What is the minimum number of coats of Portland plaster required on the exterior over masonry?

The minimum number of coats of Portland plaster required on the exterior over masonry is indeed two coats. This requirement is based on standard practices within the construction industry, which aim to ensure durability, moisture resistance, and aesthetic appeal for plaster finishes.

The first coat, known as the scratch coat, is applied to create a key for the second coat, allowing better adhesion. This scratch coat also helps to level the surface to some extent. Once it has cured, the second coat, often referred to as the brown coat, is applied. This coat provides the necessary thickness and smoothness to the plaster finish.

By using two coats, builders can ensure that the plaster has sufficient bonding to the underlying masonry and can withstand weathering, reducing the risks of cracking or delamination that might occur with fewer coats. This method is a recognized standard, particularly in climates with varying conditions that can affect building materials.

The other options suggest a lower number of coats than recommended, which would compromise the integrity and effectiveness of the plaster system.