Flagship guide · 9 min read
The Ultimate Guide to Sliding Doors
How aluminium sliding doors work, what separates a good system from a poor one, and what to expect from specification through to installed cost.
Level Thresholds for Sliding Doors
A level threshold looks like a small detail on a drawing. Getting it wrong is one of the most expensive mistakes to fix once a floor slab has already been poured.
3 min read
Written by Windows in the Lakes · A trading name of Home Glazing Ltd
A level threshold removes the step between inside and outside at a sliding door, relying on a recessed drainage channel and careful floor build-up rather than a raised sill to keep water out, and it needs to be designed into the structural and floor plans before the slab is poured, not adjusted afterwards.
This guide explains how level threshold drainage actually works, what floor build-up depth it requires, when it may not be appropriate, and the coordination needed between architect, builder and glazing supplier to get it right first time.
How a level threshold actually works
Rather than a raised aluminium sill acting as a physical barrier to water, a level threshold uses a recessed drainage channel set into the floor build-up just outside the door line, which collects any water and directs it away via a gully or soakaway before it can reach the internal floor level. The door track itself typically sits only a few millimetres proud, or is recessed to be genuinely flush.
Floor build-up and structural coordination
| Element | What needs agreeing early |
|---|---|
| Slab or floor level | Set relative to external ground level and drainage channel depth before pouring |
| Drainage channel | Confirmed with the glazing supplier's specific threshold system dimensions |
| External ground and paving falls | Graded away from the building to support the drainage channel |
| Insulation and screed depth | Coordinated so the internal finished floor still meets the threshold correctly |
Because all of these need to be resolved before concrete is poured, a level threshold is one of the clearest examples of why a glazing supplier should be involved at design stage on any project with large sliding doors, rather than brought in once the shell is already built.
When a level threshold may not be appropriate
On a site with poor natural drainage, a very exposed elevation facing driving rain, or where floor levels cannot be adjusted relative to external ground, a level threshold needs more careful engineering and may not always be achievable without additional drainage provision. A low-profile (rather than fully flush) threshold is a reasonable compromise on some sites, keeping most of the accessibility benefit with a more robust water management margin.
Accessibility benefits
Beyond aesthetics, a level threshold removes a trip hazard and makes a garden or terrace accessible to wheelchair users, pushchairs and anyone with mobility difficulty, which is a common reason for specifying one on family homes and properties designed for long-term accessible living, not purely a contemporary design preference.
Decision criteria
- Good natural drainage, sheltered elevation
- Full level threshold achievable with standard detailing
- Exposed, driving-rain elevation
- Confirm drainage capacity carefully, consider low-profile alternative
- Accessibility a priority
- Level threshold specified early, coordinated with floor levels
- Existing floor levels fixed and cannot be adjusted
- Low-profile threshold may be the practical compromise
Common mistakes
- Pouring the floor slab before the threshold drainage channel dimensions were confirmed
- Assuming 'level' means no drainage detailing is required
- Fitting a level threshold on a severely exposed elevation without adequate drainage capacity
- Failing to grade external paving away from the threshold, causing water to pool at the door
From a real Windows in the Lakes project
North West England · Full house · 4-panel slimline slider
Whole-House Aluminium Glazing, Contemporary Clad Remodel
Re-glazing a remodelled house where the cladding, canopy steelwork and pool terrace all had to align with the glazing rather than the other way round — with a full-width sliding run at ground floor and a rhythm of square apertures above that had to stay visually consistent across two elevations.
We set out the aperture sizes with the design team before the cladding battens were fixed, so every window sits centrally within the batten rhythm. The sliding run was surveyed after the canopy steel was in place and manufactured to that survey, giving a track and head detail concealed behind the soffit, and a threshold flush with the terrace paving.
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