Hi Mike

You say that the building houses the aircon units which implies that the units are INSIDE the building but then say that the units are ON the roof (as is quite typical).

So, let's assume for a moment that the building predates the first iteration of the CDM Regulations.

Then, if we ignore for a minute higher tier legislation and what might be implicit for a Client to expect under e.g. HSWA, CDM requirements on e.g. Designers did not exist. Further the CDM duties on Designers were tightened up in CDM 2015 in terms of what they were specifically required to consider in terms of the "use of a workplace".

So, you have people who will occasionally have to go up on to the roof to do this and that, often to do with the AC units.  Such work may or maty not be subject to CDM, but whether or not that is the case, the Work at Height Regulatons come in to play. 

Wirh WAH we got rid of a rather arbitrary 2m rule and requred employers and others to adopt a risk based approach and do what is reasonably practicable to prevent people from being injured as a result of falls from height.

The risk will be dependent on a number of variables including how close to the edge of the roof (or for that matter any fragile component on the roof e.g. rooflight) people need to be to do whatever they need to do.

May be the flat roof (defined as slope of maximum 10 degrees) is relatively safe to walk on unless one needs to get within body length of the edge, so say 2m. Might not be the case as the pitch might be at the upper end of the scale and/or the roof might be slippery + at certain times weather might substantially increase the risk. 

So, as an employer you need to consider what is reasonably practicable given the local circumstances. That might include providing roof edge protection as a retrospective measure, but if you do that the edge protection itself is just another reason for people to go up on the roof. So, reduce one risk and introduce another - liked to occasional need to go up to check the integrity of the edge protection. 

If there is internal access, then possibly you might have a system in place wherein someone can be in a harness and be attached to what is generically termed a Mansafe system (other suppliers are available!!).

Possibly the risk is such that the physical precautions, if any, should be determined according to the type of reason for access or even on a case by case basis. 

If this was a new build I would be expecting the Client and Design team to be thinking about future access as part of the planning process - to comply with CDM with the Health and Safety File explaining the rationale for whatever decisions are made. NOTE - this is what should happen, not what might often happen in practice. Arguably the bits of CDM that are most often breached by front end CDM duty holders are the bits that are there to consider the future life cycle of any asset.