What is laitance in a subfloor and why do we grind it?

Have you ever seen a beautiful, newly poured subfloor, perhaps in a new build house, or indeed in your own renovation?

And have you ever noticed that sometimes this lovely smooth subfloor has a milky, white chalky, sometimes slightly flaky covering?

Well this is laitance.

How do you know if you have laitance?

If you scrape with a knife, and create a powdery substance, you have laitance

And you do not want laitance on your subfloor, as it has the potential to cause the new flooring you put on top to fail.

What exactly is laitance?

Subfloors made of calcium sulphate (pouring floors) are increasingly popular as they are lightweight and cost effective, compared to say a traditional concrete subfloor.

However, when they are poured and have been mixed with too much water or haven’t had the correct ventilation in the drying process, then contaminants in it rise to the surface and form a crumbly crust; laitance.

What is the problem with laitance?

In order to be able to lay a floor on your sub floor, the subfloor needs to be smooth and solid, to allow products to be bonded over the top. If there is laitance the bond will fail and so potentially will your floor.

Is laitance the only problem for subfloors?

No. All too often we see other products mixed up in subfloors, such as paint flakes and plaster bits from decorators and plasterers. These will also cause subfloors to fail, for the same reason as the laitance.

What can be done about this?

The best way to resolve the problem of laitance or other contamination is to grind the surface of the subfloor. Ideally this will be done 7-14 days after the subfloor has gone down, but often it ends up being the job of the floor installer, who may come in months later to remove these contaminants.

We know this to be the case and so Art of Flooring has attends annual training courses provided by Uzin and Floorskills on subfloor preparation to keep abreast of correct subfloor preparation methods. We know how to deal with laitance and contaminants and use the Neo 230 Grinder to remove these contaminants.

We will always complete the correct method of subfloor preparation for the surface we are dealing with following methods learnt in these training courses. Because of this, our quote specification may differ from other companies. It is critical however to get this right, it really pays to prepare.

Just because you can’t see it, it doesn't mean a subfloor isn’t important. Correct subfloor preparation is vital; if you are investing in a new and expensive floor, if you don’t prepare correctly it won’t last.

If you do prepare it correctly it will stand the test of time!