I’ve spent the past couple of days replacing all of the original doors in a client’s mid-century ranch. The old doors were just as plain as could be and the homeowner is wanting to upgrade to a nicer look using 6 panel Masonite doors.
Modern-built homes are all about standards. Walls are standard heights and doors are standard sizes. If you have one of these homes, you can buy a door off the shelf at the home store and it will fit with no problems. However, for the house I’ve been working at, like many others in our Inglewood community, every door may be a little different.
When I have to custom fit a door to an existing door frame I like to buy what’s called a door ‘slab’. It a standard sized door, but with no hinge mortises or holes for the hardware. It’s a blank slate.
Let’s focus on getting the door sized right with the hinges in the right place:
1. If you’re replacing a door, don’t take the old one down just yet. Measure the opening that the door fits into. Subtract 1/8″ for the gap on either side and cut the door to this measurement. For example, if the door opening is 30″, then the door needs to be 29 3/4″. Measure the old door as a reference if it fit well.
I’ve just been using the circular saw with a rip guide (invaluable tool!) to take the edges off of doors. After any cuts, lightly sand the sharp edges to match the factory sides.
(Always check to make sure the door frames are level and square or your door won’t fit correctly.)
2. Let’s measure to see where the hinges will go. For this, it’s very important to make all measurements from the top of the door as a reference point. Don’t measure one hinge from the top and the others from the bottom. Remember that you want around 1/8″ gap at the top of the door too. This is the time to measure twice and cut once. 
3. For this project, I used a simple hinge mortising template that I picked up at Home Depot a few years ago. It’s just a metal guide that attaches over the hinge location. It will guide my router bit to make the inset for the hinge. I usually just secure it in place with screws so it’s easy to remove after I’ve made the cut. (Yes, you can use a wood chisel to make the cuts, but it won’t be as neat and it’s hard to get the depth exactly right.)
4. Cut the hinge mortise with your router. I was using a straight 1/2″ bit. The biggest consideration here is to NOT CUT TOO DEEPLY! If you do, your door will not fit right and you’ll be baffled. You want the surface of the hinge to be flush with the door’s edge when it’s in place, so make the cut accordingly, around 1/8″ deep, but check it with your hinge.
5. See if your hinges have rounded corners or square. Mine were square so I needed to remove the corners left by my router template. For this I used a wood chisel and carefully squared up the corners.6. Remove the old door, by taking the screws out of the hinges on the door. Leave the hinges attached to the door frame.
7. Hold the door in place so you can line up the hinges. Mark the holes and make pilot holes before you screw.
8. Check it and fix. I had a couple instances where the door stop wasn’t letting the new door close. Apparently, the new door is just a touch thicker than the old. To remedy this I carefully removed the door stop (see this post) and moved it around 1/16″ inch so the door would close properly.
Hopefully, your measurements were accurate and you’re ready to move on to doorknobs!
-Peter
