How to Create a Fillet Joint?

A fillet joint (chamfer) is an essential element that must be applied where the foundation wall meets the footing, as well as at wall-to-wall connections inside the foundation. The sharp external edges of the footing and foundation wall should be chamfered or rounded. This process ensures a controlled thickness of the waterproofing layer and helps prevent cracks at material transitions. Additionally, a fillet joint aids in directing rainwater away from the structure.

Why Is a Fillet Joint Necessary?

One of the main functions of a fillet joint is to seal the connection between the foundation slab and the footing. Without a fillet, it is impossible to apply waterproofing in an inside corner while maintaining a uniform thickness of the layer.

Applying too much waterproofing material in these areas often leads to tearing due to shrinkage. The thinner layers dry faster and shrink earlier, while the corners shrink much later, causing stress and cracks in the waterproofing layer.

How to Create a Fillet Joint?

Creating a fillet joint is not a difficult task. You will need a mineral mortar based on pure cement and a trowel. SANIER recommends using the waterproofing mortar Bostik Sperrmörtel for this purpose.

Builders often use a PVC pipe, a glass bottle, or a jar to shape the fillet joint.

Step-by-step process:

1. Prepare the surface – Ensure the substrate is stable, free of holes, grease stains, paint, plaster, and cement slurry. Clean and roughen the surface, then remove any dust.
2. Dampen the surface – Before applying the mortar, the area should be moist but not wet.
3. Apply the mortar – Take a small amount of mortar with a trowel and throw it into the corner. Spread and shape it while removing excess material. You can collect the excess and reuse it elsewhere.
4. Profile the fillet joint – Use a rounded trowel, a PVC pipe, a jar, or a bottle to shape the fillet joint smoothly.
5. Allow it to dry – The setting time is approximately 5 hours, but full load capacity is reached after 24 hours.

Important Notes

• External wall corners must be chamfered (beveled) to avoid a sharp 90-degree angle.
• If chamfering is not done, a reinforcement mesh or sealing tape should be embedded in the waterproofing layer to strengthen the joint.
• SANIER recommends using Bostik Dickbeschichtung 2K+ bituminous waterproofing for optimal protection.

By following these steps, you ensure a durable and crack-resistant waterproofing layer, improving the overall longevity and efficiency of your foundation.