Door undercut: standard clearance and fire door limits

Typical door undercuts

The undercut is set by what is under the door. The number that matters is the finished clearance: the gap between the bottom of the door and whatever the door passes over (finished flooring, carpet or threshold), not the bare slab.

• Standard commercial door, hard floor: about 3/4 in. clear is common.
• Carpet: the undercut has to account for the carpet and pad, or the door will drag. Get the flooring thickness before the doors are ordered.
• Threshold: the clearance is measured to the top of the threshold, so the door is cut shorter by the threshold height.
• Door bottom or sweep: an automatic door bottom or sweep needs its own clearance, set by the product.

Undercut limits on fire doors

On a fire-rated door, the gap under the door is part of the tested assembly. Since the 2007 edition, NFPA 80 has limited the clearance between the bottom of the door and the top of the floor or threshold to 3/4 in. Editions before 2007 set different limits depending on the floor surface, so check the edition your jurisdiction enforces. Some doors have tighter limits: chute-type doors with a sill 38 in. or more above the floor allow only 3/8 in., and bottom-latching hardware may need less than 3/4 in. to engage properly.

A fire door cannot be cut down in the field beyond what its listing allows. Trimming a rated door to clear new carpet can void the label. Because NFPA 80 measures the clearance to the top of the flooring or threshold, the accepted fix for a gap over 3/4 in. is usually a threshold rather than cutting or extending the door; on an accessible route that threshold must also stay within the ADA limit of 1/2 in. in height.

Undercuts for return air

Mechanical designs sometimes rely on door undercuts to move return air out of a room, which calls for a larger gap, often 1 in. or more. The mechanical engineer should state the free area required. When the needed undercut gets large, it creates problems: light and sound pass under the door, privacy suffers, and the gap can conflict with accessibility thresholds and gasketing.

Alternatives include transfer grilles in the wall, a ducted return, or a door louver. On rated doors, a large undercut is not an option, and louvers are allowed only where the code and the door listing permit them, using listed fire door louvers.

Thresholds, ADA and sound

At accessible doors, thresholds are limited to 1/2 in. high, with anything over 1/4 in. beveled (see ADA door requirements). That limits how much a threshold can be used to close up a large undercut at an exterior or sound rated door.

Sound rated, light seal and smoke control doors usually replace the open undercut with an automatic door bottom that drops when the door closes, paired with a smooth threshold. Those items belong in the hardware set, and the undercut has to suit the product.

Undercut checks during submittal review

• Flooring thickness at each opening matches what the door order assumes.
• Rated doors stay within the NFPA 80 clearance with the actual floor or threshold.
• Doors that the mechanical design relies on for return air are non-rated, or an alternative path is provided.
• Thresholds at accessible doors are 1/2 in. or less.
• Door bottoms, sweeps and gasketing are coordinated with the undercut.

These are exactly the conflicts a submittal review is meant to catch before doors are cut and shipped.