Construction worker leveling cement screed with a straightedge over a structural concrete slab and services. — Screed in Construction: Ensuring a perfectly level and durable base is the critical final step before laying the finished flooring materials.

Screed in Construction: Types, Mix Ratios & Floor Leveling Guide

In building construction, a smooth, level floor is critical for structural performance and aesthetic finish. Screed is the layer of cement mortar or concrete applied over the structural base to achieve a level surface. Properly executed screed not only ensures floor flatness but also enhances durability, load distribution, and the lifespan of the final flooring material.

What is Screed?

Screed is a non-structural finishing layer applied on a concrete slab, commonly ranging from 25 mm to 50 mm thickness depending on the project. Its main purposes include:

Creating a flat, level surface for finishing floors.
Covering electrical conduits, plumbing pipes, and underfloor heating systems.
Providing a solid base for tiles, vinyl, wood, or other flooring finishes.

Reducing surface cracking and ensuring uniform load transfer to the structural slab.

Unlike the structural slab, screed does not carry major structural loads. It is primarily designed for surface leveling and durability enhancement.

Close-up of a construction professional compacting and smoothing freshly laid sand-cement screed using a hand trowel. — Selecting the Right Screed: Choose from Traditional Sand and Cement, Anhydrite (Self-Leveling), or Polymer-Modified based on project requirements, such as traffic load and underfloor heating.

Types of Screed

Selecting the right type of screed is crucial for both residential and commercial projects.

1. Sand and Cement Screed (Traditional Screed)

Composition: Typically 1 part cement to 3–5 parts sharp sand by volume, mixed with clean water.
Thickness: Usually 25–50 mm.
Advantages: Economical, strong, versatile, suitable for most flooring types.
Applications: Ideal for tiled floors, stone flooring, or as a base for timber finishes.
Consideration: Requires proper curing to avoid shrinkage cracks.

2. Anhydrite or Calcium Sulphate Screed

Composition: Made with gypsum (CaSO₄·2H₂O), water, and sometimes additives for flowability.

Advantages: Self-leveling, fast application, smooth finish, compatible with underfloor heating.
Applications: Modern residential and commercial floors where a thin, uniform finish is required.
Limitation: Not suitable for wet areas unless sealed.

3. Polymer-Modified Screed

Composition: Cement screed with polymer additives, improving adhesion, flexibility, and crack resistance.

Advantages: Reduces shrinkage cracks, enhances durability, suitable for industrial floors.
Applications: Warehouses, factories, high-traffic areas where long-term performance is critical.

Benefits of Proper Screed Application

Perfectly Level Surface: Eliminates unevenness and facilitates installation of tiles, marble, or wooden flooring.

Load Distribution: Evenly distributes point loads to prevent damage to structural slabs.
Concealed Services: Electrical and plumbing installations remain hidden without affecting floor finish.
Crack Prevention: Proper mix and curing minimize shrinkage and surface cracks.

Durability: Screed acts as a protective layer, enhancing the lifespan of the floor.

Design and Mix Considerations

For civil engineers, correct design and mix ratios are critical:

Cement-to-sand ratio: 1:3 to 1:5 for traditional screed.
Water-cement ratio: Maintain low water content to ensure strength; avoid over-watering.

Thickness: 25 mm for light residential use, up to 50 mm for high-traffic or industrial areas.
Compaction: Screed must be properly compacted to eliminate voids and ensure bonding with the base slab.
Curing: Moist curing for at least 7 days to prevent shrinkage and achieve full strength.

Application Process

Step 1: Surface Preparation

Clean the structural base of dust, debris, and weak particles.
Damp the surface slightly to improve bonding with the screed.
Apply a bonding agent if necessary, especially on smooth concrete surfaces.

Step 2: Mixing the Screed

For sand-cement screed: Mix cement and sand in the correct ratio, adding water gradually.
Avoid overly wet mixes to prevent segregation and weak surfaces.

Step 3: Laying and Leveling

Pour the screed over the prepared floor.
Use a straight edge or screed board to level the surface.

Use a trowel to smooth the surface and remove excess water.

Step 4: Curing

Cover the screed with plastic sheets or wet burlap for moisture retention.
Keep moist for at least 7 days for optimal strength.

Avoid heavy traffic until the screed has fully cured.

Common Mistakes to Avoid

Using excess water – Leads to weak, porous screed.
Skipping curing – Causes cracks and uneven surfaces.
Uneven mix – Results in inconsistent strength and finish.
Ignoring substrate preparation – Weak bonding leads to delamination.

Practical Tips for Civil Engineers

For underfloor heating, use anhydrite screed for uniform heat transfer.
In industrial floors, polymer-modified screeds increase durability and crack resistance.
For large areas, consider mechanized mixing and leveling for uniformity.

Monitor ambient temperature during curing; extreme heat or cold can affect screed performance.

Screed is a critical component of floor construction, providing a durable, level, and smooth base for various flooring finishes. For civil engineers and construction professionals, understanding the types, mix ratios, application process, and curing methods is essential for achieving high-quality, long-lasting floors.

Investing in properly designed and executed screed ensures crack-free, aesthetically pleasing floors that withstand both daily use and structural loads.