Exterior joints should be designed to shed water (see Durable Exterior Trim, below). Trapped water will cause excessive wood movement and splitting, and eventually will invite rot, bugs, and fungal infestations.

Avoid Miter Joints

Only profiled moldings should be mitered; all other trim boards should have lap joints. Miter joints inevitably open as the wood shrinks, exposing the end grain of both boards to the weather.

While all trim stock should be dry (less than 12% MC
at installation), this is especially important for profiled moldings to keep miters tight. Pre-prime all sides to help keep the wood dry.

Seal and Flash Edges

To keep joints dry, first make them as tight as possible:

  • Make sure that adjoining surfaces meet uniformly at all points; no amount of glue or fasteners can compensate for a sloppy joint.
  • Seal all joints that are likely to absorb water. Horizontal joints are the most vulnerable. Apply a coat of latex paint or a wax-based sealant such as Thompson’s Water Seal® before the pieces are assembled.

Second, use metal flashing to help water find a path away from horizontal trim pieces.

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Canted Surfaces and Drip-Edges

Railings, thresholds, and other flat surfaces can be made to shed water by bowing or sloping top surfaces, and by sawing or routing a drip groove along their undersides. Around posts and columns, build ventilation channels that will allow built-up pieces to dry.

For more on Exterior Trim, visit the JLC Field Guide.