What is the Purpose of Concrete Screeding?

Types of Screed Used in Construction

Four main screed types handle different situations. Picking the right one depends on what’s underneath, how thick it needs to be, and what the floor will be used for.

Bonded Screed

Bonded screed sticks directly to concrete using special bonding agents or cement slurry. The strong connection between layers prevents movement and creates one solid floor system.

This type works great for thin applications, usually 25 to 40 millimetres thick. The bonding means you don’t need extra thickness for strength. Perfect for rooms where floor height matters, like when you can’t raise the floor much without hitting door bottoms.

Key facts about bonded screed:

• Sticks directly to the concrete base
• Needs a bonding agent or primer
• Works at 25 to 40 millimetres thick
• Great for tight height restrictions
• Requires a very clean concrete surface
• Fast to install on prepared surfaces

The concrete must be spotlessly clean for this to work. Oil, dust, paint, or dirt stops bonding cold. Grinding or shot blasting creates the rough texture needed. Do this right, and bonded screed performs beautifully for decades.

Unbonded Screed

Unbonded screed sits on a plastic sheet or building paper instead of sticking to concrete. This separation layer stops moisture or chemicals from moving between the concrete and screed. The two layers can move independently without cracking.

This type needs at least 50 millimetres of thickness to be strong enough without concrete support. The screed has to hold itself up, so extra thickness gives it that strength.

When to use unbonded screed:

• Old concrete has oil stains or contamination
• Base concrete is painted or sealed
• Substrate moisture levels are too high
• You need a vapour barrier
• Sound dampening helps
• Surface preparation time is limited

The separation layer provides bonus benefits like moisture barriers and noise reduction. You skip the intense surface prep work, which speeds up projects. The tradeoff is thicker floors and more material cost.

Floating Screed

Floating screed rests on insulation or soundproofing instead of concrete. This setup separates the floor from the cold or noisy structure below. Underfloor heating almost always uses floating screed.

Insulation boards measuring 20 to 100 millimetres thick sit between concrete and screed. Screed thickness over insulation runs 65 to 75 millimetres minimum to prevent cracking. Wire mesh or fibres inside the screed spread the weight and control shrinkage.

Floating screed features:

• Sits on insulation boards
• Needs 65-75 millimetres minimum thickness
• Requires reinforcement like wire mesh
• Moves independently from the concrete below
• Needs expansion joints around room edges
• Perfect for underfloor heating systems

Floating screed moves separately from what’s underneath. Foam strips along walls let it expand when warm without pushing against walls or cracking. Proper spacing of these joints keeps everything working smoothly.

Energy savings make floating screed worth the extra work. Ground-floor rooms stay warmer. Upper floors don’t let heat escape downward. Acoustic versions cut noise between floors in apartments.

Self-Levelling or Flowing Screed

Self-levelling screed uses special formulas and plasticisers that make it flow like a thick liquid. Pump it onto floors and it spreads itself flat with minimal manual work. Gravity and surface tension do most of the levelling.

Speed is a huge advantage here. Large areas get covered in hours instead of days. The liquid flows around pipes and obstacles without hand labour. Less labour means lower costs despite pricier materials.

Benefits of self-levelling screed:

• Spreads and levels itself automatically
• Covers large areas incredibly fast
• Flows around pipes and cables easily
• Creates super flat surfaces
• Less physical labour is required
• Often dries faster than traditional screed

This type achieves better flatness than traditional methods. No trowel marks or surface variations. Perfect for high-end flooring where substrate quality shows through.

Minimum thickness varies by product. Some work reliably at 25 millimetres over heating pipes. Faster drying speeds up schedules, though proper curing still matters. Works in bonded, unbonded, and floating setups.

Step-by-Step Process for Screeding Concrete

Professional screeding follows a clear process where each step builds on the last. Skipping steps or rushing creates problems that show up after the flooring goes down.

1. Surface Preparation and Priming

Surface prep decides whether the screed sticks properly. Start by cleaning the concrete thoroughly:

• Power wash away dust and debris
• Grind off paint and coatings
• Degrease oily spots
• Create a rough texture like medium-grit sandpaper

Test moisture levels before starting. High readings mean waiting longer or using moisture-tolerant systems. Apply primer to bridge concrete and screed layers. Most primers need screed placed while still tacky.

2. Mixing the Screed

Traditional concrete mixes combine one part cement with three to five parts sharp sand plus measured water. Use mechanical mixers for uniform batches.

Getting the mix right:

• The mix should clump when squeezed
• Should crumble when released
• Too much water weakens screed
• Too little stops proper bonding
• Never add extra water to make it easier

3. Laying and Levelling

Spread material in sections called bays, usually 4 to 5 meters wide. Screed rails set the target height. Straight edges drag excess material forward while filling low spots. Multiple passes make it progressively flatter. Complete entire bays without stopping to prevent cold joints. If breaks happen, roughen edges and add primer before continuing.

4. Compaction and Finishing

Compaction packs screed tightly and remove air pockets. Workers use tamping beams or vibrating screeds to compress material thoroughly. Good compaction ensures the screed reaches the design strength.

Finishing varies by flooring type. Smooth trowel finishes work for thin tile adhesive. Light broom finishes provide texture for thick tile mortar. Check elevation often and fix problems immediately while the screed stays workable.

Getting this phase right determines whether your floor stays flat for decades. Pro-Mix Concrete’s skilled crews have leveled thousands of floors to exact specifications. The precision work ensures any flooring type performs exactly as intended.

5. Curing and Drying

Cover fresh screed with plastic sheeting for at least 3 days, preferably 7 days. This traps the moisture needed for proper hardening. Protect from sun, wind, and heating systems. Maintain temperatures between 10 and 25 degrees Celsius.

Drying takes roughly one day per millimetre of thickness. A 50 millimetre screed needs about 50 days before flooring installation. Test moisture levels before proceeding. Most floor coverings need a screed below 75 % relative humidity. Installing over damp screed causes adhesive failure and mould growth.