There do seem to be quite a lot of planning applications where the max
height if only slightly over is allowed due to the existing structure, so
might be an idea to run it by planning anyway.
Brian

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On 11/05/2021 16:32, Chris Bacon wrote:
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?

Your new trusses would be slightly longer, they would have to be modified
to match the positions of your concrete posts. Otherwise looks a plan
though the span feels very long. There is probably no need for the
additional angle if the original span is unchanged.

I presume you're worried about your neighbours and the need for planning
permission. Why not erect as is, with the fallback of making flat if the
need comes?