How the two differ in kind
Masonry moves, and a long restrained wall that cannot move cracks. What the wall does depends on what it is made of, and the two common materials do opposite things.
CONCRETE masonry SHRINKS. Blocks lose moisture after manufacture and shrink as they dry, and they shrink further as they carbonate over a long period — so a block wall gets shorter, and a wall restrained at its ends develops tensile stress and cracks. CLAY brickwork does the reverse: fired clay takes up moisture from the air and EXPANDS irreversibly for years, so a brick wall gets longer and, restrained, goes into compression. The provisions therefore differ: shrinking material needs somewhere to shrink TO, and expanding material needs somewhere to expand INTO.
CONTROL JOINTS address the shrinking case by deciding where the crack goes. A vertical separation through the wall, at a spacing set by the wall's geometry and its openings, lets the shrinkage occur at a chosen line rather than at a random one — and because the joint is formed and sealed rather than mortared, the movement is invisible and weatherproof. The equivalent in clay brickwork is an EXPANSION joint, which is compressible rather than open, because it has to close rather than to open.
BED JOINT REINFORCEMENT addresses it differently: instead of concentrating the movement at a joint, it distributes it. Horizontal wire laid in the bed joints ties the wall together across any shrinkage crack that starts, so the movement is shared as many very fine cracks rather than one visible one. That does not remove the need for joints — the wall still shrinks — but it permits them to be spaced FURTHER APART, which is why guidance on control joint spacing is expressed with and without reinforcement.
So they are complementary. And the placement rules matter as much as the spacing: joints belong at the points where cracking concentrates — changes of wall height or thickness, at openings, at corners, and where the wall is restrained.
The factors that actually differ
Show
| Control joints | Bed joint reinforcement | |
|---|---|---|
| What it does | Decides where the crack occurs, by providing a deliberate separation. | Distributes the cracking into many fine ones instead of one visible one. |
| Relationship | Still required with reinforcement — reinforcement widens the spacing rather than removing the need. | Permits wider control joint spacing, which is how guidance expresses it. |
| Shrinking vs expanding material | A control joint opens, for concrete masonry which shrinks. Clay brickwork needs a compressible EXPANSION joint instead. | Helps in both, by distributing movement. |
| Where it goes | At changes of wall height or thickness, at openings, near corners, and where restraint changes — the points where cracking concentrates. | In the bed joints at a vertical spacing, continuous along the wall and lapped at splices. |
| How it is formed | A separation through the wall, raked and sealed rather than mortared — mortaring it defeats it entirely. | Wire laid into the mortar bed, fully embedded with cover, lapped at joints. |
| Appearance | A visible vertical line, which is why placement is an architectural decision as well as a technical one. | Invisible. |
| Structural contribution | None — it is a deliberate discontinuity, and shear across it is provided separately where needed. | Modest in-plane, and it is not a substitute for designed reinforcement such as bond beams. |
| What defeats it | Mortaring the joint solid, or running reinforcement or a bond beam straight through it. | Wire not lapped at splices, laid without cover, or stopped short of the ends. |
| Cost | Low — a joint former and a sealed joint. | Low per course, and it adds up over a large wall. |
| Used together | Yes, and that is the normal specification. | Yes. |
Which one, and when
Choose control joints when…
- Any concrete masonry wall of length — shrinkage is a property of the material, not an occasional problem.
- At the points where cracking concentrates: changes of height or thickness, openings, near corners.
- Where a crack must be controlled for appearance, on a fair-faced or exposed wall.
- In clay brickwork as an EXPANSION joint instead, which is compressible rather than open.
Choose bed joint reinforcement when…
- Where control joints cannot be spaced as closely as the material would otherwise require.
- Above and below openings, where cracking concentrates at the corners.
- Where appearance rules out frequent visible joints.
- As part of the standard specification, since it is cheap and it distributes movement everywhere along the wall.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why do brick and block need opposite joints?
- Because they move in opposite directions. Concrete masonry shrinks: blocks continue to lose moisture after manufacture and shrink as they dry, and they shrink further over a long period as they carbonate — so a block wall gets shorter, and restrained, it cracks in tension. A CONTROL joint is what it needs: an open separation that lets the wall shrink at a chosen line. Clay brickwork expands: fired clay takes up moisture irreversibly after firing and continues for years, so a brick wall gets longer and, restrained, goes into compression — showing as spalling and bowing rather than cracking. An EXPANSION joint is what that needs: a compressible filler that can be squeezed rather than an open gap. Using the wrong one is not a partial measure; an open joint does nothing for a wall trying to expand.
- Does reinforcement remove the need for joints?
- No, and treating it as though it does is the common error. Bed joint reinforcement distributes cracking — the wire ties the wall together across a crack that starts, so the movement is shared as many hairline cracks rather than concentrated into one that shows. That is genuinely useful and it is why guidance permits wider control joint spacing where reinforcement is used. But the wall is still shrinking, and the total movement has to go somewhere: past a certain length no amount of distribution absorbs it, and a crack of visible width appears. The relationship is quantitative rather than qualitative — reinforcement buys spacing, it does not buy exemption — and the figures come from the masonry guidance for the material and the exposure rather than from judgement.
- Where should control joints go?
- At the places cracking concentrates, and at a maximum spacing along the wall. The concentration points are where the wall's behaviour changes: a change of height, such as beside a stepped parapet or where a wall meets a lower one; a change of thickness; at openings, where the corners are stress raisers and cracks step away diagonally; near corners, where restraint changes; at a change of backing or support; and at movement joints in the structure behind. Between those, joints are placed at the maximum spacing the material and any reinforcement allow, which is usually expressed both as a distance and as a ratio of wall length to height. Placement is also an architectural decision — the joint is visible — so it is best located to coincide with a feature, a pier line, or a change of plane.
- How is a control joint formed?
- As a separation through the wall that permits movement but resists weather and, where required, transfers shear across itself. The masonry is stopped either side of the line, commonly with a shear key — a proprietary joint former, or a block shape that keys the two sides together out of plane while letting them move in plane — and the joint is raked out and sealed with a backer rod and sealant rather than mortared. That sealant joint is what makes it weatherproof and is a maintenance item over the building's life. What destroys it is mortaring it solid, which somebody will do because an unfilled joint looks unfinished — at which point the wall has no joint at all and cracks somewhere else, usually within a season or two.
- What ruins bed joint reinforcement?
- Not being continuous, and not being embedded. The wire works by tying the wall together across a crack, which requires it to run continuously along the wall with proper laps at every splice — reinforcement stopped short of the wall's ends or butted rather than lapped leaves the wall untied exactly where it needs to be tied. It also needs cover: laid in the middle of the mortar bed with mortar above and below it, not pressed onto the block and buttered over, or it is neither bonded nor protected from corrosion. And it must not run THROUGH a control joint, since that would tie the two sides together and prevent the joint from doing its job — the reinforcement is stopped either side, which is a detail easy to miss on site.
- Is bed joint reinforcement structural?
- Modestly, and it is not a substitute for designed reinforcement. It contributes to the wall's in-plane behaviour and it is credited in some design methods, and in seismic detailing horizontal reinforcement at a specified ratio is a requirement rather than an optional crack-control measure. But the light wire used for crack control is not carrying the loads that bond beams and vertical reinforcement in grouted cells carry, and it does not replace them. The distinction matters when reading a drawing: reinforcement shown in the bed joints at a close vertical spacing is usually crack control, while a bond beam with bars in a grouted course at specific levels is structural, and the two coexist. Substituting one for the other because both are described as reinforcement is a real error.
- How long should a block wall go without a joint?
- Shorter than most people assume, and the figure comes from guidance rather than instinct — expressed as a maximum distance and as a ratio of panel length to height, with different values depending on whether bed joint reinforcement is present and at what spacing. The reason the ratio matters as much as the distance is that a long low panel is far more prone to shrinkage cracking than a short tall one, because the restraint at its base acts over a greater proportion of it. Other factors tighten it: the block's own shrinkage characteristic, which varies with the unit; the exposure and the moisture conditions; and any restraint from the structure. On a wall of unusual proportions or in an aggressive exposure the spacing is a design decision rather than a table lookup.
- What happens if joints are omitted?
- The wall cracks where it chooses, which is at the weakest line rather than the least conspicuous one. The characteristic pattern is a vertical or stepped crack running from an opening's corner, from a change of wall height, or through the middle of a long panel — following the bed and head joints in a stepped line because that is where the mortar is weakest. It is unsightly, it is a water path into the wall, and once open it does not close. Retrofitting a control joint is possible — cutting a joint through the wall at the crack line, keying it and sealing it — and it converts an uncontrolled crack into a controlled joint, which is the honest repair. Filling the crack alone does not work, because the movement that caused it has not stopped.