Screed cracks for a reason, and the reason decides the repair. Shrinkage cracks come from water leaving the screed. Thermal cracks come from heating too early or too fast. Structural cracks come from movement below, a debonded section, or a screed laid too thin for its construction. Sealing a crack without removing its cause only buys time until the next movement opens it again.
Screed cracking is a common issue that can occur in cement or concrete floors. Cracks can appear unsightly and over time may worsen, leading to further damage. Understanding the causes of screed cracking along with preventative measures is key for homeowners and contractors alike.
What is Screed in Flooring?
Screed is the top layer of a cement or concrete floor that provides a smooth, level surface prior to the final floor covering being laid. Screed is usually a blend of sand, cement, and water that is poured over the concrete base. Once dry, it offers an ideal subfloor for the application of tiles, wood, etc.
Common Causes of Cracking
There are several potential causes of cracks appearing in screed floors:
Excessive Water Content
Using too much water when mixing screed leads to shrinkage cracks as the excess water evaporates over time. This leaves voids in the floor.
Improper Compaction
Screed needs firm compacting to force out air pockets. Insufficient compaction leads to cracks above air voids.
Thick Applications
Applying screed in layers that are too thick can often crack due to inadequate drying time for inner moisture to evaporate.
Movement
Any movement underneath can transfer up through the screed. This includes settlement of the concrete base or expansion issues.
Rapid Drying
If screed dries too quickly, surface cracks can form much like cracked and dried mud. Keeping screed damp in the first 48 hours prevents this.
Heavy Loads
The added weight of heavy equipment or materials being wheeled over screed can cause cracks.
Preventing Screed Cracks
Cracks may be inevitable as screed dries and contracts slightly but follow these tips to minimize cracks:
Maintain Correct Water Content
Only use sufficient water for workable mixes. Excess water weakens screed over time.
Thoroughly Compact the Screed
Use tamping tools to fully compact screed and remove air pockets after pouring.
Apply in Thin Layers
Screed over 40 mm thick takes too long to dry properly. Apply in layers for better drying.
Allow Adequate Drying Times
Rushing installation causes cracks. Allow at least 1 week of curing per 25 mm thickness.
Use 1 day per millimetre for the first 25 mm and 2 days per millimetre after that, at about 20 degrees C and 65 percent relative humidity. A 65 mm screed plans at 25 days plus 80 days, about 105 days. Test with a hygrometer before covering and do not rely on the surface looking dry, because the surface dries long before the body of the screed does.
Isolate Movement
Use expansion joints to isolate screed from any movement in the substrate concrete.
Control Environment
Avoid rapid early drying. Lightly dampen surface for 48 hours and avoid direct sunlight or heating.
Protect Screed
Do not overload. Avoid wheeled traffic until fully cured.
Dealing with Existing Screed Cracks
For screed floors with minor cracks or homes with chronic cracking issues from movement consider these options:
- Flexible fillers: Allow cracks to open and fill repeatedly with minimal damage
- Reinforcing mesh: A fiberglass mesh embedded into fresh screed relieves stresses
- Crack-isolation membranes: Slip layers isolate cracks from transferring to new tiles
- Expansion joints: Divides a large floor to limit transferred stresses leading to cracks
- Restoration coatings: Specialty sealants and toppings designed for hiding cracks
Major crack damage may require floors to be broken out and re-laid after analyzing the root causes.
Cause, sign and prevention
| Cause | Sign | Prevention |
|---|---|---|
| Plastic shrinkage | Fine surface crazing in the first days | Protect from draughts and sun for the first 72 hours, cure as the product data sheet directs, and do not add extra water to the mix on site |
| Drying shrinkage | Straight cracks that follow restraints and bay shapes | Perimeter joints, bay joints, and a thickness that meets the construction minimum |
| Missing or wrongly placed movement joints | Cracks through doorways and across heated to unheated boundaries | Joints at every doorway, at every change of heating zone, and splitting L-shaped floors into rectangles |
| Too thin for its construction | Cracking and curling, worst at edges and corners | Bonded 25 mm minimum, unbonded 50 mm, floating 65 mm domestic, all to BS 8204-1, measured at the thinnest point |
| Heating too early or too fast | Cracks that appear on the first heating cycle | Wait 21 days for cementitious screed, start at 25 degrees C or less, and rise by 5 degrees C per day (BS EN 1264-4 practice) |
| Debonding or substrate movement | Hollow sound when tapped, lifting sections, and cracks that keep widening | None by sealing. Find the cause, which may be a failed bond, a cracked slab, or ground movement, before covering |
Conclusion
Screed cracking is generally caused by poor installation practices that can be avoided. Ensure an adequate cement mix is used, proper water levels maintained, thorough compaction done, suitable environment for curing enabled, and loading protected. This minimizes common issues like shrinkage cracks.
FAQs:
What thickness should floor screed be poured?
Screed should be poured in layers no thicker than 40 mm, allowing adequate time for drying between layers. This prevents inner moisture becoming trapped and causing cracks.
How long does screed take to dry?
Screed drying time depends on thickness and environment. Allow 1 week per 25 mm thickness. Maintain room temperature between 15-20°C and lightly dampen the surface for 48 hours to prevent rapid early drying.
Can cracks in screed be repaired?
Minor screed cracks can be repaired by flexible fillers designed for substrates that move. For chronic issues, consider reinforcement meshes, crack isolation membranes or saw-cut expansion joints.
What causes cracks after screeding?
Common causes of cracks forming after screed has been laid are excessive water content leading to shrinkage, inadequate compaction allowing air pockets, overly thick applications, movement underneath, rapid early drying, and overloading before fully cured.
How do you reinforce screed?
Two ways to reinforce screed prone to cracking are adding a fiberglass mesh layer placed just below the screed surface to relieve stresses or installing saw-cut expansion joints to isolate sections.