Intermediate

Traditional Screed: Mix Ratios, Thickness and Installation

11 min read

The concrete slab is poured, the shell is progressing — and you’re wondering how to get a perfectly level floor to lay your tiles or timber. The answer is one word: traditional screed. This is the 4 to 6 cm layer of mortar that sits on top of the raw slab, corrects levels, and encases service pipes and cables. Master the mix ratio, the application and the curing, and you’ll have a flawless floor substrate — without calling in a specialist screeder.

CROSS-SECTION OF A TRADITIONAL SCREED ON SLAB Concrete slab (structural support) 12-15 cm 2-5 cm 4-6 cm 1-2 cm Concrete slab Acoustic insulation Mortar screed Floor finish 1 2 3 4 Pipes and conduits embedded in screed Electrical Plumbing Underfloor heating Resilient edge strip Interior of the house

What is traditional screed?

Traditional screed (also called mortar screed or cement screed) is a mixture of cement, sand and water, spread over a concrete slab to create a flat and even surface. It differs from liquid screed (anhydrite or cement) in that it is applied manually, using a mason’s straight edge.

Screed vs slab: don’t confuse the two

Characteristic Concrete slab Traditional screed
Thickness 12 to 20 cm 3 to 6 cm
Role Structural (bears loads) Levelling + encasing services
Composition Concrete (cement + gravel + sand) Mortar (cement + fine sand)
Reinforcement Yes (welded mesh) No (except special cases)
Build phase Shell Second fix

Tip — If you simply need to correct a surface with less than 1 cm of variation, a self-levelling compound will be more suitable than screed. Screed requires a minimum thickness of 3 cm to hold together properly.

The different types of screed

Before diving into mix ratios, identify which type of screed you need:

  • Levelling screed: encases service runs (electrical conduits, plumbing pipes, underfloor heating) laid on the slab. Thickness of 4 to 8 cm depending on the pipe diameter.
  • Bedding screed (or lean screed): serves as a direct substrate for tiles. Reduced cement content (250 kg/m³). Thickness of 3 to 5 cm.
  • Finishing screed: receives a flexible floor covering (floating timber, vinyl, carpet). Must be perfectly flat — tolerance ≤ 3 mm under a 2 m straight edge.
  • Floating screed: isolated from the slab by insulation (acoustic or thermal). Required on intermediate floors for acoustic comfort.

Question

flowchart TD A{Which type of screed?} -->|Services to encase?| B{Yes} A -->|No services| C{Which floor finish?} B -->|Thickness > 4 cm| D[LEVELLING SCREED] C -->|Tiles| E[LEAN SCREED
250 kg/m3] C -->|Timber, vinyl| F[FINISHING SCREED
300 kg/m3] A -->|Acoustic insulation?| G{Yes} G -->|Insulation below screed| H[FLOATING SCREED
unbonded] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style C fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style G fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style D fill:#56C6A9,stroke:#56C6A9,color:#fff style E fill:#F58220,stroke:#F58220,color:#fff style F fill:#6B5876,stroke:#6B5876,color:#fff style H fill:#CD212A,stroke:#CD212A,color:#fff

Traditional screed mix ratios

The mix ratio depends on the intended use. Here are the tried-and-tested recipes:

Standard mix (levelling or finishing screed)

Component Quantity per 1 m³ of mortar
Cement CEM II 32.5 300 to 350 kg (6 to 7 x 50 kg bags)
Sand 0/4 mm 900 to 1,000 litres (~1,400 kg)
Water 130 to 150 litres (adjust for sand moisture)

Lean screed mix (for tiling)

Component Quantity per 1 m³ of mortar
Cement CEM II 32.5 250 kg (5 x 50 kg bags)
Sand 0/4 mm 1,000 litres (~1,500 kg)
Water 120 to 130 litres

How to calculate your volume

The formula is straightforward:

Volume (m³) = Area (m²) × Thickness (m)

Example: for a 20 m² room with a 5 cm screed:

  • 20 × 0.05 = 1 m³ of mortar
  • That’s ~300 kg cement + ~1,400 kg sand

Warning — Always allow 10 to 15% extra volume to account for waste, slab irregularities and local build-ups. Better to have too much than too little: you cannot add to a screed once it has started to set.

TRADITIONAL SCREED MIX RATIOS — PER 1 m3 STANDARD SCREED Levelling / Finishing 6-7 50 kg bags CEMENT CEM II 32,5 900 litres (~1400 kg) SAND 0/4 ~ 140 L WATER Mix: 300-350 kg cement / m3 LEAN SCREED For tiling 5 50 kg bags CEMENT CEM II 32,5 1000 litres (~1500 kg) SAND 0/4 ~ 125 L WATER Mix: 250 kg cement / m3 CALCULATE THE REQUIRED VOLUME Volume (m3) = Area (m2) x Thickness (m) Example: 20 m2 room, 5 cm screed 20 x 0.05 = 1 m3 → 300 kg cement + 1400 kg sand (+10-15% margin)

Thickness: rules to follow

The screed thickness depends on its function and what it covers:

Situation Minimum thickness Recommended thickness
Screed on slab (no services) 3 cm 4–5 cm
Levelling screed (with services) Pipe diameter + 2 cm 5–8 cm
Floating screed on insulation 5 cm 5–6 cm
Screed over underfloor heating 3 cm above pipes 4–5 cm above pipes

Best practice — For a floating screed, the minimum thickness of 5 cm is required by BS EN 13813. Below that, the screed risks cracking under load. If you are encasing underfloor heating, measure the thickness above the pipes, not from the insulation.

Preparing the substrate

Before pouring, the substrate must be in perfect condition:

  1. Clean the slab: sweep, vacuum, remove all plaster, mortar or dust residues
  2. Check levels: use a laser level to identify the high and low points of the slab
  3. Lay the services: secure them to the floor with clips or adhesive mortar to prevent them floating up during the pour
  4. Dampen the slab: the evening before pouring, wet the slab thoroughly (without leaving puddles). A substrate that is too dry will draw moisture out of the mortar and cause cracking

Laying the perimeter edge strip

For a floating screed or to limit acoustic bridging, fit a resilient edge strip (polyethylene foam, 5 mm) along all walls, columns and door frames. The screed must not touch any vertical element.

Setting up the guides (level rails)

Guides are datum points that define the screed thickness. Two techniques:

  • Mortar dabs: piles of mortar set to the laser level, spaced 1.5 m to 2 m apart, on which you lay aluminium straight edges
  • Metal rails (e.g. plasterboard channel): fixed to dabs, they act as a guide for pulling the straight edge

Tip — Set your guides to the desired finished thickness minus the floor covering thickness. If you are laying 1 cm tiles with 1 cm adhesive, lower your guides by 2 cm relative to the desired finished level. This is the moment to think about alignment with door thresholds.

Step-by-step installation

Step 1: Mix the mortar

You can mix mortar in three ways:

Method Volume/hour Suitable for
Cement mixer (160 L) ~0.8 m³/h Small areas (< 20 m²)
Trough mixer ~1.5 m³/h Medium areas
Ready-mixed mortar by truck All at once Large areas (> 40 m²)

Target consistency: the mortar should be moist but not wet. Ball test: take a handful of mortar and squeeze it firmly. It should hold its shape without water seeping out. If water runs between your fingers, there is too much water.

Step 2: Pour and spread

  1. Pour the mortar between two guides, starting from the far end of the room (furthest from the door)
  2. Spread roughly with a shovel or rake
  3. Tamp lightly with your feet or a tamper (especially around service runs)

Step 3: Pull with the straight edge

This is the key step. Place your mason’s straight edge (aluminium, 2 to 3 m) on the two guides and pull it towards you with a lateral zigzag motion. The straight edge strikes off the mortar to the level of the guides.

  • Work in 30 to 50 cm strips
  • Fill any hollows as you go
  • Never go back over an area that has already been struck off and is beginning to set

Step 4: Float

Immediately after striking off (within 15 to 30 minutes), use a float to close the surface. This light finishing pass removes the small imperfections left by the straight edge.

Warning — Do not over-trowel if you are tiling: a slightly rough surface improves adhesive bond. On the other hand, for floating timber or vinyl flooring, the surface must be as smooth as possible — a mechanical power float may be needed.

Step 5: Remove the guides

Once a strip has been struck off and floated, remove the rails/straight edges by sliding them out gently. Fill the grooves left behind with fresh mortar and float again.

Conseil

Drying and curing the screed

Drying is a critical phase that is often rushed. A screed that dries too quickly will crack.

Drying times

  • Foot traffic: 24 to 48 hours
  • Laying tiles: minimum 7 days (BS EN 13813)
  • Laying timber / vinyl: 3 to 4 weeks (residual moisture content < 3%)
  • Commissioning underfloor heating: minimum 21 days before gradually raising temperature

Curing (maintaining moisture)

During the first 48 to 72 hours, the screed must remain moist for the cement to hydrate correctly:

  • In hot weather (> 25°C): mist with water regularly or cover the screed with a polythene sheet
  • In cold weather (< 5°C): do not pour. If you have no choice, cover with an insulating tarpaulin and heat the room
  • Avoid draughts (keep windows closed for the first 3 days)

Best practice — The rule of thumb for drying is 1 cm per week under normal conditions (20°C, 50% relative humidity). For a 5 cm screed, allow 5 weeks before laying any moisture-sensitive covering. A carbide bomb test gives an accurate reading of the actual moisture content.

Common mistakes to avoid

  1. Too much water in the mortar: the mortar is easier to work, but the screed cracks during drying and loses strength. This is the number one mistake.
  2. Insufficient thickness: below 3 cm, the screed delaminates or cracks. If you are short of headroom, consider a self-levelling compound rather than an over-thin screed.
  3. No perimeter edge strip: on a floating screed, the absence of debonding creates acoustic bridging and cracking at corners.
  4. Pouring in freezing conditions: frost prevents the cement from hydrating. The mortar crumbles to dust.
  5. Walking on it too soon: wait at least 24 hours. A footprint in fresh screed cannot be repaired without reworking the entire area.
  6. Forgetting movement joints: for areas > 40 m² or lengths > 8 m, a screed without joints will eventually crack.

Movement joints

Movement joints are required (BS EN 13813) in the following cases:

  • Screed area > 40 m²
  • Length > 8 m
  • Transition between two rooms (at doorways)
  • Change in floor covering

They are formed by cutting the screed to ⅓ of its thickness with an angle grinder within 24 to 48 hours of pouring, or by inserting PVC movement joint profiles before pouring.

Required tools and materials

Checklist: equipment for pouring a screed

  • Cement mixer or trough mixer (or ready-mixed delivery)
  • Aluminium straight edge, 2 or 3 m
  • Rotary laser level + receiver
  • Float and power float
  • Shovel, bucket, wheelbarrow
  • Guide rails or aluminium datum straight edges
  • Resilient perimeter edge strip (if floating screed)
  • Sand 0/4 mm + cement CEM II 32.5
  • Polythene sheet (for curing)
  • Water sprayer (for dampening the substrate)
  • Knee pads (you’ll be grateful for them)
  • PPE: gloves, goggles, safety boots