Repairing damaged stucco exterior render


As with all conservation work, it is most important to record the area to be conserved prior to commencement as well as during work.

One of the first tasks may be to remove inappropriate materials and methods of repair and replace with the appropriate stucco/render. However, removal should only be carried out if this does not put greater risk on the original fabric. It may also be necessary to cut out defective areas of original that cannot be saved.

When conserving fine or delicate decorative details it may be advantageous to face-up original material around the perimeter of earlier repairs with acid-free tissue prior to removal of crude repairs to prevent any loss of detail.

Cracks greater than 2mm in a lime-based stucco should be carefully cut out to form a slight undercut which will act as a key, and thoroughly flushed out with water to remove dust and loose debris before being filled with fresh mortar based on trial results. Obviously a finer aggregate will be required where the crack is fine or hairline and it is often deemed unnecessary to undercut as the space is easily filled especially if limewash is to be applied.

Hollow areas and voids in a lime-based stucco should be flushed out likewise, although in this case it may be necessary to form a small hole at the base of the void to allow water to escape. Acetone may be used or added to the water to assist drying. A ten per cent solution of Primal WS24 may be injected into the void prior to grouting in order to increase the bond between grout and internal face, before injecting a fine grout based on lime putty or, in the case of a mastic, a similar mix based on analysis results. It is very important to observe the surface of the stucco while grouting to check for escape holes, surface bulging and consequent loss. Vulnerable areas should be supported until the grout has set. Finally the surface of the stucco is reinstated to its original profile, where possible without causing loss.

Salt efflorescence may be dry brushed and removed from all surfaces, as should all algal growth. A suitable biocide should be applied to affected areas only, to remove remaining algae and prevent re-growth.

Friable areas of a lime-based stucco may be consolidated with repeated applications of limewater. To avoid a white bloom it is most important not to let the limewater sit on the surface but to sponge it off with clean water.

Substituting modern materials for the original should always be avoided if at all possible. Wherever a high proportion of original stucco has survived a hundred years or more in the British climate, bear in mind that the original has been proved to work. This historic material, produced by craftsmen long ago, has its own intrinsic value like any antique and, with careful consolidation, suitable repairs and thorough maintenance, it should be possible to ensure that the original stucco work can still be seen by future generations.