Render a Wall
Applying render (called plaster in the United States) to a wall is a great way to protect it from the elements as well as provide aesthetic interest. Renders made from lime or Portland cement can be applied to walls made from earth, brick, stone, or straw bale. They work by protecting the wall from wind damage and by absorbing water and allowing it to evaporate before it can penetrate the wall. Learning how to render a wall will allow you to perform this ancient technique that is still useful today.
Steps
- Prepare the wall for rendering. If the wall has previously been rendered and the plaster is deteriorating, start by sawing or raking as much of the old plaster away as possible. Once the old render is removed, wet the wall using a garden hose. Apply more water to walls made from very porous materials.
- Mix the render. Renders are a mixture of only 3 ingredients: lime, sand, and water. Portland cement can be substituted for a portion of the lime, which will result in a stronger, but less breathable plaster. Use either hydrated lime or lime putty, and mix 1 part lime to 3 parts sand in a large bucket or wheelbarrow. You must use sand with sharp, angular grains; masonry sand is the ideal choice. Add just enough water to form the mixture into a stiff, workable paste.
- Apply the scratch coat of render to the wall. Render is most effective when it is applied in several layers, and the first layer is called the scratch coat. Apply the render to the wall as forcefully as possible to ensure that it keys into the wall well; throwing clumps of render from a trowel or from your hands in the best method. Smooth the render across the wall with a trowel. Never make repeated passes with the trowel over the same area; this will bring the lime to the surface and leave you with a weak, lime-depleted section underneath.
- Scratch the first coat of render to prepare it for a second coat. Once the first coat of render is applied but not yet set, scratch the surface throughout the wall to provide a key for the next layer to adhere to. These scratches can be applied using a hand cultivator or the edges of a trowel.
- Apply the second coat of render to the wall. Once the first coat has been scratched and allowed to set most of the way, wet the wall again with the hose. Apply the second coat of render in the same manner as the first. If a third coat is desired, scratch the surface of the current coat to provide a key. If not, let the render dry slowly, and you are done rendering the wall.
Tips
- Rendering is best done on a cool, cloudy day. Hot weather will cause the render to set too quickly, which encourages cracking.
- Never use a Portland cement render on a wall made from earth, such as adobe, cob, or wattle and daub. The cement will trap moisture against the wall, which will severely weaken the earthen material.
- when rendering the sand should be free from clay, animal feces and plant matter as these may weaken the properties of the mortar. ie. adhesion or drying ability may be sped up or slowed down.
- The mix of sand to lime should be 4 to 1 not 3 to 1. Also when doing your scratch coat it is best to key across rather than down as this will stop gravity pulling the top coat down.
Things You'll Need
- Saw
- Rake
- Garden hose
- Water
- Lime
- Masonry sand
- Portland cement (optional)
- Bucket
- Trowel
- Hand cultivator
