How to repair damaged plaster

By Stuart Roberts

Learn how to make small repairs to cracked damaged plaster  

If you own a property that has ageing plasterwork,It is likely that you will come across some damage and loose plaster from time to time. Problem areas tend be where the plaster sustains a level of impact over time for example above and around door frames,below window frames, areas that have held fixings such as shelving, picture brackets and nails and general household wall decoration, mirrors, speaker brackets and such like. More serious damage maybe a sign of structural subsidence and should be investigated immediately.  

Plaster is essentially stone, whether gypsum stone or limestone and is generally a none flexible building material and even small amounts of stress and movement can easily start to cause hairline cracks and even cause the plaster backing coats to shift away from the brick or timber wall structure (Blown plaster) When this happens the plaster must be removed and repaired.


Assessing the damage and removing the loose plaster

Wearing a protective dust mask and eye protection, It is necessary to dig away the blown sections and begin to prepare the area. Identify the plaster material by inspecting the plaster and using the age the property was built (if dealing with original plasterwork. 

Today's renovating plasters are versatile and you can purchase materials suited to repair a range of different plaster backing coats. As a guide, If your plaster seems to be made up of a thick layer of grey or brown sand like material containing hairs and small pebbles,Then your walls could be made up of a lime sand mortar topped with a white or cream coloured lime skim finish, repairing lime plasters with gypsum plaster offers great results however if your property is listed ,Then it may be necessary to repair the building using only traditional materials and techniques. For repairs to listed buildings you will need to seek guidance from specialists in the repairs to listed buildings.

For help with listed buildings & heritage work, Contact  The Traditional Plastering Company  

Locate loose plaster- By tapping along the walls using a key-You will notice a change in pitch as the metal key is tapping the loose areas and this will indicate a failed bond in the plaster. Using a large paper scraper, Begin to scrape the entire surface of the wall to remove small paint flecks and other loose debris. Treat the blown pockets by gently digging these areas out using the corner of your paper scraper you should end up with a number of small areas of bare brick or lath,The edges inside these pockets must be in sound condition and by using a small hammer you can chip around the perimeter of the pocket to ensure all loose plaster is removed without causing more damage and enlarging the areas too much - These areas must now be dusted using a small dry paint brush or hoover.

Sealing & priming

Using a suitable diluted bonding agent,(The amount of water needed to dilute the bonding agent will depend on the level of suction) Take a soft paint brush and apply a generous layer of sealer to the inside and around the border of the patch. It is necessary to ensure the bonding agent is applied not only to the bare brick or timber structure,but also to the plaster which surrounds the patch-This will give a good bond when it comes to applying the first coat of gypsum backing plaster. For the treatment of small cracks, follow the guide above and apply the bonding agent. Fibreglass plaster tape (scrim) must be applied and must cover the entire crack before applying plaster.


Applying the backing plaster

Depending on which material you are using you maybe able to apply one thick layer of gypsum backing plaster to entirely fill the patch. If you are using a sand and cement product then it maybe necessary to build up several thinner coats to prevent the plaster from cracking.

Bring your mix of plaster material to the surface of your patch by completely filling the patch with plaster and ruling any excess plaster away using a suitable straight edge. If you notice areas in your patch that have not been completely filled, simply apply more plaster to these areas and repeat the process.


Once your backing plaster has set, You can apply your finish plaster. This should slightly overlap the edges to your patch and should be thinned into the existing plaster line.


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