The more acute the angle, the less effective the support will be, so what the surveyor suggests would mean replacing a sub-optimal support with an even more sub-optimal support (you know that intuitively).
But there is another way of looking at this. Because the strut is at an angle, the compressive force in the strut can be ‘split’ into a vertical component V
and a horizontal component H
Consider these separately. Assuming the connection between the base of the strut and the ceiling joist is reasonably secure, force H
will be resisted by the floor as a whole.
The chipboard deck will join all the ceiling joists together making it one big rigid horizontal plate – you could regard this as a large horizontal beam. It will effectively support H
, so the strut will not be moving horizontally.
For resisting force V
, this will depend on the strength of the ceiling joist. These are often small, eg 75 x 30, but as the connection seems to be near the end of the joist where it bears on the supporting wall, it might
not be a problem. This can be checked quantitively to make sure that the joist is not being over-stressed, but the SE would need to do some number crunching.
FWIW, my hunch is that it’s OK – not by the book admittedly - but still OK on the basis that the roof is still there. Maybe try get your SE to write a brief report, perhaps with a few numbers thrown in, to make everyone happy?
(The ‘correct’ bomb-proof alternative would be to span a steel beam directly below the purlin and prop off that = £££s