What is Screed?
Screed is a material used to provide a smooth, level surface before laying floor finishes such as tile, vinyl, or carpet. It is applied over concrete or wood subfloors to fill in any uneven areas, creating a flat base. There are two main types of screed, traditional screed and liquid screed.
Traditional Screed
Traditional screed is a mix of sand and cement, also known as mortar screed. It has been used in construction for many decades to prepare solid and stable subfloors. To make traditional screed, a semi-dry mix is prepared using a ratio of 3 parts sharp sand to 1 part cement, plus water to achieve the required consistency.
Liquid Screed
Liquid screed, also referred to as self-smoothing screed or flowable screed, has gained popularity in recent years. As the name suggests, it has an almost self-leveling consistency. Liquid screed contains cement and water, with added polymers and flow agents. Being softer and more workable than traditional screed, liquid screed can spread easily across subfloors and does not require compacting like traditional screed.
Key Differences Between Liquid and Traditional Screed
There are some notable differences between liquid and traditional sand/cement screeds:
Installation Process
The installation process differs significantly between the two screed types. Traditional screed is a thicker mix which needs to be laid down in sections then leveled out and compacted using techniques like tamping. Liquid screed can simply be poured or pumped onto the subfloor and due to its fluid texture, requires minimal intervention to form an even surface as it dries.
Drying Times
Liquid screeds dry faster than traditional screeds. An anhydrite liquid screed dries at a rate of 1mm per day, allowing floor coverings to be installed after just 7-14 days. On the other hand, traditional cement/sand screeds need at least 21 days to dry out sufficiently before other floor finishes can be applied. Fast-drying chemicals can shorten the wait for traditional screeds.
Thicknesses
Due to differences in composition and installation methods, liquid and traditional screeds have varying thickness requirements. Liquid screeds provide a smooth floor from just 30-40mm thickness. Traditional screed needs to be laid at a minimum of 40mm over concrete or 75mm over floorboards. Areas with underfloor heating call for particular screed thickness specifications too.
Cracks and Shrinkage
Shrinking and cracking as the screed dries is less likely to be an issue with modern liquid screeds compared to traditional screed mixes. Added polymers improve flexural strength and resistance to cracking. However, for larger areas traditional screed may require carefully planned bay divisions to minimize cracking, whereas liquid screeds can comfortably cover bigger uninterrupted floor spaces.
Cost Difference
Liquid screeds used to demands a higher initial investment cost than traditional screeds, but with modern liquid screed technology becoming more mainstream, costs have balanced out in many cases. The fast installation process of liquid screeds also reduces labor time significantly compared to traditional screed application and finishing. Overall cost depends largely on which exact products are used, the screed thickness, and the floor area size.
Advantages of Liquid Screed
There are several major advantages that make liquid screed an appealing choice:
Minimal Subfloor Preparation
With liquid screed being self-smoothing to fill uneven areas, extensive raking or tamping of the base concrete or insulation is not required. Just a quick sweep to remove debris is sufficient in most cases.
Ease of Installation
Pumping liquid screed onto a floor and allowing it to spread and level removes the strenuous work involved with laying and finishing traditional screed. No compaction or troweling is necessary.
Reduced Drying Times
The typical 1mm per day drying rate allows liquid screed floors to be walked on after 24 hours. Floor coverings can then be installed in just 7-28 days, compared to over 3 weeks for traditional cement and sand screeds. This avoids delays.
Fewer Joints and Cracks
With liquid screed able to be laid in large bays without planned joints, and the screed mix having improved resistance to shrinkage and cracking, the result is a seamless floor without weak joints for cracks to form at.
Underfloor Heating Compatibility
Liquid screed bonds well over underfloor heating pipes and conduction plates. Its faster drying allows heating systems to be turned on 21 days after installation to complete drying. It also quickly reacts to heating adjustments due to good thermal conductivity.
Lower Environmental Impact
Specialized liquid screeds use recycled materials and green technology to reduce environmental footprint. Anhydrite liquid screeds for example use industrial by products for a large percentage of the content.
Disadvantages of Liquid Screed
Despite its advantages, liquid screed also comes with a few disadvantages:
Higher Initial Cost
In some cases, the materials and installation costs of liquid screed may exceed traditional sand and cement screeds. However the growing popularity of liquid screeds is driving prices down.
Specialized Installers Required
While DIY application is possible for traditional screeds, professional installers with pumping equipment are recommended for quality liquid screed installation. This can add labor costs.
Susceptible to Damage When Wet
It’s important to avoid heavy loads rolling across the screed surface until fully cured, as liquid screed remains soft and impressionable during the drying phase.
Limited Thicknesses
Due to application methods, liquid screeds have more narrow thickness specifications generally between 30mm and 80mm. Traditional screed provides wider thickness options.
Not Suitable With Some Heat Sources
The chemistry of certain liquid screeds makes them incompatible with underfloor heating systems using high temperature heat sources like wood burning stoves.
Key Considerations When Choosing Screed Type
With each screed option having pros and cons, the right choice depends on factors like:
- Floor size: For large open spaces, liquid screed may suit better
- Subfloor type and condition: Liquid screed requires less prep for uneven bases
- Underfloor heating plans: Ensure system compatibility
- Drying time requirements: Liquid screeds dry much faster
- Intended floor coverings: Some coverings have screed specifications
- Environmental impact: Some liquid screeds are more eco-friendly
- Odor: Certain liquid screeds have stronger drying odors
Getting professional advice for the specific project, floor construction, and floor coverings is recommended to determine which screed solution suits best.
Conclusion
When preparing subfloors for the installation of decorative floor coverings, both traditional sand/cement screed and liquid floor screed offer practical solutions. Liquid screed is emerging as a time and labor saving option its fluid texture allows faster spreading without compacting, it dries quickly ready for floor finishes, and has good resistance to cracking. However, traditional screeds still have the advantage of lower cost in some situations. By weighing up factors like project size, construction, heating plans, and environmental impact for each unique case, the optimal screed choice can be made. With rapid innovations improving liquid screed technology and accessibility, its growing popularity looks set to continue.
FAQs
How thick should I make my liquid floor screed?
A typical thickness for liquid screed is between 30 and 40mm. Up to 80mm is possible in some applications. Always check manufacturer guidelines.
Can liquid screed be laid over underfloor heating?
Yes, most modern liquid screeds are compatible with underfloor heating systems. Their thermal conductivity and drying rates allow heating to be used efficiently.
Is liquid floor screed waterproof?
Liquid screeds are generally not waterproof membranes, but they can be used as part of a system with a damp proof membrane underneath to create a water resistant floor build up.
How long does liquid screed take to go off?
Initial set typically occurs within 3-4 hours after installation. Final drying times depend