Confusing the screw that fixes the board with the one that joins the metal frame is easy to do at a glance — they look alike — but they aren't interchangeable, and using the wrong one compromises the assembly.
TN: for fixing the board to the profile
The board-to-metal screw (TN) is self-drilling with a needle point, designed to fix the board directly to the metal profile. The length changes depending on how many boards are fitted per side:
- 25 mm: for a single board.
- 35 mm: when two boards are doubled up on the same side (for example, to improve acoustic insulation or fire resistance).
- 45 mm: in assemblies with thicker double boards or additional insulation spacers.
TM: only for joining metal profiles together
The metal-to-metal screw (TM) is self-tapping with a drill point, for joining metal profiles together (track to stud, frame joints). Unlike the TN, the TM's drill point bores through metal instead of merely self-piercing gypsum+metal, so it isn't suitable for fixing board: it is used exclusively on the joints of the metal frame itself, before any board is fitted.
Why they can't be swapped
Using a TN to join two metal profiles doesn't pierce the metal well because it has no drill point, leaving a loose joint. Using a TM to fix board can bore too far and not hold the board properly against the profile. Each is designed for its specific function.
Frequently asked questions
Which screw do I use to fix a double board per side? 35 mm TN, the length for two boards fitted on the same side.
Can I use TN screws to join the track to the stud? It isn't advisable: that joint needs TM screws, with a drill point able to bore through the metal, not the TN's needle point designed to self-pierce gypsum.
Which TN screw do I use if I add an extra insulation spacer? 45 mm, the size for assemblies with thicker double boards or additional spacers.
Conclusion
Distinguishing the TN from the TM according to what is being joined —board to profile, or profile to profile— is a small detail that determines whether the frame is firm from the first assembly. Choosing the right TN length according to the number of boards per side avoids a loose fixing that shows over time.
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