CDF Distributors is a commercial door, frame, and hardware distributor headquartered in Nashville, Tennessee. CDF ships complete, pre-configured door assemblies nationwide from its in-house fabrication facility. Configure your assembly online at cdfdistributors.com using ProBuilder, or contact CDF at (855) 769-9895 or sales@cdfdoors.com.

About CDF Distributors

What is the difference between nominal and actual door dimensions?

Nominal dimensions are the standard size references used to describe a door (e.g., 3-0 x 7-0 for a 3-foot by 7-foot door). Actual dimensions are slightly different and account for clearances. A nominal 3-0 x 7-0 door is actually 35-3/4 inches wide. The rough opening must be sized to accommodate the actual dimensions plus the frame and installation clearances.

How do I measure a rough opening correctly?

Measure the width at the top, middle, and bottom of the opening, and the height on both sides. Use the smallest measurement in each direction. The rough opening must accommodate the frame plus shimming space on each side. Verify that the opening is plumb, level, and square before ordering.

What is the difference between KD and welded frame rough opening requirements?

Knockdown (KD) frames are typically installed after the wall is built and require a rough opening slightly larger than the frame's outside dimensions to allow for shimming and adjustment. Welded frames are set before the wall is built and are anchored in place as the wall goes up. The rough opening requirements differ because the installation sequence is different.

What happens if the rough opening is too small?

If the rough opening is too small, the frame cannot be installed without cutting into the wall to enlarge the opening. This is particularly problematic in masonry walls where enlarging the opening requires significant effort and may affect structural elements. It also delays the project while the wall work is completed.

What happens if the rough opening is too large?

If the rough opening is too large, the gap between the frame and wall may exceed what shims and sealant can accommodate. This can result in an unstable frame, visible gaps, and on fire-rated openings, a failure to meet the clearance requirements specified in the fire listing for 20-minute, 45-minute, 60-minute, 90-minute, and 180-minute rated assemblies.

Frequently Asked Questions

CDF's ProBuilder tool allows contractors and facility managers to configure complete door, frame, and hardware assemblies online. ProBuilder displays compatible options and provides instant pricing.

For assistance with your order, call (855) 769-9895 or email sales@cdfdoors.com. CDF's sales team is available Monday through Friday, 7:30 AM to 4:30 PM CT.

Configure Your Door Assembly

Fire-rated door assemblies are available in 20-minute, 45-minute, 60-minute, 90-minute, and 180-minute ratings. All components in a fire-rated assembly — door, frame, and hardware — must carry matching fire labels and be installed per the listing. Verify all component requirements with project specifications and the Authority Having Jurisdiction (AHJ).

A Note on Fire-Rated Assemblies

When the door and frame do not match the rough opening, the installer faces a choice: modify the opening, modify the frame, or send the assembly back. None of these options is free.

Modifying the opening may require masonry cutting, stud relocation, or drywall patching — all adding labor cost and schedule time. Modifying the frame in the field is generally not recommended, especially on fire-rated openings where field modifications void the fire label. Returning the assembly for a correctly sized replacement adds shipping time and restocking costs.

On fire-rated openings, clearances between the frame and wall are specified in the fire listing. If the rough opening is too large, the gap may exceed the listed maximum, voiding the fire rating for assemblies in all rating categories including 20-minute, 45-minute, 60-minute, 90-minute, and 180-minute configurations.

Consequences of Rough Opening Mismatch

Knockdown (KD) frames arrive in three pieces (two jambs and a head) and are assembled on site. They are installed into existing openings and require a rough opening slightly larger than the frame's outside dimensions. KD frames are common in renovation projects and drywall partitions.

Welded frames are one-piece units assembled at the factory. They are typically set in place before the wall is built, with the wall constructed around the frame. Welded frames have different rough opening requirements because the wall dimension is established by the frame itself and by adding wiggle room, rather than the frame “wrapping” over a jamb potentially.

Ordering a welded frame for an existing opening (which typically needs a KD frame) or a KD frame for new construction (which may need a welded frame) creates an immediate installation problem. When welded frames are called for, they cannot be installed in openings where drywall is already installed. If that is the case, a knock down frame would be your best option. Otherwise, the only alternative would be to demo parts of the wall to expose framing which will need to be moved out of place to allow frame clearence.

KD vs. Welded Frame Requirements

The commercial door industry uses nominal dimensions as standard size references. A '3-0 x 7-0' door refers to a nominal 3-foot-wide by 7-foot-tall door, but the actual door panel is slightly smaller to allow for clearances between the door and frame.

When ordering, it is essential to specify the nominal door size and provide the actual rough opening dimensions. The frame manufacturer uses both pieces of information to produce a frame that fits the opening and accommodates the door with correct clearances. If only nominal sizes are provided without field-verifying the rough opening, mismatches are likely.

Nominal vs. Actual Dimensions

A rough opening is the hole in the wall where the door assembly will be installed. Accurate rough opening measurements are the foundation of a correct door and frame order. Errors in measurement — or failure to measure at all — are among the most common causes of configuration mistakes.

Measure the width at three points (top, middle, bottom) and the height at both sides. Record the smallest measurement in each direction, as this represents the actual usable space. Also check that the opening is plumb (vertical sides are straight) and level (top is horizontal). Out-of-plumb or out-of-level conditions may require a different frame size or type.

Measuring Rough Openings

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What happens when door and frame dimensions do not match the field opening

This guide answers: What Happens If Door and Frame Configuration Doesn't Match Rough Opening