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Building components · Wall Leaves & Backing

Bed Joint

Explains the bed joint as the continuous load-carrying plane of a masonry wall, distinct from the vertical joints in the same wall and from joints designed to move.

Form:Continuous layerRoles:StructureSupportAttachment

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 bed joint is

A bed joint is the horizontal layer of mortar on which a course of masonry is laid. It runs the full length and full thickness of the wall at that level, which makes it the one joint in a bonded wall that is genuinely continuous in both directions.

Its importance is that it is the load path. Compression from everything above arrives at each course through its bed joint, and the joint has to be filled across the whole bearing area for that to happen evenly. It is also the plane a wall is levelled on, so it absorbs the small corrections that keep coursing true as the wall rises.

Boundaries

How it differs from nearby elements

The distinctions that keep this entry from being a restatement of its neighbours.

  • It is not a perpend joint. A bed joint is horizontal and continuous, and it carries the compression passing from one course to the next; a perpend is the vertical joint between two units within a course, it is interrupted at every course by the bonding overlap, and it carries very little of that load.
  • It is not a movement joint. A movement joint is a deliberate discontinuity intended to let parts of a wall move relative to each other and to stay open; a bed joint is intended to act continuously and to bond the courses either side of it into one wall.
  • It is not mortar. Masonry mortar is a published material entry with its own composition and behaviour; the bed joint is the position that mortar occupies, and the same position can be formed in mortars that behave very differently relative to the units around them.

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.

  • bedding joint
  • horizontal joint
  • mortar bed

Dependencies

What it depends on

What has to be right elsewhere for this element to do its job.

  • Being filled across the bearing area rather than only at the face, since a joint that is solid at the front and hollow behind concentrates load where it was not expected.
  • The relationship between the mortar and the units, because a mortar that is harder than the units it beds changes where cracking and decay appear.
  • Remaining level and consistent as the wall rises, since coursing errors accumulate upward rather than cancelling out.
  • What else is built into it — restraint ties, a damp-proof course, reinforcement — each of which occupies part of the same plane.

Considerations

Topics that genuinely apply here

Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.

Structural transfer

The bed joint is where compression crosses from course to course, so how completely it is filled governs how evenly the wall is loaded rather than merely how it looks.

Moisture

The joint is commonly the more absorbent part of a masonry face, which is why water movement in a wall frequently follows the joints rather than the units.

Movement

Because the bed joint is continuous along the wall, it is also the plane along which horizontal cracking tends to appear when the wall is restrained from moving.

Durability

Repointing replaces the outer part of a bed joint and not the joint behind it, so the visible condition of a joint and its condition through the wall can differ.

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 LibraryMasonry mortar →The joint is the position; masonry mortar is the material family that commonly fills it, and its properties relative to the units govern how the joint behaves.

Materials

Commonly incorporates

Materials this element commonly contains as a distinct constituent part, rather than being made of.

Materials LibraryWall ties →Restraint between leaves is commonly bedded into the joint, which makes the joint the place that restraint is either present or absent.Materials LibraryDamp-proof course →A horizontal barrier within masonry commonly occupies a bed joint, so the joint is where its continuity is achieved or lost.

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.

Primary supportJointing and sealingLoad-Bearing Masonry →The plane through which compression passes from each course into the one below.Primary supportJointing and sealingSolid Wall →In a single-thickness wall the bed joint runs through the whole wall, so it is both the load path and a route for moisture movement.Jointing and sealingAttachmentTwin-Leaf Cavity Wall →Each leaf has its own bed joints, and the restraint between the leaves is commonly bedded into them.Jointing and sealingDamp-Proof Continuity →A horizontal barrier within masonry commonly occupies a bed joint, which makes that joint the place continuity is either achieved or lost.Primary supportJointing and sealingFreestanding Wall →Exposed on both faces, so the joints weather from both sides rather than one.

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 a bed joint and a construction joint are the same kind of break. A construction joint is where work stopped and restarted and is intended to act as though the section were continuous across it; a bed joint is a designed, repeating plane present throughout the wall whether or not work paused at that level.
  • That the joint is simply the gap between units. It is a structural plane, and how it is filled and how the mortar behaves relative to the units govern how the wall carries load and how water moves through it.

Conversations

Questions for qualified professionals

Questions worth asking someone with the project in front of them. This page cannot answer them.

  • What mortar was used in this wall, and how does it behave relative to the units it beds?
  • Are the bed joints solidly filled through the thickness of the wall, or only at the face?
  • Is any horizontal cracking along the bed joints related to restraint, to loading or to the mortar itself?
  • What else is built into the bed joints of this wall, and does any of it need to be located before work starts?

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 wall. It states no joint thickness, no mortar mix, no strength and no load capacity, and none can be inferred from it.
  • Whether the bed joints in a particular wall are performing as intended is a matter for inspection and structural assessment, not for 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 →