A smooth subfloor is essential for successful flooring installations, but smoothness means far more than achieving a flat surface, says Richard Renouf in the fourth part of his guide to flooring inspections
IN last month’s issue we looked at the first of four basic requirements for any subfloor over which we’re going to lay flooring – it must be sound. The second is our topic for this month’s article – it must be smooth.
This characteristic is often misunderstood, especially when it’s assumed the purpose of those bags and bottles or ply overlays are simply to create a flatter floor surface.
The most common subfloor we encounter is a sand and cement screed. Even when the aggregate (the sand) is quite fine, the surface will be like sandpaper. If you run your fingers over it firmly, you’ll lose your fingerprints because of the abrasion.
If you try to glue flooring directly onto the rough surface the bond strength will be greatly reduced because very little of the screed is actually touching the back of the flooring. A smoothing compound fills the roughness and provides a surface that is smooth and will give a superior bond.
Calcium sulphate screeds always need to be sanded or ground off to remove laitance, but the surface may still require a suitable smoothing compound before using adhesive because of the sanding or grinding marks.
There are some overlaps between soundness and smoothness. There are two types of subfloor which appear and feel extremely smooth and sound, but which aren’t suitable for bonding flooring directly to.
Particle boards (chipboard and oriented strand board – OSB) consist of wood particles and/or shavings glued together with resin under high pressure. The surfaces tend to be the finer particles of the mix to give smoothness, but the resin can’t hold the small particles well and so glueing directly to the manmade board will lead to failure of the bond.
This has been clear in the British Standards for flooring and tiling, and in recent years the tiling standards have moved towards requiring a mineral backing board over chipboard and OSB, but flooring standards have clear requirements for flooring grade materials which may include plywood and hardboard to suit the application.
The second subfloor surface is power-floated concrete. This has a very dense and smooth surface, but this surface does not provide a ‘key’ to allow anything to be bonded to it. If you’ve ever visited a new warehouse with crisp yellow lines painted onto the power-floated surface, then revisited a month or two later, the lines will already be disappearing. Even paint won’t stick without a mechanical key, even a primer isn’t effective.
It means power-floated concrete has to be roughened and the smooth glossy surface has to be taken off. This can be done by grinding, shot blasting, scabbling or scarifying. Once this has been done, a new smooth surface has to be added using a suitable smoothing compound and then the flooring can be laid, as long as the other subfloor requirements are met.
A few months ago I visited a commercial food preparation unit where the flooring had been stuck down over a power-floated floor with minimal surface preparation. I could see a few grinding marks when I lifted the flooring, but most of the power-floating was intact.
So as part of any inspection the preparation of the subfloor should be checked. What is it? Does it meet the requirements for soundness and smoothness? These questions may give vital clues to help understand the cause of issues which may have arisen.
See you next month when we’ll be looking flatness.
www.richard-renouf.com
Richard Renouf is an independent flooring consultant
