This is an easy project that gives a lot of pop for the effort. Nick built the cornice board frame, and I finally got it covered and hung last week. It's one of the finishing touches in my bedroom re-do (which will be my next blog post so check back to see the result!).
Because our windows are a standard length, he was able to build it out of a single 1x12 board. This made construction much simpler. Ultimately, a cornice is a 3 sided box (no back on it, as the wall it hangs against becomes the back).
The height of the cornice board should be about 1/5 the length of the window. Just measure the window and divide the height by five. If you are using floor length or puddling drapes, you might want a taller cornice, but for most windows, the one-fifth ratio is ideal.
How far the finished cornice box extends from the wall depends on your other window treatments. If, like me, you are only using blinds or shades, it only needs to come out 4 or 5 inches from the wall. About.com and DIY Network both have good step-by-step instructions for how to measure and build one.
I sprayed the board with spray adhesive, and covered it with one layer of polyester batting. The batting should extend around over the front edges but should not cover the thin back edge that will be against the wall, nor is it needed on the top.
The fabric is upholstery-weight Waverly, a pattern named "Claremont". Having a pattern to match in the fabric meant a bit of creative cutting and very careful measuring to be sure the pattern matched along the full length of the board.
When measuring and cutting the fabric, allow a good 4 to 6 inches extra so that you will have plenty of selvege to staple into. Remember you are wrapping the edges too, all the way around to the back. Excess can be trimmed later, but there's no way to fix it if you cut too much off beforehand.
This fabric also required a seam in the length (it goes over a double window). The seam is located in the exact center, so as not to annoy us every time we look at it, and pattern matched exactly along the widest point in the pattern. That little trick also helps hide the seam from the eye.
I didn't iron the center seam until after the cornice was otherwise completed, then used an iron to set the crease in the seam - don't slide the iron as that might stretch the fabric.
I stapled the fabric to the back sides of the board, starting in the center of the top edge, alternating stapling one side of the top edge, then the other, and eyeing up the pattern with each staple, to be sure it matched its opposite. It will be done faster if you go slowly during this process.
The reason for starting with the top edge is that the top is the first thing you will see when it is hanging. A little error on the bottom edge might go unnoticed but the top needs to be perfect.
Do the same with the bottom edge, being careful (1) not to stretch the fabric (you don't want puckers), and (2) to use a straight edge to be sure the pattern runs straight from top to bottom as well as side to side.
Lastly, staple the sides, folding and tucking the corners. After trimming excess, I used white glue to smooth down the cut edges inside the board. If your board is going to be in a location where the inside will be visible, you could finish the inside fabric edge by gluing ribbon or seam binding over the cut edge.
Paul used L brackets to hang it for me, first attaching the brackets to the wall, using a level, then settling the cornice on them, marking the holes, and drilling small pilot holes in the board before setting it in place perfectly.
How many L brackets you use, and how carefully you hang depends a lot on whether you want to merchandise the top of it or not. If you plan to set things on it, be sure to hang it into studs or with anchors, like you would any shelf.
Isn't it great? We are very happy with the results. Next post: better photos of this bedroom, and how pretty it turned out!
Sept 2011 UPDATE: Per request, here is a close up of the middle seam, which is nearly invisible from an ordinary vantage point in the room. Details in the comments. Click to enlarge so you can see how the seam looks up close.