Tag: can you make a door of the wrong swing fit

How to Measure a Prehung Door


Do You Know How To Measure a Prehung Door?

We’ll go over everything you need to measure on your next replacement project.


In this article, we want to go over the different ways you can measure a prehung door. Depending on the application, you’ll need to know how and what to measure so you can order the correct replacement door from the Lumberyard.

How To Measure a Prehung Door Interior

Prehung Door Measurement Terms:

Door Handing –

First, if you’re planning to measure a prehung door yourself, you’ll need to know how to tell what the “handing” or “swing” of the door is. The best way we’ve found is to put your nose to the hinge pins, (with the door shut) and see which of your hands can grab the door handle. Since the hinge pins are only visible from one side of the door (interior or exterior,) it allows for a fool-proof way of determining swing. The door in the picture would be called a LH inswing.

Note: Exterior doors can also be specified as inswing or outswing – essentially, does the door swing into the room or out to the exterior?

Slab Size –

Next, the most common way to measure a prehung door is to call out the measurements of the door slab only (translated into lineal feet.) Look at the door in the example picture above – the Blue measurements are the door slab size. This door would be called out as a 3068 (3’ x 6’8”) when ordering a new prehung unit for replacement.

Rough Opening (RO) –

Another industry standard is to remove the interior or exterior casing and measure the 2×4 stud framing or “Rough Opening” (RO). Back to our example picture – the Orange measurements show the RO of this door at 39” x 82 ½”. (Industry standard is to call out the width before the height when ordering a door or window.)

Frame Size –

Some manufacturers allow you to order a door by “Frame Size” or the outside measurement of the door jamb assembly. This allows for an exact fit when trying to save interior and exterior casings. The Red measurements show the door in the example has a frame size of 38” x 82”.

How To Measure a Prehung Door Exterior

Outside Measurement of Exterior Casing (OSM) –

Finally, if you’re trying to replace an exterior door and save the existing siding, you’ll want to measure by what’s called the “Outside Measurement of Exterior Casing (OSM). You’ll also need to measure and specify the width of the existing Brickmolding or casing. This will give you an exact replacement that fits into the existing siding and RO. The Green measurements above show this example door has an OSM of 41” x 83 ½” with 2” Brickmolding.

These terms are hard to differentiate at first, but by gaining a good understanding of how to properly measure your next door – you can avoid costly mistakes caused by miss-orders.

DID YOU LIKE THIS ARTICLE?

DID YOU LIKE THIS ARTICLE?

Here are some more you might be interested in! We’ve taken everything we’ve learned about door installation and wrote it down for your use

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The Door Is Installed Level & Plumb, but it still doesn’t work!


Has this ever happened to you?

We Can Help!


You’ve tried to do everything right, taking your time to find level & plumb perfectly.  Even shimming properly at each hinge location, but no matter what you try – the door frame will not line up correctly with the slab.  (learn some shimming tricks here)

Let’s trouble shoot some of the possible causes below:

1.  The door slab is level, but the sill is not

This is one of the most common issues when you’re facing the problem the door being installed level & plumb, but it still doesn’t work.  Look at the picture below to get a better idea of what’s going on here. 

picture-correct-door-sill-vs-an-incorrect-door-sill

A level sill is the foundation for a good door installation

As you can see, even if the door is level & plumb – if the sill isn’t, you will have a hard time getting your reveals to line up correctly.  (learn more about getting a level sill here)

2.  The door’s frame legs are not installed true with one another

Here is a picture of another trouble-spot that we’ve seen on job-sites.

Doors 101 - True

Note how the door legs don’t align with one another

It is possible to have a door slab level & plumb and one side of the door frame level & plumb. But if the other side of the door frame isn’t true with its partner – the door slab can appear warped because it will pop-out from the frame in one corner.  (you can learn more about what installing a door true means here)

3.  One of the door’s components was manufactured incorrectly

Even though it’s rare and door manufacturer’s try very hard to produce every unit correctly, it is possible for a part to be made incorrectly and find its way into the field.  Fortunately, you can check that your door was made correctly by measuring both frame legs to make sure they match and both head and sill to make sure you have a matching pair.  As for the spacing between the door and frame (usually called the reveal) it is recommended that there is at least 1/8” on both the Strike and Head sides of the door and 3/32″ on the hinge side of the door.  We’ve put together a diagram below to help explain this.

Proper reveals are required to maintain Energy STAR® ratings on exterior doors

4.  The door slab is warped

Again, this is rare for a new door. But it is possible for the door slab to warp before it has been installed.  If this has happened, the door will pop-out from the frame in one corner as mentioned above. It will also not match with its frame.  An easy way to test for this is to use a string pulled tight from the top of the bottom of the door corners. (as shown in the picture below) If the gap is larger than ⅜” from the string to the door, it is warped.

DID YOU LIKE THIS ARTICLE?

DID YOU LIKE THIS ARTICLE?

Here are some more you might be interested in! We’ve taken everything we’ve learned about door installation and wrote it down for your use

Read more

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