Track 4mm Wider
If you've ever ordered SFS and noticed that a 90mm stud comes with a 94mm track, you've met the "4mm rule". It's one of the most fundamental — and most universally consistent — rules of SFS construction. But why is it 4mm? What happens if you use the wrong track? And why doesn't every manufacturer explain this? This article answers all those questions.
The 4mm rule explained
In SFS construction, the track (both base track and head track) is always 4mm wider than the stud it accommodates. So:
- A 70mm stud fits in a 74mm track
- A 90mm stud fits in a 94mm track
- A 100mm stud fits in a 104mm track
- A 146mm stud fits in a 150mm track
- A 150mm stud fits in a 154mm track
- A 200mm stud fits in a 204mm track
- A 250mm stud fits in a 254mm track
- A 300mm stud fits in a 304mm track
This is universal. Every UK SFS manufacturer follows the 4mm rule. It's what allows a stud to fit inside a track with enough tolerance to be installed and adjusted, but not so much that the stud is loose.
Why exactly 4mm?
The 4mm is not arbitrary. It's the result of the geometry of the SFS profile and the tolerances needed for installation.
The geometry of a C stud
A C stud has a flat web (the back) and two flanges (the sides). The flanges turn inward at the edges to form short return lips. The total width of the stud, from flange face to flange face, is the "stud width" (70mm, 90mm, etc).
The track needs to fit over the top of the stud — the flanges of the track go around the flanges of the stud. So the track's internal width must be at least the stud's external width.
Tolerance for fitting
If the track were exactly the same width as the stud, they'd be a perfect friction fit — which sounds good but isn't. It would be very difficult to slide the stud into the track on site, and any small manufacturing variation (a fraction of a millimetre) would make it impossible.
The 4mm gives about 2mm tolerance on each side. That's enough for the stud to slide in easily, but not so much that the stud is loose and prone to rattling or misalignment.
Accommodating the flange lips
Both the stud and the track have return lips on the flanges. The track's lips need to fit around the stud's lips. If the clearance were too tight, the lips would interfere. The 4mm provides the right amount of clearance for the lips to fit cleanly.
The track types
SFS uses several types of track, all following the 4mm rule:
Base track (also called floor track or starter track)
The track at the bottom of the wall, fixed to the floor. Usually has a 25mm or 40mm flange — the flange width is the part of the track that fixes to the floor. Base tracks are typically 1.2mm gauge.
Head track (also called ceiling track or top track)
The track at the top of the wall, fixed to the soffit above. Similar profile to the base track, but for infill walls, it's slotted (deflection head — see our Deflection Heads article). Head tracks are typically 1.2mm gauge, but may be 2.0mm for heavy-duty or deflection applications.
Deep track (50mm flange) and Extra Deep Track (70mm flange)
These have wider flanges — 50mm or 70mm — used where more fixing surface is needed. The width (in the stud direction) still follows the 4mm rule. A 94mm deep track fits a 90mm stud, same as a 94mm standard track.
Slotted track
Track with vertical slots cut in the web, used for deflection heads. Available in standard, deep, and extra-deep flanges. Again, the width follows the 4mm rule.
The standard size pairings
| Stud Width | Track Width |
|---|---|
| 48mm | 52mm |
| 50mm | 54mm |
| 60mm | 64mm |
| 70mm | 74mm |
| 90mm | 94mm |
| 100mm | 104mm |
| 120mm | 124mm |
| 146mm | 150mm |
| 150mm | 154mm |
| 200mm | 204mm |
| 250mm | 254mm |
| 300mm | 304mm |
Note: 146mm and 150mm are both valid stud sizes, with different track widths (150mm and 154mm respectively). Similarly, 48mm and 50mm are both valid, with 52mm and 54mm tracks. Don't assume 150mm track fits a 150mm stud — it fits a 146mm stud. For a 150mm stud, you need a 154mm track.
Why this matters on site
Ordering correctly
The most common problem is ordering the wrong track. If you order "70mm track" expecting it to fit your 70mm studs, you'll receive 74mm track (because the manufacturer uses the standard convention) — or if you order 70mm track from an unfamiliar supplier, you may receive 66mm track, which fits a 62mm stud, not a 70mm.
Always order track by its actual width (74mm), or explicitly state "74mm track to suit 70mm stud".
Mixing with existing stock
If you're adding to an existing wall, you need to match the track size to the existing stud size. If the wall was built with 70mm studs and 74mm tracks, you need 74mm tracks for any new work.
Fire and acoustic systems
Fire and acoustic tested systems specify the track and stud sizes that were tested. Substituting a different track size (e.g. using 74mm track with 70mm studs — that's correct; but using 70mm track with 70mm studs — that's not) may invalidate the tested system.
Deflection head slots
The 4mm width is critical for the deflection head detail. The stud fits inside the slotted track with the right tolerance to slide vertically. If the track is too narrow, the stud jams and the deflection head can't function.
What happens if you use the wrong track
Track too narrow
If the track is narrower than the stud (e.g. 70mm track with 70mm stud), the stud won't fit into the track. You'll have to force it, which may distort the stud or the track, or leave a gap that compromises structural and fire performance.
Track too wide
If the track is wider than the stud + 4mm (e.g. 94mm track with 70mm stud — that's a 24mm gap, not 4mm), the stud will be loose in the track. It won't align reliably, the deflection head won't function correctly, and the wall may be unstable. Boards may crack because the studs aren't held firmly in line.
Mixed sizes
If you use 74mm tracks with 90mm studs, the studs won't fit. If you use 94mm tracks with 70mm studs, the studs will be loose. Mixing track sizes (e.g. 74mm base track with 94mm head track) causes misalignment at the top and bottom of the wall.
Fire and acoustic implications
Tested systems specify the exact track size. Using a different track size may invalidate the fire or acoustic rating. Always match the tested system.
Ordering correctly
To avoid the wrong track:
- Order by track width — 74mm, 94mm, 104mm, etc, not by stud width
- Specify clearly — "74mm track to suit 70mm stud" removes all ambiguity
- Match base and head tracks — both should be the same width as each other and the same as the stud + 4mm
- Confirm with the manufacturer — if you're unsure, ask. Better to ask than to install the wrong product
- Check fire/acoustic system — if the system specifies track size, use that size exactly
Summary
The 4mm rule is universal in SFS construction: the track is always 4mm wider than the stud it accommodates. This gives the right tolerance for installation while keeping the stud held firmly. It's why a 70mm stud uses a 74mm track, a 90mm stud uses a 94mm track, and so on.
Order tracks by their actual width, not by the stud width they accommodate. Match base and head tracks to the same size. And for fire or acoustic systems, use exactly the track size specified in the tested system.
Key Takeaways
- Track is always 4mm wider than the stud — 70mm stud → 74mm track, 90mm stud → 94mm track, etc
- 4mm gives tolerance for fitting — 2mm on each side
- Order tracks by track width — 74mm, not "70mm track"
- 146mm stud → 150mm track. 150mm stud → 154mm track — don't confuse them
- Match base and head tracks to the same size
- Don't force a track that's too narrow, or use one that's too wide
- Use exactly the tested size in fire or acoustic systems
- Our SFS Order Builder does this automatically — no risk of ordering the wrong track
Specify the right track automatically
Our SFS Order Builder adds 4mm to your stud size and specifies the correct track automatically. No more ordering mistakes.