What Are Metal Stud Knockouts and Why Are They Provided?
Metal stud knockouts are standardized, pre-punched openings located along the centerline of a C-shaped steel stud's web. They are manufactured during the cold-roll forming process.
Knockouts serve two primary construction functions:
- Utility Service Passage: They allow electrical cables, non-metallic sheath (Romex), metal-clad (MC) conduit, PEX, and copper water pipes to run horizontally through framed walls without damaging the steel or requiring manual field cutting.
- Mechanical Lateral Bridging: They allow 3/4-inch or 1-1/2-inch cold-rolled U-channels (CRC) or specialized spacing bars (such as Spazzer® bars) to pass horizontally through stud webs to brace members against flexural-torsional buckling.
+-------------------------------------------------+
| |
Flange | +---+ +---+ | Flange
=====| | | PUNCHOUT | | |=====
| | | /-------------\ | | |
| | | | Centerline | | | |
Web | | | | Knockout | | | | Web
| | | \-------------/ | | |
=====| | | | | |=====
Flange | +---+ +---+ | Flange
| |
+-------------------------------------------------+
Knockout Locations, Alignment, and Spacing
Industry specifications — including Steel Framing Industry Association (SFIA) standards, American Iron and Steel Institute (AISI) S220/S240 standards, and manufacturer submittals — mandate precise positioning for factory punchouts:
- Web Centerline Positioning: Knockouts are always aligned along the exact longitudinal centerline of the stud web to maintain symmetric structural stress distribution.
- Indexed End Distance: The first knockout is placed exactly 12 inches (305 mm) from the indexed starting end of the stud member.
- Intermediate Spacing: Subsequent intermediate knockouts recur at 24-inch center-to-center intervals (minimum 24 inches per SFIA/AISI standards) along the length of the stud.
The Importance of Wall Alignment During Framing: To ensure that service pathways remain continuous across a wall run, framing contractors must install all studs with their indexed ends oriented in the same direction (e.g., all indexed ends resting in the bottom floor track). If a stud is flipped upside-down, its knockouts will be offset by several inches, blocking horizontal conduit, piping, and bridging channels.
Stud Web Depth vs. Knockout Dimension Relationship
Removing steel from a stud web reduces the member's cross-sectional area and shear capacity. Consequently, maximum allowable knockout dimensions are governed by total web depth.
1. Standard SFIA / AISI Specifications (AISI S220 & S240)
Under generic SFIA and AISI technical specifications:
- For Web Depths Greater Than 2.50 Inches (e.g., 3-5/8", 4", 6", 8", 10", 12"): Maximum punchout width is 1.50 inches (38 mm) wide × 4.00 inches (102 mm) long.
- For Web Depths 2.50 Inches and Smaller (e.g., 1-5/8", 2-1/2"): Maximum punchout width is 0.75 inches (19 mm) wide × 4.00 inches (102 mm) long.
2. ClarkDietrich Product Line Specifications
ClarkDietrich structural and drywall framing studs utilize standardized rectangular punchouts:
- Web Depths 3-1/2 Inches and Larger: Pre-punched with 1-1/2-inch wide (38 mm) holes.
- Web Depths Less Than 3-1/2 Inches: Pre-punched with 3/4-inch wide (19 mm) holes.
3. Marino\WARE ViperStud® Drywall Framing Specifications
Marino\WARE's proprietary ViperStud® drywall system utilizes flattened, rounded-corner knockouts:
- 1-5/8" Web Studs: Knockout size is 3/4" × 1-3/4".
- 3-5/8", 4", and 6" Web Studs: Knockout size is 1-1/2" × 2-1/2".
4. Bailey Metal Products Specifications
Bailey cold-formed steel C-studs feature flat-bottom oval knockouts:
- Maximum Width: Cannot exceed half the member depth or 2-1/2 inches (63.5 mm), whichever is less.
- End Distance: Minimum distance from the center of the last knockout to the member end is 12 inches (305 mm).
Practical Metal Stud Knockout Size Chart
The following chart synthesizes verified manufacturer data, SFIA guidelines, and AISI standards for cold-formed steel stud web punchouts:
| Manufacturer / Standard | Stud Web Depth | Knockout Width | Knockout Length | Center Spacing | First KO Distance |
|---|---|---|---|---|---|
| SFIA / AISI Generic | 1-5/8" (162S) & 2-1/2" (250S) | 0.75 in. (3/4") | 4.00 in. | 24 in. o.c. | 12 in. from end |
| SFIA / AISI Generic | 3-5/8", 4", 6", 8", 10", 12"+ | 1.50 in. (1-1/2") | 4.00 in. | 24 in. o.c. | 12 in. from end |
| ClarkDietrich | 1-5/8" & 2-1/2" | 0.75 in. (3/4") | 4.00 in. | 24 in. o.c. | 12 in. from end |
| ClarkDietrich | 3-1/2", 3-5/8", 4", 5-1/2", 6"+ | 1.50 in. (1-1/2") | 4.00 in. | 24 in. o.c. | 12 in. from end |
| Marino\WARE ViperStud® | 1-5/8" (162VS) | 0.75 in. (3/4") | 1.75 in. (1-3/4") | 24 in. o.c. | 12 in. from end |
| Marino\WARE ViperStud® | 3-5/8", 4", 6" (362VS–600VS) | 1.50 in. (1-1/2") | 2.50 in. (2-1/2") | 24 in. o.c. | 12 in. from end |
| Bailey Metal Products | All Member Depths | Max 1/2 depth or 2.50" | Varies by profile | 24 in. o.c. | 12 in. min. |
Routing Electrical Cables & Pipes Through Knockouts
Passing utility lines through steel framing requires strict adherence to physical protection rules to satisfy electrical, plumbing, and building codes.
RUBBER / PLASTIC GROMMET FOAM PIPE INSULATION
+--------------------+ +------------------+
| +------------+ | | +----------+ |
Steel | | Electrical | | Steel | | Copper | |
Stud ==> | | Cable/Wire | | Stud ==> | | Pipe | |
Web | +------------+ | Web | +----------+ |
+--------------------+ +------------------+
(Prevents Chafing) (Prevents Corrosion/Rattle)
1. Electrical Cable Protection Requirements
- Chafing Prevention: Cold-formed steel edges created during factory punching can slice through plastic wire insulation during cable pulling.
- Snap-In Grommets: Electrical codes require CSA/UL-approved plastic snap-in grommets or bushings installed in every knockout prior to pulling non-metallic sheath (Romex) or low-voltage communication wiring.
2. Plumbing Penetrations & Dissimilar Metal Isolation
- Galvanic Corrosion: Direct physical contact between copper water pipes and zinc-coated galvanized steel studs creates a bimetallic reaction, leading to rapid pipe corrosion and leaks.
- Thermal Expansion Rattle: As hot water pipes expand and contract, unisolated pipes rubbing against steel knockouts generate loud clicking or rattling noises inside wall cavities.
- Protection Methods: Plumbers must isolate copper lines using plastic grommets, specialized pipe clamps, or pre-slit sections of foam pipe insulation wrapped around the pipe where it penetrates the knockout.
3. Lateral Bridging Channel Passage
- For wall height stability, 3/4" or 1-1/2" cold-rolled channel (CRC) is passed horizontally through stud knockouts.
- CRC bridging is secured to each stud web using clip angles or mechanical friction-fit bridging clips.
Knockouts in Nonstructural vs. Structural Metal Studs
Nonstructural (Drywall) Interior Framing
- Wall Function: Used for interior non-load-bearing partitions carrying drywall weight only.
- Code Standard: Governed by ASTM C645 and AISI S220.
- Knockout Impact: Standard factory knockouts do not diminish partition limiting height tables when gypsum board is attached continuously on both sides.
Structural (Load-Bearing & Exterior Curtain Wall) Framing
- Wall Function: Used for load-bearing exterior walls, floor joists, roof trusses, and axial framing.
- Code Standard: Governed by ASTM C955 and AISI S240.
- Engineering Calculations: Structural engineering calculations must distinguish between gross shear strength away from punchouts (Vag) and net shear strength at punchouts (Vanet).
- End Bearing & Web Crippling: Per AISI S240 Section A5.9, unreduced end shear and web crippling capacities assume that factory punchouts are located at least 10 inches away from member bearing ends.
- Field Modification Rules: Cutting, field-punching, or enlarging knockouts in structural load-bearing studs without written approval from the engineer of record is strictly prohibited.
How to Identify Knockout Dimensions from Manufacturer Submittals
When reviewing architectural plans or product submittals, follow these steps to verify knockout specifications:
- Locate the Product Nomenclature: Identify the full SSMA callout (e.g., 362S125-30 or 600S162-54).
- Check the Member Depth: Determine if the web depth is ≤ 2.50" or > 2.50".
- Verify Product Line Type: Determine whether the member is standard flat-steel framing or a proprietary high-strength system (e.g., Marino\WARE ViperStud® or ClarkDietrich ProSTUD®).
- Consult Submittal Data Sheets: Confirm the knockout width, length, and edge clearance in the manufacturer's physical property tables.
Common Mistakes When Using Metal Stud Knockouts
- Omitting Plastic Grommets: Pulling bare electrical cables through raw steel knockouts without grommets violates electrical codes and creates short-circuit fire hazards.
- Contacting Copper Pipe to Steel: Allowing copper plumbing to touch galvanized steel studs directly leads to galvanic corrosion.
- Flipping Studs During Installation: Installing studs upside-down misaligns knockouts horizontally, preventing the installation of continuous CRC bridging bars or utility conduits.
- Cutting Knockouts in Load-Bearing Studs: Field-cutting or enlarging punchouts in structural framing compromises shear capacity and web crippling resistance.
- Placing Knockouts Too Close to Tracks: Placing web punchouts closer than 10 inches to track end-bearing points reduces structural end-reaction capacity.
Frequently Asked Questions
What is a metal stud knockout?➕
A metal stud knockout is a pre-punched hole located along the centerline of a cold-formed steel stud's web, designed for routing electrical wiring, plumbing pipes, and lateral bridging channels.
How far apart are metal stud knockouts spaced?➕
Knockouts are positioned starting 12 inches from the indexed end of the stud, with intermediate knockouts spaced at 24-inch center-to-center intervals along the length of the member.
Do I need grommets for electrical wire in metal studs?➕
Yes. Building codes require plastic snap-in grommets or plastic bushings in metal stud knockouts to prevent the sharp steel edges from damaging wire insulation.
What size is the knockout hole in a 3-5/8" metal stud?➕
In standard SFIA and ClarkDietrich 3-5/8" studs, the knockout is 1.50 inches wide by 4.00 inches long. In Marino\WARE ViperStud® 3-5/8" studs, the knockout measures 1.50 inches by 2.50 inches.
Can I cut new holes in a load-bearing steel stud?➕
No. Cutting, field-punching, or enlarging knockouts in structural load-bearing studs without written approval from the engineer of record is strictly prohibited by AISI S240 and building codes.
References & Authoritative Documentation
- Steel Framing Industry Association (SFIA): Technical Guide for Cold-Formed Steel Framing Products
- American Iron and Steel Institute (AISI):
- AISI S100-16 / S2-20 — North American Specification for the Design of Cold-Formed Steel Structural Members
- AISI S220 — North American Standard for Cold-Formed Steel Framing (Nonstructural Members)
- AISI S240 — North American Standard for Cold-Formed Steel Structural Framing
- Manufacturer Technical Submittals:
- ClarkDietrich Building Systems: Intro to Cold-Formed Steel Framing Catalog
- Marino\WARE: Structural Studs & Track Technical Guide & ViperStud® Submittals
- Bailey Metal Products Limited: Cold-Formed Steel C-Studs Data Sheet
Disclaimer: Knockout dimensions, spacing rules, and allowable penetrations presented in this guide are provided for product identification and estimation purposes only. All mechanical penetrations, structural web modifications, and utility routing must be verified against the specific manufacturer's submittal data, applicable building codes, and project engineering specifications.