Joints in the screed
[ The complete guide to avoiding cracks ]
The invisible hero of your floor
For the layman, a joint in the screed is often a mystery. Sometimes it is even misunderstood as a planned weak point. The truth is exactly the opposite: a correctly planned and executed joint is the invisible, indispensable hero that saves your screed and your expensive floor covering from certain ruin.
Don't think of a joint as a separation, but as an intelligent pressure relief valve. A screed is not a dead material. It moves. It shrinks when drying and expands when heated or contracts when cold. These forces are immense. Without a system that absorbs and dissipates these stresses in a controlled manner, the screed will release them in an uncontrolled manner - in the form of ugly, unpredictable cracks in the middle of your living room.
This guide decodes the function and necessity of joints and shows you why a professional joint plan is not an "extra", but a non-negotiable part of any high-quality construction project.
Contents at a glance:
Chapter 1: The law of physics - Why your screed needs joints
Two unstoppable physical forces act on every cement screed:
- Shrinkage (hydration): As the screed hardens, it loses water. This chemical and physical process leads to a reduction in the volume of the screed - it "shrinks". This shrinkage process generates massive internal tensile stresses.
- The thermal movement (temperature): A screed, especially a heated screed, is subject to constant temperature changes. It expands when it heats up and contracts when it cools down. This movement has to go somewhere.
The brutal reality is that if these stresses are not relieved in a controlled manner by means of specifically designed joints, the force seeks its own path. This path is always the weakest point in the surface - and the result is an uncontrolled crack.
Chapter 2: The anatomy of joints - The 3 key types
A professional screed layer works with a system of three different joint types, each of which fulfills a specific task.
Type 1: The movement joint (the main artery)
- Function: It completely separates two screed areas from each other, including insulation and (if present) underfloor heating. It is the thickest of all joints.
- Purpose: It absorbs the greatest movements, both through shrinkage and thermal length change. It acts like the expansion joint of a bridge.
- Where it is absolutely essential: In doorways, in large areas (over 40 m²), in complex room geometries and between heated and unheated areas.
Type 2: The dummy joint (the predetermined breaking specialist)
- Function: Here the screed is only weakened in the upper third by a cut.
- Purpose: It serves as a predetermined breaking point. The unavoidable shrinkage crack forms in a targeted and controlled manner on this weakened line instead of migrating uncontrollably through the surface.
- Where it is used: In large, square fields to divide them into smaller, manageable sections.
Type 3: The edge joint (the crumple zone)
- Function: This is the space created by the edge insulation strip between the screed and all rising components (walls, columns, stairs).
- Purpose: It gives the screed the necessary "breathing space" to expand without pressing against the wall and prevents the transmission of impact sound.
- The deadly sin: If this edge insulation strip is bridged later, the joint is ineffective. This leads to forced tension and sound bridges.
Chapter 3: The joint plan - the mark of the true professional
How do you decide where to put which joint? By guesswork? By experience? No.
An absolute professional creates a joint plan before installation. This is a technical drawing based on the floor plan and specifies the exact position and type of each individual joint.
A joint plan is your proof that your screed installer thinks before he acts. Don't ask your contractor if he does joints. Ask him if you can see the joint plan. The answer to this question will separate the wheat from the chaff.
Chapter 4: From theory to practice - the link to the big picture
You now understand the crucial role that joints play in the durability and freedom from damage of your floor. Joints are not a minor matter, but a central component of process reliability.
This knowledge of joints is an in-depth insight into one of the five fundamental areas of screed installation. To understand the whole picture and fully understand all the risks for your construction project, this detailed knowledge must be seen in the context of the overall process.
We have created our main work for this purpose: The Cement Screed Protocol. It is the overarching guide that shows you how this and other critical details interlock to guarantee a perfect and damage-free result.
Your next step
If you are looking for a partner who not only cuts joints, but also understands the physics behind them and works according to a precise plan, then let's talk.