Roof flashing are at least as important as the roof covering itself. A covering can be brand new, but unless the flashing are in a similar condition and properly installed, the roof is going to leak.
Basically, any interface between a roof plane and any other component, including another roof plane with a similar covering, needs flashing. If it is missing or incorrectly installed, or has already failed, there may likely be problems with the whole roofing system. Complicating matters is the fact that it's impossible to see most flashings during a visual-only home inspection.
The only thing better than one flashing is two flashings doing the same job, which is where counter-flashings come into the picture.
Materials
Flashing should be made from corrosion-resistant metal, the most common metals for this use are galvanized steel, copper, aluminum, lead, and stainless steel. Aluminum should not be used in coastal areas, as the salt air rapidly corrodes any flashings made of this material.
The main types of flashing include:
edge flashing;
ridge and hip caps;
valley flashing;
roof-wall flashing;
roof-roof flashing;
chimney and vent flashing; and
skylights.
Edge Flashing
According to the 2018 International Residential Code (IRC), Section R905.2.8.5, all eaves and rake edges need flashing. These prevent rainwater from wicking into the roof sheathing or decking as it leaves the roof covering. The profile of this flashing ensures that any drips from the edge fall away from the sheathing of the roof deck (the structure supporting the roof covering). It is for this reason that edge flashing is also commonly referred to as drip edges or drip flashing.
A drip edge flashing should be installed at the eaves and rake edges of shingle roofs (eaves: along the gutter; rake: along the gable edge). Adjacent segments should overlap. The drip edge flashing should extend at least 1/4-inch below the roof sheathing and extend backward up onto the roof deck at least 2 inches. The drip edge flashing should be fastened to the roof deck at every 12 inches, maximum.
Eaves and Underlayment Cover Flashing
The underlayment should be installed over the drip edge along the eaves, meaning that the metal drip edge flashing is covered by the underlayment. Another way of saying it is that at the eaves, the underlayment should overlap or cover the metal drip edge flashing.
Rake and Flashing Cover Underlayment
Along the rake edges of the sloped roof, it's the opposite installation. The underlayment at the rake edge should be installed under the metal drip edge. Another way of saying it is that at the rake, the metal drip edge flashing should overlap or cover the underlayment.
Again, along the rake edges, the drip edge flashing should be installed over the underlayment, covering the underlayment. At the rake, the metal flashing covers the underlayment primarily to prevent wind-driven rain from getting under the underlayment at an exposed edge.