Chroma Global Flooring Solutions

Relative Humidity & Aspecta


Floor Preparation and the 10% rule explained

In reality there are numerous sources of information available as to the guidelines and recommendations and types of preparation, and there isn’t anything too complicated about the subject. The complications arise when reality doesn’t follow theory, well whenever does it, and flooring materials have to be installed in line with the build programme, and so as not to delay other trades, sound familiar?


However, Floor preparation can be one of the most confusing and has to be one of the most talked about topics in the world of flooring installation. Also sometimes the floor preparation is referred to as latex. Which is ‘latex screed’. Invariably the subject of the ‘moisture content of the slab’ is eventually raised. The slab is what the floor has normally been prepared from and most times some form of screed is used.


But what is ‘screed’ and what is ‘dry enough’?

Floor Preparation and the 10% rule explained

What is Screed?

There are many types of screed but when it comes to flooring installation they all have to be dry enough.


Levelling screed is screed suitably finished to obtain a defined level and to receive the final flooring. It does not contribute to the structural performance of the floor.


Wearing screed is screed that serves as flooring. This term was formerly known as high strength concrete topping. It is also used to refer to structural toppings as well as wearing surfaces.


Bonded screed is screed laid onto a mechanically prepared substrate with the intention of maximising potential bond.


Cement sand screed is a screed consisting of a screed material containing sand up to a 4mm maximum aggregate size.


Unbonded screed is screed intentionally separated from the substrate by the use of a membrane.


Floating screed is screed laid on acoustic or thermal insulation. This is a type of unbonded screed.


Fine concrete screed is a screed consisting of a concrete in which the maximum aggregate size is 10mm.


Pumpable self-smoothing screed is a screed that is mixed to a fluid consistency, that can be transported by pump to the area where it is to be laid and which will flow sufficiently to give the required accuracy of level and surface regularity. This is what is sometimes referred to as a self levelling screed, and commonly used in the building industry.

There are many types of screed

All screed preparations need time to cure or dry before any further work can take place. Drying times vary enormously from 3-6 days to 8 weeks, based on a 50mm thick screed, on a dry substrate or isolated from damp by a dpm, and that is in ambient drying conditions, which in layman terms means no rain or damp! Getting this right is imperative as most flooring manufactures guarantees ALWAYS refer to testing the screed for relative humidity and seek proof that this was low enough before any installation was started.


Now imagine, in reality waiting for the floor to dry throughout. If it isn’t dry enough sooner or later that fact will present itself in unsightly bubbling or lifting of floor covering. This causes all manner of problems and can also be rather smelly! But what is dry enough?


Well industry guidelines in the soft flooring specification document referred to as the M50, should, but don’t always, suggest that floors to be installed with a moisture sensitive flooring material should have a tested relative humidity RH value that does not exceed of 75%.


Aspecta TEN a totally new dimension in Luxury Vinyl Tile Flooring due to its Isocore technology and Ultra fresh treatment can be installed in areas with a tested relative humidity RH value that does not exceed 85%.


The 10% rule explained that saves both time and money!



The 10% rule explained that saves both time and money!
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