— Billie Jean, Summersville, West Virginia
A: Tom Silva replies: First you have to make sure of what's inside the wall that you want to cut the doorway in. Are there any pipes, are there any wires, are there any ducts? If you have a basement, the way to find out is to go down to the basement and look for any obvious ductwork or pipes that go through the ceiling heading up in the direction of the wall you want to cut. Of course, any electrical outlets on the wall tell you that there are wires in that wall. Bear in mind that often, wires run around a room parallel with the floor, so if you cut into that wall, you could be cutting straight through a number of wires. A register somewhere in the floor or on the wall indicate that you may have a duct in that wall as well.
Next, having checked that out, you then have to find out if the wall is load-bearing. It's a whole different process altogether if it is a bearing wall, and you'll have to support the space above when you remove the structure below. This involves figuring out what is going on with the direction of the floor load. Once you know that and you've supported the load above, you can cut a hole out of the wall using a drywall saw or a keyhole saw. Remember: it's going to be very dusty, and you want to make sure to allow for that. Cover up everything, block all the doorways, and use a vacuum. We take a shop vac and follow along the edge of the blade when we're cutting to suck up all the dust as we go.
On the door opening itself: If, for instance, you're going to use a 30" by 6'6" door, then you want to cut the hole in the wall 32" by 6'8" — two inches bigger than the door size that you want. This is because if the hole you cut turns out not to be exactly plumb, you'll have to have room for shimming the doorjamb plumb before trimming it. Having done all that, you slide your structure into the wall opening, so that you can begin to nail in your frame.