The message
from The Natural Philosopher contains these words:

The moment you put a binder across a couple of rafters, you have a sort
of truss.

This does suggest a neat way of doing a loft conversion with a 'massive
propped purlin' type roof.

Convert it to truss.

Simply add rafters - alongside or below existing - and cross link them
at new ceiling height with 'loft ceiling joists' bolted through as
binders for real rigidity..nails work, but bolts and spider plates are
better.

Then bolt new joists (if needed fior floor rigidity: otherwise use
existing)) at floor level to new rafters. Hey presto - you have a center
span braced truss and you can remove the purlins altogether apart from
lightweight stuff to stabilise the rafters laterally. Since you will be
lining the rafters, after insulation that could be no more than sheets
of ply screwed to the rafter undersides.

If the bottom of the A is tied then I don't doubt that ceiling height
braces would provide some rigidity if the purlins were removed but I
doubt very much whether it would be sufficient to prevent the roof
collapsing even in still conditions. Wind loading would case the
windward side to bow in and the leeward to bow out if the roof did
survive still conditions and on top of that any roof would require an
adequate safety factor over and above the most extreme conditions which
might include a foot or more of snow as well as galeforce wind.

I am not at home atm so haven't the time to even take a look at John's
hymn sheet much less expand on what I have written above.

It may actually turn out that existing rafters and joists are adequate -
the purlin function being replaced by spreaders at ceiling height to
stop the rafters bowing inwards. And existing joists enough top prevent
outward loads on the upper walls at the rafter eave level.

In your dreams.

--
Roger Chapman