Hi Andy
I have investigated 10 fatal accidents where people have fallen whilst working on roofs or when accessing to or from them.
None with the same set of causes, with the only common feature being falling.
So, you need to consider each job on its own merits to have a chance of assessing what would be reasonably practicable and what NOT reasonably practicable.
Any suggestion that this is not a problem for the people contracting out work is a non-starter! Well before anyone started drafting CONDAM a short lived acronym that didn't even survive the Consultation before the first iteration of what is now known as CDM came into force, I investigated an accident where a scaffolder fell through a rooflight on a flat roof and wrote the prosecution report that resulted in Client, Main Contractor and the scaffolding subbie ending up in Court.
Your starting point is the HSE guidance, HSG33, now in its 5th edition.
So, you need to know what the engineers are doing on the roof, how they get there (e.g. permanent access from within the building) and how close they might be to any place from which they can fall whilst doing the work or getting to or from that place or places + anything that might make it likely that they will end up closer to the point from which they can fall.
A flat roof should NOT be flat - if designed as such it is a recipe for water pooling and leaking through the roof. So "flat roofs" are designed with a "pitch" and the term flat roof means one with a pitch of maximum 10 degrees. The closer you are to that 10 degrees, the more chance of slipping towards an edge, and of course the nature of the roofing material also influences the likelihood of that slip.
In terms of where they could fall from, obviously the open edges, but also any material that might be fragile, such as the roof lights that have already been mentioned.
Putting in permanent edge protection might NOT be reasonably practicable as it might just be another reason to access the roof, just to check that it is still in sound condition, but also as one of the considerations that might need to be taken into account is its impact on the aesthetics of the building - in ye olden days ornate edge protection was common on the roofs of buildings, but numerous accidents have occurred when old railings have failed.
You have a similar issue if considering permanent running lines ("Latchway" or similar) - another bit of kit which needs access for inspection and maintenance if you don't want the lines to provide a false sense of security - and such systems need to be carefully designed so that someone is attached to some anchorage at ALL times when they are at risk.
Not going to tell you where, but I once climbed a very high ladder with a running line on the LEFT hand side. Got to the top and needed to pass through a hatch on the RIGHT hand side of the ladder. Between the ladder and the hatch was a gap of over 300mm and nothing to stop me falling tens of metres! So, somebody hadn't thought the whole access arrangments through.
Hence the overall message is that somebody needs to sit down and work out the various reasons why people need to go on the roof and come up with a mix of possibly some permanent protections and some done for each specific job. If this is a modern school this should all have been done as part of the design process!
.....but may be these engineers are accessing plant which is well away from any potential points of fall and could go up in dry conditions with little wind and be safe, so far as reasonably practicable, with no special additional measures.
....or may be temporary protection is needed but that brings its own risks - in terms of setting up in a rational sequence and then removing, ditto.