NHL Renders Diagnostics Chart

St Astier Natural Hydraulic Limes (NHL)

Defect Causes Remedies
Shrinkage &
Cracking greater
than 2mm

Less than 2mm



Hairline cracks


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 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. (Engineer to
check). If building stable,
repair cracks / areas.

Depending on extent, open
out crack and fill with same
mortar.



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. 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, ( pror 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.
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For further Guidance, contact your St Astier Distributor.
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