Eliminating moss and lichen from a shade-covered roof, repairing a leaky valve and planting shade-loving groundcover under a 100-year-old beech tree.

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, plumbing and heating expert Richard Trethewey, landscape contractor Roger Cook and host Kevin O'Connor ask, "What is it?" in the ASK THIS OLD HOUSE loft. Next, Richard shows Kevin how to repair a leaky valve by replacing the washer and valve seat. On a HOUSE call, Roger helps a homeowner plant shade-loving groundcover under a 100-year-old beech tree.