Help and Advice on How To Render Internal & External Walls
Help and advice on rendering internal and external walls by a qualified plasterer and renderer.
What is Render & why use it ?
Render is a measured mixture of water, sand, and cement. Render is applied similar to plaster, but has a more durable and longer lasting finish.
The mixture remains hard for many years and helps protect the buildings structure from the elements.
Internal render is usually used when there has been a damp problem. Normal plaster or plaster boards may rot and begin to crumble quite quickly if the damp continues to perforate the external brick or block work.
Internal rendering permits a solid inner skin on the damp walls which is mainly impervious to moisture from outside.
The render can be skimmed over with plaster afterwards to create a perfectly even finish which is more moisture resistant.
How To Mix Render
Correct measurements are the key to a durable and long lasting render. The same as most mortar mixes, the mixture is three measurements of sand to one measurement of cement. 3 to 1.
Use a bucket, spade, or other large measuring implement to ensure correct quantities are used. Local qualified plasterers or renders will mix render in the same way.
Ideal For Home Mixing
A simple method of mixing all types of plaster, mortar or render is the drill attachment mixing implement. Cheap enough to keep in the garage for when ever it is needed. Simply fit into a drill and begin whisking.
Mixing in a cement mixer will take most of the labor out of the job in hand, or even hire someone to mix on your behalf. If using a mixer, turn the machine on, add some water, then add the 3 to 1 mix.
Add PVA to the water and stir well prior to adding sand or cement. This will act as a waterproof barrier within the set render.
The mixture needs to be seen 'folding' whilst the mixer is turning. This means not to stiff, but wet enough to spread without falling all over the floor.
Whether rendering inside or outside, ensure that the render remains above the damp proof course (DPC)
With external render, ensure a bell cast bead is placed on the wall JUST above the DPC.
Internal render can stop just above where the skirting board will be.
Most render is applied to a height of 1100mm above floor level.
Help with rendering begins with preparing the wall to be rendered. Remove any loose pieces and locate the piece most protruding from the wall.
This protruding piece is called the highest piece and and will be the shallowest part of the rendered wall.
Coat the entire wall with a 50/50 mix of PVA Bonding Agent and water. This will help with suction and remove more dust and particles. Allow the PVA mix to go tacky before applying any render.
The scratch coat is the first layer of render to be placed on the wall. This is typically up to 10 mm thick. Once the render has been applied to the wall, use a straight edge (about 1.8 - 2 m long) to smooth over the render.
Use the straight edge both vertically and horizontally. Gently scrape off the lumps and mounds of render protruding out. This should leave a more or less flat wall. Now scratch it.
The scratching will give a 'key' or 'biting point' for the top layer of internal or external render. Always scratch horizontally, this helps prevent any moisture from forming at the bottom of the wall.
Scratcher for Render or Plaster
The top coat of render is usually about 5 mm thick. This coat is simply to cover over the scratch coat and leave a smooth finish. Use a long level to ensure that the wall is straight both vertically and horizontally.
Rubbing up render means using a plastic float in circular motions to leave a sandstone effect. Whilst this is being done, any small dents can be filled in with moist render mix.
The render is to wet to rub up, if when rubbing up the render is being dragged. On hot days, only render small areas and then rub up before applying render to another part of the wall.
Skimming Over Render
Using finishing plaster, it is possible to skim over render. Apply a liberal coat of PVA onto the render, then skim over with plaster.