Why Does Screed Crack? (Common Causes & How to Prevent It)

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Learning why screed cracks

Screed cracking is one of the most common concerns on any new floor — especially when you're dealing with fast-track schedules or underfloor heating. But the truth is, not all screed cracks are failures. Some are superficial and easy to fix, while others signal a problem with prep, materials, or drying.

Whether you’re laying traditional sand and cement or a flowing liquid screed, understanding what causes cracks (and how to prevent them) is key to long-term performance. In this guide, we’ll break down the types of screed cracks, why they happen, and what you can do to avoid costly repairs later on.

Common Causes of Screed Cracking

  • ✔ Rapid Drying

    If screed dries too quickly — especially in warm weather or with poor curing — it shrinks unevenly and cracks form as it pulls apart.
  • ✔ Poor Substrate Preparation

    A dusty, oily, or unstable base can lead to debonding and movement underneath, causing cracks to open in the screed above.
  • ✔ Lack of Expansion Joints

    If you skip joints or don’t place them correctly around perimeters, doorways or large rooms, screed has no space to move as it cures.
  • ✔ Incorrect Mix Ratio

    Adding too much water or using poor-quality sand weakens the mix and reduces strength, making cracks more likely under load or stress.
  • ✔ Underfloor Heating Expansion

    If screed around UFH isn’t reinforced or allowed to dry properly, thermal movement from heating up can cause wide stress cracks.

Surface-Level Hairline Cracks

  • Usually caused by drying shrinkage or laitance
  • Often just cosmetic and won’t affect flooring
  • Can be filled with resin or flexible compound
  • Still safe to tile or finish over once repaired

Full-Depth Structural Cracks

  • Usually go through the entire screed thickness
  • Caused by movement, poor sub-base, or thermal stress
  • May lead to debonding or hollow spots
  • Needs resin stitching or full replacement in severe cases

✔ How to Prevent Screed Cracking (Before & After Pouring)

  1. 1

    Start with a Clean, Stable Sub-Base

    Before you pour any screed, make sure the sub-base is compacted, level, and free from loose debris or contaminants. Use a DPM and insulation if needed, and prime bonded slabs. Any movement underneath the screed will lead to cracks as it dries or is walked on.

  2. 2

    Use the Right Screed Mix

    Stick to tried-and-tested ratios for sand and cement (usually 1:4 or 1:3) and don’t overwater the mix. For liquid screeds, make sure you’re using the correct flow grade and follow the supplier's curing instructions. Always check for compatibility if applying over underfloor heating.

  3. 3

    Install Proper Movement Joints

    Movement joints are essential for rooms over 40m², long corridors, changes in direction, and anywhere there's underfloor heating. Edge foam and separation joints allow the screed to move slightly during drying and prevent uncontrolled cracking across the floor surface.

  4. 4

    Control the Drying Environment

    Screed should dry gradually. Avoid direct sunlight, strong draughts, or switching on underfloor heating too early. Use a curing agent if needed. The ideal rate is 1mm per day, so a 50mm screed needs at least 5 weeks to dry fully in stable conditions.

Screed Crack Types, What Causes Them, and How to Fix

Crack Type Main Cause How to Fix
Hairline surface cracks Drying shrinkage or laitance Resin filler or flexible screed repair
Cracks around edges No expansion foam or joint Cut in joints or add expansion edging
Diagonal stress cracks Thermal movement or UFH expansion Stitch with resin or fibre reinforcement
Cracks over substrate joints Sub-base not isolated or primed Re-prime and patch or re-lay
Wide structural cracks Excessive load or poor mix Break out and re-lay area
🔹 Is it normal for new screed to crack?
Small cracks or hairline crazing is quite normal during the drying phase. It doesn’t always mean the floor has failed — especially if expansion joints and curing were done properly. If in doubt, tap-test the area or speak to a screed specialist before installing flooring.
🔹 Can I tile over cracked screed?
Yes, as long as the cracks are surface-level and have been filled properly with a resin-based repair. For deeper or structural cracks, you should repair with stitching resin and mesh, or consider re-laying the affected area before tiling.
🔹 Do liquid screeds crack more than traditional screed?
Not necessarily — but they do require better curing and preparation. Liquid screed is more sensitive to movement and heat (especially with UFH) so cracks usually come from missing edge joints or rushed drying.
🔹 Can you fix screed cracks without digging it up?
Often, yes. Resin crack repair kits, epoxy stitching, and flexible fillers are widely used to fix cracks without full removal. But if the screed is debonded or crumbling underfoot, it’s safer to break out and re-lay that area.

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