In this illuminating column, James Rosher answers key questions some contractors ask regarding subfloor preparation.

HOW much subfloor preparation product do you really need?
Before applying any subfloor preparation products, we always advise a visual inspection of a subfloor in the first instance, as this can highlight concerns a floorlayer’s expert eye will see. The builder’s cry of, ‘The floors are dry – they’ve been down for ages and only need a skim coat,’ is, from experience, rarely the case.

Many flooring contractors are now finding subfloors onsite to be higher in texture, with water damage and aggregate protruding from the surface, or with an undulating surface or quilting effect. In these instances, the usual minimum requirement of 3mm of smoothing compound may not achieve the desired surface on which to install your floorcovering.

What will the floor be used for?
The standards we need to work to can vary between projects. These are often specified by the architect or client and will depend on the end use of the building. When smoothing a floor, we’re looking at removing abnormalities that may affect the finished surface. Some textile finishes can hide a multitude of sins, while thin LVTs or sheet vinyl will highlight discrepancies from the screed underneath. Levelling the surface can require more smoothing compound compared to smoothing the surface.

In domestic and specialist commercial environments, the subfloors are laid to SR1 (±3mm over a 2m straight edge). To simply smooth the floors, a 3mm application of smoothing compound will be adequate, however a 3mm coat will follow the contours of the floor so may not fully level the surface. If a totally flat surface is required eg in an operating theatre, more than 3mm will be needed.

On larger projects such as schools, a number of floors are laid to SR2 (±5mm). This is mainly where concrete has been installed and during the curing period a quilting effect has occurred on the surface, mirroring the re-bars situated within the floors’ construction. Again, a 5mm application of smoothing compound will follow the contours of the floor and won’t be enough to fully level it.

Several years ago, I was involved with a project at a specialist analytical laboratory.

Based on the architect’s plans, the floors’ build-up was to be 250mm of concrete with a 25mm topping screed, achieving a surface greater than SR1. The flooring contractor was then to prepare and smooth the floors for the installation of sheet vinyl, so this is what the contractor did, with the end result looking good.

It was only when the client inspected the floors that a problem arose. The floors weren’t flat enough for the installation of the analytical laboratory equipment and needed to be rectified.

The flooring contractor did what he was asked: ‘prepare and smooth the floors for the installation of sheet vinyl’, while the builder was expecting the floors to be made level. As this wasn’t stipulated in the original request, who was at fault?

It turned out the builder didn’t understand the relevance of the topping screed, omitted it from the build-up of the floor, and replaced the depth of the topping screed with more concrete, which is how the problem was created.

Why a minimum of 3mm?
Manufacturers of smoothing compounds don’t just say a minimum of 3mm to sell more product. The main reason is to provide a suitable surface in which to bond your floorcoverings.

When adhesives dry, the water within the formulation needs to dissipate. This is easy when the floorcovering is fully breathable. However, with resilient floorcoverings, moisture within the adhesive cannot dry upwards as the floorcovering is impermeable, therefore it can only go down and migrate through the smoothing compound.

If the smoothing compound is applied too thinly (especially over a surface-applied DPM), there will be an insufficient amount of material for the adhesive to dry in to, making it questionable whether the adhesive will fully cure.

We were once called to inspect the flooring in a corridor at a doctor’s surgery where the sheet vinyl was starting to discolour. The flooring contractor was adamant that is wasn’t a moisture-related issue as he had used a two-part epoxy DPM. On inspection, the discolouration was only in the middle of the corridor and localised to a small area. When the vinyl was lifted we could see the adhesive hadn’t fully cured and the smoothing compound was between 1-2mm in thickness. The cause of the discolouration was owing to the adhesive being sandwiched between two impervious substrates (DPM and vinyl). As the smoothing compound was too thin, there was nowhere for the moisture to dissipate in to, which would have allowed the adhesive to cure.

When you could need more product
Sloping floors and rough or textured floors can both conform to SR1, so it’s important to understand the client’s needs. Your typical everyday latex can smooth a floor up to 10mm in thickness – possibly up to 20mm when bulked out. There are many specialist products on the market now which can cope with much thicker applications, such as fibre-reinforced smoothing compounds or products with accelerated drying times. These aspects should be considered when levelling or raising the height of any floor. When levelling or smoothing a floor, a minimum of 3mm of smoothing compound will always be required on the highest point.

Sometimes the subfloor may exceed 75%RH and therefore be deemed too wet for the installation of floorcoverings, so the use of a two-part epoxy DPM will be required. Should the floor be rough or textured, the consumption rate of the DPM will increase in order to obtain a uniform application throughout: a liquid DPM will pool in the lower spots. In these cases it’s prudent to pre-smooth the subfloor with a moisture tolerant smoothing compound; this will then enable you to apply the DPM in a more uniform and cost-effective way.

If you’re unsure about whether you should be using a thicker application or have doubts about a product’s suitability for an installation, you can consult with the manufacturer’s technical team, who will be best placed to offer advice and solutions.
james.rosher@bostik.com
www.bostik-profloor.co.uk/the-edge/
James Rosher is technical service consultant, Bostik