Steel Studs and Cutting Into Them
Why this matters
Steel studs show up in finished basements, condo interiors, mixed-use buildings and the occasional custom home, and a tech who treats them like wood gets three things wrong at once. The holes are already there, and making new ones is a different question from wood. The edges of every hole are sharp enough to cut cable insulation and hands. And the fasteners that hold a cabinet or a grab bar to wood pull straight out of a 25 gauge stud. None of these is hard once you know it, and each one has produced its own callback.
This card covers identifying what you are cutting into, using the prepunched holes, protecting cable and pipe through steel, the different rules for load-bearing cold-formed steel, and fastening.
Two kinds of steel stud
Non-structural studs frame interior partitions that carry nothing but their own drywall. They are light gauge, commonly 25 gauge (about 18 mil base steel thickness) or 20 gauge drywall studs (about 30 mil), and they flex noticeably when you push on a single stud.
Structural cold-formed steel (CFS) frames load-bearing walls, exterior walls, floors and roofs. It is heavier, commonly 33 mil (20 gauge structural), 43 mil (18 gauge), 54 mil (16 gauge) or thicker, and a stud feels rigid. It is designed by an engineer or under a prescriptive code section, and its holes and cuts follow those rules.
Read the marking. Steel framing members are typically stamped or labeled along the web with the manufacturer, the base steel thickness in mils, the coating, and often the yield strength. Mils are thousandths of an inch of steel, and they are the number that matters; gauge is a traditional label that does not map one-to-one across manufacturers. If you find 43 mil or heavier in a wall, or any steel in an exterior wall, treat it as structural until the plans say otherwise.
The shape. A steel stud is a C-section: a flat web across the wall's depth, a flange on each face where the drywall screws in, and a small return lip on each flange that stiffens it. The flanges and lips are what keep the stud from twisting and buckling. The web is where the holes belong.
Use the prepunched holes
Steel studs come with holes punched in the web at regular spacing, usually aligned when the studs are installed the same way up. They are there for cable, pipe and bridging, and on most residential-depth studs they are an oval or slotted shape around 1-1/2 in wide. Using them is the whole point. A run of cable or PEX through the punchouts needs no cutting at all.
Check that they line up before you count on them. A stud installed upside down, cut from a longer length, or swapped during framing can put its punchouts at a different height from its neighbors. Look down the wall cavity with a light before committing to a route.
New holes in non-structural studs are less restricted, not unrestricted. A hole saw through the web of a 25 gauge partition stud, centered and well clear of both flanges, is common practice. Never cut a flange or a return lip, and never cut a hole that reaches the flange, because the stud loses most of its stiffness at that point. Keep new holes away from the stud ends where it seats in the track.
New holes in structural CFS follow the designer's limits. For prescriptive CFS framing, the IRC (R603 in the 2021 edition, for wall framing, as adopted by your jurisdiction) limits web holes in width, length, spacing and distance from the end of the member, with web holes generally not wider than 1-1/2 in or 40 percent of the member depth, whichever is less. Larger holes need a reinforcement detail from the code section or the designer. For engineered CFS, the drawings govern. A field hole in a load-bearing steel stud without either is the same mistake as an unapproved hole in an LVL.
Protecting cable and pipe through steel
The edge of a punched or cut hole in steel is a blade. Cable pulled across it, or pipe that rubs on it as it expands and contracts, gets cut.
Cable. Where nonmetallic-sheathed cable passes through holes in metal framing, the NEC (300.4(B)(1), in the edition your state adopted) requires the cable to be protected by listed bushings or listed grommets covering all the metal edges, securely fastened in the opening before the cable is installed. Snap-in plastic grommets made for the punchout size are the usual answer. A grommet put in after the cable is pulled, or left out of one stud because it was hard to reach, is the defect. Where a nail or screw is likely to reach the cable, 300.4(B)(2) requires a steel sleeve, plate or clip not less than 1/16 in thick, the same protection idea as nail plates in wood.
Pipe. PEX and other plastic pipe also need a grommet or isolating sleeve at each stud, because the edge abrades the pipe as it moves. Copper pipe in direct contact with steel framing invites galvanic corrosion where moisture is present and rattles where it is not, so isolate it with a plastic grommet or a clamp with an isolating insert. Keep pipe off the flanges where a drywall screw can reach it.
Low-voltage cable gets grommets for the same reason, even where the code does not demand them for that cable type; a data cable cut on a punchout fails intermittently, which is the most expensive kind of failure to find.
Cutting steel framing
The cut produces hot metal and sparks. Snips and a nibbler cut light gauge without sparks and are the first choice indoors. An abrasive wheel or a metal-cutting saw throws sparks and hot chips that ignite dust, insulation facings and sawdust; clear the area, keep a fire extinguisher at hand and watch the area for a while after the cut. Wear eye protection rated for impact, cut-resistant gloves for handling cut edges, and hearing protection with an abrasive wheel.
Never cut galvanized steel with a torch. Heating the zinc coating releases zinc oxide fume, which causes metal fume fever; there is no reason to torch-cut light framing in a house.
Deburr what you cut. A file or a deburring tool on every field-cut edge that cable, pipe or a hand will touch.
Fastening to steel
Use screws made for steel. Fine-thread screws, self-drilling (with a drill point) for heavier steel, self-piercing (sharp point) for light gauge. Coarse-thread wood screws strip out of thin steel.
Light-gauge studs do not hold heavy loads from a screw. A screw into a 25 gauge flange holds by about 18 thousandths of an inch of steel. Cabinets, TV mounts, grab bars, shelving and handrails need backing: a strip of fire-retardant-treated wood or a steel backing plate fastened across studs behind the drywall, installed when the wall is open. Where the wall is already finished, a toggle-type anchor rated for hollow walls, into drywall and stud together, is the fallback; know its published rating and stay well under it.
A worked example: a basement circuit and a TV mount
An electrician is adding a 20 A circuit along a finished basement wall framed with steel studs, and the customer also wants a 60 in TV mounted on the same wall. Before opening anything, the electrician switches off the circuits feeding that wall and proves the existing receptacles dead with a tester checked on a known live source before and after.
Identifying the framing. A small inspection hole beside an existing receptacle shows a 3-5/8 in steel stud. The stamp on the web, read with a light and a mirror, gives the base steel thickness as 18 mil, a non-structural partition stud. The wall is an interior wall dividing the family room from storage and carries nothing above it, so it is a non-structural partition by both tests.
The route. The new cable runs horizontally 16 in off the floor, through the punchouts. Looking down the cavity from the inspection hole with a light, the electrician sees the punchouts line up on six of seven studs. One stud, the fourth, has its punchout about 4 in higher than its neighbors; it was installed upside down. The electrician routes the cable up to that stud's punchout and back down in the next bay rather than cutting a new hole. A punchout centered in a 3-5/8 in web leaves the cable a little over an inch back from each face, clear of an ordinary drywall screw. Every punchout gets a snap-in grommet before the cable is pulled, all seven.
The TV mount. The mount's instructions specify fastening into studs. Into an 18 mil stud, that is not enough: the screws would hold by the thin flange. The wall is already open at the receptacle, so the electrician and the customer agree to open a strip 16 in tall at the mount height across three studs, and the installer fastens a 3/4 in fire-retardant-treated plywood backer across the three studs with fine-thread self-piercing screws, two per stud. The mount is lagged into the backer, which spreads its load across three studs instead of a few screws in thin flanges.
What would have changed the answer. Had the stamp read 43 mil, or had this been an exterior wall, the stud would be treated as structural. The punchouts would still carry the cable, but the electrician would not cut a new hole through that stud without the drawings or the code section's limits, and the backing would be steel attached as the framing designer details it.
The patch. Drywall back over the backer strip, and the record notes the framing type and thickness, the grommets, the reversed stud and the backer location for the next person who wants to hang something on that wall.
Where steel framing changes the fire question
Steel stud walls are often fire-rated walls. Condo demising walls, corridor walls and walls around stairs in multifamily buildings are frequently steel framed precisely because they are rated assemblies. A hole through one is a breach in a rated assembly and needs a listed firestop system for that penetrant and that wall. See related: Dwelling Unit Separations in Townhouses and Duplexes, and How to Firestop a Penetration So the Rating Survives.
References
- International Residential Code, 2021 edition, R603 (cold-formed steel wall framing), as adopted by your jurisdiction
- NFPA 70, National Electrical Code, 300.4(B) (protection of cable through metal framing), in the edition adopted by your state
- The framing manufacturer's product literature and the structural drawings for engineered cold-formed steel
- See related: Nail Plates and Protecting What Runs Through Studs
- See related: Before You Cut: The Four Things a Hole Can Break