A flexible material used to seal a gap between two surfaces e.g. between pieces of siding or the corners in tub walls.

Caulking is Great, but it's not Magic!

Caulking and roof patching materials are wonderful products but they can also cause problems. For example: most roofing and siding systems are designed to shed water from one surface to another. Ridge caps drain onto shingles, one shingle drains onto a lower shingle, metal flashing moves water from the chimney onto the roof, etc. When caulking is used to "repair" a defect, it must be used in conjunction with the system in place. In most cases, the defective part needs to be repaired or replaced, only then can caulking be used as a secondary level of protection.

If caulking or roof patching products are used incorrectly, they can nullify the original design and cause leaks and other problems. Such incorrect usage often results in preventing water from properly shedding from one surface to another. It can result in the water being wicked sideways or even up and under surfaces.

See also: Q&A: Bathrooms