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 pose a danger to passers-by should the wall fall over. Don’t overlook the importance of expansion joints!
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!
Furthermore, the work done on these boundary walls will result in more cracking in the walls!
Many boundary and retaining walls may fail prematurely due to insufficient provision for movement. However, this is usually not a material fault, but a lack of proper design. Even when the design is correct, the construction of the boundary wall and expansion joints is 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
Expansion joints relieve stress on building materials caused by thermal expansion and contraction. They are therefore specifically provided in boundary walls to prevent cracks from occurring in the wall.
Temperature changes and seasonal changes mostly cause movement in the boundary walls. However, expansion joints also experience 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 mortar strength also play 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 insufficient brickwork to restrict thermal movement. Consequently, the distance between expansion joints should be 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 to construct an expansion joint
Firstly, they must start at the foundation level and continue vertically through coping or capping courses. At the same time, joints should be more than 6mm or wider but less than 25mm. Furthermore, never leave the gap open. A joint should be watertight by filling it with a compressible material. Notably, the compressible material is built into the wall joint 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 expansion joint should be watertight.
Joint fillers:
The joint filler should be a tight-fitting compressible material in the gap. However, don’t use Softboard or similar boards for brickwork expansion joints. Builder’s merchants stock purpose-made foam rubber or sheets of polystyrene used for this purpose. Specialised products such as Polyethylene, Polyurethane, and Polypropylene joint fillers are available.
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 in the joint. In addition, it should be insoluble and readily workable. The use of mastic or silicon sealing compounds is ideal for this.
How does an expansion joint work?
For example, when a boundary wall heats up, the building materials expand, causing 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, the joint material is designed 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 cracking and boundary wall failure is the lack of expansion joints. Walls with butt joints can fail as well.
Incorrectly constructed expansion joints are another cause of 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 filled with mortar. Likewise, lack of sealant adhesion and elasticity can result in tearing over time. This may allow debris and dust to enter and fill the joint.
Cracked walls, leaning walls, and spalling or shattering of the brickwork at the joint are 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 such as Dust masks, eye protection, etc.
The process of repairing expansion joints
Most sealant manufacturers recommend the following process:
- If the wall is plastered, 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.
- If the wall is a fair-faced (face brick or semi-face brick), the process is somewhat different. Broken and shattered bricks will have to be carefully removed and replaced with the same or similar face bricks.
- 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.
- Thoroughly apply a good-quality bonding liquid to the V-shaped portions of the joint.
- Apply diesel or a release agent onto the slightly tapered wooden batten and position it in the joint.
- 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.
- Once the plastering has cured, carefully remove the wooden former from the joint.
- Insert the foam backing strip into the joint
- Cover the sides of the joint with masking tape.
- Using the sealant primer (if required), condition the internal parts of the joint up to the backing strip.
- Fill the joint with the recommended sealant using a caulking (silicone) gun.
- Smooth out the surface, leaving it slightly concave, and allow the joint sealant to cure.
- Remove the masking tape. At this stage, fair-faced expansion joints are complete.
- For painted and plastered boundary walls, use a conventional paint roller to apply a plaster primer, followed by an undercoat over the expansion joint repair area.
- The repaired areas of the wall can now be repainted after the rest of the wall has been 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 a result of defective or a lack of expansion joints will have to be repaired. Cracks will allow rainwater to penetrate the wall, causing moisture damage and requiring additional maintenance.
Repair cracks as follows:
Plastered brickwork
Reinforce the wall across the cracked area.
- Remove the plaster from 75 and 150 mm wide on each side of the crack, and remove all debris and dust.
- Nail a strip of expanded metal or chicken mesh the width of the area to the wall, then wet the wall.
- Force the plaster mortar into the area and work it to match the surrounding plastered area.
- Allow a drying period before repainting the wall between expansion joints.
Fair-faced brickwork
Cracks in fair-faced walls pose a different challenge!
- Here, you will need to remove the mortar joints across the crack about 50mm deep and between 150mm and 200mm on either side.
- Drill holes another 50mm deep, one on each side.
- Then insert an epoxy and a reinforcing bar with the ends bent up in a U shape into the holes.
- Repoint the joint with mortar.
Alternatively, remove the brickwork around the crack and rebuild it with the same or similar face bricks, or reuse some of the existing bricks.
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