Ask the Builder: Divide French doors, hang separately

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? Tell me what’s involved. I can handle the truth. –  Candace S., Calgary, Alberta

DEAR CANDACE: I feel you can successfully complete this crazy job if you possess moderate finish-carpentry skills and have attention to detail.

Here’s the good news: The hardest part of the job – the hinge mortising of the doors and two door jambs – is complete. This was done at the factory since your door is prehung.

To complete the job, you’re going to need two new long side flat jambs and one top jamb. These long jambs are where the door latch striker plate will be installed. You’ll also need enough door stop molding for all three jambs.

In the prehung French door unit, you already have enough top jamb material and door stop for one of the two single doors you plan to create. All you have to do is cut it down for one door.

I would start by making a critical mark on the edge of the top door jamb. Have someone help you lay the entire 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.

The mark you make on the long top jamb 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 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. Start by taking off the door stop molding from the door jambs. Use a stiff putty knife to gently pry the molding from the jambs. Mark the pieces so you know which piece goes on which jamb. The door stop molding on the long top jamb 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.

All you have to do now is cut two new long side jambs. Make them the same length as the existing side 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. This may be 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 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 cellphone and capture close-up photos to help you remember what your goal is.