NHBC Standards & Your Render: Building Design for Longevity

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Why design matters: We explain NHBC Section 6.11.7 and why correct movement joints and coping overhangs are critical for a render that lasts.

It’s Not Just About the Render. It’s About the Design.

Why even the best render can fail if the building isn't ready.

Applying render to a building is a great way to improve its appearance and protect it from the elements. However, even the best render system in the world can fail if the underlying building design is flawed.

As your local Silicone render specialist, I want to ensure your investment lasts as long as possible. That is why it is important to understand the "Hidden Rules" of building design that determine the success of your project.

The "Rule Book": NHBC Standards 2023

The NHBC (National House Building Council) Standards are the authority on good building practice. Specifically, Section 6.11.7 provides critical guidance on how to prepare a wall for render.

Here are the four key factors we assess before we start a job:

diagram showing an expansion joint from rendering - infographic

1. Movement Joints (Stopping the Cracks)

Buildings are not static; they naturally expand and contract due to temperature changes and moisture.

The Risk: If this movement isn't accommodated, the tension builds up until it snaps the render, causing ugly cracks.
The Solution: We ensure "Movement Joints" are built into the system. These are strategic gaps that allow the wall to flex without damaging the finish. (If you already have cracks, see our crack repair solutions).

2. The "Overhang" Rule (Protective Wall Tops)

This is the most common issue we see in South Shropshire. Elements such as coping stones, flagstones, or cappings placed on top of garden walls or parapets are designed to stop water entering the wall from above.

The Rule: To be effective, they must overhang the wall sufficiently so water drips away from the render face, rather than running down it.
The Spec: NHBC standards specify a minimum overhang of 40mm after the render has been applied.
Note: If your current copings are flush with the wall, we will advise you to replace them before we can render.

infographic show the correct overhang for coping stones to protect silicone rendering from water ingress

Abode plastering hacking off old render with a Hilti Breaker

3. Background Preparation

The "Background" (the brick or blockwork) is the foundation of your entire system. It must be chemically and structurally suitable to hold the render, or the bond will fail.

Clean & Stable: We ensure the surface is free from dust, grease, algae, or loose material. If the brickwork is crumbling or "friable," we must stabilize it first.

Suction Control (Absorbency): This is critical.
- High Suction: If a block is too thirsty (like aerated concrete), it sucks the water out of the render too fast, causing it to crack. We treat this with a high-suction primer to slow the absorption.
- Low Suction: If a surface is smooth or dense (like engineering brick or concrete), the render can't grip. We apply a bonding agent or a specialised basecoat to create a mechanical key.
Material Matching: Different substrates (thermalite blocks vs. red brick) move at different rates. We select a specific basecoat designed to flex with your specific wall type.

4. Drainage and Protection

Proper drainage is vital to prevent water from accumulating behind the render, which leads to "blown" patches. Design features like adequate cavity widths and correctly installed Damp-Proof Courses (DPCs) are non-negotiable.

What This Means for You

As your render specialist, I am committed to providing a high-quality finish that lasts. However, the long-term performance of the render relies on these underlying design factors.

If you are planning a new build or renovation, it is crucial to work with an architect or builder who understands these standards. By getting the design right—especially those coping stones!—we can create a beautiful result that stands the test of time.

Worried about your walls? If you aren't sure if your property is ready for render, upload your photos to our estimate form. We can assess your coping stones and brickwork remotely.

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