Author Topic: Measuring a door jamb for a door.  (Read 3863 times)

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Offline mrFinpgh

  • Posts: 282
Measuring a door jamb for a door.
« on: March 26, 2016, 11:24 AM »
I think this might be pretty rudimentary, but I want to make sure I get it right:

I have a doorway that does not have a door in it.  There used to be a door -- there are hinges that are covered in paint.  When I bought the house, the doorway had one of those accordion doors nailed to the jamb.

I'd like to get a proper door in there.  Buying a prehung isn't an option - removing the old jamb would mean disrupting a lot of surfaces and likely damaging the plaster surrounding it.

So let's say the jamb measures 32" x 79.75".  When I buy a door, what should the measurements of the door be?   I've read that it should be 1/8" narrower on the left and right, 1/8" shorter on the top, and 1/4" higher on the bottom.  Does that mean I'm looking for a 31.75" x 79.375" door?  Seems unlikely I'd have success finding that.   Is my best bet to buy a 32x80" and use my tracksaw to fit it?

Anything else I need to watch out for?


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Offline waho6o9

  • Posts: 1430
    • Garage Door
Re: Measuring a door jamb for a door.
« Reply #1 on: March 26, 2016, 11:30 AM »
Practice on a free door from Craigslist or make one would be my suggestion.

Doors from the big box store are good to practice on as well.

Get a door that's a little bigger than the opening and scribe away.

Good times!
« Last Edit: March 26, 2016, 01:36 PM by waho6o9 »

Offline demographic

  • Posts: 425
Re: Measuring a door jamb for a door.
« Reply #2 on: March 26, 2016, 12:45 PM »
You can remove some off the width, depends on if the door has a lipping on it as to how much you can take off without it showing through to the cheaper wood they put underneath.

If you look at the top or bottom of the door you can usually see how thick the lipping is, often about 5 or so mm each side.

Used to be (before many doors had smokeseals and the hinges with the correct offset for them) that I would leave 2mm gap around the top and each side of the door.
Nowadays most 100mm hinges have a 3mm offset and that's correct for doors that have a smokeseal.

I firmly believe that doors should have a slight leading edge also.

Oh and I give them maybe 10mm on the bottom above a hard floor and maybe less above carpet cos I hate to see the scratches when someone has brought a tiny pebble in the grip of their shoe that gets trapped under the opening/closing door and scabbles the floor.

Offline tjbnwi

  • Posts: 5895
  • Cedar Tucky Indiana
Re: Measuring a door jamb for a door.
« Reply #3 on: March 26, 2016, 01:16 PM »
You will most probably end up with a 2/8-6/8 slab.

You will have to fit the slab to the opening.

You must determine the lock side before you do any thing if it is a flush hollow core door. The top may have an LS imprinted with an arrow towards the lock side. If there is no marking, place the slab on a solid surface and push down along the stiles in the area where you would expect the lock set to be. The lock side will have solid blocking in it.

The top revel is determined by hinge placement. Place a quarter flat against the top jamb, measure down to the top of the top hinge, this is your starting point for the hinges.

Bevel the lock side a couple of degrees, the door will be narrower towards the stop.

Bottom allow for finish floor clearance----if there is a cold air return in the room I'll leave about 3/8", no cold air return up to an inch.

As someone mentioned, scribe it in.


Offline SgtTVD

  • Posts: 1
Re: Measuring a door jamb for a door.
« Reply #4 on: March 26, 2016, 02:29 PM »
I have made custom doors for a local pre-hanger for 15 years, I understand the pre-fit requirements. Do not start with your slab, but start with the jamb set in the opening. Assumption here could lead to massive disappointment downstream.
First check your opening for Square. The larger the square the more accurate your results will be. Make sure both hinge and strike side is indeed square. Next, you mentioned that you had painted over hinges on your jambs. scribe around the edges and remove them. This will allow you do several things. You can boil them and the paint will come right off. It will allow you to find matching leafs and hinge pins at a local salvager. Replace the old slotted screws with new if you can find the correct ones. Modern hinges are thinner and use crappy 3/4" screws.

After confirming what the opening size is and that your header is square, measure down from top of header to top of each hinge location. Note the size of hinge and the depth it penetrates into the opening, as well as thickness of the hinge plate. Subtract the 1/8 from you measurements to allow for head gap. Repeat on the strike side for lock location. Undersize the bottom of the slab to match other doors around it in appearance. Take into account any obstructions that will be unique to this opening though like flooring transitions, (thick carpet or a mat etc), then trim accordingly.
For modern molded skin or flush hollow core doors there is a standard 2" blocking in the bottom. Solid cores are normally constructed this way also, but it varies from manufacturer. Stile and rail doors are easier to work with, but are put together with dowel pins. Cutting to much off makes the door weak and likely to fail.

Good luck! Measure twice and cut once!

Offline Jim Kirkpatrick

  • Posts: 1065
    • Jim Kirkpatrick Woodworking
Re: Measuring a door jamb for a door.
« Reply #5 on: March 27, 2016, 08:17 AM »
I just recently did the very same thing.  A track saw will make this task a breeze, particularly for beveling the lock side 2 degrees as someone else mentioned

Offline mrFinpgh

  • Posts: 282
Re: Measuring a door jamb for a door.
« Reply #6 on: March 27, 2016, 10:53 AM »
Thanks, guys, for your advice.  I've got some tiling to do today, but then I'm going to be moving on to this door.


Offline mkasdin

  • Posts: 121
Re: Measuring a door jamb for a door.
« Reply #7 on: April 27, 2016, 04:45 PM »
For mortising the hinges you can line up the hinge points on the door using the installed jamb. I haven't used this, but I hear it is good.,43000,40219

The hollow doors are basically cardboard? I would cut a little from the top and bottom if you need to trim let's say an inch, more than that I would just trim from the bottom and then cut a new piece of wood to sit in the place of the foregone wood.

Get the slab and trim accordingly. Bevel the back edge of the strike play side and possibly back bevel the Hing side. So two back bevels. You can use an electric hand planer if you have it