Conservation of render and plaster

Conservation of render

Up to the mid-19th century, lime was the single most-used building material. It was used with or without additives to increase strength. All renders have a restricted service life, and need to be replaced once in a while, either due to age or wrongly used repair materials.

The lifecycle of a render; when to consider age and repairs

Lime has been used as a binder for over 8000 years. Portland cement was patented by Vicat in 1824, and introduced slowly into construction throughout the 19th century. Over the next 180 years, cement replaced lime as a binder in bedding mortars and renders, both for new use and renovation. Some natural cements were also on the market as people sought stronger and faster mortars. Portland cement mixtures result in mortars that are brittle and stronger than the lime-based products, and in combination with lime mortars this induces damage

When building a new render on an old brick wall, it is essential to use the same material as the original, both to respect old building traditions and to yield the correct compressional and flexural strength. In this way, the repairs will not induce new damage. Damage on render is due to a combination of age, wrongly used repair materials, lack of maintenance, etc. Lack of adhesion to the substrate occurs when repairing mortars that are too strong are used or a system inhibits the damp diffusion necessary to maintain strength.

Remove carefully and replace with adapted renders

Build up render

Building a new render can be done partially or completely. The choice of method depends on the amount and type of the damage. The 25% rule of thumb is often used: if more than 25% of the flat area (not including modlongs and decorations) has damage then a total renovation is often economically sensible. In either case, the principles for repairing a render follow these steps: *Remove all damaged render as carefully as possible. *Repair damage in the joints and replace damaged bricks. *Refill deep joints. *Rebuild the render to the sought thickness, usually in three layers. These should consist of a strong and coarse primer, a less strong and finer base coat and a weak, fine top render. A maximum of 1 cm per layer per day to avoid shrinking and ensure carbonisation starts. *The renders should be of equal or lesser strength than the mortar in the masonry. Frequently used materials are lime-based, NHL/lime-based and lime-cement-based renders. We advise you to follow the prescribed solution, local traditions and proper investigations before starting work. Please also follow local regulations.

Partially filled render

As described earlier, the choice of binders and composition should be done taking into consideration what has been used earlier. Mortars based on hydrated lime are still the most commonly used in renovation throughout Europe. Complying with the original material, modern premixes have shown to be a good match with regards to strength and physical behaviour. Ensuring a stable quality of the binders and mixes helps to reduce uncertainty regarding their physical behaviour and durability. The properties of a hydrated lime mortar is controlled by the lime-to-sand ratio, the maximum size, size distribution of the aggregates and the lime-to-water ratio. Different additives used historically in construction can also be used to regulate its properties.