Keeping things watertight

Martin Cummins on the drying of subfloors and what testing tells us

SITE work over the past two years has thrown up some significant changes in terms of the subfloors that we’re likely to encounter. This includes anything from coated chipboard, to screed boards, to proprietary screeds.

All these offer advantages in design and flexibility to the builder, but often produce headaches for those interested in laying a quality functioning floor. However, the biggest shift I’ve seen is in the array of screeds and flowing concretes now being utilised.

The understanding of how these differ from the traditional subfloors (let’s think of cementitious screeds and power floated concretes for this scenario) is key, particularly the way they dry/cure and what moisture testing tells us.

We all know the rule-of-thumb quoted by main contractors: ‘Screeds dry a mm a day for the first 50mm, then a couple of days per mm beyond this.’

This adage is regularly thrown back into our face after we’ve demonstrated a floor is wet, whether it be by the British Standard Method of surface hygrometers, the plug ‘n’ probe method, or non-evasive methods such as the Tramex.

The key here is the drying profile quoted pretty much everywhere in the industry (not just for floors, I may add) is based on three key criteria:

  • There’s no additional moisture coming in. This basically means a watertight building, no contribution of moisture from other trades, namely plasterers and plumbers, and crucially a correct design incorporating a base DPM under the screed or concrete.
  • An ambient temperature averaging 20deg C.
  • Airflow in the building

The criteria is basically saying that for materials to dry, the airspace above them also needs to be dry and the air regularly exchanged.

Additional moisture added or materials stored on the subfloor preventing moisture escaping will mean drying times are extended. The temperature element is included as a standardisation so materials can be compared like-for-like, but if lower temperatures are present (dew point and lower) then condensation can occur resulting in moisture that may have evaporated simply returning to the substrates.

It’s very easy to see that all the factors above aren’t particularly building site or UK friendly. How often do we get such temperatures? How many plasterers cover the floor before working? The questions go on.
None of the above is new to us in technical, but it certainly can be to main contractors and builders, especially when costings for DPMs aren’t included.

The important thing to remember is that the rule-of-thumb adage should never be taken as gospel, and therefore floors should always be tested for moisture levels.

When we carry out our test regime and give a reading to say the subfloor is wet and that drying or moisture mitigation is needed, it can inevitably lead to further discussions.

The discussions with main contractors on older systems are usually straightforward. For example, a power-floated concrete has been highly polished and therefore results in a closed surface layer.

Logic tells us moisture will struggle to escape from a dense closed surface in comparison to an open textured screed, or indeed a pan-floated concrete.

Also, the depths of substrates, with power-floated concrete being anything up to 300mm and standard concrete being typically 150mm to 225mm, enables them to appreciate (even when using the old adage) that the floor will take some time to dry so a DPM is required.

With screeds, we simply discuss the three criteria above as the requisites to get the drying time down, so again we can usually get an understanding from the main contractors.

However, the understanding on newer systems can sometimes result in the need to get further involvement from the screed installers as the moisture test results can often be far from what’s expected.

My experience to date is that further to the comments above, I’m finding the flowing concrete, and to some degree the low laitance calcium sulphate screeds, are areas where knowledge is poor. 

We’re finding that on occasions these products are giving very dense upper surfaces (less than a mm) which are effectively causing a seal on the floor, almost like a very thin DPM.

This is resulting in a slow release of moisture as it’s held below the surface. The reasons for the dense finish don’t appear to be one of design, but more from application.

The likely cause is overwatering during install resulting in flotation of finer particles to the surface giving the ‘seal’.

When abrading away this thin layer the subfloors begin to dry out at an inordinate rate, suggesting the moisture is actually close to the surface.

Depending on the test you use to assess moisture, you may find contrary results. If for instance you’re using an encounter meter, this may pick up a high reading owing to moisture collecting and concentrating near the surface, whereas a carbide bomb that takes an average of moisture from the depth of the screed may show a perceived lower level.  

This brings us back to a key point; understand what the equipment tells you and don’t blindly accept its reading. If in doubt, use the British Standard test to confirm your findings.

Most importantly, offer advice to the contractors as to what’s needed to enable subfloors to dry - an open texture, a dry and warm environment, an uncluttered floor and some nice warm British weather… (I wish!)

01785 272625
Martin Cummins is UK technical support manager, Bostik