With so many render options out there, how do you choose the one you need? Peter Mayer of Building Performance Group examines the key issues and outlines the whole-life costs of three alternatives
when they are correctly designed and applied, traditional renders are a durable wall cladding option and can be expected to perform with little input over the design life of a building. From a whole-life cost point of view, this means that the initial cost will be the principle factor, determining best value over a given period. However, whole-life performance is also about risk management; reference service lives (see table below) reflect the relative risks associated with longevity rather than absolute performance.
Where renders are overpainted, whole-life costs will depend as much on the costs associated with the redecoration cycle as initial costs.
Proprietary and insulated render systems do not have the same track record as traditional renders. Third-party assessments tend to quote service lives in excess of 20 years. However, evidence of the durability of these systems in use is increasing as time progresses – in Europe insulated renders have performed well for over 40 years.
The service life of a component usually indicates when the component needs complete replacement. It is relatively rare for a facade or render to fail completely. The pattern of failure tends to be gradual; particular areas have a propensity to fail earlier than others. For example, exposed parapets or cracking at junctions and corners of openings, due to stresses caused by variations in thermal or moisture condition. Service lives can, and usually are, maintained by carrying out timely repairs rather than wholesale replacement.
- Factors affecting durability
- Wall type, exposure, substrate and render systems</b>The wall type must be suitable to the exposure at the site (BS 5262: 1991, Figure 2 and BS 8104).
- Micro-climate influences may result in an increase or decrease of the exposure.
- The render system must be suitable for the substrate and exposure zone for the site, in particular driving rain. BS 5262 clauses 31, 32 and 36; Tables 1, 2, 3 and 4 advise.
- Dissimilar substrates should be carefully detailed either with stop beads or lathing to minimise cracking.
- Blockwork commonly gives rise to pattern cracking in renders. Blocks should be adequately cured and stable before rendering.
- Many premature failures and early repair requirements are the result of poor installation. Applying renders is a skilled operation. Use of approved and specially trained contractors, particularly for proprietary and insulated render systems, reduces the risk of early repair costs. General detailing issues
- Strength and thickness of render coats; subsequent mixes should be weaker and thinner than previous coats.
- The final coat should be slightly thicker than the maximum sand particle size.
- Movement joints are essential to control cracking. They should be at centres appropriate to the movements expected of the render and substrate. The maximum spacing should be no more than 5 m but a more reliable solution would be to have movement joints in the render at half the centres of movement joints required in the blockwork.
- Rendered parapets should be avoided if possible.
- Render may be applied to both sides of a parapet but great care is required to ensure moisture from the walling does not disrupt the rendered finish. A preferred alternative is to apply render on one side only to allow moisture in the wall to evaporate.
- Durability and whole-life cost tips
- Traditional sand–cement render</B>Use a three-coat system for severe exposure.
- Permeable mixes are more durable than impermeable mixes.
- Ready-mixed render should be specified to ensure consistency of mix and use of correct materials (to the British Standards noted in BS 5262 section 2).
- Lime-based render systems have better reliability than cement-based systems with plasticiser.
- Consideration should be given to using a proprietary coloured render, which would obviate the need to paint the render and obviate future repainting costs.
- Textured finishes are less likely to show cracks than smooth finishes. Water is more likely to "ride" over the cracks rather than be drawn into the render.
- Rough textured finishes are more resistant to rainwater penetration.
- Dry dash (pebbledash) finishes help reduce rain penetration.
- Flint (with no iron) or calcareous dry dashes have the best self-cleaning properties.
- Throwing on render is more likely to produce a finish, with better adhesion and therefore better durability, than laying on with a trowel.
- Allow adequate drying time between successive coats. Protect surface from sun, rain and wind. Shrinkage and drying out may take several days.
<B>Proprietary render systems</b>
- Polymer modified render systems with a polymer reinforcing mesh have good resistance to impact damage as the polymers have the ability to flex
- Use pre-batched polymer modified render systems to ensure performance. Site mixing is susceptible to incorrect proportions.
<B>Insulated render systems</b>
- Use thin reinforced polymer modified renders or sand-cement renders supported by metal lathing when rendering over expanded plastics insulation boards.
- Increase reinforcement mesh at areas of stress concentration such as window and door openings.
- Foam insulation gives a more stable background for rendering and offers good resistance to impact damage.
- Specify systems rather than individual components. Compatibility of the individual components is key to the durability of the system.
- Where exposure is high, tall fixing frequency may need to be increased.
<B>Corner beads and stops</b>
- Austenitic stainless steel beading, at least UK type 304, is preferred if trying to achieve long life. PVCu stop beads are at greater risk of failing before the render. Galvanised steel may be used in sheltered locations provided the exposed edges are coated with epoxy resin or a plastics coating. But there is always a risk of corrosion.
- Bellcasts should overhang rendering by 50 mm and extend either side of the opening to ensure water is thrown beyond the jamb and corner details below.
- Overhang at eaves and verges should be at least 300 mm to protect render from rainwater.
- Finish window and external door heads with a bellcast stop to throw water from the render.
- Window sills should include a drip and overhang the rendered wall by at least 40 mm. Sills should project sideways to throw water running down the jamb and rendered corners away from the wall.
- Finish render above the damp-proof course and at least 150 mm above external ground floor level with a bellcast stop.
Redecoration frequencies depend on the paint system and exposure.
- External emulsion paint: every five years
- Acrylic masonry paints: between five and 15 years
- Silicate-based paint system: between 10 and 15 years
Open textured coatings have a tendency to discolour because of algae or vegetation growth, particularly on north-facing walls or other areas where dampness may persist for long periods. Application of algaecides would be required under these circumstances, or use of a specialist paint system.
- Modes of failure
- This is the most common failure in render and may be caused by: thermal- or moisture-induced movement; drying shrinkage; frost damage; differential movement; impact damage and thermal shock in the case of insulated renders.
- This may be due to poor adhesion with the substrate or where cracks allow water penetration between the render and substrate.
- <B>Surface crazing</b>
- This can be caused by differential shrinkage in the thickness of the rendering. It is unlikely to cause water penetration.
- <B>Impact damage</b>
- Insulated renders and unmodified cementitious render systems are more prone to impact damage.
- <B>Chemical attack</b>
- Polluted urban or industrial environments with concentrations of sulfates and nitrogen oxide in the atmosphere attack the cement content of the render, resulting in crumbling and spalling. Smooth renders are particularly prone to damage.
- <B>Failure of insulated render systems</b>
- The render on these systems can fail as detailed above. Furthermore, mechanical fixings may fail if they cannot withstand wind forces. Likewise, adhesive-bonded insulated render systems depend on the integrity of the substrate and degree of contact between boards and wall.