Building components · Wall Leaves & Backing
Perpend Joint
Explains the perpend as the short, interrupted joint of a bond, distinct from the continuous horizontal bed joint and from joints made to open.
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 perpend is
A perpend is the joint standing between two units in the same course. Unlike the bed joint, it does not run through the wall from end to end: it stops at the course above and the course below, and in a bonded wall it is deliberately offset from the perpends in the courses either side of it.
That offsetting is the whole point. A vertical joint that ran continuously up a wall would be a plane of weakness and a straight path for water, so bonds exist to break it. The perpend is therefore the joint that a bond exists to interrupt.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a bed joint. Both are mortar joints in the same wall, but the bed joint is the horizontal plane carrying load from course to course, while the perpend is vertical, short, and interrupted at every course by the bonding overlap.
- It is not a movement joint. A movement joint is a full discontinuity through the wall that is intended to open and close; a perpend is one short joint in a bonded pattern and is not continuous from the top of the wall to the bottom.
- It is not a weep opening, although an unfilled perpend is one of the forms a weep commonly takes. The weep is the function — a deliberate outlet from a cavity or a tray — and the perpend is the position that outlet is commonly made in.
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.
- perp
- cross joint
- vertical joint
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The bond the wall is built in, which determines where perpends fall and how far they are offset from those above and below.
- Whether the joint is filled or deliberately left open, since the two look similar on a finished face and behave very differently.
- The mortar's ability to stay in a vertical joint while the wall is being built, which is not the same demand a horizontal bed makes of it.
- What sits behind the joint, because a perpend in an outer leaf opens onto a cavity while a perpend in a solid wall opens onto more wall.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A perpend that is not solidly filled is a direct path through the face of a leaf, and in cavity construction that is anticipated rather than exceptional — what deals with it is the arrangement behind.
Where perpends are deliberately left open they are acting as outlets, and whether that was the intention or the joint was simply never filled is a question for inspection.
Perpends on an exposed elevation take driving rain at a different angle from the bed joints, and their condition often deteriorates first.
Filling a vertical joint solidly is harder than filling a horizontal bed, which is why incompletely filled perpends are among the more commonly encountered defects in masonry.
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.
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 perpends are always filled. Some are deliberately left open to drain a cavity or a tray, and telling a designed open perpend from one that was never filled properly is a matter for inspection rather than appearance.
- That perpends should line up through the wall. In most bonds they are deliberately staggered course by course, because a continuous vertical joint is both a line of weakness and a straight route for water.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What bond is this wall built in, and how does it arrange the perpends?
- Are any open perpends in this elevation deliberate outlets, and if so what are they draining?
- Are the perpends solidly filled where they were intended to be?
- How does water passing through a perpend in this wall get out again?
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 position in a bond. It states no joint width, no spacing of open perpends, no mortar mix and no drainage performance, and none can be inferred from it.
- Whether a particular open perpend is functioning as an outlet, or is simply an unfilled joint, cannot be determined 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 →