A roller furler lets you set and reef the headsail from the cockpit. It's a drum at deck level, an aluminium foil over the forestay that the sail is fed into, and a swivel at the top that the halyard pulls on. Each part has its own way of failing, and the worst of them put the forestay, and the mast, at risk.
What fails
The top swivel. Riggers interviewed by Practical Sailor said most problems start at the top end of the swivel unit, which gets less attention than the drum because it's out of reach. When its bearings stiffen or seize, the drum turns but the top of the sail doesn't.
Twisted foil. With the top swivel stuck, furling line tension twists the foil. Practical Sailor has seen foils barber-pole this way and names upper swivel bearing failure as the usual cause.
Halyard wrap. If the halyard leads too close to parallel with the forestay, or the swivel sits more than a few inches below the top of the stay, the halyard can wind round the stay as the sail furls. Practical Sailor warns this can lead to forestay failure, and one rigger reported dismastings from it. The fix is a halyard restrainer on the mast or a diverter.
Foil joints and corrosion. Foil sections can open at their joints over time. Another rigger told the magazine that corrosion between stainless and aluminium causes most of the problems he sees.
How a surveyor checks it
Furl and unfurl the sail by hand. It should run freely; needing a winch means something is wrong. Look up the foil from the bow for twist and open joints. With binoculars, check the top swivel's position and the halyard's angle to the stay. At the drum, check the bearings, the guard, the furling line and its lead, and the fastenings to the stemhead. Look for white corrosion round stainless screws in the foil.
What decides the grade
Whether the forestay is at risk. Halyard wrap is an A, and so is a foil twisted by a seized swivel. A stiff furler, opening joints, galvanic corrosion or a badly chafed line is a B. Salt and a worn furling line are a C.