1. Condition Assessment: peeling paint, weather checks, decay pockets
(see the book, Save America's Windows, pages 21-23, http://saveamericaswindows.com/get-the-book/)
2. Remove the old sill: saw it into thirds with two cuts, make the cuts on a slight bevel and angle to make removal of the middle section easier, don't cut into the stool above or the wall siding below, drill a hole at each kerf near the outer edge of the stool, finish sawing the sill into thirds, split out the middle section with a mallet and chisel, split out the end sections, splitting off about 1-2" pieces works well. Use caution when splitting into the joint between the sill and the jamb, so the joint in the jamb is not ruined. Also, the interior apron and the edge of the stool may be nailed into the sill inner edge of the sill, so watch out for those nails, which may need to be cut off flush with the bottom of the stool with a hacksaw blade. Trim off the nails at the sill-jamb joint.
3. Remove siding below the sill if necessary.
4. Document and measure the surrounding parts and make a measured drawing of the new sill. Begin with a cross-section of the sill by take the angle of the sill with a bevel and mark two lines on the drawing, one for the top of the sill and one for the bottom. Develop the drawing to show details like the siding groove and the drip bead. Write down the dimensions. Make a top view of the sill, and write down the length dimensions.
5. Make the new sill: plane the plank to thickness, rip it to width, cut it to length, lay out the joints on the stock according to the drawing. Form the joint by cross-cutting with a saw and then splitting off the waste with the chisel and mallet. Plane off the front edge with a hand plane, or rip it on a table saw.
6. Install the new sill.
7. Attach the sill/jamb joint.
Run one or two screws through each joint to hold the joint together. Angle the screws up from the bottom of the sill, sit passes through the middle of the joint, and so it does not come out on the top of the sill. To begin drill a shank hole for the screw with 3/16" bell-hangers twist bit, through the sill to the joint. Then drive in a 3" to 4" long galvanized or stainless steel screw.
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8. Attach the sill/stool joint.
9. Re-install the siding beneath the sill.
more to come...
Awl or screwdriver to probe for decay
Drill, 1/2" twist bit, 3/16" x 6" or 9" bell-hangers twist bit
Chisel & mallet
Hacksaw blade, for cutting nails
Folding rule or tape measure