When you come to convert the loft then that is a different matter as you
may
have to move the cylinder elsewhere and then make what it is resting on
strong enough.

The location was chosen as being its final resting position with the
proposed conversion in mind. I suppose careful consideration of the runs of
pipe could mean that they only need adjusting for height when the cylinder
is moved up 10cm.

It is still tempting to build a 20cm high platform out of 2 by 4 that can
become a 10cm high platform when the joists are thickened, but then,
subsequently changing the pipe lengths is probably no more work than making
the platforms.

The cylinder will be directly above the wall not through coincidence. The
wall is directly below the ridge line of the roof. The cylinder needs to be
installed below the ridge line due to the height required. (The conversion
will be a full width dormer extension to the rear).

Would a thick lump of plywood be stronger than chipboard?

Christian.