If you've been specifying floors for a while, you've probably noticed calcium sulphate screed coming up more often in project conversations - particularly on commercial builds, multi-residential developments, and anything with underfloor heating. But for many contractors and specifiers in New Zealand, it's still a product that raises questions. How does it differ from a standard cement-based screed? When does it make sense to use it?
This guide covers the essentials.
What is Calcium Sulphate Screed?
Calcium sulphate screed - also called anhydrite screed or gypsum screed - is a flowing, pumpable floor screed that uses calcium sulphate as its binder, rather than the Portland cement used in traditional sand and cement screeds.
We supply it as pre-batched, dry powder that's mixed with water on site with a water controlled screed mixing pump. The result is a highly workable, self-levelling material that flows into place across large floor areas with minimal manual spreading.
The product has been widely used throughout Europe for decades and is increasingly specified on New Zealand projects where performance requirements go beyond what a standard cement screed can comfortably deliver.
How Does It Differ from Cement-Based Screed?
The key differences come down to chemistry, flow behaviour, and performance characteristics:
Binder type. Cement screeds use Portland cement, which hydrates to form calcium silicate hydrates. Calcium sulphate screeds use anhydrite (calcium sulphate), which reacts with water to form a dense, crystalline gypsum matrix.
Workability. Because calcium sulphate screed is self-levelling, it flows under its own weight to find a flat, even surface. This makes it significantly faster to install over large areas compared to traditional screeds that need to be laid, tamped, and floated by hand.
Drying shrinkage. Cement-based screeds shrink as they cure, which can cause curling at edges and cracking - particularly on thinner applications. Calcium sulphate screed has very low drying shrinkage, which reduces the risk of cracking and curling and can eliminate the need for movement joints on many projects.
Thickness. Calcium sulphate screed can be laid at thinner depths than equivalent cement screeds - typically 30–40mm over underfloor heating systems, compared to 65–75mm for a traditional cement screed over the same setup. This is a significant advantage where floor build-up height is limited.
Thermal conductivity. The denser, more homogeneous structure of calcium sulphate screed makes it an excellent conductor of heat - which is why it's the preferred choice for underfloor heating applications.
When Should You Specify Calcium Sulphate Screed?
Not every project needs it, but there are situations where it's clearly the better option:
Underfloor Heating Systems
This is the primary application where calcium sulphate screed excels. Its superior thermal mass and conductivity mean heat transfers from the pipes more efficiently and distributes more evenly across the floor surface. The reduced screed depth also means the system responds faster to thermostat changes. If you're specifying UFH, calcium sulphate screed should be your default consideration.
Large, Open Floor Areas
Flowing screeds can be pumped and installed quickly across large footprints - commercial floors, warehouse mezzanines, open-plan apartments - with a smaller crew and in less time than an equivalent sand and cement screed. On a big slab, the time and labour savings can be significant.
Tight Floor Build-Up Tolerances
Where you're working to strict floor-to-ceiling heights, or where the build-up between structural slab and finished floor level needs to be minimised, the thinner application depth of calcium sulphate screed gives you more flexibility.
Projects Requiring a Flat, Crack-Resistant Surface
The self-levelling nature of the product and its low shrinkage characteristics make it well-suited to applications where surface flatness is critical - under large-format tiles, LVT, or hardwood floating floors, for example.
New Build Multi-Residential and Commercial Projects
Calcium sulphate screed is commonly specified on multi-storey residential and commercial builds where consistent quality, fast installation, and a predictable finish are valued. It's particularly suited to projects using suspended concrete floor slabs.
What Are the Limitations?
Calcium sulphate screed isn't appropriate for every situation. There are a few important constraints to be aware of:
Moisture sensitivity. Anhydrite-based screeds are not suitable for areas that will be permanently wet - bathrooms, laundries, or external areas. If moisture penetrates and the screed rewets in service, the binder can rehydrate and cause the screed to soften and fail. It must always be protected by appropriate floor finishes or waterproofing membranes.
Drying time. While calcium sulphate screed can be walked on within 24–48 hours, it requires thorough drying before floor finishes are applied - typically 1mm of screed depth per day under good drying conditions. Screeds over underfloor heating should be cured on a low-temperature commissioning programme before the system is run at full temperature.
Surface laitance. A surface laitance layer forms as the screed dries, which must be removed by sanding or grinding before floor finishes are applied. This is a standard step and adds minimal time, but it needs to be accounted for in the programme.
Cement-based adhesives. Some cement-based adhesives aren't compatible with calcium sulphate screeds. Always check adhesive compatibility with your screed supplier before specifying floor finish systems.
The EzyMix Calcium Sulphate Screed
EzyMix's calcium sulphate screed is a pre-batched product designed for the New Zealand market - easy to pump, consistent to mix, and formulated to deliver the flat, high-quality finish that professional flooring applications demand.
It's suitable for use over underfloor heating, on suspended slabs, and across large commercial and residential floor areas where a fast, reliable, crack-resistant screed is required.
Want to know if calcium sulphate screed is right for your project? Contact the EzyMix team - we can talk through your specification requirements and help you choose the right product.
