A sliding door threshold is the bottom part of the frame that the panels run on, and it comes in three broad types: a standard raised threshold, a low threshold of typically 15–25 mm, and a flush threshold set level with the floor inside. The lower you go, the more the surrounding floor, patio and drainage have to be planned around the door, because the threshold is also the door's main defence against rain running in.
This guide explains the options, how drainage works, what level access means for wheelchairs and buggies, and the decisions that need making before the floor screed goes down or the patio is laid.
What are the main types of sliding door threshold?
Every sliding door has a bottom track that carries the weight of the panels on rollers and drains away any water that gets past the seals. How high that track sits above the finished floor inside is what people mean by threshold height.
| Threshold type | Typical height above the inside floor | What it needs | Often suits |
|---|---|---|---|
| Standard | Often around 40–70 mm, depending on the system | Little special preparation; fits most existing openings | Straight replacements and exposed elevations |
| Low | Typically 15–25 mm | A level floor and a careful sill and drainage detail | Extensions and replacements where a small upstand is acceptable |
| Flush | Level with the finished floor inside | Track recessed into the floor build-up and a drainage channel outside | New builds and extensions planned from the start |
Exact heights vary by system and by how the threshold is installed, so the figure that matters is the one in your surveyed specification.
How does water drain from a low or flush threshold?
A raised threshold holds water back with height. Rain that blows onto the track is caught behind an upstand and drains out through slots in the outer face of the frame. As the threshold gets lower there is less height to work with, so the drainage has to do more of the job.
With a flush threshold the outer track is often level with, or only just above, the patio. That means a drainage channel is needed directly outside the door: a slotted or grated channel running the full width of the opening, set slightly below the track and connected to a drain or soakaway so the water has somewhere to go. Without it, standing water on the patio can run back towards the frame in heavy rain.
On exposed elevations that take a lot of driving rain, a standard or low threshold can be the more reliable choice. We talk through that trade-off at survey, once we have seen the elevation and the ground outside.
What is level access and do I need it?
Level access means there is no step or trip edge between inside and outside, so a wheelchair, walking frame, buggy or wheeled bin can cross without lifting. A flush threshold gives the most level route. A low threshold with a patio built up to meet it, or a short gentle ramp outside, also works well for many people.
Think about who will use the door over the next ten or twenty years, not just now. Choosing a lower threshold when the door goes in is far simpler than altering the floor, the sill and the paving later.
Do the Building Regulations say anything about thresholds?
In England, Approved Document M deals with access to and use of buildings. For new dwellings it sets standards for step-free entrances, and the design team works out which door has to meet them and how. If you are building a new home, check the threshold detail against the drawings approved for your project before the door is ordered.
For a replacement door on an existing home, a low or flush threshold is usually a design choice rather than something the regulations call for. The replacement still has to meet the energy and safety glazing rules for controlled fittings, which are covered in our building regulations guide.
How do floor and patio levels need to be planned?
A flush threshold only looks flush if the floor inside and the paving outside are both built to suit it. That has to be agreed before work starts, because the frame is set down into the floor build-up rather than sitting on top of it.
- Inside: fix the finished floor level, including screed, insulation, any underfloor heating and the final floor covering, so the track can be recessed to match.
- Outside: the patio needs to fall away from the door, with a drainage channel across the opening and a route for the water to a drain or soakaway.
- The wall: the damp-proof course and cavity trays must still protect the building, so raising a patio up to the door needs proper detailing at the frame edges.
- Timing: on a new extension the door is often fitted before the final screed and paving, so the builder, the door fitter and any landscaper should all work from the same levels.
If you are building an extension, our sliding doors for extensions page explains how we fit around the build programme.
Does a low threshold cost more?
A low threshold with its drainage detail is usually an extra over the standard threshold, because it needs a different track profile, extra sealing and more time at fitting. Where you choose it, the cost appears as a separate line in your written quote, so you can see exactly what it adds before you decide.
A flush threshold on a new build can also involve groundwork, such as the drainage channel and its connection, which is normally part of the builder's work. The quote makes clear what is included and what your builder needs to provide.
Can I lower the threshold on my existing door?
Not usually, because the threshold is part of the frame. Lowering it means replacing the door and often altering the floor, the sill and the patio around it. If you are planning a replacement sliding door, tell us at the free survey that you want a lower threshold so we can check the levels and drainage before you commit.
On many existing homes the floor inside sits well above the patio, so a low threshold is often more realistic than a fully flush one. The survey confirms which options will work on your opening.
Common questions
How high is a low threshold sliding door?
A low threshold is typically 15–25 mm above the finished floor inside, depending on the system and how it is installed. That is low enough for most wheelchairs, buggies and wheeled bins to cross easily, while still giving a small upstand that helps keep rain out. The exact height for your door is confirmed in the surveyed specification.
Do flush thresholds leak?
A flush threshold should not leak when it is designed and installed properly, but it relies on good drainage because there is little or no upstand. A drainage channel outside the door, a patio that falls away from the house and careful sealing at the frame edges all matter. On very exposed elevations a low threshold can be the safer choice.
Can I have a flush threshold on a replacement door?
Sometimes. It depends on whether the floor inside can be built up or cut down to suit, and whether there is room for a drainage channel outside. On many existing homes the inside floor sits well above the patio, which makes a fully flush finish harder. We check the levels at the free survey and explain what is realistic.
Is a level threshold required for wheelchair access?
For new dwellings in England, Approved Document M sets step-free access standards that the design team applies to the relevant entrance. For a replacement door on an existing home it is usually your choice. If someone in your household uses a wheelchair, tell us at the start so the threshold, floor levels and outside surface can be planned together.