Accent Window Systems
Bi-Fold Door Sills & Thresholds: Flush, ADA, and Raised
The sill is the decision that determines whether the wall leaks in year three.
The overhang rule
Rule of thumb: the overhang you need above a bi-fold opening scales with the height of the opening. As tall as the opening is, you want cover over it — roughly a one-to-one overhang-to-height guide for flush and weather-exposed thresholds. Storms in this area swirl and come up the coast from the south and southwest, which is why a wall with no overhang cares which way it faces.
Sill comparison
| Flush / ADA | Standard raised | Recessed into slab | |
|---|---|---|---|
| Water resistance | Depends on overhang and drainage | More forgiving in driving rain | Strong when the slab drain path is designed; weak if water has nowhere to go |
| Overhang required | About as deep as the opening is tall — a common rule of thumb | Less dependent on overhang | Still needs cover; recess alone does not replace an eave |
| Accessibility | The point of an ADA / flush sill | A step at the threshold | Can read as flush indoors when the slab and finish align |
| Relative cost | Higher — detailing and hardware | The usual baseline | Higher on new slabs; often impractical to retrofit |
| Retrofit feasibility | Case-by-case — slab and finish must allow it | Usually the practical retrofit | Rare on existing slabs without major concrete work |
| When we recommend it | When the overhang and drainage can support it | When the wall has little cover | When the slab can be detailed for it on new work |
| When we'll talk you out of it | No overhang, weather-facing wall | When you need a flush floor and the opening can support it | When the existing slab cannot take the recess |
Exposure and orientation
Which way an opening faces matters most when there is little or no overhang. Weather in the South Bay arrives from the south-southwest. A north wall under a deep eave is a different problem from a southwest wall with nothing above it. City pages can localize this later; the mechanism is the same.
What we look at before recommending a sill
- Overhang depth
- Exposure and orientation
- Slab versus raised floor
- Interior floor finish and transition
- Accessibility requirement
- Drainage available at the sill
Will a flush sill leak?
Not if the opening is detailed for it. A flush sill with adequate overhang, a designed drainage path and the right exposure performs fine for years. A flush sill on a wall with no overhang facing into the weather is a problem you will meet in the first heavy winter. The sill is not the variable — the whole opening is.
Can I retrofit a flush sill into an existing slab?
Sometimes — when the slab can be cut and drained correctly and the finished floor can meet the new threshold. Many existing slabs are better served by a raised sill or a carefully detailed transition. We say which after we see the floor and the exterior grade.
Does an ADA sill cost more?
Usually yes — the hardware, drainage detailing, and install time run higher than a standard raised sill. The premium is the accessible threshold and the weatherproofing that has to work with it, not a label fee.
What if I have no overhang at all?
Then orientation and drainage carry the whole load, and a raised sill is usually the honest recommendation. We would rather tell you that at the quote than warranty it later.
Does the sill choice affect the warranty?
Manufacturer coverage assumes the sill was specified and installed for the exposure. We document the sill type and drainage at close so warranty claims have a clear paper trail. A flush sill on a weather wall with no overhang is a detailing problem, not a warranty surprise.
Ask about your opening.
Overhang, exposure, and floor — those three decide the sill.