I've used turbo trallskruv from XL in previous projects.
However, I think they only come in up to 75mm in length.
I suspect that's not enough when attaching the outer panel. (21+21mm)
The other screw brands I've seen in longer lengths suddenly have quite large heads compared to these. -Have you seen longer ones with small heads?
(Should one aim for 90mm or how is it? 90 for the outer, 70 for the inner?)
However, I think they only come in up to 75mm in length.
I suspect that's not enough when attaching the outer panel. (21+21mm)
The other screw brands I've seen in longer lengths suddenly have quite large heads compared to these. -Have you seen longer ones with small heads?
(Should one aim for 90mm or how is it? 90 for the outer, 70 for the inner?)
If it's V/S/B Turbo decking screws you're referring to, they are excellent for what you're describing. You drive the entire threaded section of the screw into the underlying beam. Once that's done, it won't pull further even if you drive it deeper.
Findus
Findus
Grabber PTS is probably what you're looking for. The screw has hardly any head and becomes invisible, however, it is a very dur method to get the panel up with.