How to prevent render cracking: Advice for site agents

To avoid the risk of cracking, it is essential that certain build considerations are taken into account. This section aimed at the site agent focuses on good practice when building the substrates that are to be rendered.

Storage of blockwork

The most common cause of cracking results from the initial drying out of background materials. The BRE publication Understanding Dampness (BR466) states that where a concrete block has moisture content greater than 12% by volume, action is required. This must be reduced below 8% before excess shrinkage will subside.

Blocks that are left out uncovered during inclement weather will absorb moisture and exceed 12% moisture. If the moisture content is above 12% the blocks must be allowed to dry before use. To avoid these problems the blocks should be covered with waterproof sheeting when stored on site.

Mortar mixes

A significant proportion of masonry contraction is due to the mortar. The effects of this shrinkage and the stresses caused can be reduced by ensuring mortar joints are weaker than the masonry units. Typically a mortar with a cement:lime:sand ratio of 1:1:6 will be suitable. If the strength of the mortar is too strong it will cause large amounts of stress to build and will cause the blocks to crack.

Mortar bed reinforcement

Mortar bed reinforcement should always be used above and below openings as highlighted on the opposite page. Incorporating mortar bed reinforcement will significantly reduce the risk of cracking.

Best practice presentations

A 45-minute presentation is available covering the most pertinent points of rendered masonry, ideally suited for Design/Technical teams and Site/Project Management. Weber can visit your offices to explain ‘Best Practice’ in detail, sharing recommendations covering the façade solution, design, construction & render installation in accordance with the relevant Codes of Practice and European Standards. Saint-Gobain Technical Academy facilities can be made available as required.

You can arrange for a best practice presentation to be carried out by your local Weber Application Manager

Tool box talks

Weber can visit new site starts and conduct briefing sessions for site staff, block layers and render contractors, to ensure everyone is familiar with best practice. Materials can also be made available for use on site to remind everyone of the core best practice principles. This is backed up with proactive on-site support service and periodic visits throughout the installation by a Weber Application Manager.

You can arrange for a Tool Box Talk to be carried out by your local Weber Application Manager

Building practice

Infilling or coursing with dissimilar materials should be avoided. Different types of blocks expand and contract at different rates creating increased stresses which can cause cracks to develop. Blockwork should be laid in a regular brick bond pattern and perp joints should not clash.

Protecting the substrate

Once the blockwork has been built and is awaiting rendering, temporarily guttering socks should be used to direct rainfall from areas of the wall. If one area of the wall is allowed to become soaked while the adjacent area is dry, the differential movement will cause stress and cracks will form.

Masonry with recycled content

We are aware that some blockwork is progressively being formulated with varying quantities of recycled materials. These recycled materials can dramatically affect the water absorbency of the block with extreme examples absorbing twice the water volume over a short period. For this reason it is imperative that due care is taken to protect blockwork from the weather in both storage and construction. These blocks often appear similar in appearance to standard masonry. It is therefore essential, prior to rendering, that a suction assessment should be undertaken to ascertain the level of absorbency within the blockwork. Should the level of absorbency be deemed as high, action will be required. In normal circumstances careful controlled dampening with water may suffice, however in more extreme cases an application of weberend aid would be needed. If in doubt please contact Weber Technical Services on 08703 330070.

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