Building for the great outdoors calls for a little extra thought at the design and assembly stages. Here are five things you can do to keep your project from falling prey to the rain.
2 drawings, on air circulation

Build to shed water

Birdbaths and planters should catch rainwater; your patio table should not. Instead of edge-joining lumber for a large, flat tabletop, leave 1/8" gaps between each piece. The gaps allow rain to drain, and let the wood move. Likewise, leave the spaces between slats open so water falls through, or caulk to prevent trapped water.

Minimize end-grain exposure.

Moisture moves through end grain like soda through a straw, so plug up end grain by sealing it with finish or caulk, or capturing it in a joint. For end grain that points to the stormy sky or touches the ground, consider covering it with a cross-grain cap, as shown. Also, keep the edges of laminations from facing up, and fill the edges of plywood with epoxy or primer and paint.

Labeled “Capping end grain”

Make your own plywood

For fragile fretwork or parts, such as wheels, that must endure stress from multiple directions, you can fabricate your own marine-grade plywood, as shown. Resaw and plane an odd number of thin pieces of your project stock (the more the better). Then laminate them back together using polyurethane, epoxy, or resorcinol adhesive, and rotating the grain 90° each layer. The parts will be stronger and more stable than solid stock, and will match the wood in your project.

5 flat boards floating above one another

Use mechanically sound joinery

Locked rabbets and splined miters, as shown in bottom drawing, are more durable than butt joints and non-reinforced miters and bevels because they rely less on adhesive alone to make the connection. Scarf and finger joints, like those shown in top drawing, provide lots of gluing surface for long workpieces or when gluing up an arch.

Labeled “Strong outdoor joints”
Labeled “Good corner joints”

Allow for shrinking and swelling.

Weather-exposed wood changes size more often than a lottery jackpot. Use tongue-and-groove or shiplap joints as shown for broad vertical surfaces to keep the inevitable wet/dry cycles from tearing apart your project.

Labeled “Good panel joints”