Preparation and Removal
Place a temporary support wall just beside the garage wall. Measure the height from the garage floor to the ceiling, and build the wall to the corresponding height. Tap the wall in place, and plumb the wall with a 4-foot level.
Remove the garage siding so that the rotted sill is exposed. Gently pry the siding with a flat pry bar. Remove the siding six to 12 inches above the sill plates. Once the siding is loosened, pull the nails with the pry bar, and place the siding to the side for reattachment.
Determine whether the sill is placed on a concrete slab or a concrete foundation. A concrete slab sill is always the same size as the wall framing and is attached directly to the wall. A concrete foundation sill sits beneath a framed floor and is not part of the wall framing. It is generally 8 inches wide or the width of the foundation.
Detach the sill plate from the wall framing by cutting through the framing where it is connected to the plate. Cut through the nails where the sill attaches at the wall framing and below the sill plate where the plate attaches directly to the concrete slab.
Cut the sill plate into 4-foot sections for easy removal. Pry the sill from the wall with a flat bar and remove all remaining debris from the concrete slab. Cut new treated sill plates and slide them beneath the wall framing.
Insert 3-inch wood screws through the wall framing at an angle and into the new sill plate. Drill holes through the sill plate and into the concrete with a hammer drill and insert concrete screws to hold the sill in place.
Remove the sill plate from beneath the floor framing. Drill a 1-inch hole beneath the floor framing where it meets the sill. Insert the reciprocating saw into the hole and cut through the rotted sill. Cut the sill into 4-foot sections.
Do not cut through the anchor bolts that are anchored into the concrete foundation. The anchor bolts will be used to re-anchor the new sill plates. Pry the sill from the foundation with a flat bar.
Place the new treated sill onto the concrete foundation and against the anchor bolts. Place pencil marks at the location of the anchor bolts. Cut 1-inch slots into the treated sill from the side of the pencil mark that extends to the center of the treated lumber.
Slide the sill into place and tighten the anchor bolts down onto the new treated sill. Insert 3-inch wood screws through the floor framing and into the treated sill at an angle, spacing them four feet apart. Replace the siding on the garage.