Building components · Openings & Frames
Door Threshold
Explains why the threshold is the hardest small area of an envelope, because its requirements genuinely conflict.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a threshold is
A threshold is the bottom of a door opening. Unlike a window sill it is walked over, which means everything asked of it has to survive traffic and has to be at or near floor level.
It is the point where several requirements genuinely conflict. Keeping water out favours a raised upstand; level access favours no step at all; thermal continuity requires the insulation to pass through a position that is also structural and trafficked. Resolving all of them at once is why thresholds are among the most-designed square metres in a building.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a window sill. A sill is not walked over and can project and fall freely; a threshold must be trafficable and commonly level.
- It is not a door frame. The frame surrounds the opening; the threshold is the base condition beneath and around it.
- It is not simply a floor finish transition. It is a junction through the building envelope, with water outside and conditioned space inside.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- thresholds
- door sill
- entrance threshold
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- How water arriving at the door is intercepted and drained before it reaches the opening.
- Whether level access is required, since that removes the option of a substantial upstand.
- How the insulation line continues through a position that is also structural and trafficked.
- How the waterproofing or damp-proofing of the floor connects to that of the wall at this point.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The threshold is where the outside ground and the inside floor come closest, and drainage immediately outside it commonly determines whether it works.
A continuous, trafficked, structural element at floor level is one of the hardest places to maintain insulation continuity.
Everything at a threshold is walked on, so wear and the loosening of seals over time are inherent rather than exceptional.
Level thresholds depend on external drainage being built correctly; the two are one design problem rather than two.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Materials
Commonly incorporates
Materials this element commonly contains as a distinct constituent part, rather than being made of.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Junctions
Interfaces this element takes part in
Junctions where the condition between this element and another governs how the construction behaves. These name the junction and the subjects it raises; they are not details, arrangements or approved ways of building.
Threshold to External Ground
Where a door opening meets the ground outside, and where keeping water out and letting people in are in direct conflict.
Raises: Moisture · Drainage · Thermal continuity · Buildability
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Opening
Clarifications
Commonly misunderstood
- That a level threshold is a finishing decision. It changes how water has to be managed outside the door and is a design problem from the start.
- That threshold leaks are seal failures. They are frequently drainage failures outside the door.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- How is water intercepted and drained immediately outside this threshold?
- Is level access required, and how does that change the water management strategy?
- How does insulation continue through the threshold?
- How do the floor and wall damp-proofing or waterproofing layers connect here?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- No upstand height, drainage provision, insulation arrangement or accessibility dimension is stated here.
- Level access thresholds and their drainage are regulated matters requiring qualified design.
Openings & Frame Elements
The frames, heads, jambs, sills, thresholds and closers that form and finish an opening through a wall.
Browse all openings & frames entries →