Interior wallboard is a paint substrate. Exterior gypsum sheathing is a cladding substrate. Mixing those jobs is how a shop tapes DensGlass-class joints like a Level 4 corridor, or how a GC assumes the weather-resistive barrier started when the board went on.
This page is for the drywall / wall-and-ceiling sub who hangs glass-mat or paper-faced gypsum sheathing and has to know where the WRB begins. It paraphrases public class language from ASTM C1177 (glass-mat gypsum substrate for sheathing), ASTM C1396 (paper-faced gypsum board, including sheathing types), ASTM C1280 (application of exterior gypsum panel products as sheathing), and GA-253 (application of gypsum sheathing). It is not a reprint of those documents and not a cladding how-to. Buy the current editions. Fire-rated exterior walls still follow the listed UL / GA-600 design.
Manufacturer examples: Georgia-Pacific Gypsum (DensGlass-class), USG, National Gypsum, CertainTeed Gypsum. Directory: Exterior sheathing, Products.
Two boards, two ASTM families
Glass-mat gypsum sheathing (ASTM C1177). Fiberglass mat instead of paper. This is the DensGlass-class product the commercial envelope actually wants in most current specs: a substrate for cladding, with a manufacturer weather window longer than paper sheathing. It is not interior C1396 wallboard, not a paint-grade face, and not a silent WRB unless the specific product and joint treatment are listed as a WRB system.
Paper-faced gypsum sheathing (ASTM C1396 sheathing type). The older paper sheathing still shows up. Exposure time is short. GA-253’s public handling language is blunt: cover it; if exposure will run long, get a water-resistive barrier on it. Do not prerock paper sheathing the way you prerock glass-mat and hope.
Interior glass-mat (DensArmor-type and peers) is a different SKU for interiors. Exterior sheathing is the elevation. See choosing drywall type.
Application of either as sheathing is ASTM C1280 / GA-253, not interior ASTM C840 / GA-216. Fastener type, spacing, and edge distance come from those application documents, the fire design if rated, and the board manufacturer. Interior hang habits — how to hang drywall — do not travel.
Who hangs it
On a typical commercial CFS wall:
- Backup steel may be W&C, a CFS sub, or the GC’s skin. Interior drywall stud is not the member. Exterior CFS vs interior light-gauge, Exterior cold-formed framing.
- Gypsum / glass-mat sheathing is often in the wall-and-ceiling number because the same shop already hangs gypsum. Confirm. If it is yours, it is a sheathing hang: corrosion-resistant fasteners, staggered vertical joints, snug fit at openings, no interior compound.
- WRB — building wrap, self-adhered membrane, fluid-applied, or an integrated glass-mat WRB system — may be you, the GC, or an envelope contractor. The hang of the board is not automatically the start of the barrier.
- ci and cladding rails sit outboard or as insulating sheathing. Continuous insulation.
Residential: wood-frame houses more often get wood structural panels plus wrap. Glass-mat gypsum sheathing shows up on mixed-use, some multi-family, and IRC bracing / fire-resistance details when the documents name it. A residential production hang crew that has never set glass-mat on CFS should not price it like 1/2-inch interior. Different gun, different weather, different punch.
Joints, openings, and what “treated” means
GA-253 / C1280 class rules the field lives on (confirm the current text):
- Cut by scoring and snapping or by sawing; trim cut edges so joints fit.
- Stagger vertical joints. Fit snugly around openings. Offset sheathing joints from the edge of openings — GA-253’s public language uses a 4-inch minimum offset from the opening edge.
- Sheathing is not continuous through building construction joints. Stack with the building joint; do not bridge it.
- Where shear values are not required, 4-ft-wide sheathing may run parallel or perpendicular to framing, with vertical joints on framing.
Joint treatment is not interior taping. Glass-mat edges are dimensionally stable; industry guidance (Gypsum Association / gypsumsheathing.org class notes) is that joints are not regularly finished unless the cladding or the barrier system requires it. Integrated WRB glass-mat systems do require the manufacturer’s mastic, tape, or sealant at joints, penetrations, and openings. That work is envelope work even if your taper is standing there with a knife. Paper-face interior compound on a glass-mat elevation is the wrong product.
Horizontal joints. GA-253’s public application notes send you to a water-resistive barrier or sealed horizontals per the local code when that is the path. Do not leave a raw horizontal in the weather because the verticals looked tight.
Flashings at openings are not the gypsum joint. Sheathing is fitted; flashing and WRB transitions are the barrier. If WRB is not in your number, still leave the opening in a condition the next trade can flash. Punch photographs of stained glass-mat are how that argument starts.
Where the weather barrier starts
Three common stacks — the drawings pick one:
- Sheathing + wrap or membrane WRB. You hang the board. WRB goes over it. The barrier starts when the wrap/membrane is on and detailed, not when the last screw went in.
- Glass-mat as the WRB. Only if that exact product is listed as a WRB and joints, fasteners, and openings are treated per that manufacturer. Untreated joints are not a WRB. This is the coordination fight: GC thought DensGlass-class is the wrap; spec wanted joints sealed or a separate membrane.
- Fluid-applied over sheathing. Board is the substrate. Fluid starts after hang, fastener heads, and any required joint prep. Pinholes at overdriven screws are your hang, not the coating vendor’s.
ci outboard of sheathing does not erase the WRB question. Foam can be part of a WRB system when the foam manufacturer says so; it can also be just insulation over a wrap. Continuous insulation.
ASTM C1280’s public scope is a substrate for cladding; gypsum sheathing is not intended for long-term exposure. Glass-mat manufacturers publish a longer weather window than paper — read the sheet you submitted, do not quote a number from another brand.
Fasteners and the backup
Sheathing screws are not interior bugle Type S by default. Head style (wafer, pancake, bugle), coating, and point follow the sheathing and the steel thickness. Light-gauge interiors are C1002 Type S; heavier exterior CFS is often C954 class. Type S vs Type W, Fasteners, Grabber.
Hit framing. Edge distance still matters so you do not blow the core. Depth: snug, not a punched mat. Torn glass-mat at a run of overdriven screws is a leak path and a cladding telegraph.
Rated exterior walls: fastener spacing and orientation come from the UL design, which can beat the general C1280 table. Same rule as interiors — the design number wins.
Framing has to be in plane. Oil-canning 25-ga drywall stud is not an exterior backup. Steel stud framing basics, ClarkDietrich.
Bid and punch
Price sheathing as sheathing: lifts on the elevation, corrosion-resistant fasteners, layout to openings, control of exposure. Do not use interior production rates.
Write who owns WRB, joint treatment for an integrated barrier, fluid-applied prep, and ci. If the spec says “glass-mat sheathing, joints treated as WRB” and your number was hang-only, you will donate mastic.
Commercial vs residential stays distinct. Commercial CFS + glass-mat + WRB is an envelope. Residential wood + wrap is usually not in the drywall number. Multi-family area separation and shaftliner are interior systems — not this hang.
Related directory
Exterior sheathing, Exterior cold-formed framing, Steel stud / framing, Fasteners, Products. Board makers: Georgia-Pacific Gypsum, USG, National Gypsum, CertainTeed Gypsum.
FAQ
Who typically hangs DensGlass-class board?
Often the wall-and-ceiling sub, because it is gypsum. Confirm the package. Hanging it does not automatically include the WRB.
Do I tape glass-mat sheathing joints like interior drywall?
No. Joints are untreated unless the cladding or the WRB system requires the manufacturer’s sealant/tape. Interior compound is the wrong material.
When does the weather barrier start?
When the specified WRB — wrap, membrane, fluid-applied, or a listed integrated glass-mat system with treated joints — is actually installed. The last sheathing screw is not that moment.
Can paper-faced gypsum sheathing sit in the weather like glass-mat?
Not as a class. Paper sheathing has the short exposure window. Cover it or get WRB on it. Follow GA-253 and the product data.
Is exterior sheathing a Level 4 wall?
No. It is a substrate for cladding. Paint-grade interiors are C1396 / C840 on the room side after close-in, or a listed prerock glass-mat interior — different product.