# Screed: The Layer That Decides Whether the Floor Finish Works

> A screed is the layer laid over a structural slab to produce a level, sound surface for the floor finish, and to carry services or insulation where required. It is hidden once the finish goes down, but its flatness, strength and dryness largely determine whether that finish performs or fails.

## Why it matters on a project

Nobody sees the screed. It is also the layer that decides whether the floor
above it succeeds.

Most flooring failures — resin debonding, vinyl showing every ridge beneath,
large tiles rocking, joints opening — are not failures of the finish. They are
failures of the layer underneath, discovered after the finish is laid, when the
remedy means taking it up again.

## What a screed is asked to do

| Function | Consequence if it is not achieved |
| --- | --- |
| Provide a level, flat surface | Thin finishes telegraph; tiles lip and rock |
| Provide a sound, strong surface | Finish debonds or cracks under load |
| Be dry enough for the finish | Adhesive fails, resin lifts, vinyl blisters |
| Carry services where required | Insufficient cover, or an unrecorded hazard |
| Accommodate movement | Cracks appear through the finish above |
| Provide falls where needed | Water stands in wet areas |

## The two that cause most failures

**Moisture.** Concrete and screeds release moisture for a long time. Seal an
impermeable finish over a substrate that is still drying and the moisture has
nowhere to go but into the bond. The control is to **measure** — with a
recognised method such as **ASTM F1869** — against the flooring system's stated
requirement, not to judge by appearance or by counting days.

**Flatness.** Tolerance requirements differ enormously by finish. A carpet floor
and a large-format tile floor are not asking the same thing of the substrate,
and the preparation cost differs accordingly. Agreeing the required tolerance
before the screed is laid is far cheaper than correcting it afterwards with
levelling compound.

Screed material properties are covered by **EN 13813**, with concrete practice
in **IS 456:2000**.

## The programme point

Drying is a duration, not an activity someone can accelerate by working harder.
On a tight fit-out it is one of the few genuinely fixed constraints.

That makes it worth deciding early: either allow the drying time in the
programme, or specify a moisture-tolerant system or barrier from the outset.
Discovering the conflict two weeks before handover leaves neither option
available.

## Common mistakes

- **Laying the finish over an untested substrate.** The leading cause of floor
  failure.
- **Not agreeing a flatness tolerance before screeding.** The finish then
  dictates a correction nobody priced.
- **Treating levelling compound as a screed.** It corrects flatness, not level,
  falls or strength.
- **Ignoring movement joints.** They must carry through the screed and the
  finish.
- **Embedding services without recording them.** Nobody can drill the floor
  safely afterwards.
- **Assuming all finishes need the same preparation.** They do not, and the
  difference is a real cost.

## Standards referenced

Screed material properties and requirements in **EN 13813**; concrete practice
in **IS 456:2000**; substrate moisture measurement to **ASTM F1869**. Screed
type, thickness, tolerance and acceptable moisture levels for a specific floor
must be established by the project's consultant and the flooring specialist.

## Frequently asked questions

### What is the difference between bonded and unbonded screed?

A bonded screed is laid onto a prepared slab and adheres to it, so it can be relatively thin and relies on the slab for strength. An unbonded screed sits on a separating layer and must be thick enough to work on its own. A floating screed sits on insulation or an acoustic layer and must be thicker still. Which is appropriate depends on what is beneath and what the floor must do, and the required thickness follows from the choice.

### Why does drying time matter so much?

Because most floor finishes trap whatever moisture remains. A slab or screed that appears dry can still be releasing moisture from within, and once an impermeable finish seals it in, the vapour pressure builds beneath and can debond adhesives or lift the covering. This is the leading cause of resin and vinyl floor failure, and it is a programme problem as much as a technical one — drying takes the time it takes, and it cannot be compressed by wanting the date.

### How is dryness established?

By measurement, not by appearance or by elapsed days. Recognised methods such as ASTM F1869 measure the moisture vapour emission rate from the substrate. The acceptable level depends on the finish and the adhesive being used, so the flooring specialist's requirement governs. Where the programme cannot allow full drying, a moisture-barrier system is the alternative — specified deliberately, not improvised.

### What does flatness have to do with the finish?

Thin finishes telegraph everything beneath them. Vinyl and resin show trowel ridges, joints and level changes; large-format tiles rock or lip at their edges if the surface deviates. Carpet is far more forgiving. This is why the preparation cost differs so much between finishes, and why comparing floor finishes on material rate alone is misleading.

### What is a self-levelling compound?

A flowable material applied in a thin layer over a screed or slab to correct minor deviation and produce a smooth surface for a thin finish. It corrects flatness, not level or falls, and it is not a substitute for a properly laid screed. Used to fix a badly out-of-tolerance floor, it becomes expensive quickly because of the volume required.

### Can services run in a screed?

Commonly yes — conduit, small pipework and underfloor heating are often embedded. The consequence is that the screed must be thick enough to give adequate cover above them, and the service locations must be recorded, because a screed with concealed services is a floor nobody can later drill into safely without a record.

## Sources

- [IS 456:2000](https://standards.bis.gov.in/website/published-standards/department-wise) — Plain and reinforced concrete — code of practice
- [EN 13813](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Screed material and floor screeds — properties and requirements
- [ASTM F1869](https://www.astm.org/products-services/standards-and-publications.html) — Measuring moisture vapour emission rate of concrete subfloor

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Source: https://hagerstone.com/glossary/screed