A lum is the vertical masonry structure that vents smoke and combustion gases from a fireplace to the outside. In Scottish and Northern English building trades, the term refers either to the entire exterior chimney stack or specifically to the top pot where the smoke exits. Masons and roofers working on regional builds use the word when repairing old masonry heaters or sealing the roof penetration.
Heard on the job
Get up on the roof and check the mortar joints on the lum before we pack up the scaffolding.