Drone surveys in Ottery St Mary, flown by the surveyor who writes them

Ottery St Mary is around half an hour from Exeter, sitting on gently rolling ground in the Otter valley between the Blackdown Hills and the coast eight miles south. It is inland and reasonably sheltered, which sounds like good news for a roof and only partly is.

The valley floor holds mist and damp, and that lets moss and lichen establish quickly on shaded slopes and in valleys. Shelter from wind and persistent damp is the combination that produces the thickest organic growth, because nothing ever scours the roof clean and nothing ever dries it out.

On an exposed coastal roof, moss barely gets a foothold. In a sheltered damp valley it becomes a layer, and a layer of moss does three things. It holds water against the covering. It lifts the tails of slates and tiles slightly, which lets wind get underneath. And when it comes away in a storm it goes into the gutters and the valleys and blocks them.

So on an Ottery survey the growth is photographed and assessed rather than noted in passing: how thick, where, whether it is in the valleys or only on the open slopes, and whether it has started to lift anything. The answer determines whether the recommendation is cleaning, cleaning and re-fixing, or leaving it alone, and those are very different bills.

The coverings are natural slate, clay plain tile, thatch and concrete tile on the modern estates. Thatch in a damp valley has the same ridge problem as Crediton, for the same reason.

The compact centre around St Mary's Church has Georgian and Victorian frontages, and there are cob and thatch survivals in the surrounding lanes with post-war and modern estates spreading north and west.

St Mary's itself is a very substantial parish church, and church roofs are among the most rewarding aerial surveys there are: large areas of lead, parapet gutters, tower and turret details, and a maintenance budget that has to be argued for. A photographic record that shows exactly what has failed and what has not is worth a great deal to a parochial church council trying to prioritise.

Cadhay House and the Tumbling Weir are the other local landmarks, and cob buildings in the lanes bring their own consideration: cob depends absolutely on staying dry, so the roof over it and the rainwater goods on it are doing structural work rather than merely keeping the weather out.

Gently rolling ground, reasonable shelter and a compact centre make this one of the more straightforward towns to plan a flight in. There is no significant wind problem, no coastal salt, and no steep terrain to complicate a launch point.

What it does need is the right conditions for looking at growth and damp: after rain rather than after a fortnight of sun, and in the colder half of the year rather than in high summer, so that what shows in the imagery is representative of how the roof spends most of its life.