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How to Measure a Prehung Door

Written by JENTRA on November 24, 2020. Posted in Doors 101. Leave a Comment

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.

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”.

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?

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

5 Mistakes You Might be Making Installing a Door

Written by JENTRA on June 5, 2014. Posted in Doors 101.

we’ve been there too.

Our pick of the top 5 most common door installation mistakes

Below are 5 of the most common problems seen when it comes to installing a door. Take a look below to see if you’ve ever struggled with any of these!

1. Hanging the door parallel to the wall, even if it is out of level

If the wall is out of plumb, it’s hard to install a door correctly and it may tend to swing open or closed. If the wall is plumb but the door frame is not, the door frame may need to be adjusted to ensure an even reveal on each side of the door. If the walls on either side are not parallel, extra time may be needed during shimming to ensure the best results.

Don’t Expect Good Results if you Don’t Start Right.

2. Using the wrong hinge screws or over-tightening them

The right screws, or 3 ½” screws, keep the door stable over time. Too small of screws can allow the door to migrate over time, which can cause it to rub or scrape on the floor or not shut at all. When the screws have been tightened improperly, the door will experience uneven tension and will eventually warp.

Go Slow and Steady.

3. Placing shims improperly or backwards

When installing a door, thick end of the shim should face the door’s hinge pin. This helps it swing shut properly. If the shims have been installed backwards, the door will begin to resist before it is completely shut. Putting door shims in backwards can also be a large contributing factor to a door that swings open or shut on it’s own. Read more about proper shimming techniques here: The Ups and Downs of a Door Shim.

Proper Shimming is Important.

4. Failing to check the reveals

As the industries top cause of warped doors, uneven reveals can cause significant long-term problems with a door. The reveals must be parallel to ensure proper operation and a long life for the door. If the reveals are uneven, the door will build up pressure over time and warp as the moisture level changes. If a door is hanging with even tension across all three hinges, it will operate properly for years to come.

EnergySTAR Ratings can be Voided by Improper Reveals.

5. Not starting with a level sill

As seen in the picture below, by starting without a level sill, it’s practically impossible to end up with a good result to your door installation. The sill is the door’s foundation, if it is out of level; the problems compound over the height of the door and become much more difficult to troubleshoot. Read more about proper level sill here: How Important is a Level Sill?

Installing a door with a level sill is like building a house on a level foundation, start right to end right.

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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How to Measure a Prehung Door