The flat part behind the steep part
Look at a row from the street and you see a steep, tidy front slope. Walk to the alley and you find something else: a low, wide plane over the rear ell that does most of the failing and gets none of the attention.
- 1 The ridge. The highest course, and on slate the one most often reset by hand.
- 2 The field. Slate on the older stock, shingle on the rest. Two very different clocks.
- 3 Valleys. Where leaf litter settles, and where a slate roof usually ends its life.
- 4 Wall flashing. The party wall of a row or twin, rising through a roof two houses share.
- 5 Penetrations. Holes cut on purpose. When access is awkward they get sealed instead of rebuilt.
- 6 The overhang. Where meltwater from a warm attic reaches cold framing and stops.
Two roofs on one house
The front pitch sheds water by geometry and asks little of its materials. The rear section, added or built shallow to keep the back of the house usable, has almost no slope at all. Water leaves it slowly, sits in low spots, and finds every lap and seam it can.
That means the two halves need different products and fail on different schedules. A quote that prices the whole house as one covering is either over-specifying the front or under-specifying the back, and it is almost always the second.
Where the water actually gets in
At the transition, most often: the line where the steep plane stops and the shallow one begins. It is a junction between two systems, and it has to be built rather than sealed. Then at the perimeter, where a low roof meets the party walls on both sides and has nowhere to drain except through one outlet.
That single outlet is the second common failure. Block it with leaves and the rear roof becomes a shallow pool, which is a load the structure was never asked to carry.
What to ask
Which product goes on the rear section, and how the transition to the front slope is detailed. Those two answers separate a roof that lasts from one that gets patched every few years, and neither of them is visible from the street where most quotes are written.