Windowsills on the exterior of your home prevent's leaks. You can adhere wooden sills to almost any type of material, such as brick, stone, stucco and siding. Depending on the type of wood you add, will give appeal to your windows, and also prevent any water damage. This may also help to increase the life of the window's trim.
A hardwood that can be used is Ash. A dense struture, this type is more resistant to rotting and water damage. It will work well for a exterior windowsill. It has a light brown color with a light grain making it a attractive wood to use.
Another soft wood would be Birch, a yellow, silver wood. The high density is more than pine, with a fine grain almost white in coloring. This wood is on the expensive side.
Then there is the Red or Silver Maple, which are both hardwood. This is the most resistant to water damage. Making it high quality for making a windowsill. With the high density and attractive grain, it has a swirling gold pattern. A very attractive wood.
Have another good wood for windowsills, let me know.