Render is a type of coating applied to the exterior (and sometimes interior) walls of buildings to provide a protective and decorative finish. It typically consists of a mixture of sand, cement, lime, and water. Render can help protect the structure from weather elements, improve its appearance, and enhance its thermal performance.

Most renders are impermeable, meaning they stop the absorption and release of moisture within the wall. This changes the moisture dynamics of a wall. A simple analogy would be comparing a brick wall to an overcoat, which allows some interaction with the surrounding air, as opposed to a rendered wall that acts like a raincoat, blocking the absorption of moisture but also its release.

Expansion during a hot way and contraction during a cold night will often result in cracking. Ideally large expanses of render should have an expansion gap will with a mastic type material.

Render needs to be checked annually as expansion cracks or other gaps can allow rainwater to penetrate deep into a wall. Thin cracks act like capillaries, drawing water in. Horizontal cracks are typically more water-absorbent than vertical cracks, both require attention.

The solution to cracks is to rake out and fill them with an external acrylic caulk for small gaps, similar material to the original render, probably a cement based mortar, for larger gaps (>3mm).

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