Does Membrane Thickness Matter?
Membrane thickness can influence how a roof covering responds to physical wear and certain forms of damage. However, the relationship between thickness and performance depends on the material’s composition, reinforcement, manufacturing process, and installation.
Important considerations include:
- Puncture resistance: Thickness can contribute to resistance against damage from tools, dropped objects, and rooftop activity.
- Physical wear: A thicker membrane may provide additional material to withstand certain forms of abrasion and service-related wear.
- Weather exposure: The membrane’s formulation, reinforcement, and tested properties influence its ability to withstand environmental conditions.
- Building operations: Roofs with frequent maintenance access or rooftop equipment may require different specifications from roofs with limited access.
- Long-term performance: Material selection, installation quality, drainage, maintenance, and repairs all influence service life.
A thicker membrane is not automatically the best choice for every building. Owners should compare the complete roofing assemblies and their documented performance rather than selecting a product based on thickness alone.
4. What Does Mil Mean for Roof Coatings?
Roof coatings also use mils to describe film thickness. However, coating specifications require an important distinction between wet film thickness (WFT) and dry film thickness (DFT).
- Wet film thickness: The thickness of the coating immediately after application.
- Dry film thickness: The thickness remaining after the coating has cured and the applicable water or solvents have evaporated.
Wet and dry film thickness are not necessarily the same. The relationship depends on the coating’s formulation and volume-solids content.
For example, a coating applied at 20 wet mils will generally produce a thinner dry film if some of its volume evaporates during curing. Therefore, a specification calling for 20 dry mils should not be assumed to mean that applying 20 wet mils is sufficient.
When reviewing a roof restoration proposal, confirm:
- Whether the specified thickness is wet or dry
- The manufacturer’s required application rate
- The number of coats required
- The expected dry film thickness
- The preparation and repair requirements
- The inspection and measurement procedures
- The applicable warranty conditions
The NRCA glossary explains that wet film thickness refers to a coating as applied, while dry film thickness refers to its thickness after curing.
5. Does a Higher Mil Rating Guarantee a Better Roof?
No. A higher mil rating indicates greater nominal thickness when comparing measurements expressed on the same basis, but it does not guarantee superior overall performance.
Two membranes with the same nominal thickness may differ in:
- Polymer formulation
- Reinforcement and internal construction
- Tensile and tear strength
- Resistance to weathering
- Seam-welding or bonding characteristics
- Compatibility with the roof assembly
- Manufacturer testing and approved applications
Similarly, a thicker membrane can still fail if it is installed incorrectly, has defective seams, or is exposed to conditions outside its intended application.
Owners should evaluate the product data sheet, applicable product standards, approved roof assembly, installation specifications, and warranty requirements before making a decision.
6. How Should Commercial Property Owners Compare Roofing Proposals?
When two contractors propose different membrane thicknesses, compare the complete scope rather than choosing the lowest price or highest mil rating.
Ask each contractor to specify:
- The membrane manufacturer and product name.
- The nominal membrane thickness and applicable product standard.
- The complete roof assembly, including insulation and cover boards.
- The attachment method and seam requirements.
- The proposed flashing and penetration details.
- The manufacturer’s warranty eligibility and terms.
- The installation inspection and quality-control procedures.
- The expected maintenance requirements.
Confirm that the proposed system is suitable for the building’s use, environmental exposure, rooftop traffic, and project requirements. For Florida properties, also verify applicable wind-uplift, building-code, and product-approval requirements with the project design professional and local authority having jurisdiction.
7. Does Mil Thickness Affect Roofing Warranties?
It can. Manufacturers may specify particular membrane thicknesses, roof assemblies, installation procedures, and other conditions for certain warranty options.
However, there is no single mil thickness that automatically guarantees a manufacturer warranty. Requirements vary by manufacturer, product, approved assembly, installation details, and requested warranty coverage.
Before approving a roofing proposal, obtain the applicable warranty requirements in writing and confirm that the proposed system satisfies them. Do not assume that selecting a thicker membrane automatically qualifies the building for a longer warranty or broader coverage.
Owners should also retain product documentation, installation records, inspection reports, and maintenance records to support future warranty service.
8. How Is Mil Thickness Verified?
The appropriate verification method depends on the material being measured.
For roofing membranes, owners and inspectors may review manufacturer product data, material identification, specifications, and appropriate physical measurements when needed.
For liquid-applied coatings, contractors may use suitable wet-film gauges during application and appropriate dry-film measurement methods where applicable. Measurement techniques must be suitable for the coating, substrate, and manufacturer’s requirements.
For a coating project, request documentation showing:
- The specified wet or dry film thickness
- The manufacturer’s required application rate
- The quantity of material used
- The areas coated
- Quality-control measurements
- Any corrective work needed to meet specifications
Material usage alone does not prove that the required coating thickness was achieved uniformly across the roof. Appropriate inspection and measurement records provide stronger evidence of compliance.
9. Common Mistakes to Avoid
- Confusing mils with millimeters: One mil is 0.001 inches, not one millimeter.
- Comparing different measurements: Do not compare a coating’s wet film thickness with its dry film thickness as though they are equivalent.
- Assuming thicker always means better: Consider material properties, installation, and the complete roof assembly.
- Ignoring warranty requirements: Confirm the actual manufacturer specifications rather than relying on a contractor’s general statement.
- Overlooking installation quality: Proper seams, flashing, attachment, drainage, and detailing remain essential.
- Accepting undocumented coating thickness: Request appropriate application and inspection records.
Bottom Line
A mil in roofing equals one-thousandth of an inch (0.001 inches) and is commonly used to describe membrane and coating thickness. For example, a 60-mil membrane measures approximately 0.060 inches thick.
Thickness is an important specification, but it does not determine roof quality by itself. Commercial property owners should compare membrane composition, product performance, installation details, complete roof assemblies, warranty requirements, and maintenance needs before approving a roofing system.
Related Questions
- What does mil refer to in TPO roofing?
- What mil thickness is required to secure a manufacturer warranty?
- What is a commercial roof condition report?
Sources
- National Roofing Contractors Association — Roofing Glossary
- National Roofing Contractors Association — Roofing Membrane Systems Guidance
- Roofing Contractors Association of British Columbia — Flexible Membrane Properties
Last reviewed: October 2026
Related Resources
Learn more about mil thickness in TPO roofing.
Need Commercial Roofing Help?
ShieldLine Roofing can help commercial property owners compare membrane specifications, review roof restoration proposals, and evaluate roofing systems based on building requirements, installation quality, and long-term performance.
Disclaimer: This information is provided for general educational purposes and is not engineering, construction, warranty, or insurance advice. Material specifications and warranty requirements vary by manufacturer and roofing system. Consult the applicable product documentation and a qualified roofing professional before making project decisions.
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