Introduction

Plaster or render?

While they involve almost identical skills, rendering and plastering are not the same thing.

• Render
  is 3 parts plastering sand mixed with 1 part cement, with plasticizer and waterproofing agent added to create a waterproof layer for exterior applications.
• Pebbledashing
  is an external render that has been mixed with coloured stones to create a maintenance free, textured surface.
• Plaster
  contains gypsum and is for internal applications only, as moisture will cause it to crumble.

Types of plaster

There are several types of plaster, each made for specific applications:

Finishing plaster

This is used for the top and final coat on a wall or ceiling as it produces a fine, smooth finish when it has been polished. This top coat is also known as a ‘skim’ and applying it is called ‘skimming’.

Ensure you select a finishing plaster that is suitable to cover the surface, or if in doubt choose a multi-finish plaster.

• Aim for a coat that is 3mm (⅛in) thick. Drying time will depend on ambient temperatures and airflow. Ensure all areas of plaster are fully dried before doing any decorating, usually 24 hours.

Browning plaster

Use this plaster for the initial backing coats of plaster straight onto porous surfaces such as brick, block or masonry.

• Aim for a thickness of 10mm (⅜in). Allow drying time of 2 hours.

Bonding plaster

This is used for backing coats of plaster that are applied to non-porous surfaces, such as those pre-treated with a PVA bonding agent.

• Aim for a coat that is 10mm (⅜in) thick. Allow 2 hours for it to dry.

‘One-coat’ plaster

A product that claims to be a backing and finishing plaster all in one, which can be applied in a couple of layers. Separate layers of the different types of plaster as listed above are preferred however.

Lime plaster

This is an old fashioned form of plaster, where the binding gypsum is replaced with lime. It is an environmentally friendly plaster, although it is up to three times more expensive than standard plaster, and takes longer to apply.

The plaster is a mix of 1 part lime to 3 parts coarse sand, with 4kg of horse hair added per cubic metre of plaster to form a ‘key’ between layers.

• Always apply to a well wetted wall.

Lime plaster is designed to absorb and evaporate moisture, so do not replace such materials with waterproof cement-based renders as this will encourage penetrating damp to take hold.

• If employing the services of a plasterer, be sure before you hire them that they are experienced in using lime plaster.

Daub

This is one of the earliest forms of wall finish, being made from mud, animal dung and straw. It is applied to a structure of ‘wattle’, or slim interwoven lengths of wood, which provide a structural trellis for the daub to hold on to.

How much plaster will I need?

Plaster has a poor shelf life and does not keep if stored, so you should aim to buy only what you need for the job in hand.

All types of plaster are usually available in 10kg or 25kg bags which cover as follows:

• 10kg bag covers 5m2 (54 sq ft) to a thickness of 3mm (⅛in)
• 25kg bag covers 12.5m2 (135 sq ft) to a thickness of 3mm (⅛in)
• 10kg bag covers 1.5m2 (16 sq ft) to a thickness of 10mm (⅜in)
• 25kg bag covers 3.75m2 (40 sq ft)to a thickness of 10mm (⅜in)

Alternatively you can calculate it using the following equation:

Which plaster do I need?

Plaster adds a good, smooth finish to a variety of sub-structures, which largely depend on the age of the property in question.

In very old properties, partition walls were made of vertical timber studs which were crossed with hundreds of slim lengths of wood, or ‘lathes’. This provided a structural framework for the plaster to adhere to. Such walls should be finished with lime or gypsum plaster, using a good base coat with an additional skim finishing layer on the top.

More modern homes were built with solid brick or block walls. These should be plastered with a good layer of bonding or browning plaster, followed by a layer of finishing plaster.

Nowadays, homes are built with solid exterior walls and studwork interior walls, all of which are covered in plasterboard, also commonly known as ‘drywall’. These are prefabricated panels of gypsum plaster sandwiched between two layers of paper.

Once the joins between panels have been covered with special jointing tape, plasterboard only requires a top finishing skim layer of plaster and therefore avoids bonding or browning layers and all the moisture that must dry out from these layers.

Plasterboards are also attached to the underside of ceiling joists to form ceilings, which can again be skimmed with a single layer of finishing plaster.

How to Plaster a Wall or Ceiling

Further information and useful links

• Identifying & eliminating damp & rising damp
• Successfully damp proofing your home
• Full list of painting & decorating topics
• More walls and ceilings topics

• Find a Builders Merchants near you
• Find a Plasterer near you
• Find a Masonry Repairs Service near you