Renders
with Natural Hydraulic Limes
General information
The correct specification for any render should consider the nature and condition of the background, site exposure, seasonal conditions (weather maps, rainfall and wind-driven rain indices are available from the Met Office) and type of finish required.
The success of a render depends on ensuring good background preparation and suction control, the correct choice of mortar and its application. Products should always be tested in a small area or test panel before application.
The durability of a render depends on a mortar’s adhesion to the background and its capacity to breathe and resist harsh climatic conditions.
A good bond to the substrate and between all coats is essential to the soundness of the rendered structure. Bonding is both physical and mechanical:
– A physical bond is achieved by controlling the suction correctly so that a suction bond develops. The natural surface condition can also offer a good key.
– Mechanical bonding is induced by the method of application. Ensuring good keying between layers, especially the first coat, by casting/harling or spraying is by far the most successful method.
To avoid de-bonding and cracking, each coat should not be richer in binder or thicker than the preceding one (thicker base coats are applicable on thin stipple/scratch coats).
Sands for renders
In dubbing out, stipple coats and base coats the sands should be well-graded, washed and free of clay/silt (particles below 0.075). Use sharp sands from 3 or 4mm, down to 0.075mm, with the bulk of the sand in the 1.18mm/0.6/0.3/0.15 range. Fine sands or monogranular sands (bulk in 1 or 2 grades only) are to be avoided.
In finishing coats, well-graded finer sands can be used for smooth finishes (avoid over-trowelling). Particular attention will have to be paid to finishing coats with fine sands to avoid high shrinkage due to the high amount of water that fine sands absorb. The use of a wooden float, energetically applied in small circular motions, will help. Floating with plastic floats is not suitable. Sponge floats can be used after the wooden float work is completed to achieve a particular texture in the finish. Curing will also be important. Small hairline shrinkage cracks can be healed if treated in time with a light water mist.
Note: Finer sand particles are mostly responsible for colour and therefore used for colour rendition. If clay particles are present (particles below 0.075), the NHL binder quantity should be reduced (clays are also binders!). A wet sieving analysis is recommended to check clay/silt content.
Check that any movement cracks are stable and where necessary ensure they are properly tied and if needed, grouted/pinned/pointed. Careful removal of existing renders will result in fewer remedial repairs prior to re-rendering. Removal of failed or inappropriate existing render or finishes, including many types of paint, may require the walls to be left to dry out properly before re-rendering and time should be allowed for this. Ensure all repairs to the background are completed and that loose pinning stones or defective bricks are repaired or replaced prior to the commencement of any rendering. Partial or complete re-pointing/consolidation may be required. Remove all loose and friable materials, remove and treat all organic growth and use biocides where applicable, ensuring that they will not affect the mortar.
Newly built walls should be allowed to dry properly, usually for 1 month. Application is not recommended during winter months.
Repointing before rendering
If this is necessary it should be done with a compatible mortar. See Pointing with NHL.
Detailing
Make sure to inspect all the details (e.g. copings, etc.). Check gutters and down pipes and all forms of roof drainage, ground drainage and general ground conditions. Make sure all the above items are functioning properly and where remedial action is required, ensure it is completed before proceeding with render work.
Rendering should never come into contact with soil. Renders should be kept clear of the ground or end at the base of a wall with free-draining gravel.
Dubbing out
On defaced surfaces or in areas with a large number of damaged joints, it will be necessary to apply a dubbing out coat to provide a level surface. In most cases this will be sufficient with mortar, however very deep joints or hollows should be pinned to reduce the mass of mortar. When a dubbing out coat is used, let it set sufficiently (8-10 hours) before scraping it and keying it. Apply the first coat after approx. 2 days (more if very deep recesses have been filled) and depending on weather conditions. Dubbing out should leave a relatively flat surface, keyed as necessary, on which to render.
Suction control
If needed, apply sufficient water to reduce excessive suction, especially on bricks and porous stone. Old bricks often require more water than new ones. On many occasions this is done the day before, if necessary, several times with the last damping just before application starts. Apply water starting at the top of the structure. Over saturation of the background will result in loss of bond. Never render backgrounds that have standing water on the surface. Always dampen preceding coats before applying next coat.
It should be noted that in the presence of different suction levels the degree of dampening will vary accordingly.
Keying
Provide adequate keying between the background and base coat and between each coat. Crisscross patterns are preferable to combing. Make sure that keying does not cut too deeply. Sometimes joints in brickwork are raked back (normally 10mm), this is not necessary with NHL renders if a stipple coat is cast on, harled or sprayed on.
Two-coat work
Two-coat work is suitable for renders with an overall thickness of approx. 15 mm on surfaces that provide adequate suction and a good key. On surfaces offering poor suction and keying, it is recommended to use a stipple coat (3-4mm thick) applied by casting on, harling or spraying. The main coat can be applied after sufficient hardening and finished as required. Alternatively, use 3-coat work by applying a finishing coat.
On two-coat work, the base coat will be the thickest (up to 10mm, more if applied in 2 passes) and with a binder (sand ratio of 1:1.5 or 1:2). Use mainly Saint-Astier® NHL 5 or NHL 3.5. This can be laid on or preferably cast/sprayed on. Scour back and key after the initial set. For the undercoat, see the three-coat work section.
1) Curing
Check for initial shrinkage. If found, dampen surface lightly with water and tighten back and re-key. Repeated shrinkage is usually a result of poor quality sands, poor suction control or rapid drying.
2) Finishing coat
Use Saint-Astier® NHL 3.5 or NHL 2 (see individual product sheets), 5 mm maximum for smooth or light textured finishes, 7-8 mm for coarse finishes (tyrolean, roughcast, etc).
3) Smooth and light textured finishes
Use finer, well-graded sands, 1-2 mm down to 0.075mm. Add just enough water to obtain the required workability. The more water added, the higher the risk of shrinkage. When the mortar is firm enough, proceed to float up with a cross-grained wood float. This is the most important phase of the finishing work and should be done diligently. Together with good curing and protection, it is vital in obtaining a good finish. See Protecting Lime Mortar.
4) Coarse finishes
Use coarser sands if thick/rustic granular finishes are required. The thickness of the coat depends on the final finish required. Some of these finishes, especially ones requiring skills such as the cottage/scraped finish, can also be done by using the same type of sand as smooth textured (floated) finishes. Here and in tooled renderings (patterned), if initial shrinkage takes place, lightly dampen the surface and re-float the area during the first day or two. Tooling is normally applied when the render is 5-7 days old.
5) Dry dashing
Throw the chosen aggregate onto soft mortar and leave exposed. To speed up the work a plasterer throwing the aggregate can follow the laying-on plasterer.
6) Curing
Curing by water mist over 3 to 4 days, if necessary more than once a day, is essential when weather conditions cause quick drying. See Protecting Lime Mortar.
Three-coat work
For information about background preparation, sands, suction control, keying and dubbing out, see the previous sections.
1) First coat
The thickness of the first coat depends on the nature of the background and the overall thickness required for the render. The first coat must provide sufficient bonding. Stipple or spatter dash can be used on all backgrounds, but works especially well on impervious and smooth backgrounds. Leave these coats rough to provide a key. Use a richer mix (1:1.5 preferably). The normal thickness is between 3 and 5 mm. On soft or weak backgrounds, use a mix of 1:2 or 2:5. Successive coats must be weaker than the first coat.
A laid-on scratch coat can be used on old bricks or surfaces providing a good key (greater care is required in the application to ensure good bonding with the background). It will be scoured back with a cross-grained wood float and keyed (crisscross keying pattern preferred) once initial stiffening has taken place.
2) Second coat (straightening)
The second coat has to be applied 2 days (or more, depending on weather conditions) after completion of the first coat. Its strength should be less than the first coat. The thickness will vary according to the overall thickness required but it is normally between 10 and 15 mm. It must not be over 20 mm thick. If a thickness over 20 mm is required, it should be done in successive coats not exceeding 20 mm each. The thicker the intermediate coats, the longer the waiting time before each subsequent application.
3) Ensuring a level surface
To achieve a uniform and level surface, fix vertical timber battens or dabs on the wall at 2-2.5 m intervals. If the wall is uneven, use spacers and check that battens are straight with a plumb level. Fill out to screeds, if necessary in layers. Screed off excess mortar between battens with a wooden straightedge spanning between the battens. When battens are taken down, fill in strips with the same mortar. An alternative is to make running screeds 100 mm wide at regular intervals. Scour back and key as usual after the initial set. Check for shrinkage during the first 2 days and, if necessary, lightly dampen the relevant area, tighten back and re-key. In the case of intermediate coats, this would apply to each coat. Do not apply a finishing coat until the undercoat is adequately hardened.
4) Finishing coat and curing
Follow advice for two-coat work.
Protecting NHL mortars and renders
The setting properties of NHL mortars require protection against adverse weather conditions such as heavy precipitation, strong winds and high temperatures. Please contact your Saint-Astier® Distributor to advise further. See Protecting Lime Mortar.
Early exposure to rain will cause some moisture absorption in the first few millimetres of a fresh render. If frost occurs, there is likely to be damage.
The preferred form of protection is hessian covers that, with re-damping, will also contribute to curing the mortar. Hessian covers are essential to protect against frost. Plastic sheeting is effective against rain but should be kept clear of fresh work. If too tight it will generate condensation leading to unsightly staining. Plastic sheeting will not protect against frost. Frost protection should be provided even if frost is not occurring at the moment of finishing the day’s work but is forecast during the early days of a mortar. Work should not start in frost conditions or when frost is forecast or with temperatures below 8oC. In working with NHL 2 or in rendering with fine finishing coats, the minimum working temperature should be 8oC. Protection from the quick drying effects of wind or direct, strong sunlight should be provided by using shading sheets on scaffolding.
Good work practices
A good and durable result depends mainly on the correct mortar mix, sand, dosages and workmanship, including background preparation, suction control, detailing, keying, protection and curing. One item not to be overlooked is scaffolding.
Where scaffolding is being used, make sure that the scaffolding has adequate clearance from the face of the wall to allow application, avoiding unsightly lift lines. Scaffolding should project past all areas to be rendered to allow for protection of the new work against direct rainfall. Generally, scaffolding should be capable of carrying the protective screens necessary to shade the work and prevent rapid uncontrolled drying and any covers needed to protect against frost.
NHL Render Diagnostics
| Defect | Causes | Remedies |
|---|---|---|
|
Shrinkage & cracking greater
than 2mm
Less than 2mm
Hairline crack |
General or partial movement of the background
or the building.
Thermal movement. Poor workmanship. Render too
thick. Too much water in mix. Over saturated
backgrounds. Insufficient setting between coats.
Bad preparation of background. Over saturated
background. Too much binder. Too many fines (clay particles) in
sand. Finishing coat to thick. Too much water in the
mix. Rapid drying/lack of protection.
Too much sun or wind during curing. |
Check if movement is still
active (structural engineer to
check). If building is stable,
repair cracks/deteriorated areas.
Depending on extent, clean out crack and fill with like-for-like
mortar.
If sound, apply slurry fill. Otherwise remove existing mortar and replace properly. |
|
Loss of bond |
Poor background preparation. Poor suction control.
Over saturated background. Background too
smooth. Incompatibility with existing
background. Insufficient strength in bonding
coat. Background movement. Metal corrosion. Salt
crystallisation. Excessive or late towelling. |
Repair or replace as appropriate. Consolidation by grouting may be considered. |
|
Bulging |
Poor background preparation. Incompatibility with existing background. Metal corrosion. Frost damage during curing. |
Depending on the extent of damage, either partial repair or total replacement. Neutralise and treat any rusting metal. |
|
Powdering / Friability |
De-calcification of render (loss of binder). Poor background preparation. Poor suction control. Rapid evaporation of water during application, prior to adequate set. Frost damage. Insufficient binder dosage. Variation in surface compaction/finishing. Poor quality sands. |
Partial or total repair with correct mortar with appropriate drying and curing protection. |
|
Moisture penetration |
Poor background preparation. Weak mortars.
Bad detailing. |
Partial repair. Light repairs with several coats of limewash. Rectify detailing problems. Replace if necessary. |
Some recommended mixes
| Background: Prepare background. Re-point and dub out as necessary with compatible mortar. | Stipple Coat: Cast or spray on only. Must be used on poor suction, dense or smooth surfaces. Leave as a cast cure for 2-4 days. | First Coat Cast: Spray or lay on a well-keyed background. Control suction and leave keyed cure for 4-7 days. | Second Coat Cast: Spray or lay on. Control suction. Straightening coat. Leave keyed and cure for 7-10 days. Finishing coat in 2 coats work. | Finish Cast: Spray or lay on. Control suction and finish as required. Cure for minimum 3-10 days. |
|---|---|---|---|---|
|
Cob / Earth |
Mix 1A – Sand SG3-5mm |
Mix 1B – Sand SG10-15mm |
Mix 1C – Sand SG5-10mm |
Mix 1C – Sand SF / FS3-5mm |
|
Wooden Lath |
|
Mix 2B/3B – Sand SC12-15mm (8-10 cover) |
Mix 2C – Sand SC8-10mm |
Mix 1C – Sand SF3-5mmMix 2C – Sand SM5-8mm |
|
Metal Lath |
|
Mix 3B – Sand SC10-15mm (8-10 cover) |
Mix 2B – Sand SM10-20mm |
Mix 1C – Sand SF3-5mmMix 2C – Sand SM5-8mm |
|
Soft Brick / StonePorous Blocks |
|
Mix 1B – Sand SC10-15mmMix 2B – Sand SC10-15mm |
Mix 1B/C – Sand SC/SM10-12mmMix 2B/C – Sand SC/SM10-12mm |
Mix 1C – Sand SF3-5mmMix 2C – Sand SM5-8mm |
|
Medium Brick / Stone / Blocks |
Mix 2A/3B – Sand SG*3-5mm |
Mix 2B/3C – Sand SC10-20mmMix 2B/3B – Sand SG**10-15mm |
Mix 2C/3D Sand SC/SM8-12mmMix 2C/3C Sand SG**6-10mm |
Mix 1C – Sand SF3-5mmMix 2C – Sand SM5-8mm |
|
Dense Brick / Stone/ Blocks/ Concrete |
Mix 2A/3A -Sand SG3-5mm |
Mix 2B/3C – Sand SC10-20mmMix 2B/3B – Sand SG**10-15mm |
Mix 2C/3D Sand SC/SM8-12mmMix 2C/3C Sand SG**6-10mm |
Mix 1C – Sand SF3-5mmMix 2C – Sand SM5-8mm |
| Lime | Mix A 1:1.5 | Mix B 1:2 | Mix C 1:2.5 | Mix D 1:3 | Mix E 1:4 |
|---|---|---|---|---|---|
|
NHL 2 |
1A |
1B |
1C |
1D |
1E |
|
NHL 3.5 |
2A |
2B |
2C |
2D |
2E |
|
NHL 5 |
3A |
3B |
3C |
3D |
|
| Sands | Type | Particle Sizes |
|---|---|---|
|
SG |
Sharp gritty |
5mm down to 0.075 |
|
SC |
Sharp coarse |
3.35mm down to 0.075 |
|
SM |
Sharp medium |
2.36mm down to 0.075 |
|
SF |
Sharp fine |
1.18mm down to 0.075 |
|
FS |
Fine soft |
0.8mm down to 0.075 |
* Stipple coat optional, depending on background suction and conditions.
** For harling applications.
Note: A wide variety of finishes can be achieved by adopting different binder and sand mixes to satisfy all requirements.
Coat thickness and optional mix ratios are related to exposure and background conditions and are the responsibility of the designer.
Curing and protection must follow best working practices in order to be effective.