A patio door can transform a gloomy room. Learn how to remove an existing patio door and replace it with a new one.

This project shows how to replace an existing door of the same size. Call in a carpenter if you need a new opening cut in your wall.

French doors—a pair of doors that open singly or together—certainly have charm, but can be very expensive. If a house settles even slightly, the joint where the doors meet each other may get out of alignment, making it difficult for the doors to latch and seal off outside air. Sliding doors are generally less expensive and can be easily adjusted.

Vinyl doors are inexpensive and do a fairly good job of insulating, but are easily cracked. Aluminum doors do not insulate well, and paint tends to peel from them. Wood doors, with vinyl or aluminum cladding on the outside, are usually worth the extra cost because they look great, insulate well, and are easy to care for.

Purchase a door suitable for your climate and situation. If you have severe winters, be sure to get thermally insulated glass. Low-E coating on the glass limits the passage of heat, keeping your room cooler in summer and warmer in winter. Also, be sure the door you choose seals tightly when closed and that the weather stripping is durable. If you plan to keep the door open in the summer, see that the screen door seals tightly as well.

You'll find several styles and prices at a home center; check out a door and window supplier for more options.

Installing a patio door in a new location is a difficult job, calling for extensive carpentry skills and knowledge. Before cutting the opening, the ceiling must be temporarily supported with posts and a beam to avoid structural damage to your house. The opening must then be framed correctly, with a massive horizontal header spanning the length of the door overhead. Hire a pro to do this, or at least contact your building department to find out how to correctly frame the opening.

Literature that comes with your door will tell you the exact size of the required opening. Make sure your opening is very close to perfectly square: Use a level to check the sides for plumb, and the top and bottom for level.

In most cases, you want the door's exterior molding to fit inside the house's siding, rather than sitting on top of the siding. With a helper, hold the door up against the house, check it for level and plumb, and mark your siding for cutting. Cut the siding with a circular saw, taking care not to cut too deeply. Set the door in place again to make sure the molding will fit.

Pry back the siding and slip in 10-inch-wide pieces of roofing paper (or felt). Staple the paper to the framing, so that all exposed wood is covered (see Figure 1). Cut a piece of drip edge flashing to fit over the door. Slide it under the siding. Don’t nail it, just slip it in tightly.

With a helper, set the door in the opening. Have the helper hold the door in place from the outside while you work on the inside. Check all four sides for level or plumb. Gently tap in shims every 6 inches or so, and re-check the jamb for plumb and level (see Figure 2). Do not drive shims too tightly or the jamb will bend. Test to see that the door slides smoothly.

From the outside, drive casing nails through the molding and into the house's framing. Inside, drive finishing nails through the jambs and into the framing. Drive one nail near every shim (see Figure 3). Cut the shims off, and install interior molding to cover the gaps between door and wall.

Install a piece of molding against the threshold, tight up against the bottom of the vertical molding pieces (see Figure 4). Seal all outside joints with exterior-grade caulk so there is no place where rainwater can seep in.