How to Install Exterior Corner Boards

Corners are the most vulnerable parts of a home's exterior. When siding planks meet, they rarely create a perfect seal on their own, often leaving gaps that invite moisture, wind, and pests to settle right into the wall assembly. Installing solid vertical corner boards creates a sturdy, weather-tight vertical junction that anchors the look of the house while protecting the ends of your siding. Done well, this project turns a jagged, messy intersection into a crisp architectural feature. The key lies in the layout and the flashing. If you get the water management right behind the boards, the house will stay dry for decades. If you skip the flashing, you are essentially building a funnel for rain to rot your sheathing. Measure twice, flash once, and aim for a tight fit against the siding edges.

  1. Expose the wall framing. Remove any old trim or siding ends that are damaged or rotting. Ensure the house wrap or building paper underneath is intact and extends past the corner framing.
  2. Build your water shield. Cut a strip of self-adhering membrane or metal flashing to cover the corner framing entirely. The flashing should extend at least 4 to 6 inches onto the wall sheathing on both sides of the corner.
  3. Size for settlement. Measure the height from the bottom of the siding to the soffit. Cut two boards, one slightly wider than the other if you are overlapping them, or both to equal widths for a butt joint.
  4. Lock out the moisture. Apply high-quality exterior primer to all cut ends and the back faces of your boards. This seals the wood fibers against moisture absorption that causes paint failure.
  5. Set it plumb. Place your first corner board against the wall, checking for plumb with a level. Tack it in place with a few finish nails so it stays put while you check the alignment.
  6. Anchor into the studs. Fasten the boards into the corner studs using stainless steel trim nails or exterior screws. Space your fasteners every 12 to 16 inches along the length of the board.
  7. Seal every gap. Apply a high-quality exterior-grade paintable sealant between the edge of the corner board and the siding. Do not caulk the bottom edge of the board, as it needs to weep moisture.