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NHL Renders – Guide

Defect
Causes
Remedies
Shrinkage
&
Cracking greater
than 3/16″Less than 3/16″

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 trowelling.
Repair or replace as appropriate.Consolidation
by grouting may be considered.
BulgingPoor
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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