The short answer
Rarely through the middle of a tile slope. Water usually gets in at the edges and junctions: the valley or abutment where one roof meets another, the ridge, the verge, the chimney, or an overflowing gutter. Finding which one is the whole job, and the repair is often smaller than the stain on the ceiling suggests.
A stain on a bedroom ceiling rarely sits directly under the hole in the roof. Water runs along rafters, down walls and across felt before it shows. Tracing it back is detective work, and it is best done with a look from the loft as well as from the roof.
Q1.1Why check the junctions first?
Where a porch roof meets the main wall, where a garage roof butts against the house, or where two pitched roofs form a valley, there is a joint that has to be sealed with metal or mortar. Mortar fillets crack as buildings move. A lead flashing, dressed into the wall and over the covering, lasts far longer, which is why it is our usual repair at these places. Our roof repairs page explains how we scope a junction fault.
Q1.2What goes wrong at the edges and eaves?
The roof edge faces the weather head on. A verge tile that has lifted, a fascia board that has rotted behind the gutter, or a gutter bracket that has pulled away all let water behind the covering. Gutters on cul-de-sac houses often run along a short span, so a single blocked outlet can overflow onto a wall below. The cause is simple, the damage slow, and the repair cheap if it is caught early. Left for several winters, the same blocked outlet can soak the fascia, the soffit and the brickwork below.
Q1.3When is the tile slope itself to blame?
The open tile slope is the last place we look, not the first. Individual cracked tiles are quick to replace. If tiles are failing in a pattern, such as a line across the slope, the cause is usually a batten or underlay problem beneath, and replacing tiles alone would hide it.
Ask for the photographs. A survey that records the junctions, the edges and the slope separately lets you see where the water came in, and where it did not. It also gives you something concrete to compare if a second roofer quotes for the same house, which is a sensible thing to do before agreeing to a large job. Gaps in a quote usually show up when two surveys are put side by side.