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I have plaster which is falling off an interior wall below a sash window. I believe this is caused because the waterproof membrane has failed on the exterior window sill and now the damp / water is seeping to the inside wall? I never through there would be a waterproof membrane but it seems like where it has cracked, it has multiple layers of more than just paint. It has also cracked in a straight line, which looks to be like it has cracked along a seam?

How would I go about repairing this?

1. Scrap off all paint / membrane from the exterior window sill
2. Prime / replace membrane (what membrane / primer should i be using?)
3. Paint (what exterior paint should I be using?)
4. Chip away interior plaster from under the window
5. Replaster and paint

Repairing a damaged interior wall caused by a failed waterproof membrane on the exterior window sill can be done by following these steps:

Prepare the area: Start by protecting the surrounding area with drop cloths or plastic sheets. Use a scraper or putty knife to carefully remove any loose or peeling paint or plaster from the interior wall below the window. Be gentle to avoid causing further damage.

Assess the exterior window sill: Inspect the exterior window sill thoroughly. Remove any remaining paint or membrane from the sill using a scraper or sandpaper. Check for cracks, gaps, or damage in the sill that may have caused the water infiltration. If necessary, repair or replace the sill to ensure it is in good condition.

Apply a waterproof membrane: Once the exterior window sill is clean and repaired, it's crucial to apply a new waterproof membrane. There are various options available, such as liquid waterproofing membranes or self-adhesive membranes. Choose a product specifically designed for the purpose of waterproofing exterior surfaces. Follow the manufacturer's instructions for application, including any recommended priming steps.

Prime the exterior surface: After applying the waterproof membrane, it's essential to prime the exterior surface. Select a high-quality exterior primer suitable for the material of the window sill (e.g., wood, concrete, or metal). Follow the manufacturer's instructions for proper application, such as surface preparation and drying times.

Paint the exterior surface: Once the primer has dried, apply a suitable exterior paint to the window sill. Choose a paint that is specifically formulated for exterior use, as it will provide durability and protection against weathering. Consider the material of the window sill when selecting the type of paint (e.g., latex paint for wood, masonry paint for concrete). Follow the paint manufacturer's instructions for application, including the number of coats and drying times.

Repair the interior wall: Chip away any damaged or loose plaster from the interior wall beneath the window using a scraper or chisel. Be careful not to cause additional damage to the wall. Clean the area thoroughly to remove any debris or dust.

Replaster and paint the interior wall: Once the damaged plaster has been removed, apply a new layer of plaster to the area. Follow standard plastering techniques or consider hiring a professional if you're not experienced in this type of work. After the plaster has dried, sand it down to achieve a smooth surface. Then, prime and paint the repaired area to match the existing wall.

Remember to follow all safety precautions and use appropriate personal protective equipment when undertaking any repair work. If you're unsure about any step or feel uncomfortable with the process, it's recommended to consult a professional for assistance.