DEAR TIM: I’ve decided to install some interior french doors. Since I’m a rookie carpenter, I’m intrigued by some prehung interior french doors I saw at a lumber yard. What’s your opinion of these doors? What else can you share that will help me install interior french doors like you might do it? Surely there are some secret tips that make the job go faster and easier. Michael G., Burbank, CA

DEAR MICHAEL: Installing interior french doors, assuming you’re a real rookie carpenter, is going to sap you of every bit of skill and patience you have at this point in your DIY career. It’s hard enough hanging a single prehung door, much less a set of french doors. I’ve installed many prehung french doors with great success.

These interior french doors just need trim, paint and hardware to be complete. PHOTO CREDIT: Brent Walter

The first step is to inspect the doors at the store. If you can see the hinge side of the doors, look 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 just about one eighth inch everywhere. If the jamb is bowed or the gap is wildly different, look at other doors that may be in stock. You want everything in your favor before you take the door home.

The first thing to get right as you begin to tackle your double interior french doors is the rough opening. This is the hole in the wall where the door fits. I prefer to have my rough openings at least one-half inch wider than the overall unit size of the door. The unit size is the overall height and width of the frame that the doors hang from. The same is true for the height. I like having the rough opening one-half inch higher than the the door frame’s overall height.

Remember, this height distance is measured from the top of the finished floor. It’s always better to have the door jambs sit on top of a finished floor instead of installing the door and then trying to butt up flooring to the door jamb.

Since the actual doors in your new french doors are undoubtedly perfectly square, this means that if the reveal or spacing in between the doors and the frame are equal, then the overall door frame is square. It’s imperative that the two door jambs are installed plumb, parallel and in the same plane. If you can make the rough opening so the vertical rough studs are plumb it will really help you.

Not only does the rough opening need to be square and plumb, it’s also very important that the opening is not a helix. This means that the opening has to be straight and plumb in both directions. If one leg of the rough opening is not plumb then the two doors, when closed, will not be even at the bottom. Are you still wanting to do this install? It’s now just getting interesting.

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 door jamb on the high side of the floor 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 an ugly gap. Your margin of error here is less than one-sixteenth of an inch.

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

After you’re satisfied that the door is installed and the spacing is perfect, I recommend installing a hidden screw under the top hinge of each door. I prefer to use a 2.5-inch-long screw that’s driven through the door jamb into the rough opening framing lumber. This screw ensures that the doors stay in position for years. There’s tremendous tension on the top hinge, and the screw will anchor the doors to the rough framing.

You’ll also need a nail set tool to drive the finish nails below the surface of the door jamb. Be very careful as you hammer the nails so the head of the hammer never touches the surface of the wood. You don’t want any rookie beauty marks on the door jamb.

Once your french doors are installed to your satisfaction and that of your significant other, you need to be sure they don’t warp. It’s imperative that you paint the top and the bottom of your wood doors with a minimum of 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 it.