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Here’s why screeds fail

Padraic McGrath looks at why some screeds fail and, more importantly, how you can correct it when it happens.

AT first glance, floor screeds present themselves as a relatively simple part of the building, yet they’re an essential component, providing a durable and long-lasting foundation for the floor finish. All we ask of them is that they should provide a flat surface that’s suitable for a floor finish. Of course, in order to do this, we need them not to crack, curl or otherwise fail once the installation is completed.

When laid correctly, the screed is rarely given a second thought and work continues on the building. However, if improperly installed, it has the potential to compromise the integrity of the entire floor structure. So, why do some screeds fail and how do we avoid this?

Early-stage issues would include cracking, curling or just general low strength either of the entire screed or its surface. Reasons for this can include but aren’t limited to the following:

  • Problems with manufacturing of the screed
  • Problems with the screed itself, such as cracking, curling, or low strength

    Incorrect manufacture of the screed
    Incorrect manufacture of the screed can be either the wrong ‘mix design’, in that the constituent materials weren’t used in the correct proportion, or, that the screed was insufficiently mixed. It’s important to remember that one of the components of a screed mix is water and that this too needs to be used in the correct amount.

    The addition of too much water, which is a temptation since it can make materials easier to work with, can damage screeds by making them weaker and much more prone to shrinkage, cracking, and curling.

    Incorrect installation of the screed
    Incorrect installation of the screed would mean, essentially, not following the ‘rules’ for the type of screed that is being used. These rules would determine the condition that the building should be in and, at the least, would stipulate that it should be weatherproof. There would be specific requirements as to the maximum size of bays that could be installed without joints, minimum and sometimes maximum thickness, and even temperature limit values for installation.

    In the case of traditional, hand applied, screeds there would be a minimum density to be achieved when the material is compacted onto the floor and, while this is very difficult to achieve, it’s vital since a lower density from lack of compaction will greatly reduce strength.

    One of the bigger advantages of liquid applied screeds is that they require no physical compaction; as a liquid they’re delivered at the correct density.

    Once a screed is successfully installed, there are certain conditions that need to be followed. Screeds shouldn’t, at an early stage, be exposed to severe drying conditions such as draughts or direct sunlight. Rapid drying will greatly increase the risk of shrinkage and cracking. Newly installed screeds will not yet have gained their full strength, so while light foot traffic may be acceptable after 24/48 hours, care must be taken to reduce the risk of physical damage with heavy site traffic.

    In buildings where underfloor heating is present, it’s important the UFH system is commissioned before any floorcoverings are laid, to prevent the risk of cracking.

    Most, though not all, of these early-stage problems, if not extreme, can be repaired before floor finishes are needed, so while they are unwelcome, they are reparable. Later stage problems with screeds can be much more expensive and difficult as they may involve replacement of floor finishes or even fitted furniture and may require the building to be taken out of use for repairs to be completed.

    Very late-stage cracking, while not completely impossible, is rare. Most problems that occur after floor finishes are installed, or when the building is in use, affect the floor finish rather than the screed itself. Of these, by far the most common are that either the floor surface wasn’t suitable for the floor finish or that the screed wasn’t sufficiently dry before floor finishes were installed. In either case this represents a lack of checking/testing of the surface or the moisture content or incorrect surface preparation.

    Ideally, the condition of the surface of the screed, particularly with respect to it being sufficiently dry, should be planned and managed from the day the screed is installed rather than just being a responsibility that’s ‘handed to’ the floor finishes installer just before he’s due to start.

    So how do we avoid these failures?
    1 Always use an approved and certified installer
    2 Make sure that proper preparation is completed, eg, the screed chosen is the most appropriate material, drying and curing conditions are controlled, expansion joints are used where needed and the screed is protected as needed from heavy traffic etc.
  1. Ensure all aftercare guidelines are followed, for example, commissioning the underfloor heating before applying a final floorcovering.
    In summary, screeds are a demonstrably reliable and robust material. While failures can happen, these can be avoided by following established guidelines and treating screed with care from the day it’s installed.
    Padraig McGrath is technical manager at Cemfloor Liquid Screed
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