Defects Of Plaster

Common Plaster Defects

Plaster defects have been a problem in South Africa’s building industry for decades. However, the SABS specifications and the National Building Regulations do not have any requirements covering plaster quality. Additionally, most contract documents are not clear on the subject. Therefore, it may be difficult to force the contractor to make repairs should there be any defects.

Defects most frequently noticed and plaster fall into one of the following categories:

  • Non-structural cracks
  • structural cracks
  • Debonding
  • lack of hardness
  • Grinning
  • Expansion
  • Popping

Non-Structural Cracks

“Crazing” is a network of fine cracks, usually in a hexagonal pattern, which measure between 5 and 75 mm across each hexagon. They are usually very fine and shallow and do not extend through the whole depth of the plaster.

They are usually the result of over-trowelling a rich mix (one with a high cement content) or using a sand containing an excessive amount of dust (more than 15% by mass passing a 0,075 mm sieve). plaster cracks

Crazing often occurs within a few hours of the plaster being applied to the wall and cracks may hardly be visible until dust or moisture makes them noticeable.

Craze cracks are of little importance, do not open and close with time, and can be covered using a reasonable quality paint. Glass fibre tissue is applied during the painting operation.

Cracking which results when an excessive amount of water is lost from the plaster in the first hours after application is known as plastic shrinkage cracking.

Plaster will always shrink and crack so it is desirable that is should develop a large number of fine, unnoticeable cracks at close spacings. Plasters with very high cement contents and those which are made with poor quality sands having a high water requirement will tend to develop a few, widely-spaced cracks. Plaster applied in layers that are too thick will also tend to crack in this way. These cracks are normally stable and can be filled with a proprietary filler and painted over.

 

Structural Cracks

Some cracks visible in the plaster may result from cracking of the wall. This can be caused by differential movement of the foundations, moisture expansion or drying shrinkage of masonry units, or thermal movement of the roof.

This type of crack often forms in straight vertical or horizontal lines, or in stepped diagonal lines, and may be quite unsightly.structural cracks

The crack width will often vary with the seasons. Because these cracks originate in the wall and not in the plaster, repairing the plaster is ineffective.

A specialist should be called in to establish the cause of the cracking and to recommend remedial measures. Such measures may include structural alterations which change cracks into movement joints. Visible joints can be hidden by cover strips fixed on one side of the joint or sealed with elastomeric sealants.

 

Debonding Plaster

Debonding of plaster is often noticed as a hollow sound when the surface is tapped.

Plaster is inclined to curl and debond from the wall because the outside skin of the plaster that is exposed to the air will shrink at a different rate from the plaster in contact with the wall.

This is especially true of excessively thick plaster layers. Because debonding is generally the result of inadequate preparation of the substrate, it is important to make sure that the bond between plaster and wall is as good as possible. This can be done by:Debonding

  • Cleaning dusty or oily walls thoroughly.
  • Allowing the walls to reach the correct moisture content.
  • Using a cement slurry or spatterdash coat before plastering.
  • Using bonding liquids and following the procedure recommended by the manufacturer.

Small areas of debonding (about the size of a plate) are not significant, but larger areas should be removed and replaced.

 

Lack Of Hardness

There are no specifications covering the hardness or strength of plaster, and there is no reliable way of measuring it.

Scratch the surface with a hard sharp object such as a screwdriver or a key.

It is often better to have a slightly weaker plaster that is less likely to show significant cracking or debonding than one which is too strong. However, very weak plasters will be unable to resist impacts, will have reduced resistance to water penetration and picture nails will tend to fall out.

There are five common causes of soft plaster:

  • Insufficient cement
  • The use of sand containing excessive quantities of dust (more than 15% by mass passing the 0,075 mm sieve)
  • The use of a mix with poor water retention properties
  • The addition of extra water some time after first mixing (a practice known as retempering)
  • Rapid drying due to plastering in full sun or wind.

Painted plaster can only be removed and replaced. The inconvenience of this option has to be weighed up against living with the weak, unsatisfactory plaster.

A coat of high quality exterior paint will normally reduce the risk of water penetration to acceptable levels if the plaster is strong enough to hold such a paint. Areas which are particularly susceptible to impact, such as corners, can be re-plastered with relatively little disruption.

Grinning

Grinning is the term given to the appearance of a plastered wall when the positions of the mortar joints are clearly visible through the plaster.Grinning

The difference in suction between the masonry units and the mortar causes grinning. Raking out mortar joints also causes grinning. Therefore, limit raking to soft clay brickwork. While grinning may be unsightly, it is unlikely to lead to further cracking. The choice is to live with it, or to remove and replace the plaster. Application of an undercoat or a spatterdash coat before plastering will help to avoid grinning.

 

Expansion

This includes swelling, softening, layer cracking and spalling of the plaster. Proprietary gypsum-based products in the mix causes expansion.

Under moist conditions, the sulphate from the gypsum reacts with the portland cement paste and forms compounds of increased volume which disrupt the plaster.

The only remedy for this type of plaster defects due to gypsum in the mix is to remove and replace the plaster.

Popping

Popouts are conical fragments that break out of the surface of the plaster leaving holes which vary in size. The presence of contaminant particles in the plaster mix causes popping. It reacts with the moisture in the mix, expands and causes cavities in the plaster.plaster popping

Contaminants are usually seeds, other organic material, or particles of dead burnt lime.

The hole can be filled with a proprietary filler and painted over once the cause of the popout has been removed.

 

 

 

Plaster Defects

Contact our professional team today and we will assist you with a durable solution to your plaster problems.

Roof Tips

Tips To Keep Your Roof Safe Against The Rain

The truth is, extreme weather can prove harmful to your home or office. Therefore, potentially causing short-term and long-term damage. If your building is not prepared, it may accumulate dangerous mould and erosion as well as possible costly damage to its infrastructure.

Tip 1: Check Your Roof

Look for any current damage and check for any areas that look vulnerable to leaks and repair these immediately. Don’t be afraid to chat to us for advice if you do not want to hire a water ingress professional.

Tip 2: Trim Your Plants and Trees Hanging Over Your Roof

Plants and trees with overgrown branches can pose a danger to your home or office. Blocked valleys, blocked gutters and downpipes, mould growth are just some of the problems overgrown trees can create.

Tip 3: Waterproof Your Windows

Check the edges of your window sills to see if they need waterproof sealer. You can conduct a window flood test yourself by spraying your window with water to see if any leaks occur.

Tip 4: Clear Out Your Gutters & Valleys

blocked gutters

It is very important that these are clear in case of heavy rain to prevent flooding and other roof damage. Debris reacts with water to form a sludge in gutters. Not only does this reduce the life of your gutters, but it is also a fire hazard on hot, dry days. Blocked gutters and downpipes are also the perfect spot for nesting insects and rodents.

 

Tip 5: Inspect Your Chimneys & Skylights

Chimneys, skylights and other protruding roof objects are usually weak points on roofs. Ensure you inspect these areas along with the rest of your roof area.

At some point, sooner or later, you will experience water ingress in your home. Whether it is a roof leak, rising damp, leaking windows or flooding. Identifying a potential problem will save you money and further damage to your home.

Get in touch with our waterproofing professionals should you need any advice on how to keep your home safe against damaging precipitation.