How to Shim an Exterior Window Trim


Most often the structure and framing of a house is not perfectly square. For this reason, it is often necessary to adjust trim elements to conform to subtle irregularities by inserting shims. Shimming the trim of your exterior window is easy and worth doing to create trim that is level, has aligned and flush miter joints, and has a tight, secure connection to the wall. Shims can be made out of pieces of scrap wood.

Things You'll Need

  • Wood scraps
  • Saw
  • Hammer
  • Chisel
  • Finishing nails or brads
  • Latex painter's caulk
  • Cut a thin wedge shape from a piece of wood. Use a jig saw, band saw, or table saw. It should be very skinny on one end and then widen out just a bit on the other end. Make the shim longer than the width of the trim you are shimming. Shims should be at least 1 1/2 inches wide.

  • Insert the shim into the necessary spots underneath the trim. Use more than one shim side by side if necessary to create a secure backing. Lightly tap the shims into the gap with a hammer until the trim is aligned and level or until the miter joint is flush.

  • Secure the trim and affix the shims by finish nailing or screwing through the trim, the shims, and into the window trimming studs that are in the wall.

  • Hide the visible ends of shims with a bead of latex painter's caulk.

Tips & Warnings

  • Pre-cut shims can be bought in bulk at hardware stores.

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