External finishes

Render specification is determined not only by the nature of the substrate, but also by the location of the building. General purpose sand/cement renders are defined by mix types (i) to (iv) in accordance with Table 1 in BS 5262*. Table 2 in BS 5262* confirms that mix type (iii) (1 : 1 : 6) is suitable for application to aerated concrete blockwork in areas of sheltered, moderate and severe exposure conditions.

If a cement-rich mix is used for rendering Thermalite®, the amount of water necessary to hydrate the cement gives rise to a higher rate of drying shrinkage in the render than in the blockwork. This differential shrinkage can lead to a shearing action building up between the two materials, resulting in failure. However, this issue can easily be avoided by adopting a compatible render specification.

General
BS 8104 describes a method of establishing the local spell index for walls subject to wind-driven rain. This takes account of meteorological data and expresses the worst expected conditions prevailing in a spell of bad weather during any three year period.

Since such data is variable, the definitions of the exposure categories overlap and a range of indices is recommended for each category. Although the severe and very severe categories apply to much of the West of Scotland and Wales, for example, the exposure rating also takes account of local features which afford protection to a building. It is therefore possible for a 'severe' site to exist within an area deemed to be in the 'very severe' category.

On request, the Technical Advisory Service will assess any building to determine the suitability of block thickness and external finish, with reference to exposure categories.

Specification
It is normal practice to apply render in two coats. The first coat (undercoat) should be trowelled on to a thickness of 10mm to 16mm and scratched. An adequate period of time should elapse between the application of coats in order to allow the undercoat to dry out thoroughly, but not too quickly, before the next coat is applied.

The thickness of the final coat will be governed to some extent by the texture required, but will normally be 6mm to 10mm, as finished. The final coat should be thinner and no stronger than the undercoat and this, in turn, no stronger than the substrate. The finishes suitable for external renderings onto Thermalite® blocks are  classified by BS 5262* as follows:

a) Float finish.
b) Scraped or textured, hand-applied.
c) Dry-dash, dry-thrown by hand.
d) Machine-applied, Tyrolean or power spray.

Mixes
When selecting a suitable render specification it is important to ensure that the composition is compatible and not too strong (or weak) in relation to the strength of the substrate. The following recommendations - mix type (iii) - take this into account and are based on the requirements of BS 5262* and BS EN 13914-1.

To gain benefit in both accommodation of movement and resistance to rain penetration, the preferred render mix is:
 
a) 1 : 1 : 6 cement : lime : sand by volume.
 
The addition of lime improves workability and cohesiveness. It improves the render's ability to accommodate movement, thereby reducing the tendency to crack. The lime should be hydrated and comply with the requirements of BS EN 459-1:2001.

b) 1 : 6 cement : sand by volume, with the addition of an approved plasticiser used in accordance with the manufacturer’s instructions.

c) 1 : 5 masonry cement : sand by volume.

The sand in all coats should be sharp, clean and comply with the requirements of BS EN 13139. The grading of the sand is important as it will affect the quality and performance of the finished render. Sands with excessive proportions of very fine material should be avoided as the finer the sand the greater the render's capacity to hold water.

In addition to inhibiting adhesion, excessive moisture retention will lead to high drying shrinkage, with consequent cracking and spalling. Conversely, a coarse sand retains insufficient moisture within the render, resulting in inadequate hydration and adhesion.

*No longer current, but still cited in Building Regulations. Use in conjunction with BS EN 13914-1.

The use of ready-mixed renders to help avoid inconsistency in site batching is highly recommended. Traditional sand/cement render should not be applied to an external leaf of Turbo material less than 215mm thick. This is to minimise the risk of failure caused by the differences in tensile strength between the finish and background material. If a ‘technical’ render system solution is proposed, the advice of the render system manufacturer must be sought.

Modern render systems
A wide range of cement-free and modified cement renders, some reinforced by fibre or mesh to resist thermal movement of the backing masonry, are readily available for application to Thermalite®. These are often referred to as ‘technical renders’. As these technically sophisticated products often require the services of an approved applicator, advice should be sought from the manufacturer prior to specification.

Preparation of background
Although the scratch finish, which is primarily an identification characteristic for Thermalite® blocks, helps provide a key, it is also possible to apply render to Thermalite® Paint Grade Smooth blocks. The scratch key should be augmented by recessing the mortar joints during construction.

The blockwork should be clean and free from dust, loose particles and any contamination which may have occurred during construction.

Thermalite® normally provides a 'medium suction background'. Given certain ambient conditions, or where the blockwork has been allowed to dry out, high rates of initial suction may be experienced. This condition can have an adverse effect on hydration and adhesion, and measures should be taken to reduce the suction. This can be done by applying water using a stock brush or a fine spray immediately prior to rendering, taking care to avoid saturation of the surface.

Alternatively, should high rates of suction persist, then a spatterdash treatment or the application of an approved bonding agent may be considered. Further information may be obtained from Hanson’s Product Services Department.

Accommodation of movement
In accordance with the recommendations of BS 5628: Part 3 and BRE Digest 342, construction joints should be provided at maximum 6 metre centres, in order to minimise the effects of movement.

External wall insulation systems
There are many external wall insulation systems available that are widely used to improve the thermal insulation performance of external walls of existing buildings. However, these systems can also be used on external walls of new build projects.

The systems vary depending on the particular manufacturer, but generally consist of a thermal insulation such as EPS insulation board or rigid fibre board, secured to the external wall with fixings direct into the wall. A base render coat is then applied to the insulation into which a fibre or metal reinforcing mesh is placed. The reinforcing mesh is also usually fixed direct into the external wall. The system is then completed with the application of a proprietary render.

It is advisable to ensure that the system to be used is supported by independent certification and full technical information is obtained from the manufacturer.

External tiling
External tiling should only be fixed to render finishes which have been applied over anchored re-enforcing mesh. It is also important to follow the guidance contained in BS 5385 : Part 3, in particular Table 3 of the standard, where information relevant to external tiling is contained.