Mortar & render
Rendering masonry in layers
Substrate, pre-wetting, spatterdash, scratch coat and finishing coat
Render is built up in coats on a prepared, low-absorption substrate: spatterdash, base coat and finishing coat. The guide covers substrate requirements, pre-wetting, coat thicknesses and curing time between coats. The geometric render quantity is area × coat thickness — the actual kg/bag consumption is taken from the supplier's data sheet.
Assess and prepare the substrate
The substrate must be clean, firm, reasonably even and free of grease and loose particles, and it must have slight suction. Adhesion is determined by the substrate — a glossy, greasy or crumbling substrate must be remedied before you render.
Pre-wet the substrate
Wet the substrate from a hose with a spray nozzle, and start as soon as the surface is matt-damp and has slight suction. A strongly absorbent substrate will otherwise draw the water out of the render so it does not cure properly.
Rendering surfaces that are glossy with water will not succeed — wait until the surface is matt-damp.
Throw out the spoil
First apply a spatterdash coat (coarse bonding layer) with a grain size of 0–4 mm. The spatterdash creates the rough key that the subsequent render coats adhere to.
Ejection, grain0–4 mmApply the base coat
Apply the base coat of render at a thickness of normally 8–12 mm and rule it off flat with a straightedge. The base coat straightens the surface and forms the substrate for the finishing coat.
Rough plaster8–12 mmObserve the grain size rule
The rule of thumb is that the largest aggregate particle should be at least a third of the layer thickness. Too thin a layer with too coarse an aggregate tears up the surface; too fine an aggregate in a thick layer increases shrinkage and the risk of cracking.
Grain≥ ⅓ of the layer thicknessLet the scratch coat cure before the finishing coat
The finishing coat must not be applied until the base coat is 1–5 days old. A finishing coat applied too early on a wet, unset base coat bonds poorly and cracks.
Hardening1–5 days before final plasteringApply the finishing coat thinly
Apply the finishing coat/fine render in a thin layer on the matt-damp, pre-wetted base coat and trowel/float it into the surface. The finishing coat determines the fineness and appearance of the surface.
Level and finish
Level and choose the finish according to the project — roughcast, bagging or float finishing give different appearances. Curing times and consumption figures depend on the product and follow the supplier's instructions.
From volume to bags
Calculate the geometric render quantity as area × average coat thickness (e.g. 20 m² × 12 mm = 0.24 m³ ≈ 240 L net coat volume) and add your own shrinkage/waste allowance. Convert to bags using the supplier's consumption figure — typically kg per m² per mm of coat.
Example20 m² × 12 mm ≈ 240 L
Source
Based on Discebo bricklaying «Rendering — coat thickness, substrate and consumption» + the render coat calculator; Mur & Tag (mur-tag.dk/udfoerelse/overfladebehandling/tykke-pudslag-udfoerelse).