The structure: trusses and rafters
The shape of the roof is formed by timber, usually factory-made trusses on a new build, or traditional cut rafters. This frame carries the weight of everything above it and gives the roof its pitch.
The underlay or membrane
Over the timber goes a breathable roofing membrane. This is a secondary barrier against wind-driven rain and any water that gets past the tiles, while still letting moisture escape from inside so the roof space stays dry and ventilated.
Battens
Thin horizontal timber battens are fixed over the membrane. These give the tiles or slates something to hook onto and set the spacing, or gauge, that keeps the courses even and weathertight.
The covering: tiles or slates
The visible surface is usually concrete or clay tiles, or natural or fibre-cement slates. This is the main weatherproof layer, and the material affects both the look of the house and how long the roof lasts.
Flashings, ridges and ventilation
Finally, the details: lead or modern flashings seal the joins around chimneys and walls, ridge and hip tiles cap the top edges, and vents keep air moving through the roof space. Get these right and a roof stays dry for decades; get them wrong and they are the first place leaks appear.