Why the Width Measurement Is the Ramp Threshold Decision Nobody Confirms Until It Is Too Late
This article is for contractors, facility managers, and project architects who specify or install ADA-compliant interlocking wheelchair ramp thresholds at commercial door openings. The subject is narrow but the problem is common: the threshold gets ordered based on a door schedule dimension that does not reflect what is actually between the jambs after framing, flooring, and finish work are done. The result is a threshold that either does not fit, leaves a gap at one jamb, or forces a return trip that delays the certificate of occupancy.
What an Interlocking Wheelchair Ramp Threshold Actually Does
An ADA interlocking wheelchair ramp threshold is an extruded aluminum assembly designed to bridge a change in floor elevation at a door opening while keeping the crossing compliant with ADA Standards for Accessible Design and ICC/ANSI A117.1. The profile rises on the exterior side and ramps down on the interior, eliminating an abrupt vertical edge that a wheelchair, walker, or rolling cart cannot clear smoothly.
The interlocking feature matters: the threshold body engages a hook strip or a door bottom sweep so that the assembly seals the sill against air and water infiltration at the same time it handles the grade change. That mechanical relationship between the threshold and the door bottom means both components must be coordinated, not just the threshold alone.
ADA code requires that threshold height at accessible routes not exceed 1/2 inch. Where height exceeds 1/4 inch, the threshold must be beveled at no more than a 1:2 slope. A ramp threshold is specifically used when the floor height differential is larger than a standard saddle threshold can handle within those rules.
The Width Problem: Where the Ordering Error Happens
A threshold listed as 27 inches wide is designed to fit a specific rough opening width. That dimension sounds straightforward. The problem is that the rough opening, the door frame opening, and the finished clear width are three different numbers on most commercial projects, and the person ordering the threshold frequently uses whichever one is most convenient rather than the one that is correct.
What Gets Measured (and What Should Be)
- Rough opening width -- the stud-to-stud or masonry-to-masonry dimension. This is often what the framing crew records and what ends up in a preliminary door schedule.
- Frame opening width -- the inside dimension of the hollow metal or wood frame, after the frame is set. This is typically 2 inches narrower than the rough opening on each side, depending on the frame profile.
- Finished floor-to-floor span at the sill -- the actual distance between jambs at the floor level, after flooring, tile, or concrete topping is installed. This is the number the threshold must fit.
On a renovation project, a ceramic tile installation at an exterior threshold can add 3/8 inch or more per side to the jamb reveal. On a new commercial building, the floor screed or leveling compound can change the sill geometry after the frame is set. Neither change gets communicated to the person who already submitted the hardware order.
Why Interlocking Ramp Thresholds Are Less Forgiving Than Saddle Thresholds
A flat saddle threshold can sometimes be shimmed, caulked, or trimmed in the field with acceptable results. An interlocking ramp threshold has a profile that must seat fully between the jambs and align precisely with the door bottom sweep or hook strip. A threshold that is even 1/2 inch too narrow leaves a gap at the jamb that voids the weather seal and creates a tripping edge. A threshold that is 1/4 inch too wide will not seat flush or will force the jamb stop out of position when fastened.
The correct approach is to measure the finished sill width on the day the threshold is ordered, not the day the door schedule is assembled. On projects where flooring is not yet down, the ordering decision should be deferred or a verified field measurement should be built into the schedule as a required confirmation step.
The Height Dimension Is Also a Two-Stage Confirmation
Ramp thresholds are manufactured to specific rise heights. A 2-1/4-inch-high profile is not interchangeable with a lower-rise assembly on a given opening. The floor height differential at the sill -- exterior finished grade or landing elevation versus interior finished floor elevation -- must be measured after both floor surfaces are complete.
Specifying the rise height before the exterior paving, concrete apron, or landing surface is poured is one of the most consistent sources of rework on accessible entrance projects. The exterior landing may be designed to a specific elevation but field conditions, drainage slope, or a last-minute grade adjustment can shift that number by 3/4 inch or more. That shift changes whether the selected threshold profile is still within compliant slope limits.
Project Types Where This Error Repeats
- School building renovations -- existing door openings being brought into ADA compliance often have floor assemblies that were modified in previous renovations; the current finish floor dimension at the sill is rarely what the original drawings show.
- Healthcare facility upgrades -- exterior vestibule and entry thresholds on occupied buildings are often ordered off old as-built drawings that do not reflect previous threshold replacements or floor resurfacing.
- Retail tenant improvement -- the base building sill condition at a storefront opening may not match the leasing plan dimensions, especially if a prior tenant modified the flooring.
- Industrial and warehouse entries -- dock-level or grade-level entries with concrete aprons are subject to seasonal heave and settlement; the threshold height that was correct at installation may not be correct at replacement.
What to Confirm Before the Order Ships
Build these four checkpoints into the threshold order process for any ADA ramp threshold on an accessible route:
- Finished sill width -- measured between jambs at the floor level, with flooring in place or a verified as-built flooring allowance confirmed in writing.
- Finished floor height differential -- interior finished floor elevation versus exterior landing or paving elevation, measured after both surfaces are complete or with a confirmed tolerance from the flooring contractor.
- Door bottom sweep or hook strip compatibility -- the interlocking threshold profile must match the door bottom assembly that will be installed; confirm these are from compatible product families before ordering either component.
- Lead time against the construction schedule -- ADA ramp thresholds in non-stock widths or heights carry fabrication lead times that can exceed standard stock items; ordering late against a flooring completion date is the most common reason a threshold delivery becomes a schedule problem.
Fastening and the Finished Floor Sequence
The threshold must be anchored to the floor substrate with fasteners appropriate for the floor construction type -- concrete, wood subfloor, or steel deck each require different anchor choices. The fastening should happen after the flooring transition is confirmed to be stable on both sides. Anchoring a ramp threshold into an exterior concrete apron that has not fully cured, or into a tile installation before the mortar bed has set, is a field shortcut that shows up as a loose or rocking threshold within the first season of use.
At accessible entrances, a loose threshold is not just a maintenance issue -- it is a tripping hazard and a potential ADA compliance deficiency that will appear on the next accessibility audit.
Specifying and Sourcing the Right Assembly
ADA interlocking wheelchair ramp thresholds are available in aluminum with a range of rise heights and widths to suit standard and custom commercial openings. Preferred materials for exterior applications are extruded aluminum alloy profiles that resist corrosion and maintain dimensional stability across temperature cycles. Slip-resistant surface treatments are available for high-traffic exterior entries in schools, healthcare, and retail environments where wet conditions are a regular factor.
DoorwaysPlus carries ADA-compliant ramp threshold assemblies suited for accessible route openings across commercial project types. If the opening dimensions require a configuration outside standard stock widths, confirming those measurements early -- before the hardware order is submitted -- is the single most effective way to keep the accessible entrance on schedule.