Building components · Wall Leaves & Backing
Sheathing Board
Explains sheathing as the element that turns an open frame into a continuous plane, and what depends on that plane.
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 sheathing is
Sheathing is board fixed across a frame's face so that the frame becomes a continuous surface rather than a series of members and gaps. Everything outboard — membranes, battens, insulation, cladding — then has something continuous to be applied to or fixed through.
On a roof the same idea appears as boarding over rafters, closing the plane beneath the covering. The position differs; the function of making a frame continuous does not.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a racking panel, although a racking panel is a sheathing board. The difference is whether the board is specified and fixed to provide structural resistance, and that is a fixing-specification difference rather than a board one.
- It is not a lining board. A lining goes on the inner face for finish, fire or acoustic reasons; sheathing closes the outer face.
- It is not a breather membrane. The membrane is applied over the sheathing and does the water-shedding; the board provides the surface.
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.
- sheathing
- external sheathing board
- sarking board
- sheeting board
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- What is applied over it, since the membrane or insulation outboard relies on a continuous, regular surface.
- Its behaviour if it gets wet, which is a build-up design question because the board sits within the wall rather than on its face.
- How joints between boards are treated, particularly where the board itself contributes to air or water control.
- Whether it is also carrying a structural duty, which changes the fixing requirements entirely.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A board within a wall build-up will experience some moisture, and how it responds — and whether it can dry — is part of designing the build-up rather than a property to assume.
Sheathing is commonly exposed to weather during construction before the outer layers go on, which is a different exposure from its permanent one.
Where insulation sits relative to the sheathing determines whether the board is warm or cold, which governs condensation risk within the build-up.
Board choice in an external wall build-up is a fire matter as well as a structural and moisture one, and is subject to regulation.
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.
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.
Sheathing to Frame
Where boarding meets the frame behind it, and where fixing pattern decides whether the board is structural.
Raises: Structural transfer · Moisture · Buildability · Fire continuity
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.
- Building Envelope
- 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 sheathing keeps water out. The membrane over it does; the board provides the surface and, in some build-ups, contributes to air tightness.
- That all sheathing is structural. Whether it provides racking resistance depends on how it is specified and fixed.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this sheathing providing structural racking resistance, or closing the frame only?
- How does the board behave if it becomes wet, and can the build-up dry?
- How are the board joints treated, and does the air barrier rely on them?
- What exposure will the board see during construction before the outer layers are complete?
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 board type, thickness, fixing pattern, fire classification or moisture performance is stated here.
- External wall build-up design, including board selection, is subject to regulation and requires qualified design.
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 →