The right crack filling solution depends on the type, size, location, and cause of the crack.
Cracks can appear on walls, ceilings, terraces, concrete surfaces, window frames, and construction joints. Some are small surface cracks. Others are caused by movement, shrinkage, moisture, or changes in temperature.
A small surface crack may need a simple filler. A moving construction joint may need a flexible sealant. Cracks that allow rainwater to enter may need a waterproof and weather-resistant solution.
This guide explains the common types of cracks, how to choose a suitable solution, and which Supex products can help.
What Is Crack Filling?
Crack filling is the process of repairing or sealing a crack with a suitable material.
The purpose is to:
Stop water and moisture from entering
Reduce air leakage
Protect the surface from further damage
Seal construction joints and gaps
Improve the appearance of walls and concrete
Reduce the risk of cracks becoming larger
Protect suitable areas from weather exposure
However, filling a crack does not always solve its root cause.
If a crack is caused by structural movement or another serious building problem, the cause should be checked before repair.
What Causes Cracks?
Cracks can develop for many reasons.
Common causes include:
Concrete shrinkage
Temperature changes
Building movement
Settlement
Poor surface preparation
Excess moisture
Poor construction practices
Movement between different materials
Expansion and contraction of joints
Water entering through unsealed areas
A crack should be assessed before selecting a filler. A flexible product may be needed when the joint continues to move.
Types of Cracks and the Right Solution
Not every crack should be repaired in the same way.
| Crack type | Common location | Suitable solution |
|---|---|---|
| Fine surface cracks | Plaster and interior walls | Acrylic filler or suitable sealant |
| Non-moving wall cracks | Interior and exterior walls | Suitable crack filler or sealant |
| Exterior wall cracks | External walls | Flexible, weather-resistant PU sealant |
| Concrete cracks | Concrete walls, slabs and floors | Product selected based on crack cause and movement |
| Beam-wall cracks | Beam and wall joints | Flexible PU sealant or suitable PU foam system |
| Expansion joints | Concrete and building joints | Flexible joint sealant |
| Window-frame cracks/gaps | Windows and surrounding walls | Acrylic, PU, silicone or MS sealant based on exposure |
| Terrace cracks | Roofs and parapets | Suitable PU sealant plus waterproofing system where required |
| Deep and irregular gaps | Construction openings | Expanding PU foam where suitable |
The table is a starting point. Always check product compatibility and the manufacturer’s application instructions.
Crack Filling for External Walls
External walls are exposed to rain, sunlight, temperature changes, and moisture.
Small cracks can become entry points for rainwater. Once water enters the wall, it may lead to damp patches, peeling paint, and further surface damage.
For suitable external cracks, a flexible polyurethane sealant can help create a durable seal.
Recommended solution: Supex PU 25
Supex PU 25 is a polyurethane sealant designed for sealing and bonding applications. It offers flexibility and adhesion and can be used for suitable internal and external crack-filling applications.
A typical crack-filling process is:
Identify the crack and check its cause.
Remove loose material and dirt.
Open the crack into a suitable groove where required.
Clean the groove.
Apply the sealant as recommended.
Tool the sealant to create a smooth finish.
Allow it to cure.
Apply a compatible coating or paint if required.
The original Supex application guidance recommends opening suitable cracks into a V-groove, cleaning the crack, applying PU 25, and tooling the excess sealant away before finishing the surface.
Crack Filling for Internal Walls
Not every indoor wall crack is caused by the same problem.
A fine crack in plaster may only require surface repair. A crack that keeps returning may indicate movement or moisture.
Before filling an internal crack, check for:
Dampness
Plumbing leaks
Water coming from an adjoining bathroom
Cracks around doors or windows
Cracks that are getting wider
Cracks that continue through several surfaces
If moisture is the cause, fix the water source first.
Simply filling a damp crack may only provide a temporary result.
Crack Filling for Ceilings
Ceiling cracks can develop because of shrinkage, movement, moisture, or leakage from an upper floor or terrace.
If the ceiling is damp, first identify where the water is coming from.
For example, water may enter through:
A terrace crack
A parapet wall
A bathroom above
A plumbing joint
A roof joint
Seal the source of the water before repairing the interior surface.
For terrace or exterior cracks, a suitable PU sealant can be used as part of the repair system. Waterproofing may also be required for long-term protection.
Crack Filling for Terraces and Parapet Walls
Terraces are exposed to rain, sunlight, and repeated temperature changes.
Cracks in a terrace or parapet can become paths for water.
For suitable cracks:
Clean the area.
Remove loose material.
Open the crack if required.
Apply a compatible flexible sealant.
Allow it to cure.
Apply the recommended waterproofing coating or membrane.
Crack sealing and waterproofing work together, but they are not the same thing.
A sealant can close a crack. A waterproofing system protects a larger surface area.
Crack Filling Around Windows
Water leakage around windows is often caused by an unsealed joint between the window frame and the wall.
This is slightly different from a crack in the wall itself.
The solution should match the joint and the materials involved.
Depending on the application, Supex recommends products such as:
Supex 500
Supex 501
PU sealants
The existing Supex guidance recommends removing old sealant and resealing suitable window-frame gaps with Supex 500 or Supex 501.
For window and frame sealing, the surface must be clean and the selected sealant should be compatible with the frame and surrounding material.
Crack Filling at Beam-Wall Joints
Beam-wall joints can develop small gaps because different building materials may move at different rates.
Temperature changes and building movement can make rigid fillers crack again.
Traditional materials such as cement mortar may not provide enough flexibility for some moving joints.
A flexible sealant or suitable PU foam system can be considered for appropriate applications.
When to use PU foam
Expanding PU foam is more suitable for deep cavities and larger irregular openings than for fine surface cracks.
For suitable beam-wall gaps, Supex 100 PU Foam can be used as a filling and insulation solution.
The cured foam can help fill the cavity and provide thermal and acoustic insulation. The exposed foam should be protected from sunlight with a compatible covering.
Important: PU foam should not automatically be used for every crack. Fine cracks and moving surface joints usually require a different approach.
Crack Filling vs Gap Filling
Cracks and gaps can look similar, but they are not always the same problem.
Crack filling
Crack filling focuses on a break or opening that has developed in an existing surface.
Examples include:
Wall cracks
Concrete cracks
Plaster cracks
Terrace cracks
Gap filling
Gap filling focuses on an intentional or accidental space between two surfaces.
Examples include:
Window-frame gaps
Door-frame gaps
Pipe openings
Construction cavities
Beam-wall gaps
The right product depends on the application.
For more information, see the Supex gap-filling guide.
Crack Filling vs Waterproofing
Crack filling and waterproofing are related but different.
Crack filling seals a specific crack or joint.
Waterproofing protects a larger surface or building area from water penetration.
For example, a terrace may require:
Crack filling
Joint sealing
Surface preparation
Waterproofing coating or membrane
If you only fill the visible crack but leave other water-entry points untreated, leakage may return.
How to Choose the Right Crack Filling Solution
Before choosing a product, ask these questions.
1. How wide is the crack?
Measure the approximate width and depth.
A fine surface crack may need a different material from a deep cavity.
2. Is the crack moving?
If the crack expands and contracts, use a flexible solution designed to handle movement.
3. Is the crack exposed to rain?
Exterior cracks need a product that can handle outdoor conditions.
4. Is water entering through the crack?
If yes, identify the source of the water first.
5. What material is cracked?
The surface could be:
Concrete
Plaster
Brick
Cement
Stone
Metal
A window frame
The filler should be compatible with the substrate.
6. Does the crack need to be painted?
If the repaired area will be painted, choose a compatible and paintable product where required.
7. Is the crack structural?
A crack in a wall is not always a simple surface problem.
If a crack is wide, growing, diagonal, deep, or accompanied by movement or structural damage, consult a qualified building or structural professional before repair.
Recommended Supex Products for Crack Filling
Supex offers several products for sealing cracks, joints, and construction gaps.
Supex PU 25
Supex PU 25 is a polyurethane sealant suitable for applications that require adhesion, flexibility, and sealing.
Suitable for:
External and internal cracks, construction joints, and other suitable sealing applications.
Supex 500
Supex 500 can be used for suitable sealing applications around windows and building surfaces.
Suitable for:
Window-frame and wall joints and other compatible sealing applications.
Supex 501
Supex 501 is another option for suitable window and frame sealing applications.
Suitable for:
Window edges, frame joints, and compatible exterior sealing work.
Supex 100 PU Foam
Supex 100 PU Foam is an expanding foam designed to fill suitable cavities and gaps.
Suitable for:
Deep and irregular construction openings, beam-wall cavities, pipe openings, and insulation-related applications.
It is not a direct replacement for a flexible crack sealant in every application.
For more products, explore the Supex sealant range.
Step-by-Step Crack Filling Process
The exact process depends on the product and crack type. A typical sealant repair follows these steps.
Step 1: Inspect the crack
Look at the width, depth, direction, location, and surrounding surface.
Check whether water or movement is involved.
Step 2: Find the cause
Do not fill the crack before addressing an active leak or other obvious cause.
Step 3: Prepare the crack
Remove loose material, dust, old sealant, and weak surface material.
For suitable cracks, create a clean groove to give the sealant enough surface area for adhesion.
Step 4: Clean the surface
Use a suitable method to remove dust and debris.
The surface should meet the product’s preparation requirements.
Step 5: Apply the product
Apply the selected filler or sealant according to the manufacturer’s instructions.
Step 6: Tool the sealant
Smooth the sealant before it forms a skin, following the product’s working time.
Step 7: Allow it to cure
Keep the repaired area undisturbed during the recommended curing period.
Step 8: Finish the surface
After curing, apply paint or waterproofing treatment if the selected system requires it.
Common Crack Filling Mistakes
Avoid these common problems:
Filling without finding the cause
If water or movement continues, the crack may return.
Using rigid material on a moving joint
A rigid filler may crack again when the joint moves.
Applying sealant to a dirty surface
Dust, oil, and loose material can reduce adhesion.
Using foam for every crack
PU foam is useful for cavities and deep gaps. It is not the right solution for every fine crack.
Leaving exterior PU foam exposed
Cured PU foam should be protected from direct UV exposure.
Treating a structural crack as a cosmetic repair
Some cracks require professional inspection before filling.
Frequently Asked Questions
What is crack filling?
Crack filling is the process of sealing or repairing cracks in walls, concrete, ceilings, terraces, joints, and other surfaces with a suitable material.
What is the best material for crack filling?
There is no single best material for every crack. Acrylic fillers may suit some small interior cracks, while flexible PU sealants are better suited to many moving or exterior joints. Deep cavities may require expanding PU foam.
Which sealant is best for wall cracks?
For suitable wall cracks, a flexible polyurethane sealant such as Supex PU 25 can be considered, especially where movement and weather exposure are factors.
Can PU foam be used for crack filling?
PU foam can be used for suitable deep and irregular cavities. It is generally not the first choice for fine surface cracks or joints that require a flexible finished seal.
How do you fill cracks in an external wall?
First identify the cause of the crack and remove loose material. Prepare and clean the crack, apply a suitable exterior-grade sealant, allow it to cure, and apply a compatible protective coating where required.
How do you stop water from coming through a wall crack?
First find the source of the water. Then prepare and seal the crack with a suitable waterproof or weather-resistant sealant. If the water is coming from another area, such as plumbing or a terrace, that source must also be repaired.
Can crack filling stop wall seepage?
Crack filling can stop water entering through a suitable crack, but it will not fix every cause of seepage. Plumbing leaks, rising damp, poor waterproofing, and other water-entry points must be addressed separately.
Should cracks be filled before waterproofing?
In many repair systems, cracks and joints are treated before the waterproofing layer is applied. The exact sequence depends on the waterproofing system and manufacturer’s instructions.
Can cement be used for crack filling?
Cement mortar can work for some rigid construction repairs, but it may not be suitable for joints that move. Flexible sealants can be a better choice where expansion and contraction are expected.
When should a crack be professionally inspected?
Seek professional advice when a crack is wide, growing, deep, repeated, diagonal, or associated with structural movement, settlement, significant water damage, or visible damage to beams, columns, or slabs.
Conclusion
Effective crack filling starts with identifying the type and cause of the crack.
Fine surface cracks, moving joints, exterior wall cracks, terrace cracks, and deep construction cavities all need different approaches.
For suitable external and moving cracks, a flexible PU sealant such as Supex PU 25 can provide a durable sealing solution. For deeper and irregular cavities, Supex 100 PU Foam may be more appropriate.
The goal is not simply to cover the crack. The goal is to choose a solution that matches the crack type, surface, movement, moisture exposure, and long-term requirements.
Explore the Supex polyurethane sealants or Supex PU foam solutions for suitable crack, joint, and gap-filling applications.
4 Responses
how to stop crack between two building attached
Great tips, noted
This post is extremely radiant. I extremely like this post. It is outstanding among other posts I’ve read in quite a while. Much obliged for this better-than-average post. I truly value it!
Wall leakage wall crack