The Stopwater

Where the keel meets the stem of a wooden boat, there is a scarf joint — two timbers overlapping and bolted together. The joint is strong in shear. But it is also a seam, and seams let water in. A stopwater is a softwood dowel driven through the scarf joint across the seam. When the boat is launched, the softwood swells, expanding to fill the bore hole and blocking the path water would take along the joint. The stopwater does not strengthen the joint. It is there only to prevent the joint from leaking. Its purpose is the one thing the structural fasteners cannot do.

Gaskets in piping work the same way. Two flanges are bolted together with tremendous force, but the metal surfaces are never perfectly flat — microscopic valleys and ridges remain even on machined faces. The gasket — a soft material compressed between the flanges — deforms into those valleys, filling the paths that fluid would otherwise follow. The bolts carry the structural load. The gasket carries the sealing load. If the gasket were as hard as the flanges, it would not conform. Its softness is its function.

Caulking between window frames and masonry is a stopwater at the building scale. The window frame and the rough opening are two rigid structures that expand and contract at different rates. A rigid seal would crack as the materials move. Silicone or polyurethane caulk remains flexible, accommodating the movement while maintaining the seal. The caulk does not hold the window in. Shims and fasteners do that. The caulk only stops water from following the gap between two things that are already held together by other means.

The stopwater is the acknowledgment that structural integrity and environmental integrity are separate problems. A joint can be strong and still leak. A connection can carry every load asked of it and still allow passage through the gap it creates. Strength and sealing require different materials, different properties, different thinking. The bolt wants to be hard. The stopwater wants to be soft. Both occupy the same joint, and neither can do the other's work.