Understand the roof before you buy one.

A built-up roof (BUR) assembles waterproofing from multiple field-installed plies of felt bonded in bitumen — redundant layers. A single-ply membrane is factory-fabricated and installed as one layer with welded laps. BUR gives redundancy; single-ply gives speed and lighter deck loading.

For built-up roofing, each ply is said to represent roughly five years of service — a 3-ply roof is a 15-year system, a 4-ply a 20-year system, if properly installed and maintained. Single-ply and metal service lives depend on the membrane, coating and detailing.

Repair when the deck is sound and leaks trace to details. Restore when the roof is mid-life, dry, and the insulation is not saturated — coating resets the clock at a fraction of tear-off. Replace when the insulation is wet across large areas or the membrane has failed in the field.

Details, not field membrane. Base flashings at parapets, equipment curbs, pipe penetrations, drains and scuppers, and copings on top of walls account for the majority of leaks on low-slope commercial roofs.