Calculating screed: volume, weight and material needs
Cement, anhydrite and liquid screed – the key figures for building projects
The screed calculation starts with the volume: m³ = area (m²) × thickness (m). For 20 m² at 5 cm thickness, that comes to 1.0 m³. With the bulk density of cement screed (approx. 2000 kg/m³), the total weight follows: 1.0 m³ × 2000 kg/m³ = 2000 kg. Split across 40 kg bags, that is 50 bags. This simple chain is the basis of every screed calculation – whether for bagged material or ready-mix delivery.
Screed types and their properties: Cement screed (CT) is the most common type in Germany – affordable, robust and suitable for almost all applications. Anhydrite screed (CA), based on gypsum, dries faster but is more moisture-sensitive and not suitable for wet rooms. Liquid screed is poured as a self-levelling flow screed – it has lower internal stress and is therefore ideal for large underfloor-heating areas. Magnesite screed is rarely installed anymore, but is still found in the renovation of older buildings.
Standard thicknesses per DIN 18560: The German standard distinguishes between load classes. In residential areas on an insulation layer, cement screed requires a minimum thickness of 45 mm. With underfloor heating, the covering increases to at least 45 mm above the top of the pipe, which results in a total thickness of approx. 60–65 mm for 15 mm pipes. The thickness also determines the curing time: cement screed needs roughly one day per mm of thickness.
Structural load during installation: Screed is heavy – 2000 kg per m³. On a residential floor with 80 m² of area and 6 cm of screed, that adds up to almost 9.6 tonnes of extra load. When renovating old timber-beam ceilings or historic building structures, a structural engineer must check the load-bearing capacity before screed is poured. Lightweight screed (e.g. with expanded clay added) can reduce the bulk density to 1200–1600 kg/m³ – at the cost of compressive strength.