These interior French doors just need trim, paint and hardware...

These interior French doors just need trim, paint and hardware to be complete. That's the easy part; installing the doors properly isn't as easy as it looks. Credit: Brent Walter

1. Inspect the doors at the store

Look at the hinge side of the doors to see if the "reveal," or gap, between the doors and the jambs is pretty close all the way around the door. You want a gap of about 1/8 inch everywhere. If the jamb is bowed or the gap is wildly different, look at other doors in stock. You want everything in your favor before you take the door home.

2. First things first

The next thing to get right is the rough opening in the wall where the French doors will fit. The unit size is the overall height and width of the frame the doors hang from. Some builders like having the rough opening 1/2 inch higher than the door frame's height. Remember: This height distance is measured from the top of the finished floor. It's always better to have the doorjambs sit on top of a finished floor than to install the door and then try to butt up flooring to the doorjamb.

3. About the frame

Since the doors of the prehung unit are undoubtedly perfectly square, this means that if the reveal, or spacing, between the doors and frame is equal, the overall door frame is square. It's imperative that the doorjambs are installed plumb, parallel and in the same plane. Try to make the rough opening so the rough studs are plumb.

4. Straight shooting

Not only does the rough opening need to be square and plumb, it's also important that the opening is not a helix. This means the opening has to be straight and plumb in both directions. If one leg of the rough opening is not plumb, the two doors, when closed, will not be even at the bottom.

5. A level floor

You'll also have an easy time if the floor under the door unit is level from one side of the opening to the other. If the floor is out of level, you'll have to precisely cut down the doorjamb on the high side of the floor by the amount it's out of level from jamb to jamb. If you don't do this, you'll have to shim up the one jamb off the finished floor, creating a gap. Your margin of error is less than 1/16 inch.

6. 10-penny thoughts

Once you get the frame into the opening, tack it in place with 10-penny finish nails. Be sure to use thin shims to ensure the jamb is plumb. You'll have to open and close the doors numerous times as you nail to check the spacing between doors and jambs.

7. Hidden screws

After you're satisfied the door is installed and the spacing is perfect, install a hidden screw under the top hinge of each door. Some builders prefer a 21/2-inch-long screw that's driven through the jamb into the rough opening framing lumber. This screw ensures the doors stay in position for years. There's tremendous tension on the top hinge; the screw anchors the doors to the rough framing.

8. Warp protection

It's important to paint the top and bottom of wood doors with at least two coats of paint. This paint significantly slows the absorption of water vapor into the long vertical stiles of the doors. If too much water gets into the door, it can warp.