Things You'll Need
Marine grade ½ inch plywood sheet
Standard plastic or wood corner molding
Self threading stainless steel screws
Circular or table saw
Paint or stain
You can convert any window that you have into a gorgeous box bay window. Half of the construction is already there since you will be using your original window to do it. Make sure that you work over the course of several days that have a sunny weather forecast in case you cannot finish the entire project in one day.
Remove the old nails from the window frame you are going to alter. Use the pry bar to pull the nails gently out of the wood. You will also need to remove any other types of staples or fasteners that are holding the window in place.
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Remove the old window frame completely from the window. It should come away fairly easily since there will be nothing left holding it in place. You may have to wiggle it gently from side to side a few times to pull it out.
Cut your plywood to the size and shape of your window frame. You should take the measurements carefully with the tape measure and cut the length to 12 inches after cutting the out the dimensions of the window.
Put a heavy bead of silicone over the edge of the frame. This will be your attachment point for the plywood.
Attach your plywood to the window frame using the screws and screw gun. The silicone they you have just applied will act like a glue and a water proof seal. Use a screw every several inches to make sure that everything is attached securely. The frame of plywood that now surrounds your window should perfectly fit inside of the house frame.
Lay down another heavy bead of silicone all around the inside of the original frame in the house. Be careful not to drip it on the inside of the house, and wipe up any spills immediately with a rag.
Screw down the box inside of the house frame. Use the stainless steel screws, and put down a screw at every 12 inches or so in the top, bottom and sides.
Cut your decorative molding to fit around the edges where the house meets the plywood. You can measure how long the molding needs to be using the tape measure. Then, using your miter saw, angle the blade for a 45 degree angle and cut off the end of the molding where you have marked it during the measurements. Turn the molding completely around, and cut the other end to the same 45 degree angle. This will enable the ends to fit together.
Install the molding. Line the molding up on the house and the box. Put a heavy bead of silicone on both the house side and the box side. Screw the molding down using a screw about every 12 inches.
Install the corner molding. Cut the corner molding to size, then apply a heavy bead of silicone over the corners of the plywood, and screw down the molding. This will make sure that you have a watertight enclosure.
Paint or stain your new box bay window. You will need to paint the inside of the window to match the rest of your house. The outside will need to be painted or stained to avoid rot.
This is a somewhat complicated project. If you have not done similar work before, then allow yourself several days to complete it. You might want to get a tarp to cover the hole at night.
Be careful when using all power tools. Wear safety goggles and clothing that fits snugly so that it will not snag in the saw.