Ask This Old House

General contractor Tom Silva shows a homeowner how to remove moss and lichen from his shade-covered roof and keep them from coming back. Then Tom, along with plumbing and heating expert Richard Trethewey, landscaping contractor Roger Cook, and host Kevin O'Connor ask, "What is it?" Up in the loft, Richard shows Kevin how to repair a leaky valve by replacing the washer and valve seat. Then, Roger helps a homeowner plant shade-loving groundcover under a 100-year-old beech tree.