Author: T. Rondou
Natural stone facade base moldings coming loose is a common problem. This issue often occurs due to improper bonding or an unsuitable connection to the pavement. In this fact sheet, we discuss the causes and provide recommendations for repair and prevention.
Context and Problem Statement
The building’s facades were finished with natural stone baseboards that were adhered to the masonry using dots of mortar. Afterward, permeable paving was installed with joints filled with crushed natural stone aggregate (gravel). The level of the paving is higher than the bottom of the baseboards.
A few months later, several baseboards come loose.
Findings
Loose baseboards tilt outward from the bottom. The joint between the baseboard and the paving consists of a narrow gap filled with gravel. As a result, the joint is virtually unable to accommodate any deformation in practice.
Causes of natural stone facade baseboards coming loose
Delamination occurs when the stresses exceed the adhesive strength of the glue. Shear stresses develop in the adhesive joint due to the skirting boards’ own weight and due to hygrothermal deformations. In addition, forces perpendicular to the bonding surface are also at work due to the pressure of the paving against the baseboard.
When the temperature rises, the paving expands. Because the joints are filled with gravel, this movement can hardly be accommodated. As a result, the paving begins to press against the baseboards.
When installing with mortar dots, air pockets form behind the baseboard. Adhesive is often missing on the underside. When the paving presses against the baseboard, a lever effect occurs, causing the baseboard to come loose and tilt outward. The less adhesive contact there is, the greater the risk of detachment.
An important distinction
This damage is not the result of a single clear construction error. The problem arises from a combination of factors. Both the bonding with mortar dots and the rigid connection to the paving are, in and of themselves, standard construction methods, but in this case, they reinforce each other in an unfavorable way.
Prevention
The most effective solution is to install the bottom of the baseboard above the level of the pavement. The baseboard can be mechanically fastened—which is the preferred method—or glued. If gluing is used, full-surface bonding is preferred. This reduces the risk of the baseboard coming loose and also minimizes the chance of staining.
When a visible joint is desired and the baseboard is flush with the flooring, it is best to leave a 1.5- to 2-cm joint. This joint should be filled to the bottom with a compressible material and finished at the top with an elastic, non-staining sealant.
In situations where there is no control over the paving—for example, in public areas—it is best to take into account potential pressure on the baseboard. In such cases, a curb stone can be placed between the baseboard and the paving, separated by an elastic joint. For adhesive-bonded baseboards, full-surface bonding is generally the safest choice.
In addition, it is important to keep water away from the baseboard. This prevents damage caused by freezing. In the case of a cavity wall, the cavity drainage must be located above the baseboard.
Repair
Loose baseboards can be reattached, preferably using full-surface adhesive or mechanical fasteners. When doing so, it is important to follow the correct installation details to prevent the problem from recurring.
Conclusion
The detachment of natural stone foundation plinths is usually caused by a combination of factors. By paying close attention to the bonding and the connection to the pavement, this problem can be prevented in most cases.