Sid Bourne on wood flooring over an asphalt subfloors

ONE of the most common complaints that I attend is failed flooring adhered to asphalt subfloor regardless of a smoothing compound applied over the asphalt.

Asphalt is in most cases 10-20mm thick in domestic situations. When used as a moisture suppressant it is normally poured over a fibre glass type material and technically is a floating subfloor.

Although the asphalt is known as a dpm (damp proof membrane) quality product it must be noted that a dpm is linked with the dpc (damp proof course). You will find that asphalt is not linked, so I question the use of asphalt as a dpm, a moisture suppressant I can live with.

The reason I am raising this here is because when I checked different manufacturers’ technical data sheets for compounds over asphalt, I could not find a single one which advised caution when gluing wood flooring to asphalt and that this practice is best avoided.

I recently went to a site where finger block wood flooring had been fixed to a prepared asphalt subfloor with a latex smoothing compound. I did not know at that stage exactly what had happened. But on my way there I popped into Paddy Power and put on a bet that the asphalt floor had lifted completely with the wood floor still attached. It was a virtual certainty.

And I wasn’t disappointed. On entering the property I immediately saw that the asphalt subfloor was completely off the base with the finger block still attached. The cause was that the blocks had picked up moisture, and once the expansion had filled with cork the asphalt floor could not withstand the forces on it. As a result it lifted and broke.

I asked the installer what he had done by way of preparation. He explained that he had sourced technical advice from the smoothing compound manufacturer who specified what he needed to use. I contacted the company in question who replied that it was not their job to fully check the subfloor, something with which I agree.

However, it became clear to me that the technical guy at the manufacturer did not understand wood. And with the manufacturer not taking any responsibility for their advice, the installer was given some of the most ridiculous answers I have ever heard.

I have seen many failures with wood flooring, both solid and engineered, where it was glued to asphalt subfloors.

There are many things to consider. One of which is that the actual stability of the asphalt. Are there any cracks minor or worse? How thick is it? A minimum of 15mm is required for fixing blocks, according to the Mastic Asphalt Council. I often measure subfloor thickness when there is failed asphalt. I frequently find it is less than 15mm.

The other thing you must take into account is the type of compound and whether it has a greater strength than the asphalt can take. For gluing wood flooring to a compound, especially solid wood, we need at least a Nm of more than 25. So taking all these factors into account, the chances of failure are high.

I know there will be doubters who will tell me that they have never had an issue with asphalt and have installed wood floors for 500 years with no issues. But not everyone complains.

For example, I went to a site and was asked to inspect a wood floor, only to discover that it was glued to an asphalt subfloor. The wood had lifted about 30mm. The consumer said that he would not complain to the retailer as he had lost faith in them because of this. He just wanted a solution. Unfortunately; the solution was that the floor need to be uplifted and a new subfloor poured (not asphalt).

The consumer accepted this solution and said he would go back to carpet, but he did not complain to the retailer who may have been very helpful. Ask any carpet fitter what happens when they attempt to nail gripper around the perimeter of an asphalt subfloor. You may as well be nailing to a digestive biscuit; it breaks away with the slightest blow from a hammer.

To summarise, it is my professional opinion that you should not glue any wood flooring to an asphalt subfloor regardless of any type of smoothing compound. If you do, you are most likely to have a failure.