Why Width Is the Measurement That Gets Assumed Instead of Taken
This article helps contractors, facility managers, and architects avoid one of the most common and expensive mistakes on accessible-route openings: ordering an interlocking wheelchair ramp threshold to a width that fits the door rather than the opening. The two numbers are not the same, and the difference can hold up a punch-list item or force a reorder with a 10-to-15-business-day lead time.
An ADA interlocking ramp threshold is the type of threshold most commonly specified when a floor height difference at a door exceeds what a standard saddle can legally accommodate. Instead of a simple beveled saddle, it uses two mating aluminum extrusions: one anchored to the floor and one attached to the door bottom, which interlock when the door closes to create a weathertight, code-compliant transition. The result is a low-profile ramp profile that satisfies the ADA and IBC requirement that thresholds on accessible routes not exceed one-half inch in height.
The interlocking design is what creates the measurement trap.
What an Interlocking ADA Ramp Threshold Actually Is
A standard saddle threshold sits entirely on the floor between the jambs. Measuring the clear opening width and ordering to that dimension is usually close enough, with minor field trimming if needed.
An interlocking threshold is different. The floor-mounted sill piece runs the full width between jambs. The door-mounted hook strip or sweep component runs only as wide as the door itself. If the door is a 3-foot leaf in a frame with a quarter-inch gap on each side, the door component is narrower than the sill component by roughly a half inch. Order the wrong overall width and one piece will be too short, too long, or simply wrong for the configuration.
The height on these units, often around 2-1/4 inches at the ramp peak, is achieved through the interlocking profile geometry, not by stacking material under the door. That geometry is why fit precision matters: a sill that is even a quarter-inch too short can leave an exposed gap at the jamb that fails both weatherstripping and inspection.
The Three Dimensions You Need Before You Order
Before specifying or ordering an interlocking ramp threshold, take all three of these measurements at the job site. Do not pull them from the door schedule or the architectural drawings without verifying in the field, especially on renovation and retrofit work.
- Clear opening width: The distance between the inside faces of the two jambs at the floor level. This governs the sill piece length. Measure at the floor, not at mid-height; jambs are not always perfectly plumb, and the floor dimension is the controlling one.
- Door leaf width: The actual door width, not the nominal size. A nominal 3-foot door is typically 35-1/2 to 36 inches. Confirm the actual dimension, especially on older openings where the door may have been replaced with a non-standard leaf.
- Undercut and door-bottom clearance: The interlocking hook strip or sweep component mounts to the door bottom. If the door has an existing automatic door bottom, a surface-applied sweep, or an unusual undercut, the threshold height and interlock engagement depth will be affected. Measure the gap between the bottom of the door and the finished floor with the door closed before finalizing the threshold height specification.
Where the Width Mistake Happens on Real Projects
The most common scenario: the hardware schedule lists a 27-inch-wide ADA ramp threshold because that is what the spec writer pulled from a prior project or a manufacturer default. The actual opening on this project is a 36-inch door in a 37-inch frame. The threshold arrives, the sill piece is 9 inches too short, and it goes back.
A second scenario: the threshold is ordered to the clear opening width, which is correct for the sill, but nobody accounts for the door-bottom component. The door component arrives at door width, which is correct, but the installer discovers the door already has a surface sweep installed from a prior renovation. Now the interlock does not engage cleanly and the door drags.
A third scenario unique to interlocking styles: the project specifies a threshold width that matches the door, not the frame, because the estimator was looking at the door schedule, not a field measurement. The sill is cut short of the jambs, leaving an exposed metal edge at each side that the inspector notes as a tripping hazard and a weatherstripping failure.
Code Context: What the ADA and IBC Actually Require at the Threshold
Under the ADA Accessibility Guidelines and ICC A117.1, thresholds on accessible routes must not exceed one-half inch in total height. Any threshold between one-quarter inch and one-half inch must be beveled at a maximum slope of 1:2 (one-half inch rise per inch of run). Interlocking ramp thresholds with ramp profiles achieve compliance through a gradual transition rather than a vertical face, which is why they are the standard specification when the floor height change at the sill approaches the limit.
The IBC echoes these requirements. For existing or altered openings, a threshold up to three-quarters of an inch may be permitted if beveled, but new construction holds to the one-half inch maximum. Verify which standard applies to the project before finalizing the threshold height specification.
Neither code specifies a minimum threshold width directly. The width requirement flows from the clear opening width requirement: accessible doors must provide at least 32 inches of clear width, and the threshold must span the full sill to seal properly and avoid a code-notable gap at the jamb.
Lead Time and the Cost of a Re-Order
ADA interlocking ramp thresholds in specific widths and heights typically carry a production lead time. A wrong-width order is not a quick fix. On a project where the accessible entrance is the last open punch-list item, a reorder lead time measured in business weeks can hold up certificate of occupancy review.
The practical answer is to treat the threshold measurement the same way experienced subs treat a door bottom sweep or an automatic door bottom: field-verify the three dimensions above before the order is placed, not after the threshold arrives on site.
Aluminum Material and Long-Term Performance
Most commercial interlocking ramp thresholds in this category are extruded aluminum. Aluminum is the standard exterior threshold material because it resists corrosion, holds its profile under foot traffic, and is available in mill or anodized finishes that blend with most frame and floor conditions.
One practical note on aluminum at exterior sills: aluminum is an efficient conductor of heat and cold. At exterior openings in cold climates, condensation can form on the interior face of the threshold, particularly at slab-on-grade conditions. Some interlocking designs incorporate a vinyl thermal break or rigid vinyl insert to reduce this effect. If the opening is exterior and the project is in a climate where condensation or frost at the sill is a concern, confirm whether a thermal break is available for the profile specified.
Specifying on Multi-Opening Projects
On school and healthcare projects with multiple accessible entrances, it is common to find that not every exterior door opening has the same rough opening width. A single threshold model at a single width gets carried through a schedule without checking each opening. Verify each opening individually when the schedule covers more than one door. Accessible routes through clinics, gym lobbies, and cafeteria entries frequently mix door widths, and an interlocking threshold ordered to the wrong width on any one of them becomes a field problem at closeout.
DoorwaysPlus carries ADA-compliant interlocking ramp thresholds and related accessible hardware for commercial and institutional projects. If you are measuring for a retrofit or specifying for new construction and need help confirming the right width and height combination, the team at DoorwaysPlus can help you get the right product on the first order.