Drone survey in Sherborne

Sherborne's historic building stock has a very high proportion of lead, parapet and valley detailing. The abbey and the school buildings carry substantial complex roofs, and the Georgian and Victorian townhouses around them share much of the same vocabulary.

Lead fails slowly, at the joints and the fixings, and the cost of putting it right is high. That combination makes it exactly the kind of thing worth photographing regularly, because a split found at the beginning is a repair and the same split found after two winters is a much bigger job with damage underneath it.

The town centre is unusually consistent in material and character, which means the same details recur across many buildings. Stone slate on the historic roofs, natural slate and clay plain tile elsewhere.

Stone slate is heavy and depends on the structure beneath it, so its condition is partly a structural question. A photograph square onto the slope shows sagging, spreading and displaced slates in a way that is difficult to judge from the street.

This is a sheltered inland valley position with lower rainfall than Devon. Wind and salt are not the drivers here. What fails instead is the slow stuff: lead fatigue, mortar softening, valleys silting up and vegetation establishing where nobody can see it.

That kind of decay suits an annual or biennial record much better than a one-off inspection, because the change between visits is what carries the information.

The abbey, the old castle and the school are significant structures with their own arrangements, and any flight near them is planned and coordinated rather than assumed. For domestic work in the town that is rarely a factor.

The town has reasonable space, the ground is gentle, and take off positions are usually available in a garden or a yard.

The character of Sherborne comes from a small number of very large historic roofs and a much larger number of domestic ones built in the same vocabulary. Pageant Gardens and the streets around the abbey carry townhouses with parapets, valleys and lead detailing that echo the institutional buildings at a smaller scale.

That matters for a first survey because the details on a modest DT9 townhouse can be as demanding as those on a school building. A parapet is a parapet whether it is protecting a chapel or a three storey house, and it fails in the same way and hides the same evidence.

The other local factor is trees. Mature planting in the older gardens and around the castle park drops a great deal of material into valleys and gutters every autumn, and a blocked valley on a lead-lined roof is a slow and expensive problem rather than a quick one.