The build era, because it sets the material.
Pre-1950s, typically vitrified clay. Durable material, but it's laid in short sections with a joint between each one — and every joint is a potential entry point. Predicted finding: root intrusion at joints, offset joints from ground movement, and cracked sections. Roots are the classic clay-era story.
Mid-century, cast iron. Strong, long-lasting, and it corrodes from the inside. Predicted finding: internal corrosion, scaling that narrows the bore, and failure along the bottom of horizontal runs where flow sits. Rough interior walls also catch everything that passes.
Roughly the 1940s to early 1970s, bituminised fibre pipe. A period material with a documented history of deforming and delaminating over time. Predicted finding: an oval or collapsed cross-section rather than a discrete blockage. Where it's present, inspectors and insurers tend to take an interest, and repair is rarely the answer — replacement usually is.
1970s onward, PVC and ABS. Generally reliable and joint-tight. Predicted finding: where there's a problem it's usually installation-related or the result of ground movement rather than material failure — bellies from poor bedding, offsets, or damage from later work.