Field note
Type X vs Type C gypsum: what the design actually named
“5/8 firecode” is not a core. Type X is a defined gypsum class. Type C is a mill designation for an enhanced Type X-style core. Hanging the cheaper one when the listing named the other is how a drywall sub inherits a failed inspection.
This page is takeoff language for estimators and PMs. It restates public class language from ASTM C1396 (gypsum board), the Gypsum Association’s Type X / Type C notes, GA-600, UL Product iQ, and mill explainers from USG, Georgia-Pacific, National Gypsum, and peers. It is not a reprint of a UL design and not a substitution license. See choosing drywall type for the rest of the board family and reading UL designs for how the sheet is read.
Type X is in the ASTM
ASTM C1396 defines Type X gypsum board. Regular board is not Type X. Thickness is not Type X. The core is formulated with glass fiber so the panel holds together as gypsum calcines and shrinks in a fire test.
Public association language for the class:
- 5/8-inch Type X is defined around a 1-hour result in a specified wood-stud wall (load-bearing 2×4 at 16 inches o.c., named nails, staggered joints).
- 1/2-inch Type X is defined around a 45-minute result in a similar specified test.
- 1-inch Type X language exists for liner / coreboard products — that is shaftliner, not ripped wallboard.
Those definition tests are not a license to claim any 5/8 Type X wall is 1-hour. The hourly rating lives in the listed design: layers, orientation, fasteners, studs, and insulation as named. Type X is the core class the design is allowed to call for.
Use: garage separations, multi-family demising, commercial partitions and ceilings wherever the design or the code names Type X. Commercial interiors often stock 5/8 Type X even on unrated walls so the floor carries one thickness. That is a production choice. It is still Type X, not regular 5/8.
Not automatic: leftover 5/8 regular is not Type X. Lightweight 5/8 is Type X only when that mill’s product is classified as Type X in the design you are building.
Type C is not a separate ASTM letter
Type C is an industry designation. It is not a second letter in C1396. Mills and UL use “Type C” for proprietary enhanced Type X cores: more glass fiber and, typically, a shrinkage-compensating additive (often vermiculite) that expands as gypsum loses water. The panel stays more dimensionally stable in the furnace. That is why some 2-hour single-layer walls, some floor-ceilings, some column wraps, and some shaft faces call for Type C by name.
Association language that estimators actually use: all Type C is Type X; not all Type X is Type C. A 5/8 Type C panel generally may replace 5/8 Type X in a design that called for Type X of that thickness — confirm the mill. A Type X panel may not replace Type C where the design named Type C. You cannot invent a 2-hour wall by upgrading the core on a 1-hour Type X listing.
There is no generic “Type C.” Composition varies by mill. The product on the submittal is the named Type C (or the mill’s equivalent proprietary enhanced core) that the UL / GA-600 design lists. American Gypsum, CertainTeed, PABCO, and the mills above publish which of their cores sit in which designs.
How the mix-up shows up on a takeoff
The drawing says “5/8 Type X both sides.” Hang Type X (or a mill-listed Type C upgrade of that thickness). Do not hang 1/2 Type C and call it even.
The UL design names Type C, a proprietary core, or a mill type designation. Hang that product. Type X from the other truck is not a silent substitute. Reading UL designs is the sheet-reading order.
The design names a mill UL type (a letter code on the board edge). That code is the product. “Type X” on the takeoff that does not match the edge stamp is still the wrong board.
Ceiling vs wall. Some floor-ceiling designs reach the hour with Type C in a single layer where Type X would have needed two. That is a tested construction, not a field invention. Do not copy a ceiling core onto a wall listing.
Shaft and column. Occupant-side shaft faces and column wraps often name Type C. The liner is a different 1-inch Type X product. Do not mix those lines.
Hang still follows ASTM C840 / GA-216 unless the listing is tighter. Fastener class does not change because the core letter changed. Framing is still the steel-stud package the design named.
Commercial vs residential
Commercial wall and ceiling. Type C is a real line item on 2-hour corridors, some shaft faces, and some floor-ceilings. Price it separately. A GC who says “it’s all firecode” is describing Type X. Qualify the design number.
Residential production. Everyday garage separations and townhouse partitions that name Type X are Type X. Type C shows up when the listed design or a floor-ceiling assembly actually calls for it — not as a production default. Texture does not rewrite the core.
Bid notes
- Take off product name and thickness, not “fire-rated 5/8.”
- Separate Type X, Type C, lightweight Type X, glass-mat Type X, and shaftliner. Five cores, five trucks if the yard treats them that way.
- Substitutions go through the design professional and the AHJ. The lumber counter does not reissue a UL design.
- Type C as an upgrade to Type X is a mill confirmation, not an estimator habit.
- Product door: Products. Mills: Manufacturers.
Related directory
Board family: Choosing drywall type, Products. Designs: Reading UL / GA-600 designs. Shaft liner as a different 1-inch Type X: Shaftliner types, Shaftwall installation. Mills: USG, National Gypsum, Georgia-Pacific Gypsum, American Gypsum, CertainTeed Gypsum, PABCO Gypsum.
FAQ
Can I swap Type C for Type X to save a layer?
No. Layers come from the listed design. Type C is not a license to omit a sheet.
Can I swap Type X for Type C to save money?
Not where the design named Type C. Where the design named Type X, Type C of the same thickness is often an allowed upgrade — confirm the mill, do not assume.
Is 5/8-inch the same as Type X?
No. Regular 5/8 exists. Type X is the core class.
Does Type C have its own ASTM type letter?
No. C1396 defines Type X. Type C is a proprietary enhanced Type X used in UL and mill language.
Why does commercial stock Type X on unrated walls?
One thickness, one fastener length, one rated SKU on the floor. Still hang the core the rated walls named.