All about Screed
A quality screed is the foundation of any stable floor. In this article, we´ll explore key insights about screed, including its uses, and the correct methods for installation.
- Zoya Glushtrom
Table of Contents
What Types of Screed Are There?
Screed is used in construction as a leveling and filling material to cover uneven surfaces like floor slabs and ceilings. It´s most applied in liquid form (flowing screed) rather than as dry screed, as the liquid distributes itself evenly to create a level surface. Different types of screeds are tailored for various applications.
Cement Screed
Cement screed is a mixture of sand and cement in a ratio of approximately 3:1. It´s highly durable, resistant to temperature fluctuations, and water-resistant, making it ideal for humid or warm environments. Once professionally dried, cement screed becomes exceptionally solid and can withstand heavy loads.
Calcium Sulfate Screed
Unlike cement screed, calcium sulfate screed doesn´t contain cement. Instead, it uses sand mixed with limestone chippings for strength and calcium sulfate or similar materials as a binder. This type of screed dries quickly and with minimal tension but is unsuitable for permanently damp environments, as it can absorb water and swell. While it can´t bear as much weight as cement screed, it´s an excellent heat conductor and heat retainer, making it ideal for specific applications.
Where Is Screed Used?
Because of the different properties of cement and calcium sulfate screed, they are suited for different purposes. Screed can be used as bonded screed, unbonded screed, floating screed, or heated screed.
Bonded Screed
In spaces like basements or garages, where insulation and aesthetics are less critical, screed is often applied directly to the supporting floor and left exposed. This is known as bonded screed, and cement screed is typically used for this application.
Unbonded Screed
If the subfloor is unsuitable for direct screed application, a separating layer can be used. The screed acts as a functional base or can be covered with decorative flooring. Both calcium sulfate and cement screeds are used for this purpose, depending on the room´s requirements.
Floating Screed
In rooms requiring sound or thermal insulation, a layer of insulation is added to the floor before applying the screed. Screed poured over this insulating layer has no direct contact with walls or structural elements, making it “float”. Cement screed is ideal for this type, as it also serves as a load distribution layer over the insulation.
Heated Screed
When a floating screed is combined with underfloor heating, it is referred to as heated screed. Thanks to its thermal retention properties, calcium sulfate screed is commonly used in this application. The screed is poured around and over the heating pipes. However, specific drying requirements must be met when dealing with heated screed, as outlined below.
How to Apply Screed?
Applying screed is a complex task best left to professionals. The process starts with mixing dry mortar with water, which is then spread across the floor using a pump or pouring hose. Proper drying is crucial to prevent issues like swelling, which can render the screed unusable.
Drying Time
The drying time for screed depends on several factors, including the type of screed, its thickness, and environmental conditions like temperatures and humidity. Flowing screed, in particular, requires longer drying times. A general rule of thumb is:
Thickness of screed layer2 x 1.6 = drying time in days
For example, if your screed is 4 cm thick, the drying time is approximately 26 days.
Here are some additional calculation examples for the drying time of flowing screed:
- Screed thickness = 6 cm: Drying time = approx. 8 weeks
- Screed thickness = 7 cm: Drying time = approx. 11 weeks
- Screed thickness = 8 cm: Drying time = approx. 14–15 weeks
However, walkability and readiness for covering are achieved earlier:
- For calcium sulfate screed: Walkable: after 1–2 days; Ready for covering: after 2–3 weeks
- For cement screed: Walkable: after 2–3 days; Ready for covering: after 3–4 weeks
To accelerate the process, heating the space can help. If the heating system isn´t operational, renting mobile heaters or fan heaters is a practical alternative. Regular ventilation is also essential to facilitate drying.
How Is Screed Heated?
If screed needs to be dried over underfloor heating, it is important to additionally ensure that the screed is properly heated. According to a specific heating protocol, craftsmen must gradually heat the screed using the underfloor heating system over several days. This process helps prevent cracks caused by temperature fluctuations later on. A distinction is made between functional heating and ready-for-laying heating.
After an initial drying period, the first heating phase begins. This involves running the underfloor heating at maximum output for several days. Proper ventilation during this phase is crucial to remove moisture released by the heating process. Once functional heating is complete (after about four days), the second phase begins.
After about four days, the functional heating phase is completed and the ready-for-laying heating begins. This phase continues until only the required residual moisture remains. During this period, the heating system should not be turned off, and the flow temperature must remain constant.
For cement screed, the permissible residual moisture is 1.8%; for calcium sulfate screed, it is 0.3%.
Learn more about our screed heating services.
How often Should I Ventilate when Heating Screed?
To remove moisture and support the drying process, it is best to air out the rooms three times a day using shock ventilation. To do this, simply open all available windows and doors for 10 to 15 minutes.
What Happens if Screed Is not Heated?
When installed over underfloor heating, screed must withstand significant temperature fluctuations and must not develop warping, cracks, or internal stress. Failure to properly heat the screed can lead to excessive strain on the underfloor heating system and result in significantly increased heating costs.
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