What is ghosting and what’s the best way to deal with it? Saint-Gobain Weber

‘Ghosting’ is a phenomenon whereby the outline of the blockwork is visible through the render, as a result of penetrating dampness from the underlying masonry. Saint-Gobain Weber explains why it happens and how it can be fixed.

It quite often appears following rainfall at the start of the drying process, as the outline of the block shows the differing levels of absorbency of the masonry and its bedding mortar. In order to prevent ghosting, careful consideration should be given to what thickness of render is used.

The location, orientation and surrounding environment of the building to be rendered, exposure to wind and rain, and the potential protection from surrounding buildings all have an impact on the thickness required to protect the build.

For sheltered to moderate exposure, the minimum recommended thickness of render is 15mm, and this rises to 20mm in the case of severe exposure. The visual impact of ghosting is worsened in new construction, if the building components have not dried out fully. If this is the case, the effect should lessen over time – Saint-Gobain Weber recommend leaving the render for a couple of months, in the hope once the blocks are fully dry, and the render fully cured, ghosting will no longer be issue. If after waiting, the problem persists, there are two options to remedy it.

1. Paint 

A high-performance, water-resistant paint, such as webersil P, may be applied to the building to overcome the issue of ghosting. webersil P is a mineral paint with exceptional long-term durability and performance, and comes in a range of 160 colours. This paint is suitable for use with our weberpral renders, provided the elevation is clean, sound and dry.

2. Re-render

In some circumstances, it may be advisable that rather than painting, the thickness of the render should be increased. Before re-rendering, the entire area to be rendered should be stripped of any paint or coating, and should be clean, dry and sound. If necessary, a coat of weber CL150 can be used to ensure no algae is present on the surface. A keycoat of weberend aid should be applied, followed by a pass of weberpral M (or weberpral D in Ireland) render at 10mm which is scraped back to a thickness of 6mm.

As with any project, Saint-Gobain Weber will be there every step of the way. If unexpected problems occur such as ghosting, Weber is happy to advise on how best to deal with the issue. For more advice and guidance visit the Saint-Gobain Weber website.