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French Door Hanging Instructions Modified

two french doors

These two french doors can be hung separately as single units with some work. Photo Credit: Tim Carter

Quick Column Summary:

  • Separating double french doors
  • Doors already have hinge mortise
  • New side jambs are needed

DEAR TIM: I bought a prehung double french door because it was so beautiful and the price was great. I want to separate the two doors and use each one at different openings. Is this possible to do without destroying one of the doors? Is this a project a mere mortal can accomplish or is this far above my DIY pay grade? Tell me what's involved and I can handle the truth. Candace S., Calgary, Alberta - CA

DEAR CANDACE: When I first read your question I thought that it was a folly for you to attempt this. But then I paused and reconsidered. I feel you can successfully complete this crazy job if you possess some moderate finish carpentry skills and have attention to detail.

Here's the good news. The hardest part of the job is complete because the hinge mortising of the doors and two door jambs is complete. This was done at the factory since your door is prehung. Mortising hinges and aligning them in door jambs does require great skill and I doubt a first-timer DIYr would get it right. You're over that hump.

To complete the job, you're going to need a few supplies. You'll need two new long side flat jambs and you'll need one top jamb. These long jambs are where the door latch striker plate will be installed. At this point there is no mortising required to be able to hang the door.

You'll also need enough door stop moulding for these two long vertical jambs and the smaller top jamb. You have enough top jamb material and door stop now on the longer double door top jamb that you can cut down for one of the two single doors.

I would start the process by taking off the door stop molding from the flat door jambs. But before you do this, it's important to make a critical mark on the edge of the top door jamb. Have someone help you lay the entire prehung door unit flat on the floor so the exposed rounded hinge ends face the ceiling.

Make certain the gap between the two doors is consistent and equal down the entire length of both doors. You want to make a mark on the long top jamb that will be your cut line so you end up with a new top jamb that's the proper length for your new single door.

The gap between the edge of the door and the jamb where the hinges are should be about 1/8 inch. You'll want the same gap on the other long side of the door once your new frame is assembled.

The new flat jamb material you will use will probably be 3/4-inch thick. This means you need to make a mark on the edge of the long top door jamb 7/8 of an inch away from the edge of one of your doors where they meet in the center. Carefully make this mark. Double check it for accuracy.

Now it's time to take the door apart and make the needed cuts. Use a stiff putty knife to gently pry the door stop moulding from the jamb. Mark the pieces so you know which piece goes on which jamb. The long top doorstop doesn't need to be marked because you'll be cutting this down in a few hours.

Remove the screws or nails that fasten the top jamb to the long side jambs. With the top jamb free, use a square to make a square cut line across the top of the jamb and cut this wood nice and square. A miter box saw does this job well. The shorter piece of the top jamb left over after the cut should be set aside for some future project.

Measure the length of the remaining top jamb. Cut an identical piece for the other door. This assumes the two french doors are the exact same width. I can't imagine for a moment they'd be different measurements.

All you have to do now is cut two new long door jambs. Make them the same length as the existing long door jambs that have the hinges attached to them. With all the cuts made, you can now reassemble the door frames. Drill pilot holes into the edges of the top jambs so you don't split the wood when you drive the screws.

The last step is to install the door stop material. Believe it or not, this may be far easier to do once you've hung the single doors in your new openings. The door stop should be installed so it's about 1/32-inch away from the door when the door is flush with the jamb edges. This narrow gap allows for paint buildup over time. Trying to achieve this narrow tolerance is fairly hard to do with the door flopping around in a jamb that's not square and nailed in place in a rough door opening.

The key to success of this project is visualizing what your finished door frame should look like. If you can't do this from my description, visit a store that sells prehung single door units. Take one off the rack and look at the consistent gap around the door on the hinge side. Then look at how the top jamb overlaps the two longer side jambs. Take out your cell phone and capture close up photos to help you remember what your goal is. Good luck!

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