Can You Screw Concrete Board After Thinset Has Dried?

Thinset Shelf Life

• Once thinset has been mixed with water, it has a limited shelf life that depends on the manufacturer. Most brands last around 45 minutes before the mixture is no longer usable due to the polymers and cement losing their adhesive properties. Screwing down the concrete board within this time is vital to the stability of the tile project or the thinset won’t stick.

Importance of the Bond

• The bond of the concrete board to the concrete mixture of the thinset is what provides the basic foundation layer for the tile installation. Without the bond, the board has the potential simply to float on top of the wood, allowing for excessive movement that will eventually crack out any tile installation put on top of the concrete board. Screwing the board down when the thinset is fresh locks it in place and allows the concrete to form around the screws and bond the board.

Concrete

• The bonding process of concrete occurs only when the concrete is fresh. Once it has time to cure and begin to harden, the adhesive properties of the cement and lime (as well as whatever polymers are in the mixture) are locked in place; if you break that bond, you ruin the structural integrity of the entire installation surrounding that area. Using fresh thinset ensures that you lock the board in place before the curing process.

Proper Installation

• Part of the installation process with concrete boards is to provide a flat surface for the tile installation, and you cannot provide a flat installation if you are attempting to screw the concrete board down on top of hardened thinset that has ridges from the trowel. Some will crumble under the pressure of the screws, but the more pressing concern is that you have zero bond between hardened thinset and concrete board because the thinset is already dry.