Telescopic Sections avoid the need to cut channel sections preceisly to fit.

The upstream telescopic section which is slightly narrower, "telescopes" into the adjacent downstream section.   

Example: 4.52 Metre Section 

1st. Step Cut Telescopic section + ~100mm overlap 

2nd Step. Insert Telescopic section into adjacent downstream section.
The wall of the downstream section is cut to fit in the telescopic section. 

3rd Step. Channel Sections are installed ready for other components to be attached.

Next: E2 Corners Installation