Building components · Terminations
Linear Gap Seal
Explains the sealed linear junction as a position in its own right, distinct from the joints and barriers it is most often confused with.
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 linear seal is
Where a partition meets the underside of the floor or roof above, or a floor edge meets the back of a facade, the two elements do not arrive tight against one another. A gap is left along the whole length of the junction — for tolerance, for deflection, or because the two were built by different trades at different times — and that gap is then closed. The sealed gap is the element this entry describes.
It is worth naming because it is linear and continuous. A crossing is a point; this is a line, so whatever the two elements were relied on to do is interrupted along the entire junction. It is also the junction most often finished by a trade whose concern is the appearance of the line rather than the reason the gap was left.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a cavity barrier. A barrier is placed inside a concealed cavity to stop that cavity running continuously; a linear gap seal closes a gap at the boundary between two elements that meet, and it exists whether or not there is any cavity involved.
- It is not a movement joint. A movement joint is a discontinuity cut into a single construction at a position where its own movement accumulates, so that the construction can crack where someone chose; a linear gap seal is the boundary at which two separately built elements arrive against each other, and it exists because the two were never one construction — whether or not either of them moves.
- It is not a sealant. Sealants are published material entries and are one of the things that may close such a gap; the junction is the gap, its backing and its closure together, and different junctions are closed by very different means.
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.
- head-of-wall seal
- perimeter seal
- linear joint seal
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- What the two elements either side are each relied on to do, since the gap interrupts those properties along the whole length of the junction.
- Whether the junction is expected to move, because a gap left for deflection has to be closed by something that can take that movement repeatedly.
- Being continuous along the whole junction, since a gap is closed only where the closure actually reaches, and the ends are where it commonly does not.
- The condition and regularity of the surfaces each side, because a closure is only as continuous as what it bears against.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A head-of-wall gap commonly exists because the structure above deflects, and a closure that cannot accommodate that movement is compressed or torn by the very thing it was left to allow.
A continuous unsealed gap along the top of a wall is an open air path between the spaces either side, and a wall's mass does very little about an air path.
Where the two elements are relied on by the building's fire strategy, what is required at the junction between them is a regulated determination made by qualified designers against tested arrangements, and nothing here states what it is.
The junction is commonly reached last, by a different trade, after both elements already exist — which is why it is the position most often closed for appearance rather than for the reason the gap was left.
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.
Clarifications
Commonly misunderstood
- That a filled gap is a closed one. What was used, how far into the gap it reaches and whether it is continuous determine what has been achieved, and a neat line says nothing about any of them.
- That the gap is a defect. It is commonly deliberate, left for movement and for tolerance, and building the two elements tight against each other replaces one problem with another.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Why is there a gap at this junction — tolerance, deflection or sequence — and what does that mean for how it can be closed?
- What are the two elements each relied on to do, and which of those properties does the junction interrupt?
- How much movement does this junction have to accommodate, and can the closure take it repeatedly over its life?
- Is the closure continuous along the whole junction, including at its ends and where it meets other elements?
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
- This entry describes a junction. It states no gap width, movement allowance, closure arrangement, fire rating or period, and none can be inferred from it.
- What is required at a junction between separating or fire-resisting elements is determined by qualified designers and by local requirements; this entry determines nothing.
Terminations & Weathering
The elements where a plane stops, turns up, sheds water or is deliberately interrupted — parapets, upstands, flashings, drips, weeps and movement joints.
Browse all terminations entries →