Level-access and low-threshold doors can provide a clean and accessible transition between inside and outside.
But achieving that reliably requires more coordination than simply selecting the height of an insulated door upstand.
The final position of the door threshold depends on several elements that are often designed, supplied or installed by different trades:
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the structural floor or slab;
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the internal floor build-up;
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the final floor finish;
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the door or sliding-door threshold;
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the insulated structural upstand;
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external paving or terrace levels;
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drainage;
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waterproofing;
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airtightness and floor membranes.
For this reason, the required upstand height should not be determined from the nominal floor build-up alone.
Start With the Finished Floor Level
The key internal reference is normally the intended finished floor level.
The complete build-up should be considered, including:
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floor insulation;
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screed or concrete;
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adhesive;
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tiles, timber or other floor finishes.
A nominal floor build-up does not always result in exactly the same finished level on site.
Subfloor tolerances, insulation installation, screed thickness and floor finishes can all introduce small variations.
At a door threshold, even a few millimetres can become significant.
The Door Threshold Geometry Is Equally Important
Two doors with the same overall dimensions may still require completely different upstand arrangements.
Relevant dimensions include:
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overall frame depth;
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underside of the threshold profile;
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position of the load-bearing points;
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internal threshold height;
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required door-leaf clearance;
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position of the frame relative to the insulation layer.
The actual threshold profile should therefore be checked before the final upstand height is confirmed.
External Levels Must Be Considered at the Same Time
For a level-access threshold, the external construction cannot be treated as an afterthought.
The design should consider:
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finished paving or terrace height;
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falls away from the building;
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drainage channels;
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threshold drainage provided by the door system;
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splash-water exposure;
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required waterproofing height;
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the connection between waterproofing and the door frame.
A low internal threshold can create a demanding external waterproofing condition if the external level is also raised close to the door threshold.
Drainage Is Part of the Threshold Design
Where external paving approaches threshold level, surface water needs a controlled route away from the building.
Depending on the project, this may involve:
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a linear drainage channel;
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falls away from the threshold;
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drainage incorporated into the door system;
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a combination of these measures.
The upstand, drainage and external waterproofing should therefore be coordinated as one junction rather than treated as separate decisions.
Waterproofing Must Remain Continuous
An insulated structural upstand addresses the structural and thermal requirement below the door.
It does not replace the need for a properly designed waterproofing system.
The external waterproofing should remain continuous across the junction:
Structural substrate → Insulated upstand → Door threshold / frame → Adjacent construction
A conventional loose DPC should not automatically be treated as the complete waterproofing solution for a level threshold.
Where appropriate, a dedicated external waterproofing system allows the upstand, frame, fixings and adjoining construction to be sealed as one continuous detail.
Airtightness and Floor Membranes
The internal side of the threshold may also need to connect to:
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airtightness membranes;
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the DPM;
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a radon membrane;
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floor insulation;
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internal screed and floor finishes.
These layers perform different functions and should not be confused with the external waterproofing requirement.
The important principle is continuity.
Each functional layer should remain continuous through the threshold junction.
Structural Loads Still Matter
Low thresholds and large glazed elements can involve substantial loads.
The upstand therefore has to provide reliable structural support while remaining within the thermal envelope.
The complete load path should be considered:
Door or glazing element → Threshold → Support points → Insulated upstand → Structural substrate
For heavier sliding or lift-and-slide doors, particular attention should be paid to concentrated loads and the position of the support points.
Information That Should Be Confirmed Before Ordering
Before the insulated door upstand is finalised, the following information should ideally be available:
1. Structural Floor Level
What is the actual level of the slab or structural substrate?
2. Finished Internal Floor Level
What will the completed floor level be, including the final floor finish?
3. Door Threshold Geometry
What is the actual dimension from the underside of the frame or threshold to the required internal finished floor level?
4. Frame Depth and Support Area
How much structural support is required beneath the threshold?
5. External Finished Level
Where will paving, terrace construction or external landscaping finish in relation to the threshold?
6. Drainage
How will rainwater and surface water be directed away from the door including threshold?
7. Waterproofing Strategy
How will the external waterproofing connect between the structure, upstand and door frame?
8. Airtightness, DPM and Radon Strategy
How will the internal and floor membranes connect to the threshold junction?
9. Structural Loads
Where are the load-bearing points and how are those loads transferred into the supporting structure?
Why Early Coordination Matters
Threshold problems often become difficult to correct once the door has been manufactured or the floor build-up has been completed.
If the upstand is too low, the door may sit too close to the finished floor.
If it is too high, the intended level-access transition may no longer be achievable.
If the external level is established without considering drainage and waterproofing, additional remedial work may later be required around the threshold.
The safest approach is therefore to coordinate the door threshold, finished floor levels, structural upstand, waterproofing and drainage before the upstand is ordered.
The Important Point
The required insulated door-upstand height cannot be determined from the floor build-up alone.
It must be coordinated with:
Finished Floor Level + Door Threshold + Structural Support + External Level + Drainage + Waterproofing
When these elements are considered together, the insulated upstand becomes part of a coordinated level-threshold junction rather than simply a support block beneath the door.