Cement and sand for plaster and render, by area and by the thickness you build.
Painting, tiling and finishes. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Plastering and Rendering tool.
Plastering is a thickness business wearing an area costume. The area decides how much trowel work there is, and the thickness decides how much sand and cement arrives on site, which is why two walls of the same size with different specifications are different orders. The specific difference is that plaster and render are not one material. A gypsum plaster on a wall inside and a cement render on a wall outside have different mixes, different thicknesses and different preparation, and the calculator keeps them apart so a wall is not priced at the other's rate. Beads, scrim and bonding agents are counted separately where the specification calls for them.
A wall of thirty square metres in a fifteen millimetre cement render is zero point four five cubic metres of mortar, which in a one to four mix is about a tenth of a cubic metre of cement, roughly a hundred and forty kilogrammes, or three bags, and a third of a cubic metre of sand. The same wall in a three millimetre gypsum skim is four and a half square metres of a bagged product at a small coverage per bag, which is a very different order. Beads at the door reveals and the external corners come off the lengths you enter, not off the area.
Internal plaster and external render, and within those the difference between a gypsum finish and a cement based one. They differ in mix, thickness and preparation, so the calculator asks which you are applying before it calculates anything. Where a wall takes a render base coat and a plaster skim, measure the two as their own surfaces, because the products and the thicknesses are not interchangeable.
From the area and the thickness, which gives a volume, then from the mix you specify. A one to four mix splits that volume into one part cement and four of sand, and the cement is then converted to the bag size sold in your market. A bagged product instead asks for its coverage, because a manufactured plaster states how many square metres a bag will cover at a stated thickness, which is a more direct route than a mix.
Yes, and more than most people expect, because material follows volume. Doubling the thickness doubles the sand and cement on the same wall, and a two coat render at eighteen millimetres total uses well over twice the material of a single ten millimetre coat. That is why the thickness is an input and why the page shows the volume: a specification change that looks like a detail on a drawing is a lorry load of sand.
Beads, scrim and bonding agents are counted where the specification calls for them. Beads follow the corners, reveals and stops you enter, measured as lengths, since a stop bead at a ceiling line is a metre of bead rather than an area. Scrim follows the joints and the changes of material. Bonding agent follows the area you treat, at its stated coverage, and it is the item most often left out of a plaster estimate.
Only where you enter it. Surface preparation is a site judgement: a dusty wall needs a bonding agent, a smooth concrete wall needs a key, and a painted wall may need more than either. The calculator asks for the area you are treating and adds the product at its coverage, so the preparation is priced as its own line rather than folded into a trowel rate that assumes a perfect substrate.