Masonry Expansion Joints — Why Brick Buildings Crack Without Room to Move

Quick Answer

Those vertical lines of caulk running up a brick wall are masonry expansion joints — intentional gaps filled with a flexible sealant (a backer rod and elastomeric sealant, not mortar) that give the wall room to move. Brick expands over its lifetime and with heat; concrete and block shrink as they cure and cool. Without movement joints — expansion joints in brick, control joints in block — that movement has nowhere to go, and the wall cracks instead, usually vertically near corners and openings. And because the sealant in these joints wears out, resealing movement joints is routine masonry maintenance — neglect it and water gets in and cracking returns.

Most people assume a solid, unbroken brick wall is the ideal. With masonry, a wall that cannot move is a wall that cracks.

Masonry Is Always Moving

Brick and concrete look permanent, but they are dimensionally restless materials:

• Brick expands. Fresh from the kiln, clay brick slowly absorbs moisture from the air over its life and grows — a permanent, irreversible expansion called moisture expansion. On top of that, brick expands and contracts with temperature, swelling in summer heat and shrinking in winter.
• Concrete and block shrink. Concrete masonry units and poured concrete lose volume as they cure and dry, and contract further in the cold.

A long masonry wall, baking on a south face in July and freezing in January, is under constant dimensional stress. The wall will move. The only question is whether the building gives that movement a planned place to go — or lets it tear the wall apart.

Two Kinds of Movement Joint

Because brick and block move in opposite directions, they get opposite solutions:

• Expansion joints (for brick). Since brick expands, the joint is an open gap that gives it room to grow without crushing against the next section. It is filled with a compressible backer rod and a flexible sealant that squeeze as the brick expands and rebound as it contracts.
• Control joints (for concrete and block). Since block and concrete shrink, the joint creates a deliberate weak line so the inevitable shrinkage crack forms in a straight, planned, sealable location instead of wandering jaggedly across the wall.

Both are movement joints — flexible by design — and in everyday conversation people lump them all under "expansion joints." The key point is the same: they are the wall's relief valve.

What Happens Without Them — or When They Fail

When a wall has no movement joints, or when the joints have been compromised, the stress finds another exit. That looks like:

• Vertical cracks, often near building corners, where expanding brick has nowhere to go.
• Cracking around windows and door openings, common stress-concentration points.
• Stair-step cracks through mortar joints.
• Spalling or displaced brick where expansion has crushed masonry against a rigid restraint.
• Cracks at offsets and parapets, where long runs of brick meet a fixed point.

Here is the failure mode that quietly causes the most trouble in existing buildings: the sealant in the movement joints wears out. Sun, weather, and years of flexing harden it until it cracks or pulls away from the masonry. Once that happens two things go wrong at once — water pours into the open joint and runs behind the wall, and the joint can no longer do its job, so the cracking it was preventing starts to come back.

The Most Damaging Mistake: Filling a Movement Joint with Mortar

A well-meaning but destructive "repair" we see often: someone notices a gappy, failed expansion joint, assumes it is missing mortar, and packs it solid with mortar to stop the leak. Mortar is rigid. Filling a movement joint with it removes exactly the flexibility the wall needs, and the trapped expansion simply cracks the wall somewhere else — frequently straight through the brick rather than the joint. A movement joint must always be restored with the correct flexible materials, never pointed solid.

How Movement Joints Are Maintained and Repaired

The good news is that maintaining movement joints is straightforward and routine:

1. Identify the joints. We locate the building's expansion and control joints and assess the condition of the sealant.
2. Remove the failed sealant. Old, hardened, or detached sealant and the old backer rod are cut out cleanly.
3. Reinstall the backing. A correctly sized closed-cell backer rod sets the proper sealant depth and shape.
4. Apply the right sealant. A quality elastomeric sealant, in a color matched to the wall, is tooled into the joint so it bonds to both faces and can flex.
5. Add missing joints when needed. On walls cracking for lack of relief, movement joints can sometimes be cut in to give the wall the room it should have had.

Done on a sensible cycle, this keeps water out and keeps the wall's relief system working — cheap insurance against the cracking and water damage that failed joints invite.

The Takeaway

Expansion and control joints are not defects or afterthoughts — they are the engineering that lets a rigid material survive a climate that makes it move. On a Chicagoland building swinging from summer heat to deep winter cold, those flexible joints are doing quiet, essential work every day. Keep the sealant in good shape, never fill a movement joint with mortar, and watch for vertical cracking near corners and openings that can signal a wall fighting movement it cannot release.

Emerald Masonry LLC is a family-owned, licensed and insured masonry contractor serving Chicago and the Chicagoland suburbs with 40+ years of experience in tuckpointing, chimney repair, brick repair and replacement, lintel and parapet repair, foundation and limestone/sill repair, caulking, sealing, and commercial, residential, and historic masonry restoration. We reseal and repair masonry movement joints on homes and commercial buildings. Free on-site estimates — call (708) 288-1696.

For related reading and services, see our caulking and joint sealant service, our commercial masonry restoration work, and our guides to tuckpointing vs. caulking in masonry and hairline cracks vs. structural cracks in brick.

Frequently Asked Questions

What is a masonry expansion joint?

It is an intentional vertical gap built into a brick wall, filled with a compressible backer rod and a flexible elastomeric sealant rather than rigid mortar. It gives the wall room to expand and contract with temperature and long-term material movement. Without it, that movement has nowhere to go and the wall cracks instead.

What's the difference between an expansion joint and a control joint?

They solve opposite problems. Expansion joints are for brick, which tends to expand over its life and with heat — the joint gives it room to grow. Control joints are for concrete and concrete block, which shrink as they cure and with cold — the joint creates a planned, straight place for that shrinkage crack to form instead of a random jagged one. Both are flexible movement joints; people often call them all "expansion joints" loosely.

Why does the sealant in my expansion joints keep failing?

Sealant is the one part of the joint that wears out. Sun, temperature swings, and constant movement degrade it over years until it hardens, cracks, or pulls away from the masonry. Once it fails the joint leaks and the wall loses its movement relief, so cracking can return. Resealing movement joints on a schedule is normal building maintenance, not a sign something is wrong.

Can I fill an expansion joint with mortar to stop the gap leaking?

No — that is a common and damaging mistake. Mortar is rigid; filling a movement joint with it removes the wall's room to move, and the wall will simply crack somewhere else, often through the brick. A failed expansion joint should be re-sealed with the correct flexible materials, never pointed solid with mortar.

Get a Free Masonry Inspection

If your building has failed joint sealant or vertical cracking near corners and openings, the movement joints may need attention before water and cracking spread. Request a free on-site estimate through our contact page or call (708) 288-1696 today.