The same mill will deliver interior drywall stud and exterior cold-formed steel to the same site. They are not the same bid. Interior light-gauge is the partition the commercial drywall sub already carries. Exterior CFS is backup, curtain wall, or load-bearing — wind, axial load, sheathing, a different ASTM family.
Mixing them is how a shop hangs 25-ga habits on a wind-bearing wall, or how an estimator lets a structural CFS package hide inside a TI stud count. This page is for the drywall / wall-and-ceiling sub who sees both on one job. It is not an engineering stamp. Member selection still comes from the project documents and the manufacturer’s tables — ClarkDietrich, MarinoWARE, and peers.
Two families of steel
Interior nonstructural (light-gauge drywall stud). Members manufactured to ASTM C645; installation ASTM C754. Newer code language also points at AISI S220 for nonstructural CFS. These are the 3-5/8 and 6-inch interior partitions, the soffit, the drywall-side furr. Lateral design in the interior tables is a partition load (the 5 psf class of interior wall), not a wind load. See steel stud framing basics. Directory: Metal stud / light-gauge framing.
Structural / exterior CFS. Members manufactured to ASTM C955 (base steel not less than 0.0329 inch in that spec’s structural-member scope); design AISI S240 / AISI S100. Curtain-wall backup, load-bearing interior, exterior walls, floor joists in a CFS structure. Transverse and axial loads that exceed the nonstructural limits. Directory: Exterior cold-formed framing.
The shape can look the same in a photo. The standard, the coating, the mils, the flange, the clip, and the fastener do not.
IBC editions vary in whether they still lean on C645/C955 or have moved the CFS references to AISI S220/S240. Read the spec section on the job. Do not argue the wall from a blog post.
“20-ga” is not one product
The trailer still says gauge. The tag says mils. Interior 20-ga drywall is commonly ~30 mil. 20-ga structural is commonly ~33 mil and lives under C955. Those are not interchangeable stickers.
EQ / equivalent-gauge interior studs are rolled so a mill can rate them as a 25-ga or 20-ga drywall equivalent. They are not a free upgrade to structural 20-ga and they are not an exterior backup member unless that exact EQ product is in an exterior listing — which, as a class, it is not. Use EQ only where the interior spec and the height table allow EQ.
Coatings differ as a class: C645 interiors often sit at a G40-class minimum; C955 structural language is commonly a heavier G60-class coating. Exterior walls also see specified G60 / G90 because the backup gets wet before the cladding is on. Do not send interior G40 drywall stud to the envelope because the depth matched.
Color codes are mill-specific and change. Read the bundle tag for mils and spec (NS vs structural designator), not the color you memorized last year.
What changes on the exterior line
Loads. Wind and, on bearing walls, gravity. Limiting-height tables for interior 5 psf / L/240 do not size a curtain wall. The structural engineer’s drawings and the mill’s exterior / curtain-wall tables do. Bridging, clip connectors, and headers are part of that system, not optional drywall bridging.
Sheathing, not Level 4 board. Exterior gypsum and glass-mat sheathing (Georgia-Pacific DensGlass and peers) close the backup. Different ASTM (glass-mat sheathing is not C1396 interior wallboard), different fasteners, different joint treatment. Application of gypsum sheathing is ASTM C1280 / GA-253, not the interior GA-216 hang pattern. It is a substrate for cladding, not a paint finish. See choosing drywall type and Exterior sheathing.
Fasteners. Interior hang into thin drywall stud is ASTM C1002 Type S. Heavier exterior and structural steel wants ASTM C954 (Type S-12 class) self-drilling points, plus the mill’s structural framing screws for steel-to-steel. Sheathing screws are yet another data sheet. See Type S vs Type W and Fasteners.
Openings and deflection. Exterior heads, jambs, and sills are engineered. Interior slotted deflection track is a partition detail under a deck; it is not a curtain-wall bypass. Do not copy a TI door-frame habit onto a punched window wall.
Weather and sequence. Exterior CFS often stands before the building is closed in. Interior drywall stud wants a closed, conditioned box before gypsum goes on (manufacturer environmental notes). Prerock interiors and glass-mat liner are the exceptions the mill actually listed — not a license to hang paper-faced Type X on an open elevation.
Who owns which package
On a typical commercial job:
- Interior metal stud, board, finish, often insulation and ACT sit in the drywall / wall-and-ceiling number sold to the general contractor.
- Exterior CFS, sheathing, and sometimes the WRB-adjacent board may sit in that same W&C number, in a separate CFS subcontract, or with the GC’s skin package.
Read the bid invitation. If both are yours, split the estimate anyway: different members, different screws, different labor, different punch. If exterior is not yours, still coordinate where interior partitions die into exterior backup — that joint is where fire, air, and deflection details fight.
Residential production shops meet light-gauge on townhomes, mixed-use, and garage demising. They meet exterior CFS less often; when they do, it is not “the metal-stud package we already know.” Do not toekick a structural stud like a 2×4, and do not hang Type W into it.
Shaftwall is a third interior system (liner + C-H / H-stud), not exterior CFS and not a 6-inch drywall partition. Keep it on its own takeoff line.
Layout habits that do not travel
Interior drywall sequence — snap tracks, 16 or 24, jacks at doors, board both sides for composite height — is in the steel stud guide. Exterior CFS adds clip layout, bridging that stays, engineered openings, sheathing patterns, and often a survey of the slab and structure the interior crew never does.
Finish-sensitive interior walls still care about L/240 vs L/360 because Level 5 photographs waves. Exterior backup cares about cladding tolerance and wind. Using the interior deflection limit on an exterior wall, or the exterior wind table on a 9-foot office partition, is how both get sized wrong.
Related directory
Interior package: Steel stud / framing. Exterior package: Exterior cold-formed framing. Sheathing: Exterior sheathing, Georgia-Pacific Gypsum. Mills: ClarkDietrich, MarinoWARE, Manufacturers. Screws: Fasteners, Grabber.
FAQ
Is exterior CFS usually in the drywall sub’s number?
Sometimes. Interior metal stud usually is. Exterior backup and bearing CFS are a separate question on every bid. Confirm the package.
Can I use interior 25-ga or EQ stud on the envelope if the depth matches?
Not as a class substitution. Exterior / structural members are C955 / AISI S240 class, with the mils, coating, and connectors that job named. Depth is not the spec.
Why does 20-ga drywall not equal 20-ga structural?
Different mil thickness and different ASTM. ~30 mil interior DW vs ~33 mil structural is the usual split. Read the tag.
Do I hang interior board on exterior studs?
You hang what the elevation calls for: sheathing on the outside, and on the inside either prerock glass-mat, delayed paper interiors after close-in, or a furr-out. Paper-faced interiors on an open, wet backup are a manufacturer environmental problem, not a production win.
Is shaftwall exterior CFS?
No. Cavity shaftwall and area separation are interior rated systems with liner panels. Different members, different sequence, different directory term.