CMU Fire Rating Chart: Equivalent Thickness for Concrete Masonry Units
When your project needs to meet a specific fire resistance rating, choosing the right CMU is not guesswork. This page gives you the equivalent thickness values for concrete masonry units manufactured by Ernest Maier and Parker Block, referenced against the 2014 edition of ACI/TMS 216.1. Use this chart to specify the right block before the job starts and avoid costly rework during inspection.
Why Fire Ratings Matter for CMU Selection
Building codes require structural assemblies to resist fire for a minimum period before failure. For concrete masonry units, that resistance is measured in equivalent thickness, which is the solid material cross-section that blocks radiant heat transfer. Specifying the wrong unit can result in a failed inspection or rework that stalls a project.
The 2015 and 2018 International Building Code both reference the 2014 edition of ACI/TMS 216.1 for concrete and masonry fire resistance calculations. The Concrete Masonry and Hardscapes Association documents these calculations in TEK 07-01D. Ernest Maier CMU is tested and documented against current industry standards, so the values on this page are ready to cite in your submittals.
About Ernest Maier CMU Aggregates
Ernest Maier and Parker Block CMU use quarried stone, expanded shale, and/or pumice as primary aggregates. These lightweight aggregates increase the thermal resistance of each unit and are a key factor in the equivalent thickness calculations below. For unit dimensions and configurations, see our CMU shape guide.
Ernest Maier CMU Complying with the 2014 Edition of the ACI/TMS 216.1
Equivalent Thickness Table 6.0 – Tek Note 7.01D
| Fire Resistance | Nominal Size | ASTM | Solid Materials | Equivalent Thickness (in) |
| ½ Hour | 2x8x16
3x8x16* |
C-145
C-90 |
100% 73% |
1.9 1.9 |
| ¾ Hour | 4x8x16
3x8x16 |
C-90
C-145 |
65% 100% |
2.4 2.6 |
| 1 Hour | 4x8x16
4x8x12 4x8x16* |
C-90
C-90 C-145 |
73% 73% 78% |
2.7 2.7 2.8 |
| 1 ½ Hours | 6x8x16
8x8x16 Normal Weight |
C-90
C-90 |
54% 50% |
3.0 3.8 |
| 2 Hours | 2-1/4×3-3/4×8**
4x8x16 4x8x12 8x8x24*** 6x8x16*** 8x8x16 Medium and Light Weight*** |
C-55
C-145 C-145 C-90 C-90 C-90 |
100% 100% 100% 52% 64% 50% |
3.6 3.6 3.6 3.7 3.6 3.8 |
| 3 Hours | 8x8x16
10x8x16 |
C-90
C-90 |
63% 50% |
4.8 4.8 |
| 4 Hours | 8x8x16
8x8x12 12x8x12*** 12x8x16 6x8x16 8x8x12 8x8x16 |
C-145
C-145 C-90 C-90 C-145 C-145 C-145 |
75% 75% 55% 50% 100% 100% 100% |
5.7 5.7 5.4 5.6 5.6 7.6 7.6 |
The 2014 edition of the ACI/TMS 216.1 is referenced by the 2015 and 2018 IBC for concrete and masonry materials
* BMS 92 – National Bureau Standard
***Above Blocks Plastered on One Side with 1/2 inch 1:3 Gypsum and Sand Plaster adds 1/2 Hour to Fire Rating
Primary Aggregates – Expanded Shale and/or Pumice
Plaster Addition for Fire Rating Improvement
Plastering one side of a CMU wall with 1/2 inch of 1:3 gypsum and sand plaster adds 1/2 hour to the fire resistance rating of the assembly. This is a practical option when a project is close to the next rating threshold and changing block size is not feasible.
How to Use This Chart for Project Submittals
Start with your required fire resistance period from the building code or structural drawings. Locate the applicable rating row and identify block sizes that meet or exceed the required equivalent thickness. Cross-reference the ASTM standard and percent solid against the block you are planning to order. To estimate quantities for your wall, use our masonry wall calculator. For spans over openings, review the concrete lintel load tables. Confirm selection with your project architect or structural engineer before finalizing the submittal.
For large projects or specialty applications, contact Ernest Maier directly. Stock varies by location and high-solid units may require advance ordering.
Contact Ernest Maier to confirm availability for your project size and timeline.
FAQ
Q: What standard does Ernest Maier use for CMU fire ratings?
A: Ernest Maier CMU complies with the 2014 edition of ACI/TMS 216.1, which is referenced by both the 2015 and 2018 International Building Code for concrete and masonry fire resistance.
Q: What is equivalent thickness and why does it matter?
A: Equivalent thickness is the calculated solid material cross-section of a CMU unit. Building codes use this value to determine how long a wall assembly will resist fire. A higher equivalent thickness means a longer fire resistance period.
Q: Can adding plaster to a CMU wall improve the fire rating?
A: Yes. Plastering one side of a CMU wall with 1/2 inch of 1:3 gypsum and sand plaster adds 1/2 hour to the fire resistance rating of the assembly.
Q: What aggregates are used in Ernest Maier CMU?
A: Ernest Maier and Parker Block CMU use quarried stone along with expanded shale and/or pumice as primary aggregates. These lightweight aggregates contribute to the unit’s thermal resistance properties.
Q: How do I choose the right CMU for a 2-hour fire rating?
A: Several block options meet a 2-hour rating, including 4x8x16 C-145 units at 100 percent solid and 6x8x16 C-90 units at 64 percent solid. Review the equivalent thickness values in the chart above and confirm selection with your project architect or structural engineer.
Q: Where can I order fire-rated CMU from Ernest Maier?
A: See our e-store or contact Ernest Maier directly for availability and pricing. Stock varies by location, and specialty or high-solid units may require advance ordering for large projects.
VP of MasonryTed Kelly
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