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SFS studs come in two main profiles — C section and I section. To a casual observer they look similar, but the difference in shape changes how the stud performs, how it's used, and what it costs. This guide explains the difference, what each profile is for, and how to choose between them.

What is a C stud?

A C stud is a cold-rolled steel section shaped like the letter C — a flat back (the web), two perpendicular sides (the flanges), and two short return lips at the edges of the flanges. It's formed from a single strip of steel through a roll-forming machine, so it's a continuous profile with no welds.

C studs are the standard vertical framing member in most drylining and SFS applications. They're used in:

• Internal partitions (drylining)
• SFS infill walls
• Wall linings
• MF ceiling systems (in smaller sizes)

The "width" of a C stud — 70mm, 90mm, 100mm etc — is the depth of the web, measured between the outer faces of the flanges. The gauge (1.2mm, 1.5mm, 2.0mm) is the thickness of the steel strip.

What is an I stud?

An I stud (sometimes written I-stud or I section) is also a cold-rolled steel section, but the profile is shaped like the letter I — a flat web with flanges on both sides and a narrow central web connecting them. It's formed by folding and crimping the steel strip into an I shape, giving the stud a different set of structural characteristics.

I studs are used where the performance requirements push beyond what a C stud can deliver:

• Taller walls (over 4m typically)
• Higher wind loading
• Loadbearing applications
• Acoustic separating walls where the I shape helps break sound transmission

I studs are also more common in larger widths — 146mm, 200mm, 250mm, 300mm — where the profile is structurally more efficient per kg of steel.

The key differences

The main differences between C and I studs are:

When to use a C stud

C studs are the default choice for most applications. Use them when:

• The wall height is under 4m and loading is standard
• The application is a drylined partition in a domestic or light commercial setting
• You're building SFS infill where the stud width is 100mm or less
• The acoustic target is Rw 45 or lower
• Budget is the primary driver

For a typical internal partition (studs at 600mm centres, 2.7m height, no wind loading), a 70mm × 1.2mm C stud is sufficient. For heavier domestic loads (a bathroom wall carrying tiles, for example), a 70mm × 1.5mm C stud is more appropriate.

When to use an I stud

I studs are used when the loads or performance requirements exceed what a C stud can provide at the same width. Use them when:

• Wall height exceeds 4m
• Wind loading is significant (exposed locations, high rise, coastal)
• The wall is loadbearing (carries loads from above)
• The acoustic target is Rw 50 or higher
• The fire rating is 90 minutes or above
• You need maximum stiffness per metre of steel

For a 6m SFS infill wall in a moderately exposed location, an I stud at 150mm width with 1.5mm gauge may be specified where a 150mm C stud of the same gauge would not span the height. The I profile gives the extra stiffness without increasing the wall thickness.

Which is better for acoustics?

For acoustic performance, I studs have a small advantage in theory because the narrow central web breaks the direct path of sound through the stud. In practice, the dominant factors in acoustic performance are:

1. Board mass and layers — 2 × 12.5mm board each side performs far better than 1 × 12.5mm, regardless of stud profile
2. Cavity insulation — full-fill mineral wool is essential for good acoustic performance
3. Cavity depth — a 90mm stud cavity outperforms a 70mm cavity
4. Isolation — resilient bars or twin-frame construction make the biggest difference

So while I studs have a theoretical acoustic advantage, in practice the choice of board, insulation and isolation matters far more. If you're aiming for Rw 55+, use a 100mm stud (either profile), 100mm insulation, and 2 × 12.5mm board minimum — or go twin-frame.

Which is better for fire?

Fire performance is determined almost entirely by the board layers, not the stud profile. The stud only needs to remain in place during the fire test — it doesn't need to be an I stud to achieve 60, 90 or 120 minutes.

That said, some fire test build-ups are tested with specific stud profiles. If the tested system specifies an I stud, you must use an I stud. If it specifies a C stud, you must use a C stud. The tested construction is the tested construction — no substitution without checking.

Our full Fire Tested Build-Ups register lists the tested profile for each construction.

Cost comparison

Per metre, I studs cost more than C studs at the same width. But at the same level of performance, the costs are much closer — sometimes I studs are cheaper.

For example:

• A 90mm × 1.2mm C stud might span 4.0m at 600mm centres
• A 90mm × 1.2mm I stud might span 4.3m at 600mm centres
• A 100mm × 1.2mm C stud might span 4.2m at 600mm centres

If you need 4.3m of span, the I stud at 90mm is cheaper than the next width up of C stud at 100mm (assuming both meet the requirement). But if 4.0m is sufficient, the 90mm C stud is cheaper than the 90mm I stud.

Summary

C studs and I studs are both cold-rolled steel sections used in SFS framing. The choice between them depends on what the wall needs to do:

• C studs — the default choice for standard partitions, infill walls and drylining. Cheaper per metre, universally available, and sufficient for most applications.
• I studs — used where the loads or performance requirements exceed what a C stud can provide. Higher strength and stiffness per metre, particularly useful at larger widths (146mm and above), or where wall height, wind loading or acoustic performance is critical.

For most projects, start with a C stud and check whether it meets the height, load and performance requirements. Only move to an I stud if the C stud is genuinely insufficient. And always check against the manufacturer's span table for the specific project conditions.