I'm replacing an old windowsill with a 2x8 piece of redwood. On the old one the angle (slope) was created by using a strip of 1x2 under and across the inner width of the windowsill. This seems a bit unstable and I'd like to know if there are better ways to do this.

Also are there any other woods that can be used instead of the redwood?

 

It's likely that originally, the jambs of the window were nailed to the ends of the sill, which would have been what held the sill at the proper angle. Sometimes the bottom legs of the jamb will have a dado that accepts the sill, other times not. When I replace sills, I usually use treated wood (unless it will be clad with aluminum), but redwood is also a good choice. Oak would also work, but is a bit spendy. What I like to do, since the rough opening is seldom level, is shim the sill up with a 4 piece shim.

I start with a 2x12, which is wider than needed. I'll rip the proper angle on each side of the board (so that once the sill is installed, the front edge and back edge will both be plumb) which will leave me with 2 scraps that also have angles on them. I'll determine where the sill needs to go, and cut about 6" off each of those scraps to make 4 shims, which will each end up being cut a little different from each other. I'll cut those shims down to the right size and place them on each end of the sill... 2 in back, to support near the window stool, and 2 in front, out toward the siding. Once I have the shims cut to the right size, and am happy with where to locate them, I'll nail them down so that they don't move when I go to install the sill. The sill needs to be able to slip in smoothly (test-fit it) without having to beat it to death, because you're doing finish work when you replace sills that are to be painted. As I slip the sill in, I'll tuck fiberglass under the sill to fill the outer edge. Then when the sill almost reaches the inner shims, I'll run a bead of expanding foam (don't over-fill!) and push the sill into place. 16d galv. casing nails through the ears, and down through the shims are generally enough to hold it in place.