The short answer
The exposed parts give way first: ridge and hip tiles, verge cement, eaves courses and flashings. Open country lets wind lift loose edges, and a small slip at the ridge can feed water into the loft long before a stain appears on a ceiling. Every survey here begins at those edges, working inwards towards the middle of each slope.
Shipton Bellinger is a Hampshire village on the edge of Salisbury Plain, close to the River Bourne. Open country around a village does not cause roofs to fail, but it does decide where the trouble starts. Wind reaches the edges of a roof long before it reaches the middle of a slope, and rain driven sideways finds any gap at a flashing or a cracked verge.
Q1.1Why do ridges and verges let go before the main slope?
At the crest of the roof sits the ridge, a line of capping tiles, while the verge is where the tiling meets the open edge of a gable end. Both were traditionally bedded on mortar, which cracks and loosens with age. BS 5534 was revised in 2014, and since then mortar is no longer treated as sufficient holding for a ridge or a hip, and screws or clips go in as well. Plenty of older roofs here still have the earlier bedding, so re-bedding with clips, or swapping to a tested dry system, comes up often in our survey reports. A village edge with no neighbouring buildings to break the wind is exactly where that upgrade earns its keep. Our ridge, verge and dry-fix page explains how those systems are tested.
Q1.2Does moss matter on a roof close to a river?
It can. A thick mat keeps the tile surface wet, and a hard frost then flakes the face of a clay or concrete tile. A thin green film on a sound roof is cosmetic. A heavy growth that dams the gutter or lifts the tile edges is worth taking off by hand, never by forcing a pressure lance up under the laps.
Q1.3Is one loose tile a repair or a warning?
Sometimes it is simply one tile. When several have slipped along the same line, or bare felt shows beneath, the real cause lies in the nails or battens, and swapping tiles one at a time only postpones the larger job. Photographs let you see which case you have.
Our roof repairs page describes how a patch is weighed against something bigger.