Bonded Screed vs Unbonded Screed: Which is Better for Your Project?

There are two main types of screeds used in construction today:

Bonded Screed

A bonded screed, also known as a bonded concrete screed, uses a bonding agent to adhere the screed mix directly to the concrete slab. The screed bonds tightly to the substrate, forming an integral part of the floor.

Unbonded Screed

An unbonded screed, also referred to as a floating screed, sits on top of a separating or damp proof membrane, completely independent from the substrate below. There is no bonding to the concrete slab.

Key Differences

Below we outline 5 key differences between bonded and unbonded screeds:

1. Speed of Installation

Bonded screeds take longer to install as the surface preparation is more extensive, and time must be allowed for the bonding agent to cure.

Unbonded screeds can be installed more quickly as less surface preparation is needed and curing times are faster.

2. Surface Regularity

Any variations or undulations in the concrete slab will be reflected in a bonded screed. The finished surface will match the slab below.

An unbonded screed acts as an independent layer “floating” over the substrate. A smooth regular surface can be achieved regardless of minor variations in the slab.

3. Cracking Resistance

Bonded screeds are more prone to cracking as stresses transfer through from the substrate below. Any movement or cracks in the slab risk propagating up through the bonded screed.

Unbonded screeds are more crack resistant given the “floating” action and isolation from the substrate. Cracks are less likely to reflect through the unbonded layer.

4. Sound Insulation

Unbonded screeds generally provide better sound insulation and acoustic performance given the isolated floating action over the slab. Impact sound waves have difficulty transferring through from the substrate to the finished layer.

Bonded screeds offer less sound insulation as there is a continuous bridge allowing impact sounds to travel from the slab through to the surface.

5. Leveling Compounds

With a bonded screed, the use of leveling compounds prior to installing floor coverings is not normally required as the screed itself acts as the wearing surface.

Unbonded screeds may require a compatible leveling compound to smooth any variations before floor coverings can be successfully installed.

Bonded Screed Applications

Bonded screeds are well suited to situations where:

• Regular foot traffic is expected e.g. residential homes
• Leveling of the substrate is not required
• Loads and impacts are low e.g. bedrooms
• Floor coverings with limited thickness can be used e.g. vinyl
• Fire resistance is important e.g. apartments

Common Residential Uses

Typical residential applications of bonded screeds include:

• Ground floor slabs
• Upper level floors
• Wet areas like bathrooms and laundries
• Bedrooms and living spaces
• Under vinyl, carpet, or tile finishes

Unbonded Screed Applications

Unbonded screeds work well where:

• Heavy loading or rolling loads are expected e.g. factories
• Vibration resistance is needed e.g. gymnasiums
• Leveling over poor substrates is required
• Acoustic performance is important e.g. home theatres
• Limited construction heights are available

Common Commercial Uses

Some common commercial uses for unbonded screeds include:

• Retail shops and malls
• Office buildings and towers
• Hospital and aged care facilities
• Schools and education facilities
• Apartment buildings and hotels
• Warehouse and factory floors

Screed Binders

The binder used in the screed mix impacts strength and performance properties.

Bonded screeds typically use cement-based binders.

Unbonded screeds can use cement or gypsum binders depending on loading requirements.

Cement Binders

Cement binders produce high strength screeds capable of withstanding heavy loads. A concrete screed with cement binders is suitable for both bonded and unbonded applications.

Gypsum Binders

Gypsum binders provide faster curing, better sound insulation, and a smoother finish. Gypsum screeds are well suited to unbonded screeds under lighter loads.

Screed Reinforcement

Screeds often incorporate reinforcing elements to enhance strength, control cracking, and improve impact resistance:

Steel Mesh

Welded steel wire mesh reinforcement is commonly used in cement-based bonded screeds over large areas to help control shrinkage cracks.

Fibers

Short discrete fibers help manage plastic cracking in unbonded screeds. Synthetic macro-fibers provide the best results.

Underfloor Heating

Both bonded and unbonded screeds can effectively incorporate underfloor heating systems:

• Electric underfloor systems can be cast within either screed type
• Hydronic piping suits sand & cement bonded screeds on ground slabs
• Foil heating elements easily fit under unbonded gypsum screeds over floors

Proper commissioning ensures the screed cures safely around heating pipes or wires.

Installation Best Practices

Some key best practices apply when installing either bonded or unbonded screeds:

• Ensure adequate concrete strength before applying bonded screeds
• Fully cure substrates as per manufacturer instructions
• Use proper mixing equipment and procedures for consistent results
• Employ recommended jointing approaches for crack control
• Allow proper curing times before loading or applying finishes

Following manufacturer guidelines helps achieve successful long-term screed performance.

The Future of Screeds

Advances in materials and technology aim to enhance screeds in the future:

Self-Leveling Properties

Self-leveling screeds that flow out and smooth without needing compaction or trowels could become more prevalent. These help achieve flatter floors faster.

Composite Materials

Hybrid screeds utilizing the best properties of cement, gypsum, and polymers might take over from traditional mixes. Lighter materials could also gain traction.

Smart Automated Installation

Robotic screeding machines and IoT sensors could help automate and optimize screed installation, saving time and costs while improving quality.

Conclusion

In summary, bonded screeds interlock tightly with the slab below while unbonded screeds float over substrates. Both serve an important role depending on expected floor usage and performance requirements. Material choice, reinforcement, underfloor heating integration, best practice installation, and emerging technologies all influence modern screeds. When selected and constructed appropriately, screeds provide an efficient way to create smooth, durable surfaces for long-lasting floor coverings. Assessing your project needs is key when deciding between using a bonded vs unbonded screed.

FAQs

What are the pros and cons of bonded screed?

Pros: Bonds tightly to substrate, reflects fewer cracks from below, suitable for thinner floor coverings, good fire resistance.

Cons: Slower to install, transfers slab imperfections to surface, prone to cracking from substrate movement, less sound insulation.

What are the pros and cons of unbonded screed?

Pros: Faster installation, achieves flat regular surface, resists cracking from substrate, provides better acoustics and soundproofing.

Cons: Requires compatibility with substrate, may need leveling compound, not suitable under very heavy loads, limited fire resistance.

Can underfloor heating be used with either screed type?

Yes. Both bonded and unbonded screeds are compatible with hydronic heating pipes or electric wire/foil systems. Proper commissioning ensures the heating integrates safely.

What thickness of screed is typical?

Common thicknesses are 35-50mm for bonded screeds and 40-60mm for unbonded. Thinner sections may be used over heating pipes or insulation. Local standards specify exact minimum depths.

How long before floor finishes can be installed onto screeds?

Curing times vary by screed binder from around 7 days for cement-based to 48 hours with quick-set gypsum. Moisture testing ensures finishes can be successfully installed without issues.