Damp Proofing a Wall Before Plastering: A Complete Guide
When preparing a wall for plastering, ensuring that it is properly damp-proofed is crucial to maintaining the integrity and longevity of the surface. Damp walls can lead to structural issues, peeling plaster, and unsightly mould. In this guide, we’ll explore the key aspects of damp proofing walls before plastering, including methods, products, costs, and frequently asked questions.
How to Treat Damp Walls Internally
Treating damp walls starts with identifying the source of moisture. Common causes include rising damp, penetrating damp, or condensation. Here are the steps to treat damp walls internally:
- Fix the Damp Source:
- Address the original cause of the dampness. This could involve repairing leaking gutters, fixing plumbing issues, or addressing faulty roofing.
- Remove Old Plaster:
- Remove any old plaster or render to expose the original masonry. Damaged plaster can contain salts that attract moisture, so replacement is essential.
- Clean and Prepare the Wall:
- Scrub away any mould or efflorescence (salt deposits) using appropriate cleaning solutions. Ensure the surface is clean and dry before proceeding.
- Apply a Damp Proof Membrane (DPM):
- Cut the membrane to size, place it on the wall, and fold it around corners for a snug fit.
- Drill into the wall and secure the membrane using special fixing plugs. This creates a waterproof barrier.
- Use a Fleece Over Strip to seal any joints or overlaps in the membrane.
- Apply a Damp Proof Plaster:
- Use a plaster containing a salt retardant solution to prevent moisture-related damage.
- Optionally, apply a layer of PVA to the wall before plastering to help the plaster dry evenly and adhere properly.
- Wet the Wall Surface:
- Slightly dampen the wall to ensure the mortar adheres well. Avoid over-saturating the surface.
Illustration tip: Show a diagram of a cross-section of a wall with layers of waterproofing and plaster.
Wall Damp Proofing Products in the UK
The UK market offers a wide range of damp proofing products tailored to different needs. Here are some popular options:
- Injected Damp Proof Courses (DPC): These are gel or cream-based solutions injected into brickwork to form a water-resistant barrier. Popular brands include Dryzone and Safeguard.
- Damp Proof Membranes (DPM): Available as sheets or liquid coatings, these membranes prevent moisture from reaching the surface. Liquid DPMs, such as those by Sika or Ronseal, are user-friendly and effective.
- Waterproof Renders and Plasters: Specialized plasters like sand-cement mixes with water-repellent additives are ideal for damp-prone areas. K Rend and Weber are reputable brands.
- Anti-Mould Paints: Products like Dulux Easycare or Ronseal Anti-Mould Paint provide a decorative finish with added protection against mould growth.
- Waterproofing Paints and Sealants: These can be applied directly to walls to create an additional waterproof layer. Brands like Zinsser and Ronseal offer effective options.
- Textured Wall Panels: These can be used to hide wall seepage while providing an aesthetic finish.
Damp Proofing Internal Walls Costs in the UK
Costs vary based on the severity of the damp and the chosen treatment. Here is a detailed breakdown:
- Injected DPC:
- Cost: £150-£300 per wall (including labour and materials).
- Suitable for treating rising damp.
- Damp Proof Membrane (DPM):
- Cost: £20-£50 per square metre.
- Ideal for larger areas requiring comprehensive moisture protection.
- Waterproof Render or Plaster:
- Cost: £25-£50 per square metre.
- Commonly used in basements or highly affected areas.
- Ventilation Systems (PIV):
- Starting cost: £500 for basic installations.
- Helps to mitigate condensation issues and improve air quality.
Remember that costs can rise if extensive plaster removal, wall repairs, or additional waterproofing layers are required.
How to Treat Damp Walls Before Painting
Painting over damp walls without proper preparation can lead to peeling and discolouration. Here’s how to do it right:
- Dry the Wall Completely:
- Use dehumidifiers or heaters to accelerate drying. Ensure the wall feels dry to the touch and test with a moisture meter if possible.
- Seal the Surface:
- Apply a waterproof primer or damp seal paint. Products like Zinsser B-I-N or Ronseal Damp Seal are ideal for this purpose.
- Apply Anti-Mould Paint:
- Use paint containing mould inhibitors to prevent future growth. Anti-mould paints are particularly useful in kitchens and bathrooms.
- Test for Residual Damp:
- After sealing and priming, allow the wall to sit for 24-48 hours and inspect for any signs of persistent moisture before applying decorative paint.
Waterproof Plaster for Damp Walls
Waterproof plaster ensures that your walls remain moisture-resistant while providing a smooth finish. Follow these steps for best results:
- Choose the Right Product:
- Opt for pre-mixed waterproof plasters or prepare a sand and cement mix with water-repellent additives.
- Prepare the Wall:
- Remove all loose or damaged plaster and clean the substrate to ensure good adhesion.
- Mix and Apply the Plaster:
- Follow the manufacturer’s instructions to mix the plaster correctly. Use a hawk and trowel to apply it in thin, even layers.
- Work in sections to ensure consistency and smoothness.
- Cure and Finish:
- Allow the plaster to cure fully before applying decorative finishes. This can take 2-3 days depending on humidity levels.
Illustration tip: Show an application sequence with a trowel spreading plaster over a damp-proofed wall.
Technical Considerations for Damp Proofing Walls
Do You Need to Remove Plaster for an Injected DPC?
Yes, removing plaster is often necessary when installing an injected DPC. Moisture-damaged plaster can contain salts that attract water, compromising the new finish. Removing and replacing the plaster ensures a clean, dry base for damp-proofing.
How to Waterproof and Spread Plaster?
- Use Waterproof Additives: Mix these into the plaster to enhance its moisture resistance.
- Apply Evenly: Spread the plaster in thin layers to avoid cracking.
- Seal Edges and Joints: Ensure these areas are well-covered to prevent moisture ingress.
Ventilation Systems Like PIV:
PIV systems improve air circulation and reduce condensation-related damp. They work by introducing filtered air into the property, forcing out stale, humid air. Installing PIV systems is particularly beneficial in older homes with poor ventilation.
Frequently Asked Questions
Do You Damp Proof Before Plastering?
Yes, damp proofing is essential before plastering to prevent moisture-related issues such as peeling and mould.
How to Waterproof a Wall Before Plastering?
Apply a damp proof membrane, liquid waterproofing solution, or waterproof render to create a protective barrier against moisture.
Can You Plaster Over a Damp Proof Membrane?
Yes, but ensure the plaster mix is compatible with the DPM and follow the manufacturer’s guidelines for best results.
Is It Necessary to Wet the Wall Before Plastering?
Slightly wetting the wall can help the plaster adhere better, but avoid over-saturating it, as this can weaken the plaster’s bond.
Damp proofing walls before plastering is a vital step in ensuring a durable and moisture-free finish. By using the right products, techniques, and tools, you can protect your walls and enjoy a smooth, long-lasting plastered surface. If you’re unsure about the best approach, consult a damp and mould specialist for expert advice.