This article provides additional guidance on the use of rendering on different substrates. It is important that all workmanship carried out during construction be completed in accordance with the relevant tolerances. The article covers:

 

1. General requirements (all substrates).
2. Rendering directly onto masonry outer leafs to cavity walls.
3. Render cladding systems onto timber / steel framed structures.
4. Direct rendering onto Insulated Concrete Formwork Structures (ICF).
5. Rendering on External wall insulation systems (EWI or EWCS).
6. Rendering below horizontal DPC level – all cases (i.e. within 150mm of the adjacent ground level).

 

General requirements (all substrates)

 

Site made render

 

Is only acceptable on well-prepared masonry subsrates where strict control over workmanship can be demonstrated and the correct selection of materials can be assured. The developer will be required to provide a site-specific 'design specification and quality process' document to detail the proposed render specification, application requirements and detailing proposed.

 

Site made render solutions will not be acceptable on projects where the render is to be applied on the following substrates:

• Render board
• Render carriers
• Hollow clay brick /block units
• Insulated concrete formwork (ICF) or;
• Chimneys, cappings, copings or parapets.

 

Pre bagged and blended (factory made) render

 

Pre bagged factory made renders are the preferred solution as this ensures consistent mix of render components during installation. However it is still important to ensure correct specification, detailing and application. Pre bagged and blended render must be BS EN 13914-1 for the design and application and be CE/UKCA marked in accordance with EN 998-1 or EN 15824 for specification of the render. Evidence of the CE/UKCA marking and declaration of performance should be provided upon request.

 

General render notes (all substrates)

 

• All render must comply with BS EN 998-1:2006 for the specification of render and BS EN 13914-1:2016 for the design, preparation and application of external rendering
• Workmanship for rendering should be in accordance with BS8000
• Render failures are generally due to poor workmanship, inadequate preparation of substrates, poor detailing or inadequately trained personnel applying the render or failing to follow the correct render specification by using alternative inappropriate / substandard materials or mix ratios
• Gypsum based renders are not covered by BS EN 13914 (see Section 1 Scope) and should not be used unless the product holds a third party product approval (BBA or BDA) for the intended use and confirms adequate durability will be achieved
• Render is generally not considered 'waterproof', therefore water seepage can be expected through the render to the substrate/cavity i.e. The render is only considered as contributing to towards the ‘weather resistance’ of an external wall and not considered as providing an impermeable cladding. Therefore, all substrates must be constructed to prevent moisture reaching the internal finishes. Examples of this are the rendering applied to:
• Masonry outer leaf of a masonry cavity wall.
• A suitable render board (third party approved) or carrier system with a drained cavity (and vented for framed structures) provided to separate and protect the substructure.

 

All substrates must be in a sound condition and be suitable to receive the render. A substrate should not consist of differing material e.g. concrete blockwork with clay facing bricks.

 

Rendering directly onto masonry outer leafs to cavity walls

 

The following is acceptable in all exposure zones:

 

Site made render (subject to the general requirements above) and pre bagged and blended (factory made) render may be used on cavity masonry walls with a minimum 90mm thick outer masonry leaf, minimum 50mm wide clear residual cavity and a 100mm minimum inner leaf.

 

Note: For cavity insulation requirements, please refer to our Technical Manual.

 

For the purpose of warranty:

 

• Render on an external leaf of clay bricks (F2, S1 or F1, S1 designation bricks BS EN 771) in severe or very severe exposures is not permitted where the cavity is to be fully filled with insulation.
• Direct rendering is not acceptable on high absorption materials e.g. lightweight blockwork, autoclaved aerated concrete blocks, common brick and/or smooth dense substrates e.g. engineering bricks, as the moisture can be extracted by the substrate from the wet render which affects its curing and bonding capability.

 

Where render is applied to a masonry outer leaf to a timber frame or light steel frame structure:

 

• A drained (and vented, if timber/ SIP framed) cavity will be required between the frame and out leaf and,
• A breather membrane must be provided to protect the main structural frame as a second line of defence.

 

For both factory made and site made renders the walls which are to be rendered should be examined for excessive moisture content prior to rendering. This is particularly important where the masonry background has no upper limit on its soluble salts content, e.g. N designation clay bricks. 

 

To minimise the potential for differential thermal movement and effects that the different suction that each type of background material may create; the section of walling to receive the render should be constructed using the same type and density of material throughout.

 

When rendering is required to be applied to wet masonry substrates, a specialist sealer key coat prior to applying the main coat of render should be applied, to control suction and reduce the impact of lime blooming occurring through the render. The key coat should provide a sound substrate and be compatible with the subsequent render system.

 

To control suction, always apply a specialist sealer key coat or suitable render preparatory coat. Allow a minimum of 48 hours for the key coat to fully dry before applying the next coat.

 

For our warranty purposes, all externally rendered cavity walls (masonry outer and inner leafs, or masonry outer and framed inner structures) must be provided with a suitable cavity tray, stop ends and weep vents over all external wall openings, flashing abutments and horizontal cavity barriers. The weep vents must be provided as a specific Warranty requirement (regardless of the render type, British Standards or manufacturers guidance) and must not be blocked/covered over by the render.

 

Render cladding support systems onto timber/steel framed structures

 

Where render is specified for timber/steel frame structures, the following must be satisfied:

• Only pre bagged and blended (factory made) render complying with BS EN 13914-1:2016 must be used and evidence of UKCA/CE mark provided (site made renders are not acceptable)
• A drained (and vented, if timber/ SIP framed) cavity must be provided between the external cladding and the structural frame of the building
• A breather membrane must be provided to protect the main structural frame as a second line of defence
• A vertical DPC should be inserted between the metal render carrier and any vertical rail / batten support

 

In addition, the render must be supported by either:

 

A) Render carrier boards

 

• The render boards must be proven to meet a relevant BS EN standard (specific to that board type) UKCA/CE marked, and
• The performance criteria of the board (based on testing from the relevant BS EN standard) must be declared to prove it is suitable for use as a direct render board. For example, a fibre cement board should be to a level category A weather resistance to BS EN 12467, unless it has been tested by a UKAS accredited third party product approval body as a system i.e. the render and board together, where a category B board may be acceptable (see note 2 below), and
• A third party product approval certificate for the board must also be provided to demonstrate the render board will achieve:
  • A 15 year minimum life expectancy, and,
  • Proven to be suitable for use in the expected wind driven rain exposure zone of the project.

 

Notes:

1. BS EN standards or UKCA/CE marks alone are unlikely to confirm the durability period and maximum wind driven rain exposure category a board can achieve – and will not be shown in the CE Therefore, a third party product approval certificate (BBA or other accepted by Warranty) is required to confirm these performance requirements.
2. A third party product approval certificate for a combined render and render board ‘system’ may be acceptable providing the scope of the certificate clearly states the limitations of use. Please note, only the ‘products’ specified in the third party product approval will be considered. Mixing and matching to different boards or other renders than those shown on the certificate, will not be covered by the
3. Evidence from the pre blended and bagged (factory render) manufacturer is required to confirm the render is compatible with the render
4. MgO boards are not accepted for use on warrantied projects

 

B) Metal render carrier systems

 

• The carrier system must be stainless steel in accordance with EN 10088-1 (austenitic steel) or zinc coated steel in accordance with EN ISO 16120-2 and EN 10346.
• Where sited in a coastal location a higher grade A4 stainless steel should be used (see Appendix B of the Technical Manual).

 

Direct rendering onto insulated concrete formwork structures (ICF)

 

A render directly applied to an ICF structure to provide a weather resistant cladding to the ICF, will not be acceptable and alternative solutions must be made. Typical alternative solutions are:

 

• A pre bagged and blended (factory made) render complying with BS EN 13914-1:2016 on a carrier system with a drained cavity behind and a breather membrane provided as a second level of defence to the ICF structure, or,
• An impervious cladding system provided (with appropriate cavity provision as required by the impervious cladding system), or,
• An independent masonry external cladding with a 50mm minimum cavity is provided, or,
• An independent cladding system, with a separating cavity complying with our Technical Manual requirements and a breather membrane provided as a second level of defence to the ICF structure.

 

Rendering on external wall insulation systems (EWI or EWCS)

 

Where render is be applied to an EWI, a third party product approval such as BBA, BDA is required for the combined render and insulation system and must clarify which substrate has been assessed for the EWI / EWCS system to be applied too (i.e. for a masonry or framed substrate).

 

Please note: ETA documents, do not generally state the maximum UK exposure zones they have assessed the system for. The ETA documents usually only say the system (including the render) improves the thermal performance of the wall and contributes towards weather resistance.

 

Therefore for Warranty purposes:

 

• For installing on a masonry substrate the masonry wall should be thick enough to meet PD6697 for a given exposure zone.
• For installing on a framed structure a drained (and vented) cavity will be required and a breather membrane installed to protect the framed structure.

 

Caution, EWI / EWCS may not be acceptable for use in external walls in buildings over 11m / 18m high (Scotland / England-Wales).

 

Rendering below horizontal DPC level - ALL cases (i.e. within 150mm of the adjacent ground level)

 

• Render should not bridge the DPC and should be finished onto a durable render stop profile such as a proprietary uPVC bead or stainless steel bead.
• Renders used below the DPC will only be considered if the render manufacturer provides a site- specific specification for this. Where site made render is used, a strong mix (M4) that is sulfate resisting should be used.
• Renders / boards to be used within 150mm of the adjacent ground level, to have third party approval for use in this location.
• There should be appropriate drainage installed along the perimeter or the ground should fall away from the building.
• Adjacent ground surfaces should not promote splashing

 

 

Please Note: Every care was taken to ensure the information in this article was correct at the time of publication (updated February 2024). Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of our Technical Manual.