Accessible thresholds for terrace doors.
What a low sill does to the water rating, and how to specify around it.
Vistaza Engineering · Mar 7, 2024 · Updated Sep 20, 2026 · 5 min read
An accessible threshold is low enough for a wheelchair to cross. A low threshold also gives water less to climb. So an accessible sill lowers the water rating of the door it sits under. That is true of any door from any maker, and it is better decided on the drawings than discovered at the water test.
What the ADA says about a threshold.
The 2010 ADA Standards for Accessible Design deal with thresholds in two places.
- Section 404.2.5, Thresholds. A threshold at a doorway may be 1/2 inch high at most. An existing or altered threshold may be 3/4 inch high if it has a beveled edge on each side, with a slope no steeper than 1:2.
- Section 303, Changes in level. A rise of up to 1/4 inch may be vertical. Between 1/4 inch and 1/2 inch it has to be beveled, again no steeper than 1:2. Anything higher than 1/2 inch is a ramp.
Read together: a new threshold is 1/2 inch at most, and the top 1/4 inch of it is beveled.
Which standard governs a terrace door depends on the building and on the room behind the door. A door on an accessible route in a public or common area falls under the ADA. A door inside a dwelling unit may fall under other rules: the Fair Housing Act design guidelines, and ICC A117.1 as the local building code adopts it. The Fair Housing Act guidelines allow a threshold of up to 3/4 inch [19 mm (3/4″)] at an exterior door, sliding door tracks included, beveled no steeper than 1:2, and where the balcony or terrace is an impervious surface such as concrete, it may sit up to 4 inches [102 mm (4″)] below the floor inside. ICC A117.1 allows the same 3/4 inch [19 mm (3/4″)] at exterior sliding doors in Type A units. Settle this with the code consultant before the door schedule is fixed.
The sills we make.
| Sill | Door | Height | What to know |
|---|---|---|---|
| Standard sill | Vi6 in-swing terrace door | 3/4 inch, stepped | The sill the door's ratings were earned on: CW-PG65, water tested at 12.11 psf |
| Accessible sill | Vi6 in-swing terrace door | 1/2 inch | Flat, ramped on both sides. Tested on its own, to a lower water pressure |
| Standard sill | Vi5 in-swing terrace door | Stepped | CW-PG45, water tested at 9.82 psf. No accessible sill is drawn for the Vi5 |
The adaptable sill.
Not every door needs the accessible sill on day one. Many apartments only have to be adaptable: built so that they can be made accessible later without rebuilding the opening. For those doors Vistaza details the standard 3/4 inch threshold with the finished floor brought up to it on the room side, and notes on the shop drawings that a ramp can be added later, by others, to make the sill accessible. The door keeps the standard sill and the water rating that goes with it. The ramp is not part of the window package, so its slope, length and anchoring belong to the architect's detail.
The accessible sill is a flat aluminum threshold, 1/2 inch high, that takes the place of the raised bottom frame. It finishes 3/16 inch above the finished floor on the room side and is 6-7/8 inches deep. The door does not drag on it: a seal in the threshold lifts to meet the underside of the door as it closes and drops as it opens. A weatherboard on the outside face of the door throws rain clear of the joint.
It is made for the Vi6 in-swing terrace door, and it is ordered door by door.
Why the water rating drops.
In a water test the door is sprayed while air pressure pushes the water inward. The upstand of the sill is a large part of what holds that water back. Take the height away and the door resists less pressure before water reaches the inside. Drainage and gaskets win some of it back. They do not win all of it back.
A low sill is a choice, not a default.
So the water rating on a test report belongs to the sill that was tested. The 12.11 psf of the Vi6 in-swing door was earned on the standard stepped sill. The accessible sill was tested separately, and its figure is lower. Ask us for the accessible sill's tested result. When you compare doors, ask every maker the same question: which threshold was on the door when it earned that number?
The threshold is one item of several.
A compliant sill does not make a compliant door. The same standard covers the rest of the opening.
| Item | 2010 ADA Standards | Section |
|---|---|---|
| Clear width | At least 32 inches, measured with the door open 90 degrees, from the face of the door to the opposite stop | 404.2.3 |
| Maneuvering clearance | Clear floor space to approach, reach and open the door, level within 1:48 | 404.2.4 |
| Threshold | 1/2 inch at most | 404.2.5 |
| Hardware | Works with one hand. No tight grasping, pinching or twisting of the wrist. Mounted 34 to 48 inches above the floor | 404.2.7, 309.4 |
| Opening force | 5 pounds at most for interior hinged doors, and for sliding or folding doors. The Standards set no figure for exterior hinged doors. Some state codes do: California's sets 5 pounds for them too | 404.2.9 |
| Door surface | Smooth on the push side for the bottom 10 inches of a swinging door | 404.2.10 |
| Glazing | Where a door or sidelight has vision glass, the bottom of at least one panel is no higher than 43 inches | 404.2.11 |
| Floor surface | Stable, firm and slip-resistant | 302.1 |
On a terrace door the clear width and the hardware are set by the product. The maneuvering clearance and the floor surface on both sides are set by the plan. Check the terrace side as carefully as the room side. Pavers, drains and planters often end up inside the clear floor space.
How to specify around it.
- Decide door by door. Mark which terrace doors must be accessible and which need not be. Do not apply the low sill to every door by default.
- Put the sill type on the door schedule next to each door. Carry it onto the shop drawings, so the rating that goes with each sill is visible to everyone who signs them.
- Check the required water resistance for each elevation and height. Compare it with the rating of the door with the sill you chose, not with the best rating in the catalog.
- Shelter the doors that carry a low sill. A balcony above, an overhang or a recessed opening keeps wind-driven rain off the threshold.
- Drain the terrace away from the door. The finished surface outside should slope away from the sill, and water should have somewhere to go before it reaches the door.
- Detail the waterproofing of the opening under the threshold for the low sill. There is less height to work with, so the sequence matters more.
- Level the slab where the sill bears. A low sill has almost no tolerance: set out of level, it leaks at the corners before it leaks anywhere else.
Operable windows have rules too.
Where a room has to be accessible, its operable windows have requirements of their own (sections 229 and 309):
- The operating hardware sits between 15 and 48 inches above the finished floor.
- Unlocking, opening, closing and locking each take no more than 5 pounds of force.
- The window works with one hand, without tight grasping, pinching or twisting.
Not every window in the room has to comply. Section 229.1 asks for at least one, plus every window the local authority requires to be operable. Windows in dwelling units covered by section 809, and in some hotel guest rooms, are exempt from it. Handle height is the item to check early. It follows from the sill height and the size of the sash, and both are fixed once shop drawings are approved.
Sill options and ratings for Vistaza doors are on the terrace doors page. The place to settle the sill for each door is the shop drawings.