What This Guide Covers and Who It Is For
When a new building goes up, the ADA threshold spec gets locked in during design. When an existing building needs to come into compliance, the threshold offset dimension becomes a field decision that depends on conditions the original spec never anticipated. This guide is for facility managers, commercial contractors, and project architects who are sizing and ordering an offset wheelchair ramp threshold for a door that is already hung, already framed, and already floored.
What Is an Offset Wheelchair Ramp Threshold?
An offset wheelchair ramp threshold is an aluminum or similar-material saddle installed at the door sill to create an accessible transition between two floor surfaces. Unlike a standard centered saddle, an offset profile positions the apex of the ramp closer to one edge rather than the center. This lets the threshold accommodate a floor height difference on one side of the opening without requiring demolition or floor buildup on the other side.
The key dimension is the offset measurement — the horizontal distance from one edge of the threshold base to the high point of the ramp. On a product such as the NGP RO225 family, that offset is 2-1/4 inches, placing the peak well toward one side of the base. The height of the ramp itself at its peak, and the total base width, determine whether the threshold meets ADA requirements for slope and maximum rise.
The Code Baseline: What ADA and IBC Actually Require
Before measuring anything in the field, confirm the code limits that govern the finished installation:
- Maximum threshold height (new construction): 1/2 inch (13 mm) per ADA Section 303 and IBC.
- Maximum threshold height (existing or altered openings): 3/4 inch (19 mm) if beveled on each side, slope no steeper than 1:2.
- Bevel slope: Any change in level between 1/4 inch and 1/2 inch must be beveled at a maximum 1:2 slope (1/2-inch rise per inch of run).
- Changes over 1/2 inch: Must be ramped at 1:12 or shallower.
The 3/4-inch existing-condition allowance is a code accommodation, not a preference. Specify to the lowest achievable height that still bridges the actual floor differential. An inspector may accept 3/4 inch at an existing opening, but a 1/2-inch or lower threshold is always the cleaner solution.
Why the Offset Dimension Gets Decided in the Field
At a retrofit opening, at least three conditions combine to determine which offset configuration is correct:
1. Floor Height on Each Side of the Door
Measure the finished floor elevation on both the interior and exterior sides of the threshold seat. The difference between those two measurements is the height differential the threshold must bridge. A 1/4-inch step does not require an offset ramp at all — a flat saddle covers it. A differential greater than 1/4 inch typically requires a beveled or ramped profile. An offset profile is specifically useful when the differential sits almost entirely on one side, because the offset apex puts the slope where the height change actually occurs rather than splitting it symmetrically.
2. Existing Door Sweep or Door Bottom Assembly
A retrofit threshold must clear whatever door bottom hardware is already on the door — or whatever is being specified alongside it. If a door shoe or sweep drops lower than the threshold's highest point at the peak, the door will drag or fail to close properly. Confirm the clearance between the bottom of the door (with hardware attached) and the top of the threshold profile before ordering. The threshold height and the door bottom drop height are not independent decisions.
3. Available Base Width in the Opening
The base width of an offset ramp threshold — for example, 33 inches on a threshold sized for a standard 36-inch door — must fit within the opening width without interfering with the door stop, the frame profile, or any sealing gasket on the frame. A threshold that is cut too short leaves a gap at the jambs. Confirm the rough opening width and the door stop profile before cutting to length.
Measuring for the Offset: A Step-by-Step Approach
- Remove the existing threshold if one is present and photograph the sill condition before anything is disturbed.
- Measure the floor differential at the sill: interior finished floor elevation versus exterior finished floor or landing elevation. Record in fractions of an inch.
- Determine which side is higher and note whether the door swings in or out. The offset peak should be positioned toward the higher floor side so the ramp slope faces the lower side and aids wheelchair approach.
- Confirm the door swing clearance over the sill area. Measure from the bottom of the door to the floor on the side the door sweeps across.
- Check the width from jamb to jamb at the threshold seat and verify the threshold base width fits within that dimension.
- Verify closer force once the threshold is installed: ADA requires interior non-fire doors to open with no more than 5 pounds of force. A high threshold profile adds resistance to wheelchair approach even when it meets height limits — closer adjustment is often needed post-installation.
Where Facilities Most Often Get This Wrong
- Ordering the threshold before measuring the differential. The offset dimension that works at one opening may not bridge the gap at the next, even in the same building if floor finishes vary.
- Installing a centered saddle on an asymmetric floor differential. A symmetric threshold creates a steeper slope on one side than required and may not clear the door sweep on the other.
- Ignoring the door closer setting after installation. A compliant threshold height does not guarantee a compliant opening force. Adjust the closer after the threshold is in.
- Cutting the threshold length before checking the frame stop projection. The base must seat flush against the frame stop on both jambs. Some frame conditions require a notch cut at each end.
- Assuming the exterior landing is at a consistent elevation. Settled concrete, pavement joints, and weather damage commonly create elevation variance across the sill width. Measure at multiple points, not just the center.
Application Contexts: Where This Comes Up Most Often
Retrofit offset ramp thresholds appear on a short list of recurring project types:
- School facilities undergoing ADA transition plan updates, where older buildings have mismatched floor finishes at entry vestibules.
- Healthcare clinics and medical offices converting existing doorways to accessible routes without full sill reconstruction.
- Retail and commercial tenant build-outs where the base building slab elevation differs from the tenant's interior finish floor by a fraction of an inch.
- Industrial and warehouse entries with heavy floor coatings or anti-fatigue mat systems that raise the interior floor elevation post-construction.
Specifying the Right Threshold at DoorwaysPlus
DoorwaysPlus carries ADA-compliant offset wheelchair ramp thresholds from manufacturers with stable product lines and consistent dimensional tolerances. When quoting, have the following ready: floor differential in fractions of an inch, door width, swing direction, and whether a door bottom assembly is being installed at the same time. The threshold and door bottom are a system, not two independent line items.
Browse ADA threshold options and related door bottom hardware at DoorwaysPlus.com, or contact the team directly if the field conditions at your opening do not match a catalog profile cleanly.