Curved Window Moulding
Wood Being the First Choice for Curved
Getting wood to bend around a circle is more than a little challenging, if you need to create a half round window trim. Casing then, made to match your curved window has hopefully been included in your purchase. If not you could be in for a little surprise in trying to source such a beast. It typically runs in the neighborhood of $200-300.
Here is information on ordering a curved casing for an elliptical window.
For a half round window trim, casing can be ordered by simply giving the fabricator a diameter (B+) to fit your specific needs. A template or paper pattern is not really necessary... but be sure that it is infact a "half round" window and not elliptical. The measurement "A" (from the top to the horizontal) must be one half of the distance across "B".
Make sure that the diameter that you order for your curved casing is equal to "B" plus what relief you would like to back set the casing from the edge of the jamb (usually 1/4" on each side), therefore the diameter of your half round window trim = B + 1/4" (on the left) + 1/4"(on the right).
Also make sure that you order "Casing Legs" to match the profile of the curved casing you are ordering. Specify the type of wood AND the profile or pattern you would like your curved trim to be fabricated from.
A typically curved moulding for a common sized half round window will come in a minimum of two pieces, necessitating a splice, or seam at the top. Larger windows may come as 3 pieces. Ask.