What This Guide Covers and Who It Helps
Selecting an ADA-compliant wheelchair ramp threshold for an already-occupied building is a different problem than specifying one on a new build. The door exists. The floor transition exists. The closer is already set. This guide walks contractors, facility managers, and maintenance teams through the offset and height decisions that determine whether an interlocking ramp threshold actually solves the accessibility problem at the opening — or creates a new one.
What Is an Offset Interlocking Ramp Threshold?
An offset interlocking ramp threshold is a low-profile extruded transition piece installed at the base of a door opening to bridge a change in floor level and create a code-compliant slope. Unlike a flat saddle threshold, an offset ramp has two measurements that both matter: the height (how tall the transition rises) and the offset (how far the ramp profile extends beyond the door centerline toward the exterior or pull side). The interlocking feature allows a matching piece to be installed on the opposite face of the threshold when both sides of the opening require a ramped approach.
On accessible routes, ADA and ICC A117.1 limit total threshold height to 1/2 inch maximum. Changes in level up to 1/4 inch may be vertical; anything between 1/4 inch and 1/2 inch must be beveled at a slope no steeper than 1:2. Changes over 1/2 inch require a full ramp. An offset interlocking ramp profile addresses the 1/4-to-1/2-inch range precisely — the most common condition found when an existing threshold is too tall or a door bottom seal has compressed the available clearance.
Why Occupied Buildings Create a Different Set of Decisions
On new construction, the threshold is selected before the door is hung and the closer is dialed in. On an occupied renovation or ADA retrofit, the sequence is reversed: the door is already there, the floor finish is already down, and often a previous threshold is in place that is either the wrong height, the wrong width, or simply cracked from years of foot traffic and cart impact.
The three field conditions that change the offset and height selection are:
- Existing floor finish height: Tile-over-tile or LVP installations on top of a concrete slab frequently raise the interior finish floor above the original threshold plane. A 1/4-inch height threshold that was once compliant may now create a 1/2-inch or greater transition.
- Door bottom seal compression: Auto door bottoms and door shoes reduce the functional clearance between the door and the threshold. If the closer was adjusted to compensate for a worn seal, the door may already be dragging. Replacing the threshold without accounting for the seal creates a binding condition.
- Exterior surface condition: At entries with a concrete apron, asphalt landing, or exterior tile, the offset dimension of the ramp controls how much of the ramp profile sits outside the door. A 1/2-inch offset is appropriate when the exterior surface is very close to the sill height. A longer offset works where there is more horizontal room to transition.
The Two Numbers You Must Confirm Before Ordering
1. The Height Dimension
Measure the actual change in level at the door opening, not the nominal height of any existing threshold. Use a straightedge and measure from the lower surface to the higher surface at the door centerline. If the change is at or under 1/4 inch, a beveled threshold may solve it. If it falls between 1/4 inch and 1/2 inch, a ramp profile at the measured height is the correct call. If the measurement is over 1/2 inch, a threshold alone is not an ADA solution — a recessed ramp or platform modification is needed.
2. The Offset Dimension
The offset is the horizontal distance from the door stop to the outer edge of the ramp. Profiles like a 1/2-inch offset position the leading edge of the ramp close to the face of the door — appropriate for tight vestibule conditions or where exterior pavement is nearly flush with the sill. A wider offset distributes the slope across more horizontal distance, which reduces the effective bevel angle and can improve wheelchair rollover comfort. Confirm the offset against available maneuvering clearance: ADA maneuvering clearance requirements at the latch side of the door must not be obstructed by a ramp edge that creates a trip hazard or lip.
Width Sizing and Anchor Considerations
A ramp threshold cut to match the clear opening width should fit tightly between jambs with no gaps at the sides. A 12-inch wide ramp profile is a common stocking width that covers most standard commercial door openings, but always verify the clear opening width — not the nominal door width. An undersized piece leaves exposed floor transition at the jamb legs and fails a visual ADA inspection. Fastener penetration into concrete or existing wood sills should be confirmed before ordering; some retrofit conditions require an epoxy anchor rather than a mechanical screw.
Interaction With the Door Closer and Door Bottom Seal
This is the field conflict that gets missed most often. A ramp threshold raises the surface directly under the door. If a door shoe or auto door bottom is already compressed against the old threshold, adding even a 1/4-inch ramp height can cause the door to drag, increasing the operating force beyond the 5 lbf maximum ADA requires for interior non-fire doors (15 lbf for fire doors per NFPA 80). Before the new threshold is anchored:
- Check the door bottom clearance with the ramp in place but unfastened.
- Adjust the closer sweep speed so the door closes within the ADA minimum of 5 seconds from 90 degrees to 12 degrees from the latch — a dragging door bottom will slow this further.
- If the door shoe is worn to the point of contact, replace it at the same time as the threshold. Combining both in one scope avoids a callback.
Common Applications by Facility Type
- K-12 schools: Exterior entry doors on older campuses frequently have raised thresholds from original construction. An offset ramp installed at the exterior face brings the entry into compliance without requiring frame modification or floor grinding.
- Healthcare and outpatient clinics: Patient and visitor entries must maintain low approach force and smooth wheelchair transition. Ramp thresholds at accessible entries reduce the rollover bump that can disrupt patients with limited upper-body strength.
- Retail and light commercial: Tenant build-outs with new floor finishes often raise the interior floor height above the existing sill. An interlocking ramp pair — one on each face — levels both sides of the transition cleanly.
- Industrial maintenance: Warehouse entries with rubber floor matting or grating present similar height differentials. A ramp threshold here also serves as a water stop, keeping exterior drainage from tracking inside.
What to Bring to the Opening Before You Specify
A few minutes of field measurement prevents a mismatch. Bring:
- A tape measure and a flat straightedge to check actual level change
- A door force gauge to document existing operating force before and after installation
- The clear opening width (between jamb faces, not rough opening)
- A note on the door bottom condition — shoe, sweep, or auto bottom, and whether it contacts the existing threshold
With those four data points, the height, offset, and width of the ramp threshold can be confirmed against catalog dimensions before anything ships.
Finding the Right ADA Ramp Threshold at DoorwaysPlus
DoorwaysPlus stocks offset interlocking ramp thresholds in a range of heights and widths suited to commercial renovation and accessibility retrofit work. Whether you are closing out a school renovation punch list, upgrading a healthcare entry for ADA compliance, or replacing a cracked saddle threshold in a high-traffic lobby, the selection at DoorwaysPlus covers the common retrofit dimensions contractors encounter in the field. Browse the threshold and ramp hardware category to compare profiles, or contact the team for help confirming the right offset for your opening condition.