Basics in Building Construction

Building Components And Types

Lesson 26 : Plastering And Pointing

Types Of Plaster Finishes

The plastered surfaces may be finished in a variety of forms. The types of finishing treatments usually adopted for external surfaces are:

1. Smooth cast: It is a finish which presents leveled and smooth surface. The mortar for the finish is made by mixing cement or two coats and finished by working with a wooden float. Steel floats are not recommended for external renderings as they give a very smooth finish which is liable to crack and craze due to exposure to atmospheric conditions.


2. Rough cast: It is a finish in which the mortar for the final coat contains a proportion of fairly big size coarse aggregates. The texture desired is regulated by the size of the coarse aggregate which generally varies between 3 to 12 mm. Rough cast finish is normally 10mm thick and it is applied after one or two days after the under layer of plaster. The top layer is applied after one or two days after the under layer is laid. The mortar for the finish is made by mixing cement, fine sand and coarse aggregates in the ratio of 1:1 ½ :3. Rough finish is water – proof, durable and resistant to cracking and crazing. This type of finish is considered particularly suitable for situations subjected to heavy rain fall and strong winds.


3. Pebble dash: It is a finish in which small pebbles or crushed stones of suitable size (usually varying from 10 to 20 mm) are thrown on to a freshly applied final coat of mortar and left exposed. The mortar for the final coat is usually made by mixing cement and coarse sand in the ratio of 1:3 and is laid in about 1 mm thickness. The pebbles used for dashing should be well washed and thrown wet on to the final coat. The pebbles may be tightly tapped into the mortar with a wooden float or with the flat of trowel. This finish possesses all the advantages of the rough cast finish and is recommended in similar conditions.


4. Scrapped finish: It is a finish in which the final coat, after being leveled and allowed to stiffen for a few hours, is scrapped with a steel straight edge old saw blade or other such tool to remove the surface skin. Various types of scrapped finished can be obtained by using different types of tools in different manner. In this case, the final coat is usually 6 to 12 mm. thick of which about 3mm. is removed in the scrapping process. The scrapping is done after the setting has taken place. In the process of scrapping, the surface skin of the mortar is removed to expose the aggregates and the texture obtained depends upon the grading of aggregates used in the final coat. This type of finish is less liable to crack than plain finishes.


5. Textured finish: In this finish, ornamental patterns or textured surfaces are produced by working with various tools on the freshly applied final coat. This type of finish possesses all the advantages of the rough cast finish.