Fire-rated assembly on the plans calls for 5/8" Type C on the ceiling but the yard only has Type X.

No — do not substitute Type X for Type C without engineering approval.

Type C and Type X are different gypsum board formulations with distinct fire-performance characteristics:

• Type X contains fiberglass or other additives for enhanced fire resistance (typically 1-hour rated)
• Type C is formulated with specific core additives (like vermiculite or proprietary compounds) designed for different thermal performance and may have different charring rates

While both are 5/8" and may carry similar UL labels, the chemical composition and fire-testing are not interchangeable. The fire-rated assembly was tested and listed with Type C specifically.

Code Reference:

• IBC Section 703.4 requires fire-resistant materials to be installed per their listed assembly ratings
• ASTM E84 and ASTM E119 testing is specific to each board type

Your Options:

1. Get the correct Type C — this is the safest path
2. Contact the architect/engineer for a substitution request with engineering analysis
3. Check if the manufacturer offers a listed alternative — some brands have cross-listed assemblies

Bottom line: Even though the thickness and label rating appear identical, the core composition differs. Using Type X could compromise the assembly's actual fire rating and violate the building code. This isn't a field substitution you can approve yourself.

Get Type C or get written engineering approval before proceeding.

No. Type X and Type C are not interchangeable in a tested assembly, even at the same thickness and fire rating.

Why it matters

• Type X (ASTM C1396) meets the basic 1-hr/2-hr criteria with core additives.
• Type C (ASTM C1396, “Improved Type X”) contains additional glass fibers and vermiculite that reduce shrinkage and maintain integrity longer under fire exposure.
• UL/ULC designs (e.g., UL Design L500 series, U300 series) list specific board types. Substituting Type X for a listed Type C voids the assembly’s fire rating.

Code references

• IBC 2021 §703.3 – Fire-resistance ratings shall be determined by tested assemblies per ASTM E119/UL 263.
• IBC §2508.1 – Gypsum board shall conform to ASTM C1396 and be installed per the tested assembly.
• ASTM C1396 §5.2 – Type C is a distinct classification from Type X.

What to do

1. Order Type C from a supplier who stocks it (often a drywall specialty yard).
2. If the schedule is critical, submit a substitution request to the AHJ with the exact UL design number and manufacturer’s data showing equivalency — rarely approved for Type X → Type C.
3. Document the UL design number on the submittal; the inspector will verify the board label matches the design.

Bottom line: Same thickness ≠ same performance. Install the board the assembly was tested with.