What are the joints in my concrete floor for?
The lines cut or formed into a slab are there so it cracks where you chose rather than where it likes. Filling or covering them without thinking undoes that.
- Difficulty
- intermediate
- Time
- 15 min
- Read
- 2 min
Short answer
Concrete shrinks as it cures and moves with temperature. Joints give that movement somewhere to go and control where cracking happens. They must be respected by anything laid over the slab — a rigid covering bridging a moving joint cracks along it.
People often assume the straight lines in a garage or workshop floor are decorative or an artefact of how it was poured. They are neither. They are deliberate weaknesses, and understanding that changes what you do when you coat, tile or repair the floor.
Step by step
- Recognise the two kinds you are likely to see.A control or contraction joint is a groove cut or formed into the surface, encouraging the slab to crack in a straight line underneath it. An isolation or expansion joint runs full depth, separating the slab from a wall, a column or another slab so they can move independently.
- Understand that a cracked control joint is working, not failing.A fine crack following the line of a control joint is the joint doing its job. That is the difference between a controlled crack and a random one across the middle of the floor.
- Do not rigidly fill an isolation joint.It exists so two elements can move separately. Filling it with mortar or a rigid filler ties them together, and the movement then appears as a crack somewhere less convenient.
- Use a flexible sealant where a joint needs filling.In a garage the reason to fill a joint is usually to keep dirt and water out and to give a cleanable surface. A flexible sealant does that without preventing movement.
- Carry joints through anything you lay on top.A tiled or rigidly coated floor over a slab must have a joint in the same place. Bridging a moving joint with a rigid material cracks it along the line, usually within the first year.
- Expect a coating to show the joint.A film-forming coating over a control joint will crack there eventually. That is not a coating failure; it is the joint moving. Filling the joint flexibly first and accepting a visible line is the honest approach.
- Treat a crack that is not on a joint differently.A random crack, one that steps, one with a level difference across it, or one that is opening over time is a different question and worth understanding before it is filled or covered.
Questions people ask
Should I fill the joints in my garage floor?
Usually yes, with a flexible sealant, to keep grit and water out and make the floor easier to clean. Not with anything rigid.
My slab has no joints and has cracked — why?
Because the movement had nowhere to go. Domestic slabs are often poured without joints, and a crack is the slab making its own. Whether it matters depends on the crack.