Building components · Wall Leaves & Backing
Outer Leaf
Explains the outer leaf as the exposed layer with its own duties, which in many walls does not include carrying the building.
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 outer leaf is
The outer leaf is the layer of a wall that faces the weather. In a cavity wall it stands separate from the inner leaf, and in many arrangements it carries nothing above itself: it is a weather-facing skin supported at its base and restrained back to the structure behind.
It is deliberately not expected to keep all water out. In a cavity or drained arrangement, water passing through the outer leaf is anticipated, and what deals with it is the cavity behind and the trays and weeps within it.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not the wall. It is one layer of a build-up, and in a cavity or veneer wall it is commonly the layer that carries the least.
- It is not necessarily loadbearing. In masonry veneer over a frame it carries only itself; in solid or some cavity construction it may carry more, and the difference is not visible from outside.
- It is not a cladding panel. Both face the weather, but a masonry leaf is built of units bedded together and is self-supporting in its own plane.
- It is not the material it is built from. Facing brick, block and stone are published material entries; the outer leaf is the position they occupy.
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.
- external leaf
- outer skin
- facing leaf
- veneer leaf
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- How it is restrained back to the structure behind, which is what stops a self-supporting but slender leaf from being pushed over.
- What supports it at its base, and whether that support is the foundation or a shelf angle part-way up.
- The cavity behind it and whether water passing through can drain and escape.
- Movement provision, since a long run of masonry expands, contracts and needs somewhere to do it.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Water reaching the back of the outer leaf is expected rather than exceptional, which is why what is behind it matters as much as the leaf itself.
Masonry moves, and different materials move differently — clay and concrete units behave in opposite directions with moisture, which is why joint provision is not generic.
This is the layer taking wind-driven rain, frost and solar exposure, and its condition is the visible record of that.
Anything crossing the cavity to restrain or support the leaf also crosses the insulation line, which is a thermal-bridging consideration for the wall design.
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.
Cavity Tray to Weep Discharge
Where water collected within a wall must actually leave it, and where a collection with no exit becomes a reservoir.
Raises: Moisture · Drainage · Buildability · Weathering
Outer Leaf to Inner Leaf Restraint
Where the two leaves of a cavity wall are connected so they act together while remaining separate for moisture.
Raises: Structural transfer · Moisture · Thermal continuity · Buildability
Wall Base Damp-Proofing Junction
Where a wall's damp-proof course and a floor's damp-proof membrane must be joined so neither has an open end.
Raises: Moisture · Thermal continuity · Buildability · Structural transfer
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.
- External Wall
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That the outer leaf keeps water out. In cavity construction it is the first line, and the arrangement behind it is what actually manages water.
- That an outer leaf can be opened up freely. Whether it carries anything, and what restrains it, has to be established first.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is the outer leaf carrying anything, or is it supported and restrained only?
- What supports it at its base, and is that support intact?
- How does water entering the cavity behind it get out?
- What movement provision exists, and does it suit the units used?
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 thickness, tie spacing, cavity width, movement joint spacing or support arrangement is stated here.
- Whether an outer leaf is loadbearing in a particular wall is a structural question that cannot be answered from a reference entry.
Wall Leaves & Backing
The distinct vertical layers that make up a wall — outer and inner leaves, the cavity between them, and the backing that supports what is fixed to the face.
Browse all wall leaves & backing entries →