Flow Testing Liquid Screed: Achieving the Perfect Pour

Posted on: 25th September 2021

Contents
Last updated: July 2026
In brief: A flow test or slump test tells you whether liquid screed has the right consistency before you commit to a pour. It is done using a stainless steel flow cone placed on a flow plate: you fill the cone with screed, lift it, and measure how far the material spreads. Screed that is too wet will allow aggregates to settle unevenly; screed that is too dry will not flow or compact properly. Testing at the production plant and again on site before the pour starts is the standard approach, and it is the easiest way to catch a bad batch before it becomes a bad floor.

What Is A Flow Test?

Flow testing of liquid screed is carried out to determine the flowability / slump of the screed. To do this, a stainless-steel flow cone (AKA slump cone) is placed on the clean dry stainless-steel flow plate (AKA slump plate).

The cone is then filled with screed. Once full, the cone is removed, and the spread / slump is measured to determine the flowability of the liquid screed before it’s poured.

Worker pouring screed into the Metex Stainless Steel Liquid Screed Flow Cone on top of a flow plate ready for slump testing

There are various types of screed applications; bonded screeds, unbonded screeds, floating screeds and underfloor heating screeds.

  • A bonded screed is simply a screed that is bonded to a substrate. This would usually be to a concrete slab or a beam and block floor.
  • An unbonded screed is, exactly as its name suggests, one that is not bonded to its substrate.
  • Floating screed simply means that the screed is overlaid on something that is compressible and not rigid.
  • Underfloor heating screed is essentially screed laid over your underfloor heating system.

Why Should You Carry Out a Flow Test on Site?

Flow tests should always be carried out at the production plant first and then most importantly on site using a flow kit before starting the pour. This is very important, as it makes sure the screed has the correct consistency and is workable. The main benefit of this is less chance of drying shrinkage or any related problems.

Screed is non-workable when:

  • The mixture is too wet – this allows coarse aggregates to settle at the bottom and results in the screed becoming non-uniform in its composition.
  • The mixture is too dry – this will result in the screed being difficult to handle and place correctly in position.

Testing for flowability ensures that quality is never compromised and guarantees a high performance, free flowing, self-compacting liquid screed.

Metex Flow Cones & Flow Plates are available separately or as part of a ‘Liquid Screed Flow Test Kit’. For more information, check out our other guides! To purchase Metex screed levelling tools, click here or get in touch!

 

Frequently Asked Questions

A flow test measures the flowability of liquid screed before it is poured, to confirm the mix has the right consistency for the job. It is carried out using a stainless steel flow cone placed on a flat flow plate. The cone is filled with screed, lifted, and the diameter of the spread is measured. The result tells you whether the screed is workable or whether the mix is too wet or too dry to perform correctly once it is in the floor.

A flow test should be carried out at the production plant and again on site before the pour begins, because the consistency of the screed can change during transit. Catching a problem before the pour starts is straightforward to fix. Catching it after the screed is down is not. A mix that is too wet allows aggregates to settle unevenly, producing a floor with variable strength and composition. A mix that is too dry will not flow properly into corners and around obstacles, leaving voids and weak spots.

The target spread diameter varies by screed type and manufacturer, but most calcium sulphate liquid screeds are designed to achieve a flow of between 220mm and 260mm when tested with a standard flow cone. Always check the specific guidance from the screed supplier for the product being used, as the acceptable range can vary and going significantly outside it in either direction is a sign the mix needs to be adjusted before pouring.

The test requires a stainless steel flow cone, a stainless steel flow plate, and a tape measure or rule to measure the spread. Metex supply these as individual items or as a complete liquid screed flow test kit. Stainless steel is the right material for both components as it is easy to clean between tests and does not react with the alkaline environment of fresh screed in the way that other metals can.

Liquid screed can be laid as a bonded screed, which is adhered directly to a concrete substrate; an unbonded screed, which sits on a separating membrane; a floating screed, which is laid over a compressible layer such as insulation; or an underfloor heating screed, which encapsulates the heating pipes within the body of the screed. Each application has slightly different requirements in terms of minimum depth and preparation, so it is important to confirm which type applies to the job before specifying the product.

A flow test should be carried out at the start of every pour, using the first batch of screed delivered to site. On larger pours or where multiple deliveries are involved, it is good practice to test each new load before it goes into the pump. This is particularly important if there is any change in the appearance or behaviour of the material between deliveries, as it can indicate a variation in the mix that a flow test will quickly confirm or rule out.

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