render

Rendering is the process of applying a protective and decorative layer of plaster, mortar, or similar material to the external or internal face of a wall. The render layer typically ranges from 10 to 20 mm thick and bonds to the substrate, creating a weather-tight seal and finished surface. In external applications, render serves as the primary defense against rain penetration and thermal movement.

The most common renders are cement-based, lime-based, or gypsum-based. Cement render, mixed from Portland cement, sand, and water, offers durability and quick set times, making it standard in damp or exposed conditions. Lime render, made from lime putty and sharp sand, is more flexible and breathable, preferred in conservation work and on older masonry. Modern renders may include acrylic or silicone additives to improve water resistance and flexibility without reducing vapor permeability.

Rendering begins with substrate preparation: the wall surface must be clean, stable, and often dampened to prevent the render from drying too quickly. Multi-coat renders are common, particularly on poor or uneven substrates. A scratch coat (first coat) is applied, scored to provide mechanical key for the next layer, followed by a float coat (second coat), and sometimes a finish coat. Each coat is left to cure before the next is applied, typically 5 to 14 days depending on weather and material.

Common failures and their causes

Render fails when the substrate is too wet, too dry, or when temperature swings are too rapid during application. Thermal cracking occurs when thick renders cannot accommodate building movement; this is why modern standards limit single-coat thickness and specify movement joints. Delamination happens when adhesion is poor, often due to laitance (a weak cement layer) on the substrate or incompatible suction rates between layers. Efflorescence and staining result from salts or moisture migrating through the render.

The term render derives from the verb to coat or cover, and is standard in UK and Commonwealth construction. In North America, plaster is more commonly used for interior work, though exterior stucco (a render variant) is widespread. Understanding local material conventions and climate exposure categories is essential, as render specifications vary significantly between maritime, industrial, and inland environments.