Thresholds & Door Bottoms (ANSI/BHMA A156.21)

Most of the threshold problems a facility manager hears about are specification problems, not installation problems. An inaccessible threshold at a main entrance shows up on an ADA audit. A worn threshold at a loading dock fails to seal and lets in water, dust, and rodents. A missing threshold at a fire-rated opening blows a NFPA 80 inspection. The right answer in each case is the right threshold specified at the right opening, not a field modification to the wrong product.

ANSI/BHMA A156.21 is the performance standard for thresholds. It covers material, profile, and dimensional requirements for saddle, panic, interlock, and thermal-break thresholds used on commercial swinging doors. A156.21 is the sibling standard to A156.22 (door gasketing and edge seals), and the two are usually specified together because the threshold and the door bottom have to meet cleanly.

The standard covers thresholds made from aluminum, bronze, and brass, which are the common commercial materials. It defines the profile categories (saddle, bumper, panic, interlock, thermal-break), dimensional limits, material gauge, surface finish, and the integration points with door bottom sweeps and gasketing.

A156.21 also addresses threshold anchoring, which matters because a high-traffic threshold that lifts off the substrate is a trip hazard even if the height was specified correctly. The standard references the screw and anchor patterns that manufacturers follow.

The ADA Standards for Accessible Design and ANSI A117.1 Section 404.2.5 govern threshold height at accessible doorways. The rule is straightforward: the total threshold rise cannot exceed 1/2 inch, and any rise above 1/4 inch must be beveled no steeper than 1:2.

The bevel slope of 1:2 means one unit of rise over two units of horizontal run. A 1/2 inch rise needs a 1 inch horizontal bevel surface. Manufacturers publish the bevel as a profile dimension in the threshold data sheet, and the installer just has to make sure the threshold is seated on a flush substrate so the rise measurement stays within spec.

One note about existing buildings. ANSI A117.1 allows a slightly higher threshold (up to 3/4 inch) at exterior sliding doors in existing buildings as a narrow exception. The default limit for new construction and most retrofit work is still 1/2 inch. Check the local adoption and the AHJ interpretation before relying on any exception.

Each threshold type solves a different problem. Choosing the right profile for the opening is the single most important spec decision. Substituting a saddle threshold where an interlock was called out, or swapping a thermal-break for a plain saddle at an exterior door, creates either an ADA issue or an envelope performance issue.

Saddle thresholds are the default for most interior applications. Panic thresholds are specified where the door has a fire exit device that needs clearance at the bottom. Interlocks and thermal-breaks are reserved for exterior doors because they are taller and usually trigger ADA treatment on the exterior side, which often means a gentle ramped approach from the sidewalk.

Exterior doors lose energy and let water in through the threshold gap more than through the door itself. The test standard most manufacturers cite for air infiltration is ASTM E283, which measures how much air a complete door assembly leaks at defined pressure differentials. A properly specified exterior threshold, combined with a door bottom seal and weatherstripping, is what lets the assembly meet envelope performance targets.

An interlock threshold uses a mating groove on the top surface that captures a corresponding seal on the door bottom. When the door is closed, the seal drops into the groove and forms a continuous barrier. Interlocks are common on loading dock doors, commercial kitchens, and any exterior door where water intrusion is a known problem.

A thermal-break threshold has a thermal barrier (usually a plastic or polyurethane insert) running the length of the profile between the interior side and the exterior side. This stops heat transfer through the aluminum body of the threshold, which matters more than most people expect in climates with cold winters or hot summers.

Drainage geometry is the third item. Exterior thresholds should slope away from the building or include weep channels that move water off the sill. A threshold that traps water against the door bottom rots the bottom of the door and the bottom rail of the frame.

NFPA 80 governs thresholds at fire-rated openings. The rule is that the threshold and any door bottom seal must be listed as part of the rated assembly. Substituting a non-listed threshold under a labeled fire door voids the rating.

NFPA 80 also limits the undercut (the gap between the bottom of the door and the floor or threshold) to 3/4 inch unless the specific listing explicitly permits more. In practice most fire-rated openings use a sweep or automatic door bottom to keep the effective gap much tighter than the 3/4 inch ceiling, and smoke-rated assemblies bring that gap down further to meet UL 1784.

At exit stairwell doors, the threshold frequently integrates with a listed smoke gasket on the door bottom. The combination is what meets the UL 1784 air leakage test and keeps smoke out of the stairwell during an evacuation. See the companion UL 1784 page for the air leakage side of that specification.

What inspectors write up on thresholds and door bottoms:

• Threshold rise over 1/2 inch on accessible routes
• Rise between 1/4 inch and 1/2 inch with no bevel
• Damaged threshold at high-traffic openings (cracked, lifted, or corroded)
• Gap under a closed door exceeding 1/8 inch on a smoke-rated assembly
• Threshold not listed for the fire-rated assembly it was installed under
• Door bottom sweep missing or worn away
• Automatic door bottom not dropping when the door closes
• Water intrusion at an exterior threshold with no interlock or weep channels
• New flooring installed later that raised the effective threshold rise out of compliance

The single most common finding is a threshold that used to comply and no longer does because a floor finish was changed above it. Whenever flooring is replaced, every threshold on the affected route needs a rise measurement.

For perimeter seals, meeting stile seals, door bottoms, and material selection, see our Door Gasketing & Edge Seals (ANSI A156.22) page.

For air leakage testing, S-label requirements, and smoke barrier applications, see our UL 1784 Smoke Seals page.

For door closer grade, ADA opening force, and selection by application, see our ANSI A156.4 Door Closers guide.

For fire endurance testing and NFPA 80 assembly rules, see our UL 10C Fire-Rated Doors page.