Drone surveys in Dartmouth, flown by the surveyor who writes them

Dartmouth is built on ground so steep that some streets turn into flights of steps, stacked up the west bank of the Dart above a deep water port. It is a little over an hour from Exeter, usually via Totnes, and it is a place where conventional roof access is close to impractical on a large share of the buildings.

The steep stacking has a direct consequence for the roofs: one building's roof frequently discharges towards its neighbour, or onto a wall, or into a gutter shared between two owners. Rainwater disposal here is a chain rather than a series of independent systems, and a blockage part way up the hill becomes somebody else's damp wall further down.

That is a difficult thing to establish from the ground and a straightforward thing to photograph from the air. Following the water is often the most useful part of a Dartmouth survey.

The stock around Bayards Cove and the Butterwalk includes timber framed and jettied merchant houses of a quality that is rare anywhere in England, with Georgian and Victorian terraces climbing the hill behind and the Britannia Royal Naval College dominating the skyline above.

Timber framed and jettied buildings put roof loads and rainwater in awkward places by design. Jetties throw water clear of the wall below and then somewhere else entirely; roof planes meet at angles that were set by the plot rather than by a drawing; and generations of alteration have left lead lined valleys and gutters that were solutions to problems nobody now remembers.

Slate hanging appears here as it does elsewhere on this coast, used on the elevations that take the driving rain. From the air it reads well: individual slipped or missing slates on a hung elevation show clearly, and so does the difference between an original run and a patch.

The river mouth shelters the town from the worst of the open sea, so this is not a wind exposure story in the way that Dawlish or Exmouth are. It is a damp story. Sheltered, wooded, steep sided and beside a large body of water is the recipe for slow drying, and organic growth on shaded slate is persistent.

For a listed building that matters because the remedies are constrained. Getting to the moss is the problem, not knowing it is there, and a photographic record of exactly where it is and how thick it has got lets a conservation minded contractor plan the minimum intervention rather than the maximum.

The Dart is the middle of the town's geography and there is traffic on it. The castle sits at the mouth and the Naval College above, and between them the airspace and the ground use around the town need looking at properly before a flight rather than on arrival.

None of that makes Dartmouth difficult to survey. It makes it a place where the desk work before the visit is worth doing carefully, which is the case for the whole of this list but more so here than in most.