Ask This Old House (2002)

S4E7 Moss, Algae and Lichen; Leaky Water Valve; Planting Shade

General contractor Tom Silva demonstrates to a homeowner the best way to eliminate moss and lichen from a roof that doesn't get much sunlight, as well as how to prevent future growth. Later, Tom joins plumbing and heating specialist Richard Trethewey, landscaping expert Roger Cook, and host Kevin O'Connor for a segment titled "What is it?" In the loft, Richard guides Kevin through fixing a leaking valve by swapping out the washer and valve seat. Meanwhile, Roger assists a homeowner in planting shade-friendly groundcover beneath a century-old beech tree.