Drone services in Plymouth

The post-war city centre is a large concentration of flat-roofed commercial blocks, and that single fact makes Plymouth different from every other place on this list. Asphalt and single-ply coverings, rooftop plant, lift motor rooms, ducting and parapets, most of it out of sight and out of mind.

Flat roofs fail predictably and invisibly. Ponding in a low corner, a seam splitting under thermal movement, an outlet silted up, an upstand that has pulled away behind a parapet. From the street none of it exists; from directly above it is a five minute assessment.

Managing agents and building owners are the main callers here, and the useful product is a whole-roof record rather than a look at the corner where the water came through.

Rooftop plant adds a second layer. Condensers, ducting and cable trays sit on pads and penetrations, and every penetration is a potential entry point that nobody inspects.

Scale is the other thing worth saying. A single commercial roof here can be larger than every domestic roof on a suburban street put together, and the cost of getting it wrong is on a different order, which is why a routine record beats a reactive one.

The city is built over limestone ridges, so streets climb and fall sharply between the waterfront and the higher suburbs, and the surviving Victorian terraces at the Hoe, Stoke and Mutley sit on some of the steepest of it.

Those terraces carry slate, shared valleys and party wall abutments, with the same pattern of failure at junctions rather than in the field of a slope.

Access on the steeper streets is difficult and often means a licence and a road closure for anything conventional, which is the practical reason a great deal of survey work here happens from the air.

The city is open to the Sound and to south-westerly gales coming straight off the Channel, and salt-laden wind reaches well inland across the built-up area.

The high suburbs at Crownhill and Southway catch the weather badly for a different reason: altitude and open ground rather than proximity to water.

Post-war estates there are conventional pitched roofs, and the recurring findings are the ordinary ones, accelerated by exposure rather than changed by it.

Plymouth has more airspace and security considerations than anywhere else in the coverage area. The naval dockyard, the airport site, the Sound and the ferry port all affect what is possible, and every job here is checked properly before a price is agreed rather than after.

That is not usually a blocker for ordinary residential and commercial work away from those areas, but it is the reason an enquiry from this city asks for the exact address before anything else.

The waterfront developments at Royal William Yard and Millbay bring modern flat roofs, terraces and glazing into the mix, with the risk concentrated at junctions as it always is on contemporary construction.

Waterfront apartment blocks also bring shared ownership into it. A block with a single membrane roof and forty leaseholders needs one agreed picture of what is up there, and producing that picture is often the whole job.