Boundary Walls

Preventing and Repairing Cracks in Boundary Walls

Damaged boundary walls around your property can spoil the whole look and feel of your property. Furthermore, cracked and leaning walls can also pose a danger to passers-by should the wall fall over.

boundary wall
This was a wall that was butt jointed against the neighbour’s wall. The danger here is that when the wall falls over portions of the wall will be spread over the pavement and possibly into the road. If a pedestrian is close by serious injury could result!

This article explains the correct way to repair boundary walls and install expansion joints!

Firstly, if your walls have ugly cracks and broken plasterwork and brickwork at the expansion joints do not plaster them up as shown in the photos below!

boundary wall
This expansion joint was chopped open and plastered up. The joint will crack at the joint again. This is a waste of time and money and will cause more problems than before!

Furthermore, the work done on these boundary walls will result in more cracking in the walls!

Many boundary walls and retaining walls may fail prematurely due to the lack of provision for movement. However, this is usually not a fault in the materials used, but usually a lack of proper design. Even when the design is correct, the construction of the boundary wall and expansion joints are often faulty.

What is an expansion joint?

It is a separation between two portions of the same structure. A butt joint in a boundary wall is not an expansion joint!

Expansion joints in boundary walls

When building a boundary wall, an expansion joint is a separation designed to relieve stress on building materials caused by movement induced by thermal expansion and contraction. They are therefore specifically provided in boundary walls to avoid cracks occurring in the wall.

Temperature changes and seasonal changes mostly cause the movement in the boundary walls. However, expansion joints also permit movement due to ground settlement, seismic events and expansive soils.

Many factors dictate how much brickwork will move due to temperature changes. Firstly, the orientation, geographical location and exposure to the elements of a wall will be a major influence.

Correspondingly, the brick type and the strength of the mortar also plays a part. Each unit will expand when heated by the sun and contract at night or due to cold weather. However, over a short distance, the total movement may be insignificant.

Whereas, over longer distances, something will have to take the strain of the expansion and contraction. Preferably, this will be an easily compressible joint. Furthermore, in boundary walling, unlike larger structures, there is not the mass of brickwork to restrict thermal movement. Consequently, the distance required between expansion joints should be a maximum of 5 to 6 metres. In addition, at a corner or change of direction, a joint should be provided at a maximum distance of 2.5 – 3 metres.

How is an expansion joint constructed?

Firstly, they must start at the foundation level and continue vertically through coping or capping courses. At the same time, joints should not be less than 6mm or wider than 25mm. Furthermore, never leave the gap of expansion joints open. A joint should be watertight by filling it with a compressible material. Notably, the compressible material should be built into the joint in the wall during construction. This is to avoid mortar projections within the joint and filling of the joint with mortar which would render the joint ineffective.

The following materials are required to render the expansion joint watertight

Joint fillers:

The joint filler should be a tight-fitting compressible material in the gap. However, Softboard or similar boards must not be used for brickwork expansion joints. Builder’s merchants stock purpose made foam rubbers or sheets of polystyrene that can be used for this purpose. Specialised Polyethylene, Polyurethane and Polypropylene joint fillers are available as well.

expansion joint
This an example of a closed cell polypropylene foam joint filler that can be obtained and used.

Sealing compounds:

The function of sealants is to seal the joint against the passage of moisture and to prevent the ingress of dust, grit or other foreign matter into the joint. In addition, it should be insoluble and readily workable.  The use of mastic or silicon sealing compounds is ideal for this.

boundary wall
The sealant used in this expansion joint was a grey mastic sealant. Different colours of sealants are available.

How does an expansion joint work?

For example, when a boundary wall heats up, the building materials used to build it expand.  As a result, this causes the expansion joint to close, thereby compressing the expansion joint system installed in the gap.

Conversely, when the temperature drops, the materials cool causing the joint gap to open.  For this reason, this requires the movement of the joint material to expand to follow the joint movement.

If boundary walls do not have any expansion joints they need to be installed! Cut two joints 12 to 25mm apart on both sides of the wall with an angle grinder, with the joints spaced as discussed above. Chop out the material between the two cuts to form a clear gap through the wall.

When boundary walls or expansion joints fail

Expansion joints fail for many reasons:

The most common reason for cracks and the failure of boundary walls are the lack of expansion joints. Walls with butt joints can fail as well.

no expansion joints
There were no movement joints provided in this boundary wall at all!

Incorrectly constructed expansion joints are another reason for expansion joint failure. Either incorrect joint fillers and sealants are used, or none are used at all. Furthermore, the joint is often plastered over and a joint ruled at the joint. The result of all this incorrect application is that the joint is partially or completely filled up with mortar. Likewise, adhesion loss of the sealant and loss of elasticity of the sealant and joint filler can, over time, result in the tearing of the sealant. This may allow debris and dust to enter and fill the joint.

Cracked walls, leaning walls and the spalling or shattering of the brickwork at the joint can be the result of joint failure.

Repairing expansion joints in boundary walls

Tools and materials required:

  • Grinder and cutting blades 
  • Block brush (for cleaning out the joints)
  • Paint brushes (for applying bonding liquid and primers)
  • Masking tape 
  • Foam backing strip
  • Bonding liquid
  • Plaster sand (plastered boundary walls only)
  • Cement
  • Bricks and building sand (depending on the damage to the wall)
  • Recommended sealant (by sealant manufacturers or local hardware supplier)
  • Primer (if required)
  • Slightly tapered wooden battens
  • Caulking gun
  • Safety equipment ie Dust masks, eye protection etc.

The process of repairing expansion joints

Most sealant manufactures recommend the following process:

  1. If the wall is a plastered wall, use a grinder with a diamond blade disc to remove all existing material and vee out the edges of the nosings on both sides of the joint.
  2. If the wall is a fair-faced (face brick or semi-face brick) wall the process is somewhat different. Broken and shattered bricks will have to be carefully removed and replaced with the same or similar face bricks.
  3. Clean and wash out all the debris and dust so that the joint is completely open through the wall. With fair-faced boundary walls go to Step 8.
  4. Thoroughly apply a good quality bonding liquid to the vee shaped portions of the joint
  5. Apply diesel or a release agent onto the slightly tapered wooden batten and position it in the joint.
  6. Trowel a 1: 4  cement plaster mortar with some plaster primer mixed in and plaster up to the former to rebuild the expansion joint nosings. In addition, make sure to fill in all the empty spaces with mortar.
  7. Once the plastering has cured, carefully remove the wooden former from the joint.
  8. Insert the foam backing strip into the joint
  9. Cover the sides of the joint with masking tape.
  10. Using the sealant primer (if required), condition the internal parts of the joint up to the backing strip.
  11. Fill the joint with the recommended sealant using a chaulking (silicone) gun.
  12. Smooth out the surface, leaving it slightly concave and allow the joint sealant to cure.
  13. Remove the masking tape. At this stage fair-faced expansion joints are complete.
  14. For painted and plastered boundary walls, using a conventional paint roller, apply a plaster primer, followed by an undercoat over the expansion joint repair area.
  15. The repaired areas or the whole wall can now be repainted after the rest of the wall has been suitably cleaned.

Importantly, don’t forget to treat the top of the boundary wall in the same manner!

Fixing cracks in the Boundary wall

Any cracks that have occurred in the boundary wall as the result of defective or lack of expansion joints will have to be repaired. Cracks will allow rainwater to penetrate in the wall causing moisture damage and require additional maintenance.

fixing cracks
Don’t fix cracks by chopping the open and replastering them as shown here! They will crack at the patches again!

Repair cracks as follows:

Plastered brickwork

Reinforce the wall across the cracked area.

  1. Remove the plaster approximately 75 to 150 mm on each side of the crack and remove all debris and dust.
  2. Nail a strip of expanded metal or chicken mesh the width of the area to the wall and wet the wall.
  3. Force the plaster mortar into the area and work it to match the surrounding plastered area.
  4. Allow a drying period to repaint the wall between expansion joints in the wall.
fixing cracks
Chicken mesh or expanded metal fixed over the crack before plastering over the crack.

Fair-faced brickwork

Cracks in fair-faced walls pose a different challenge!

  1. Here you will need to remove the mortar joints across the crack for about 50mm deep and between 150mm and 200mm on either side.
  2. Drill holes another 50mm deep, one on each side.
  3. Then insert epoxy and a reinforcing bar with the ends bent up in a U shape into the holes.
  4. Repoint the joint with mortar.
Repairing face brick walls
This is a typical stairstep crack in a boundary wall. In this case, the section of the wall should be demolished and rebuilt with a proper expansion joint!

Alternatively, remove the brickwork around the crack is and re-instate the brickwork if you can find the same or similar face bricks or reuse some of the existing bricks.


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