I came by an old concrete double garage, which had an apex roof, and I
want to convert it to a flat roof.

https://i.ibb.co/2jgk4gY/DSC00126.jpg

Unfortunately, 2.5m max height applies.

I had thought it would be a good idea to cut the existing trusses (a
couple are leaning against the wall, made of ~38mm equal angle, with
~27mm equal angle racing), and re-weld them together so that the top
edges are in line, i.e. "flat" rather than apex, then cut the ends and
weld on 40mm equal angle "legs". All equal angle thicknesses are ~3mm.

https://i.ibb.co/XtC5YWJ/DSC00128.jpg

The 40mm angle legs would bolt into the vertical concrete posts of th
sectional building.There would be a new 30mm equal angle tie, which
could also support an insulated ceiling, to stiffen each truss.

Some 50 x 150 boards would also bolt to the 40mm "legs", with firring
strips 150mm to 50mm or thereabouts to give some drop over the length.

External dimensions: 5000 wide, 6000 long. Span of truss approx 4800.

Look OK, or if not, why?