- 1). Remove the old sill using a hammer and chisel to pry it up from the subfloor or joists underneath. In some cases, you may need to use a reciprocating saw to cut the nails in order to free the sill plate.
- 2). Replace any damaged flooring or subflooring in the doorway. Other parts of the doorway also may need replacing as well, and this is a good time to do it. Damage to major support units, such as floor joists, should be strengthened with supporting braces rather than replaced, as it is not practical to do so in most cases.
- 3). Measure the new sill and check installation fitting. The sill you purchased will need to be trimmed to fit into your doorway according to the measurements you will take. Go slowly and use a jig saw to cut the sill plate into the correct shape.
- 4). Apply a bead of silicone sealant to the joint where the door sill plate will contact the interior flooring. This will provide some adhesive power but primarily will seal the connection from water and debris. Open and close the door several times to ensure you have a proper fit before proceeding.
- 5). Install the new sill plate into the door frame. Seal the plate with foam insulation underneath the sill to fill in any gaps as well as holding the sill in place. Once the foam sets up it will act as an adhesive for the sill plate, replacing the nails used in older construction methods. Use wooden shims between the door and the sill plate to put downward pressure on the plate and hold it in place while the foam dries. Allow at least a few hours for the foam to set up before removing the shims.
- 6). Seal the wooden sill plate with at least one coat of varnish or finishing stain. A second coat with a moisture-resistant seal is recommended for additional protection from the elements.
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