# How To Size a Prehung Door?



## kc27 (Oct 24, 2009)

My question is about how to determine whether a pre-hung door will work for me. The rough opening for my interior door measures 34" wide x 813/8" high with a wall width of 41/2"

I stopped at a Lowes and found a door that is designed to fit a 34" wide x 821/2" opening. The door itself is 32" wide.

I'm good on all the dimensions except for the height. Is the 813/8" height of my rough opening mean that I will need a custom door?

Thanks in advance for any ideas on this


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## Arkitexas (Mar 10, 2011)

No, just cut the bottom of the frame and the door to fit the rough opening. The door needs to be undercut more than the frame to clear your floor finish. Allow 3/4" where carpet or ceramic tile will be installed. Allow 1/2" to 5/8" at sheet vinyl and vinyl tile. Exterior doors with a threshold vary depending on the threshold configuration.

Rick


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## chrisBC (Dec 28, 2010)

You should be fine with that door, although you may want to see if you can add any height to the opening, if not easily done (if there is a header you don't really feel like cutting) then you can always cut the bottom of the door a bit if there isn't enough room.

Typically a rough opening is 2-2.5 inches wider, as well as taller, than the door itself. So with a 32'' door, you should have an opening 34-34.5 or so wide, and 82-82.5 high.

Depending on how level the floor/subfloor is, and what kind of flooring is in place; or whether you are installing the door on top of finish flooring, you may need to cut the bottom of the door, and possibly jambs. Again, it might be easier to add more height to the opening first, before installing. I am assuming this is an interior door and will not have a sill/threshold, and as well is a wood door. 

Your depth should match your size of framing plus drywall, so sounds like your door is for a 2x4 wall.


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## kc27 (Oct 24, 2009)

Thanks for the advice. I checked with the salesperson at Lowes, and he said the actual height of the jamb is 813/4", so may have a shot at enlarging the rough opening, I need to take a closer look tomorrow. 

The salesperson said that if I decide to cut down the door I will need a table saw. I had been.planning on clamping a straightedge on the door and using my circular saw, because I do not own a tablesaw. Is that the wrong way to go? It's a solid core door with oak veneer, if that matters.


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## chrisBC (Dec 28, 2010)

Personally I wouldn't cut it on a tablesaw, I would cut it with a circular saw and a clamped straitedge. Use a good finishing blade on the saw, with the blade extending about a quarter inch past the door. With a veneer it is a good idea to score it with a knife first on both sides to prevent chipping. As well use masking/painters tape where the saw will run will prevent scratching.

Personally i've never seen anyone run the bottom of a door through a tablesaw on a jobsite.


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## BigJim (Sep 2, 2008)

No more than that is I would cut the header off, it is no fun but doable.


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## MikeKy55 (Nov 4, 2009)

I glued and screwed a piece of 1 X 4 to a piece of 1/4" luan (both pieces about 40 inches long) and then ran my circular across with the edge of the saw tight against the 1 X. Now when I need to cut a door, I measure the door, clamp the guide I made to the door even with the marks and cut it with the circular saw using a fine tooth blade. Works great. I do score the face (top side) of the door with a box knife before making the cut. I have never had the underside of the door chip or splinter, always the top surface.
I just cut three doors and jams for my basement and had no issues.


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## Willie T (Jan 29, 2009)

I made four like this one, all various lengths, except that I used 1/2" MDF.

http://youtu.be/CH5dW-QcgeI

You just want to make sure you DO NOT use plywood, as that will eventually 'warp' or 'cup' or twist.

While you are at it, you can make one to use with your router, too.


BTW, Never, ever try to cut a door on your tablesaw. It is dangerous for a novice, and even experts resist doing it.


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