Screeding a floor requires careful consideration of the thickness to ensure a strong, level surface that can support the flooring material. The thickness of the screed layer will depend on several factors, including the type of screed, the purpose of the floor, and any underlying heating systems. Here’s a comprehensive guide on how thick floor screed should be.
What is Screed?
Screed is a mixture of sand, cement, and sometimes additives, used to create a smooth and level base for flooring. It is typically applied over concrete subfloors or other uneven surfaces. The right thickness is crucial to ensure durability, load-bearing capacity, and a professional finish.
How Thick Should Floor Screed Be?
The thickness of the screed will vary depending on the specific project, but here are some general guidelines based on different types of screed:
Traditional Sand and Cement Screed:
- Minimum thickness: 25mm (for bonded screeds)
- Typical thickness: 40–75mm (for unbonded and floating screeds)
- Recommended thickness:
- Bonded screed (where the screed is applied directly to the subfloor with adhesive or bonding agent): Minimum of 25mm to ensure adequate bonding.
- Unbonded screed (laid on top of a damp-proof membrane or separating layer): Typically 50mm or more to prevent cracking and ensure stability.
- Floating screed (used over insulation or underfloor heating systems): Usually 65–75mm for residential floors and 75–100mm for commercial or heavy-duty areas.
Flowing or Liquid Screed:
- Minimum thickness: 20mm (for bonded applications)
- Typical thickness: 35–50mm (for floating applications)
- Recommended thickness:
- Bonded flowing screed: Can be as thin as 20–25mm due to its self-compacting nature.
- Floating flowing screed: Generally 35–50mm is required, especially when laid over underfloor heating systems. This thickness ensures that the screed effectively encapsulates the heating pipes and allows for even heat distribution.
Self-Levelling Screed:
- Minimum thickness: 2mm–5mm
- Typical thickness: 5–30mm
- Recommended thickness:
- Self-levelling screed is used for fine levelling on top of existing screed layers or concrete. It is usually applied at a thickness of 5–10mm, though some compounds can be applied as thin as 2mm for minor surface adjustments.
- For deeper areas or larger imperfections, it can be built up in layers to around 30mm.
Factors When Deciding Screed Thickness
- Type of Flooring: The thickness of the screed will vary depending on the flooring you plan to lay. Heavier or more durable floors (such as tiles or stone) may require a thicker screed layer to provide additional support.
- Underfloor Heating: When laying underfloor heating systems, the screed thickness is critical. A floating screed of around 65mm is often recommended to ensure the pipes are fully covered and that the heat is distributed evenly across the floor.
- Load Bearing Capacity: For high-traffic areas or spaces where heavy loads will be present, a thicker screed is advisable. Commercial spaces often require screeds that are 75mm or more in thickness to handle additional wear and tear.
- Type of Subfloor: The nature of the subfloor (concrete, insulation, or membranes) will influence the thickness of the screed. For example, bonded screeds can be thinner because they adhere directly to a solid subfloor, while unbonded or floating screeds need more depth for stability.
Potential Problems with Incorrect Screed Thickness
- Cracking: If the screed is too thin, especially in unbonded or floating applications, it may crack under pressure or due to shrinkage during the drying process.
- Inadequate Support: A screed that is too thin may not provide enough support for the chosen flooring, leading to unevenness or damage over time.
- Longer Dring Times: A screed that is too thick may require longer drying times. As a general rule, traditional sand and cement screeds take approximately 1mm per day to dry up to 40mm thick, and 2 days per mm beyond that. Faster-drying screeds are available, but always factor in extra time for thicker screeds.
In Summary
The thickness of floor screed is an important factor that affects both the durability and functionality of the finished floor. A general guideline is to use at least 25mm for bonded screeds and a minimum of 40–75mm for unbonded or floating screeds. When using flowing or self-levelling screeds, the required thickness will depend on the application, typically ranging from 20mm to 50mm.
Always consult with a professional or follow the manufacturer’s guidelines when deciding on screed thickness to ensure the best results for your flooring project.
