Most people picture a roof as a single skin of tiles or slate laid over the top of a house, as if the covering alone were doing all the work. It isn’t. A pitched roof is a stack of cooperating layers, each with a narrow job, and the waterproofing comes from how they overlap rather than from any one material being perfectly sealed. Understanding that stack explains why a roof can look intact from the street and still be failing underneath.
The Path Rain Takes Across a Roof Surface
The outer covering is designed to shed, not to seal. Each tile or slate overlaps the one below it and the ones beside it, so water running down the slope is always passing from a higher unit onto a lower one without ever reaching the gap between them. Gravity does the steering. The steeper the pitch, the faster water leaves and the less time it has to work sideways into a joint.
This is why pitch and overlap are specified together. A shallow roof needs longer overlaps and sometimes a different covering altogether, because water moves slowly and wind can push it back up the slope. On a steep roof the same tile can be laid with less overlap because the water is gone before it can misbehave. The covering, in short, handles the bulk of the weather by geometry, not by being watertight on its own.
How Underlay and Flashing Catch What Slips Through
No covering is perfect. Wind-driven rain, melting frost, and the odd cracked tile all let some moisture past the outer layer, and the system expects this. Directly beneath the tiles sits a layer of underlay draped over the rafters, forming a secondary slope that carries any stray water down to the gutter. It is the roof’s quiet backup, and most roofs that leak are really roofs whose underlay has perished or torn.
Flashing does the same job at the awkward places the underlay and tiles can’t cover cleanly: around chimneys, against walls, in the valleys where two slopes meet, and over abutments. Usually formed from lead or a modern equivalent, flashing is dressed into the brickwork and lapped over the covering so water is guided back onto the tiles rather than into the junction. These transitions are where failures concentrate, because they rely on careful handwork rather than repeated identical units.
Why a Roof Needs to Breathe to Stay Dry
Keeping water out is only half the task. The other half is letting water vapour get out. Warm, moist air rises from kitchens, bathrooms, and simply from people living below, and it drifts up into the roof space. If that air has nowhere to go, it meets the cold underside of the roof and condenses into droplets that soak timber and insulation from the inside — damp that no amount of new tiling will fix.
Ventilation solves this by letting air move through the roof void, typically entering low at the eaves and leaving high near the ridge. Breathable underlays add another route, allowing vapour to pass outward while still blocking liquid water. A roof that sheds rain beautifully but cannot breathe will still rot, which is why good installers treat airflow as part of the waterproofing rather than an afterthought.
How Wind Load Is Spread and Held Down
Wind does not simply push on a roof; as it accelerates over the ridge it creates suction that tries to lift the covering off. The layered system answers this by distributing the load. Each tile is held by nails or clips and by the weight and interlock of its neighbours, so no single fixing carries the whole gust. The battens transfer that pull into the rafters, and the rafters into the walls, spreading one strong moment across the whole structure.
The exposed edges — verges, eaves, and ridge — take the worst of it and are usually fixed more heavily for that reason. When a storm strips a few tiles, it is almost always along those margins, and a prompt repair there is a job worth giving to experienced hands like these roofers in Chigwell before the gap lets wind under the surrounding courses and widens the damage.
None of these layers lasts forever, and they age at different rates — underlay and flashing often tire long before the tiles do. A roof checked every year or two, with slipped tiles reset and worn flashing renewed, keeps the whole system doing the quiet work it was built to do.