Screed is a thin, workable layer of material spread over a concrete base to create a smooth, level surface ready for final flooring. It acts as a uniform cushion that absorbs slight imperfections and helps underfloor systems perform reliably.
In construction, screed definition focuses on composition, thickness, and drying behavior, because these factors determine floor flatness, sound insulation, and compatibility with finishes such as tiles, wood, or carpet.
| Aspect | Detail | Impact on Installation | Typical Range |
|---|---|---|---|
| Material | Cement sand, anhydrite, or polymer-modified screed | Adhesion, shrinkage, and load transfer | Cement sand or anhydrite |
| Minimum Thickness | Depth required for coverage and strength | Underfloor heating spacing and floor level | 15 mm to 30 mm |
| Drying Time | Curing period before floor covering can be laid | Project schedule and risk of cracking | 1–7 days for anhydrite, longer for sand-cement |
| Flatness Result | Surface evenness measured with straightedge and level | Finish aesthetics and performance of flooring | Ff numbers per standard test methods |
| Insulation Contribution | Incorporation of foam or cork into the screed layer | Floor U-value and sound reduction | 0.06–0.12 W/mK with added insulation |
Material Composition and Binder Types
Cement-Based Screed
Cement-based screed is widely specified for domestic and commercial projects. It mixes Portland cement with graded sand and water, producing a robust, dimensionally stable layer suitable for both bonded and unbonded applications.
Anhydrite and Polymer-Modified Options
Anhydrite screed uses gypsum derivatives and offers faster drying with lower shrinkage. Polymer-modified screed improves toughness, reduces crack widths, and is often chosen for high-traffic areas or when fine tolerances are required.
Application Methods and Install Practices
Screed can be poured directly onto structural concrete or applied over underfloor heating pipes and insulation boards. Bonded screed locks into a prepared rough surface, while unbonded screed sits above a damp-proof membrane to isolate from the substrate.
Proper mixing water control, timely leveling, and edge restraint are essential to avoid cracking and excessive drying shrinkage. Installers often use straightedges and laser levels to achieve the required planarity for subsequent finishes.
Performance Factors and Specification
Strength and Load Transfer
The compressive strength of screed influences how loads from furniture and foot traffic are distributed across the floor. A adequately specified screed thickness prevents localized deflection and protects sensitive floor finishes.
Thermal and Acoustic Behavior
When combined with underfloor heating, screed acts as a heat transfer medium, smoothing temperature variations across the floor surface. It also contributes to impact sound reduction, limiting noise transmission between floors in multi-story buildings.
Key Takeaways for Specification and Execution
- Define screed by material, thickness, and drying time to match floor finish and underfloor systems.
- Select cement-based, anhydrite, or polymer-modified screed based on load, insulation, and drying requirements.
- Use unbonded construction with a damp-proof membrane when installing over concrete slabs.
- Control water content and curing conditions to minimize shrinkage cracks and ensure surface durability.
- Verify flatness with industry test methods to confirm compatibility with tiling, wood, or carpet finishes.
FAQ
Reader questions
What is the minimum thickness required for a standard residential screed?
For a typical unbonded sand-cement screed over concrete, a minimum thickness of 20 mm is common, while anhydrite screed can perform adequately at around 15 mm when used with underfloor heating.
Can screed be installed directly over underfloor heating pipes?
Yes, provided the heating system is tested and protected during installation. The screed encapsulates the pipes, distributing heat evenly and reducing point temperatures that could cause floor damage.
How long should I wait before laying floor tiles or engineered wood?
With anhydrite screed, tiles can often be laid after 24 to 48 hours if the surface is dry and rigid. Cement-based screed may require up to seven days or longer, depending on thickness and ambient conditions.
What signs indicate that screed has dried excessively or too quickly?
Excessive drying can cause curling, cracking, or dusting at the surface. Controlled curing and protection from rapid moisture loss help maintain flatness and prevent early failure of the finished floor.