|Other General Guidelines documents:
Render on cob
|The correct specification for any render should consider the nature and condition of the background, site exposure, time of the year (weather maps / rainfall and wind driven rain indices) and type of finish required.
The success of a render depends on ensuring good background preparation and suction control, the correct choice of a mortar and its application. Sample panels should always be carried out. The durability of a render depends on mortars that will adhere to the background, are able to breathe and resist harsh climatic conditions that can and do occur even in relatively benign climate zones. A good bond to the substrate and between all coats is essential to the soundness of the render structure. Bonding is both physical and mechanical:
To avoid potential de-bonding and cracking each coat should be 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 finishing coats, finer sands, still well graded, can be used for smooth finishes (avoid overtrowling). 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: the finer sand particles are the ones mostly responsible for color and therefore used for color rendition. If the fines denote presence of clay (particles below #200 mesh) 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 less 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 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, use biocides where applicable, ensuring that they will not affect the mortar.Newly built walls should be allowed to dry properly, usually 1 month. This will not take place readily in winter conditions.Repointing before rendering:
If this is necessary it should be done with a compatible mortar.Detailing:
Inspect all details, i.e. 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 finish at the base of a wall into free draining gravel.
It should be noted that in the presence of different suction levels the degree of dampening will vary accordingly.
Provide adequate keying between background and base coat and between each coat. Crisscross patterns are preferred to
combing. Make sure that keying does not cut too deeply. Sometimes joints in brickwork are raked back (normally 1/2″), this is not necessary with NHL renders if a stipple coat is applied cast on, harled or sprayed on.Two coat work
Two coat work is suitable for renders with an overall thickness of approx. 5/8″ 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 (1/8″-1/4″ 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 1/2″, more if applied in 2 passes) and with a binder: sand ratio of 1:1.5 or 1:2. Use mainly NHL 5 or NHL 3.5. This can be laid on or preferably cast/sprayed on. Scour back and key after initial setting.
To ensure a flat and uniform surface see “Ensuring a level surface” under Undercoat in 3 coat work section.Curing:
Check for initial shrinkage. If found, dampen surface lightly with water and tighten back and re-key. Repeated shrinkage is usually a function of poor quality sands, poor suction control or rapid drying.Finishing coat:
Use NHL 3.5 or NHL 2 (see individual product sheets)
3/16″ max. for smooth or light textured finishes, 5/16″ for coarse finishes (tyrolean, roughcast etc…).
Smooth and light textured finishes:
A laid on scratch coat can be used on old bricks or surfaces providing a good key (greater care is required in 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.
Second coat (straightening):
Ensuring a level surface:
An alternative is to make running screeds 4″ wide at regular intervals.
Scour back and key as usual after initial setting. Check for shrinkage during the first 2 days and, if necessary, lightly dampen the relevant area, tighten back and re-key. In case of intermediate coats this would apply to each coat. Do not apply finishing coat until undercoat is adequately hardened.
|Shrinkage & Cracking
greater than 1/16″
|General or partial movement of the background
or the building.
|Check if movement is still active. (Engineer to check). If building stable, repair cracks / areas.|
|Shrinkage & Cracking
less than 1/16″
|Thermal movement.Poor workmanship. Render too thick. Too much water in mix. Over saturated backgrounds. Insufficient setting between coats.||Depending on extent, open out crack and fill with same mortar.|
|Hairline cracks||Bad preparation of background. Over saturated background. Too much binder. Too many fines in sand. Finishing coat to thick.Too much water in the mix. Rapid drying / lack of protection. Too much sun or wind during curing.||Either apply slurry fill if sound or remove 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. Neutralize 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 sands.||Partial or total repair with correct mortar applying due protection and following best practice.|
|Water penetration.||Poor background preparation. Weak mortars. Bad detailing.||Partial repair. Light repairs with several coats of lime wash.Rectify
detailing problems. Replace if necessary.