Standard curtain rods are expandable from 28 to 48 inches or from 48 to 84 inches, and for extra-wide windows, you can purchase rods that extend as far as 159 inches. If you have a window with a nonstandard width, you may want to adapt your current rod to fit it rather than buying a new one, particularly if the window is just a few inches wider than the maximum length of the rod. You can buy extenders for some rods, but there are ways to improvise a solution using wooden doweling.
Using Rod Extenders
Some companies ship extra-long rods in pieces, and they supply couplers to connect them. The coupler consists of a short length of pipe that slides over the two pieces you're connecting. It has a pair of set screws that you tighten with an Allen wrench or Phillips screwdriver to secure the pipes you're connecting. You can get the coupler and an extra length of pipe by navigating to the manufacturer's website or that of a distributor.
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To install the connector, insert one rod until the end is halfway inside it, then tighten the set screw. Insert the other rod until it touches the first one; tighten the screw, and you're done.
If you already have an extra length of rod and you're just looking for a way to connect it to the existing curtain rod, consider using a metal conduit coupler, available at any hardware store. Paint the connector with a can of spray paint, and no one will ever know it didn't come from the curtain rod manufacturer.
Connecting Rods with Dowels
Perhaps you just want to add a couple of inches onto your curtain rod. If you have an extra length of rod, you can do this with wooden doweling. This technique allows you to join the rods without the use of an external coupler.
Step 1: Purchase the Doweling
Find wooden doweling that has the same diameter as the rod you want to extend. Curtain rods are generally 1/2 inch in diameter, so 1/2-inch doweling should work. However, just to be sure, it's a good idea to bring the curtain rod with you when you buy the doweling.
Step 2: Prepare the Doweling
Sand or shave both ends of the doweling to reduce the diameter enough to fit inside the curtain rod. You can do this with a pad sander and 100-grit sandpaper, a belt sander or even a pull scraper. Remove enough material to allow you to insert the dowel to a depth of 2 to 3 inches.
Step 3: Insert the Doweling
Spread a little carpenter's glue on one end of the doweling and slide it into the end of one of the rods to a depth of 2 or 3 inches. Cut the end of the doweling flush with the end of the rod, using a handsaw. Then tap the doweling in another 1/8 to 1/4 inch, using a hammer and screwdriver. Insert the other end of the doweling into the other rod in the same way, and then wait two hours for the glue to set.
Step 4: Install a Double-Ended Screw
Drill a 1/8-inch hole in the center of each dowel. Screw a 1/4-inch double-ended screw into one of the holes and tighten it a few turns, using pliers, to sink it at least 1/2 inch into the dowel.
Step 5: Connect the Rods
Insert the other end of the screw into the hole in the dowel in the other rod, then screw the rods together by turning them with your hands. Because the dowels are recessed, you'll be able to keep tightening until the rods are actually touching.