Commercial Glass Door Thresholds and ADA Transitions: Trip Edges, Bevels, and Water

A threshold is the smallest part of the entrance and the first place it fails inspection. It is only a strip of metal a few inches wide at the bottom of the door, and most people step over it without looking at it — until a wheel catches, a toe catches, an inspector kneels down with a tape, or rain shows up inside. Then that little strip is the whole job. Height, bevel, slope, and sealing all meet there, and getting any one wrong creates a problem in a different name: accessibility, safety, or water.

The numbers that govern a threshold

For an accessible commercial entrance, the basic limits are simple to state and easy to violate by accident. A threshold at a doorway is generally limited to a half inch in height. A change in level up to a quarter inch can be vertical — a clean small step. Between a quarter inch and a half inch, the edge has to be beveled, with a slope no steeper than one unit up for every two units across. Anything over a half inch is no longer a threshold detail at all; it has to be treated as a ramp or a level change handled another way. Those fractions sound generous until you add real floors: tile on one side, concrete that settled on the other, a door sweep hanging low, and a threshold that was level the day it was installed and is not level anymore.

Why a trip edge is rarely just the threshold’s fault

When someone trips at an entrance, the metal strip usually gets blamed, but the cause is often the transition as a whole. Interior flooring was built up during a tenant improvement and nobody rechecked the door. Exterior concrete heaved or settled a quarter inch. A replacement door sits slightly higher or lower than the one it replaced. The threshold itself is fine — the two surfaces it bridges no longer match. That is why measuring only the threshold tells you half the story. Measure the level change across the entire passage, at both ends and in the middle. A threshold can pass at the hinge side and fail at the latch side if the floor or frame is out of level.

Bevels: the detail that decides whether wheels roll or stop

• A square edge stops things. A vertical face, even a short one, is what a wheelchair caster, a dolly wheel, a stroller, or a rolling case hits and stops on.
• A proper bevel converts a step into a slope. At the allowed 1-to-2 slope, small wheels climb instead of colliding, and a shoe slides up instead of catching.
• Worn and loose thresholds lose their bevel. Screws back out, the strip rocks, an edge lifts — and a compliant entrance quietly becomes a trip edge between inspections.

Saddle-style thresholds exist for exactly this reason: a low, wide profile with sloped sides on both faces, so the transition works from either direction and sheds foot traffic without a sharp lip in either path.

The landing matters as much as the strip

Accessibility is not only about the inch under the door. The floor on both sides has to be reasonably level along the approach, without a steep cross-slope pitching a wheelchair sideways, and without a mat well or grate creating its own level change. Thick loose mats bunched against a threshold undo a good installation in a day. So does a door that drags: raising the threshold to meet a sagging door is fixing the wrong part, and it usually creates the height violation you were trying to avoid. Our storefronts and curtain wall service page covers how we handle entrances as a system — door, frame, hardware, and threshold together.

Water: the threshold is also a dam, and a low dam leaks

Everything that makes a threshold easy to cross makes it easy for water to cross, and that tension is the real design problem. A half-inch beveled saddle is also a low barrier against wind-driven rain and a sidewalk sloped the wrong way. The answer is not a taller threshold, which trades a leak for a code problem. The answers are slope away from the door, a properly bedded and sealed threshold, and intact door sweeps and weather seals. Most threshold leaks we find are not coming over the top. They are coming under a threshold that was screwed down dry, on a slab that pitches toward the building.

Replacing a threshold: what a proper job includes

Pull the old strip and look underneath before ordering anything — cracked concrete and old sealant failure change the work. Match the new threshold’s width to the frame and door bottom, not just the old part. Bed it in sealant, fasten it so it cannot rock, confirm the door clears it through its full swing with the sweep making contact, and re-measure the finished level change on both sides. Do not assume a grandfathered higher threshold survives a remodel that touches the doorway.

Signs yours needs attention now, not at inspection time

A threshold that rocks, clicks, or has lifted screws. A visible lip you can feel with the toe of a shoe that was not there before. Daylight or a draft under the door. Water inside after rain that tracks from the entrance. A door that has started dragging or a sweep that is torn from striking the strip. Every one of those is the transition telling you the measurements have changed — and an entrance that fails on trip height or opening effort rarely fails for one reason. Check the threshold, the door alignment, and the closer together.

The short answer

Keep the level change at a commercial glass door at a half inch or less, bevel anything between a quarter and a half inch at no steeper than 1-to-2, keep the landings level on both sides, and solve water with slope, bedding, and seals — never with extra height. A threshold is a few inches of metal doing three jobs at once: accessibility, safety, and weather. It deserves to be measured like it matters, because at inspection, at the door with a delivery dolly, and in the rain, it does.