How to Repair a Rotted Wood Foundation

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Repairs of rotted wood foundations are best left to professionals.
Repairs of rotted wood foundations are best left to professionals. (Image: Ryan McVay/Photodisc/Getty Images)

Installed against the concrete foundation, a wood foundation or a sill plate serves as the base of the structure. Despite their durability, wood foundations exposed to warm, moist conditions rot easily, requiring immediate repair. Left untreated, wood rot causes the foundation to deteriorate and become structurally impaired. Although severely damaged foundations require replacement, you can repair small, isolated sections of foundations using epoxy filler. Before you begin, however, ask a structural engineer to inspect the foundation to determine whether the wood can be salvaged when repaired or warrants replacement.

Things You'll Need

  • Chisel
  • Hammer (optional)
  • Drill and drill bit
  • Borate wood preservative
  • Two-part repair primer
  • Plastic cup
  • Wood stick
  • Paintbrush
  • Rag
  • Epoxy repair filler
  • Putty knife
  • Facemask
  • 80-grit sandpaper
  • 100-grit sandpaper
  • Acrylic primer
  • Acrylic paint

Scrape the soft, rotted wood from the foundation using a chisel until you reach the dry interior wood. Gently tap the handle of the chisel with a hammer to remove stubborn sections of rotted wood, if required.

Bore a series of 1-inch-deep holes into the exposed dry wood, spaced 1 inch apart. If working on a vertical or level foundation, tilt these holes at an angle so they hold the wood preservative. Avoid drilling the holes through the wood surface.

Inject a generous amount of borate wood preservative into the drilled holes in the foundation. The preservative protects the wood foundation from rot.

Mix the two-part epoxy repair primer in a plastic cup, using a small wooden stick. Follow label directions for the application rates. Apply this epoxy primer over the exposed surface of the wood foundation using a small paintbrush. Allow the primer to penetrate the wood for 15 minutes before you wipe off the excess with a rag.

Mix equal amounts of the epoxy resin and hardener on a plastic board with a putty knife. These two components constitute the epoxy repair filler.

Scoop small amounts of the filler and pack each over the gouged section of the rotted wood foundation. Press the filler down against the foundation to prevent air bubbles or gaps. Smooth the top of the filler so the repair patch is level with the surrounding wood. Allow the epoxy to dry overnight.

Smooth the repaired section of the foundation with 80-grit sandpaper, followed by 100-grit sandpaper. Wear a facemask when sanding the surface to prevent inhaling the residue. Wipe the surface of the foundation with a moist rag to remove sanding dust.

Apply an acrylic primer over the patched section of the wood foundation using a small paintbrush. Allow the primer to dry thoroughly before you apply two coats of acrylic paint over it.

Tips & Warnings

  • The epoxy filler remains workable for 30 minutes after its preparation.

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