Once the old floor covering and underlay have been taken up, any remaining adhesive residue will need to be removed.

Some installers take shortcuts at this stage of the job and leave bits of old residue behind because they think it's too time consuming to strip it back.

But this can cause its own problems.

One of the potential problems is that the ridges from the old glue will affect your ability to control the spread rate of the new adhesive.

Manufacturers specify certain-sized notched trowels to ensure that the spread rate is maintained at a precise level.

If you change the spread rate, you'll affect the working time and bonding properties of the new adhesive.

You'll also void the warranty that the manufacturer provides on the performance of their adhesive.

Another potential problem is the fact that adhesives were often made with solvent-based formulations in years past.

These days, they tend to be water based, as manufacturers move towards more environmentally-friendly formulations.

So if you apply a new water-based adhesive over an old solvent-based residue, you're likely to have a compatibility problem which might affect the strength of the bond.

Below are the main methods used to remove old adhesive. We'll deal with the issue of asbestos-based adhesives at the end of this discussion.

Hand scraping

Hand scraping is very labour intensive, but it can still be the most efficient way to remove small quantities of adhesive, particularly if it's only around the perimeter of a floor surface.

It is possible to combine hand scraping with chemicals that soften the adhesive - but you need to ensure that the chemical won't leave traces behind that will react with the new adhesive or floor coverings.

Acid washing

For concrete substrates, some people like to wash the surface with acid.

However, this should only ever be used as a last resort, because most levelling compound manufacturers will not warrant their products unless the concrete surface has been mechanically ground to remove contaminants.

If you do decide to acid wash the surface, be careful to keep the solution within the confines of the slab and to completely remove any residue when you finish.

Hydrochloric acid is generally used. In commercial quantities, it's often called muriatic acid.

To prepare the solution, mix 1 part acid to 10 parts water.

Apply the solution with a watering can and use a stiff-bristled broom to spread it around the floor.

Once the reaction has taken place, wash it off and make sure that it's completely neutralised before leaving the floor to dry.

Note that acid washes are not effective in removing grease and oil. The same applies to solvent washes, which have a tendency to drive the contaminants further into the concrete pores.

Grinding

Concrete subfloors can be ground back with a machine to remove adhesive residue and other contaminants.

There are various types of machines available, with each type designed for specific applications.

These include grinders, chisel scrapers and shot blasters.

Grinders are available as hand-held power tools, walk-behind machines and ride-on machines.

They use abrasive discs, often with diamond or tungsten carbide tooling, to apply a circular rubbing action to the concrete surface.

Grinders are the most suitable for removing adhesive, because they tend to do the least amount of damage to the concrete surface.

Shot blasters are generally used when the concrete surface contains weak or loose material that needs removal.

They work by throwing out thousands of shot particles at high speed, similar to the process of sand blasting.

Chisel hammers are basically jack hammers fitted with a wide chisel-shaped blade head.

They can be used for taking up the old floor covering as well as removing adhesive residue.

Every grinding and chiselling machine has an optimal method of operation.

Some machines are designed to be moved in a circular motion as they're pushed up and down the floor in parallel lines.

Other machines have different processes. For more information on grinding techniques and the various types of tooling and machines available, go to the unit: Concrete grinding.

Sanding

Timber and wood-based surfaces generally need to be sanded back, since solvents and floor strippers don't work particularly well on these sorts of floors.

Drum sanders are best on floors that need a lot of material removed.

Belt sanders provide a finer finish but get clogged up more easily, so they're most effective when the surface only needs to be lightly sanded.

Orbital sanders and 'edgers' are used around skirting boards and in corners.

Once the sanding is completed, vacuum up the dust with an industrial vacuum cleaner.

Also check that all boards or sheets are well fixed and there are no squeaks or movement underfoot.

Re-nail or screw any loose boards or panels.

Dealing with 'cutback adhesive'

The photo at right is taken from the Asbestos awareness website, showing old vinyl tiles bonded to the subfloor with 'cutback adhesive'.

This black asphalt-based adhesive generally contains asbestos fibres.

If the cutback is still firmly bonded to the floor, it's often easiest to leave it in place and only remove the thick accumulations and brittle or loose material.

This is done by wet scraping and mopping the floor to avoid the creation of dust. A solvent-based chemical remover can also be used.

Always remember that you need to be very careful to minimise the amount of dust that's produced, and to wear the protective clothing appropriate for asbestos removal.

You must also follow proper disposal methods for asbestos based products.