Skill Builder Guides & reviews for building and construction
Sam Ward demonstrates how to repair low-level plaster.
MORTAR MIX
• 4 x Sharp Sand
• 1 x Castle Cement
• Sika 1 Waterproofer
https://www.builderdepot.co.uk/sika-1-waterproofer-5l
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For patching a plaster repair, a suitable mortar mix should ideally be compatible with the existing plaster in strength, porosity, and thermal expansion to avoid cracking or detachment.
A traditional mix often recommended for such repairs is a lime-based mortar due to its flexibility, breathability, and ability to accommodate minor movements within the building structure without cracking.
Lime-Based Mortar Mix
• Lime Putty: Pure, non-hydraulic lime putty is preferred for its flexibility and permeability.
It allows the repair to breathe, reducing the risk of trapped moisture, which can lead to damage.
• Aggregate: Fine sand is typically used as the aggregate in plaster repair mixes.
The sand should be clean, well-graded, and free from impurities.
The colour of the sand can be chosen to match the existing plaster colour.
• Mix Ratio: A standard mix ratio for lime mortar for plaster repairs is 1 part lime putty to 3 parts sand by volume.
However, this can be adjusted based on the specific requirements of the repair, such as the desired strength or finish.
Considerations
• Compatibility: Ensure the mortar mix is compatible with the existing plaster, especially if the building is historic.
In such cases, matching the original materials as closely as possible is essential.
• Application: Apply the mortar in thin layers, allowing sufficient drying time between layers.
This approach helps to prevent cracking and ensures a strong bond to the existing plaster.
• Curing: Lime mortar cures slowly and needs to be kept moist for several days after application to ensure proper carbonation and hardening.
Protect the repaired area from direct sunlight, wind, and rain during this period.
Alternative: Gypsum-Based Plaster
A gypsum-based plaster might be used for interior repairs, especially in modern constructions. It sets faster than lime mortar and can be easier to work with for minor patch repairs.
• Mix Ratio: Follow the manufacturer’s instructions for mixing, as ratios can vary.
• Application: This can be applied similarly to lime-based mortars but usually dries and sets much faster.
Final Note
For best results, especially on historic buildings or where the plaster plays a structural role, consulting with a professional conservator or a tradesperson experienced in traditional building techniques is advisable. They can offer guidance tailored to the specific type of plaster and conditions of your building.
