6.11 Render

Also see:

6.1

6.11.6 Mixes

The render mix shall be appropriate for the intended purpose, be compatible with the background and be designed to minimise the risk of de-bonding, cracking, and crazing. Issues to be considered include:

  1. sand
  2. mix design
  3. admixtures and bonding agents
  4. coat thickness of site-made render
  5. application of site-made render
  6. factory-made renders
  7. lime.

Render coats should not be stronger than the background or any previous coat to which they are applied. Weaker coats can be achieved by reducing the cement content of each coat or by using the same mix but decreasing the coat thickness.

Potable water should be used for mixing render.

Sand#

Sand for render should be well-graded category 2, in accordance with BS EN 13139. Sand with excessive fine material, clay or silt can shrink and crack so should be avoided.

A sharp gritty or coarse sand is required for strength in the backing coats, but finer sand should be used for the finishing coat.

Typical sand grades should be:

  • 5mm down to 0.075mm – undercoat(s)
  • 1.18mm down to 0.075mm – final coat.

Mix design#

Designation ii, iii and iv (strength class M6, M4 and M2) mixes are generally used for rendering.

Stronger mixes are generally more moisture resistant; however, they are also more prone to shrinkage, which increases the likelihood of the render cracking. Weaker mixes may be appropriate for weaker backgrounds in less exposed zones.

For exposure zone classification, see Clause 6.1.6.

Table 4: Designation mix proportions for cement-based mixes

NHBC technical illustration

Notes

1. With fine or poorly graded sands, the lower volume of sand should be used.
2. Where soluble salts could be present in the background, mixes should have sulfate-resisting properties.
3. Where pigments are specified, batching should be undertaken with care to ensure colour consistency pigments to BS EN 12878 can be used but should not exceed 10% of the cement weight, or 3% where carbon black is used (white Portland cement may be used).

Render mixes should be:

  • in accordance with BS EN 13914 ‘Design, preparation and application of external rendering and internal plastering’
  • appropriate to the strength of the background
  • checked against the specification
  • of adequate strength and thickness to achieve durability.

Where enhanced water-resisting properties are required:

  • Portland cement with a waterproofing agent already incorporated may be used in the undercoat, or
  • a waterproofing agent should be added to the render mix in accordance with the manufacturer’s recommendations.

Rendering mortar should not be left turning over in the mixer for longer than necessary.

Admixtures and bonding agents#

Admixtures and bonding agents should:

  • be assessed in accordance with Technical Requirement R3
  • be used in accordance with the manufacturer’s recommendations
  • be compatible with the render
  • not be used with factory-made renders without the prior approval of the render manufacturer.

The effect on the adhesion of subsequent render coats should be considered when water-repelling agents are used.

Plasticisers and air entrainers should comply with BS EN 934 and not be used in mortars containing masonry cement.

Coat thickness of site-made render#

The number of coats should be designed to take account of the background and exposure conditions of the site.

The mix and its application should be suitable for the specific background. Items to consider include:

  • the number and thickness of coats
  • the strength of the coat (subsequent coats should be weaker than the background or the previous coat).

Render should have a nominal total finished thickness of not less than:

  • 16mm for sheltered and moderate exposure zones, or
  • 20mm for severe and very severe exposure zones.

Table 5: Site-made render designation and typical thickness

NHBC technical illustration

Notes

1. Designation iii (M4) should be used for the final coat in severe or very severe exposure zones.
2. For block classifications, see Table 2.
3. Specialist advice should be sought for low density aircrete backgrounds.

Where a three-coat render is used, this should include a second undercoat that is:

  • the same thickness but a slightly weaker mix than the first undercoat, or
  • a slightly thinner coat of the same strength mix.

Application of site-made render#

When applying render, previous coats should be allowed to cure before applying the next coat (typically three to four days).

To avoid surface crazing:

  • properly graded sand should be used with limits on fine sand proportions
  • overworking (polishing) of the render should be avoided, as this causes laitance to be drawn to the surface.

Surfaces should be appropriately prepared to receive following coats. This can be achieved by either combing or scratching. The final coat should be applied to an undercoat that is suitably keyed.

The size of the background to be rendered should be assessed to determine if it can be rendered in the time available. This will help to establish the most suitable location for day joints.

The final coat should be of uniform thickness and not used to even out irregularities, which should be accommodated in previous coats.

Factory-made renders#

Factory-made renders should be applied in accordance with the manufacturer’s recommendations, including those for ancillary components.

Factory-made renders with a declared mix in accordance with Table 4, applied to the thickness recommended in Table 6, and that otherwise comply with the recommendations for site-made renders, will generally be acceptable to NHBC.

Table 6: Minimum thickness of factory-made single-coat renders

NHBC technical illustration

Notes

1. Lath backgrounds generally require two coats.
2. Alternative single-coat thicknesses may be acceptable when accompanied by appropriate third-party assessment in accordance with Technical Requirement R3.

Lime#

Render mixes containing hydrated lime can improve the ability of the render to accommodate movement, improving resistance to cracking and crazing. The use of lime should be in accordance with BS EN 459.

Natural hydraulic lime (NHL) is used without cement, which can allow greater moisture vapour movement through the structure. Specialist advice may be required for the use of NHL render.

Last updated: 2nd January 2024

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