How to remove ghosting on render

Ghosting blockwork — what is render ghosting?

‘Ghosting’ is a phenomenon whereby the outline of the blockwork is showing through the render as a result of penetrating dampness from the underlying masonry. 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.

What causes ghosting

Ghosting is caused when careful consideration is not given to what thickness of render is used. The location, orientation and surrounding environment of the building to be rendered, as well as exposure to wind and rain, along with 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.

How the cavity of the superstructure has been insulated will also need to be considered when calculating render thickness. On properties where the cavity has been partially filled with insulation, applying 15mm of render as a minimum in a sheltered to moderate exposure zone would be suitable. However, in line with the guidance from the NHBC Standards 2022, 6.11.6 Table 6, if the property has been constructed with insulation that fully fills the cavity, the NHBC recommend a minimum thickness of 20mm in sheltered to moderate locations.

Further information can be obtained from the NHBC with regards to render thickness of factory-made single coat renders

How to remove ghosting

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. We recommend leaving the render for a couple of months in the hope that 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 over 100 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 that 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.

We care

As with any project, Weber will be with you every step of the way. If you’ve unexpectedly experienced ghosting, we’re happy to advise on how best to deal with the issue. You can find our contact details here.

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