The skin and bones of your home

When renovating or maintaining a property in Spain, it is vital to distinguish between the cosmetic finish (The Skin) and the underlying structure (The Bones).

Understanding how these two elements interact with the unique Spanish climate and soil is key to achieving a dry, stable, and energy-efficient home.

The Skin vs. The Bones: Understanding Your Walls

Think of external render like the icing on a cake or the skin on a body. It provides weather protection and a neat aesthetic, but it does not support the roof or floors. You could strip the render away entirely, and the building would stand perfectly fine.

Why Cracks are Almost Always Structural

Modern sand-and-cement render is cured to be incredibly strong and hard. However, because it is so rigid, it has almost no tolerance for stretching or bending. It forms a brittle shell around the house.

This rigidity turns the render into a highly sensitive diagnostic tool:

• The Physics of the Crack: If the foundations of a house shift—even by a few millimetres—the brickwork or blocks must move with them. Because the render is bonded tightly to the bricks but cannot flex, it snaps under the tension.

• The “Stepped” Pattern: This is why you often see diagonal or “staircase” cracks. The render is literally tracing the stress line where the mortar joints between the bricks have failed and shifted.

• The Rare Exceptions: Occasionally, cracks can occur if the render is applied during extreme heat (drying too fast) or if there are minor gaps behind the mesh. These are cosmetic issues, usually spotted and repaired immediately during the application process.

The Golden Rule: If the structure stays still, the render stays smooth. If the render cracks, the “bones” of the house are moving.

The Mediterranean Challenge: Clay Soil and Foundations

In many regions of Spain, structural shifting is frequently caused by clay soil. Clay is a “reactive” soil type that behaves like a giant sponge.

• Summer: It dries out and shrinks, pulling away from the foundations.

• Winter: It absorbs water and expands (heave), pushing up against the house.

The Issue of “Floating” Foundations

Ideally, foundations built on clay should be supported by piles—long concrete pillars driven deep through the soft clay into the stable rock below.

However, because piling is expensive, many Spanish properties (especially older ones) were built with “shallow” or “bagged” foundations.

• The Reality: These foundations essentially “float” near the surface of the clay.

• The Result: As the clay expands and contracts with the seasons, it takes the foundations on a slow-motion rollercoaster. The house’s rigid structure shifts in response, and the render eventually snaps under the tension.

The Trigger: Water and Poor Drainage

While the clay soil is the underlying cause of the movement, water is the trigger. If you can control the water, you can often stop the movement. Unfortunately, traditional Spanish construction often overlooks drainage.

A. Lack of Gutters

Many properties in Spain, particularly rural or rustic styles, completely lack guttering.

• The Problem: During a storm, the entire volume of rainwater from the roof cascades directly down the walls.

• The Damage: This water hits the ground and saturates the clay exactly where the shallow foundations sit. This causes the clay to swell aggressively at that specific point, lifting the wall and cracking the render.

B. Incorrect Walkway Angles (The “Reverse Fall”)

Concrete paths, patios, or tiled terraces around the house should act as a shield, sloping away from the building to direct water into the garden.

• The Drift: Frequently, these walkways are built flat. Over time, as the clay soil swells and shrinks, the ground beneath the concrete shifts. This often causes the path to tilt back towards the house.

• The Result: This creates a “reverse fall,” funnelling rainwater and surface runoff back towards the brickwork, creating a permanent reservoir of water against the wall base.

C. The Solution: French Drains

Since retrofitting deep piles under an existing house is often impossible or cost-prohibitive, the best solution is to manage the water.

• What it is: A trench dug around the perimeter of the house, lined with geotextile fabric, filled with gravel, and containing a perforated pipe.

• How it works: It acts like a “moat.” It intercepts water flowing towards the house and diverts it safely away. This keeps the moisture level in the clay consistent, stopping the expansion/contraction cycle and stabilising the foundations.

The Thermal Upgrade: Diathonite Insulation

Once the structure is understood, we look at the “Skin.” We recommend Diathonite, a cork-based render system designed to act as a thermal blanket.

Unlike standard sand-and-cement, which acts as a “thermal bridge” (transferring heat easily), Diathonite uses the natural insulating properties of cork and clay.

• Summer Cooling: The cork granules shield the masonry from solar gain, preventing the sun’s heat from soaking into the walls and keeping the interior cool.

• Winter Warmth: It traps heat inside the home, drastically reducing energy loss.

• Covering Cold Spots: It wraps the entire exterior in a continuous layer, covering concrete lintels and corners where heat usually “leaks” out most aggressively.

The Final Layer: Silicate Paint Finishes

Just as the render matters, so does the paint applied on top. For natural renders like Diathonite or Lime, standard masonry paint (often acrylic or “plastic” based) can be disastrous.

The superior choice is Silicate Paint.

How it Works: Petrification

Unlike acrylic paints, which form a plastic film on top of the wall that can peel or blister, mineral silicate paints work by a process called “micro-silicification” or petrification.

• The paint chemically reacts with the render to fuse with it.

• It literally becomes part of the wall surface rather than just sticking to it.

• It is incredibly durable, unaffected by UV light, and will not flake over time.

Maintenance: Managing the “Wicking” Effect

It is important to remember that Diathonite is an “active” material. If your property is older and lacks a Damp Proof Membrane (DPM), the bricks will naturally draw moisture up from the ground.

The “Breathable” Function

Diathonite aggressively pulls this ground moisture out of the masonry and releases it into the air.

• The Benefit: This is excellent for the house. If you used a plastic paint here, that water would be trapped inside, rotting the bricks and causing internal damp.

• The Side Effect: As this moisture is wicked out to the surface to evaporate, it can sometimes leave surface discolouration or dark patches.

Are you in the 20%

In roughly 20% of properties we treat, we see organic growth (small patches of algae or mould) on the surface. This is not a product failure. It occurs only under specific conditions where the wall is absorbing excessive water, such as:

No Gutters: Water cascading down the wall.

Leaking Windowsills: Water is getting behind the render.

Wet Clay: “Bagged” foundations sitting in waterlogged soil.

Simple Maintenance Routine

Because Silicate paint is chemically bonded to the render, it is incredibly tough. You can clean it without fear of damaging the finish. This maintenance is easy and typically only needed every few years or after a particularly wet winter.

Apply a Cleaner: Spray the affected area with a dedicated wall cleaner containing a biocide or fungicide. This is essential to kill the spores at the root. We like and recommend Sika wall cleaner. However, every silicate manufacturer has their own preference for cleaning.

Let it Soak: Allow the spray to work for the recommended time (usually 15–20 minutes).

Rinse Gently: Wash the wall with a garden hose or a low-pressure soft wash. There is no need for aggressive scrubbing.

The Science of Spores: Why Growth Occurs

We have discussed this phenomenon extensively with Diasen (the manufacturer) and analysed the specific, rare cases in which it has occurred in Spain and Portugal. It is critical to understand that this is a natural reaction to a specific set of environmental conditions.

Why does it happen? Mould or algae spores are always present in the air, especially in warmer climates. For them to “bloom” on a wall, they need a surface with moisture that remains for an extended period.

• Saturation vs. Evaporation: Diathonite is designed to dry rapidly. However, if a wall is continuously watered (e.g., from a leaking sill or a soaked foundation) during a prolonged wet period, the material simply cannot dry quickly enough.

• The Perfect Storm: When you combine a wet surface, a warm climate, and natural spores, nature takes its course, and growth appears.

Why this is actually a benefit: While it may seem counterintuitive, this growth is a positive sign that the system is functioning correctly.

• The Alternative: If we had sealed your wall with a plastic, waterproof paint, that moisture would have been trapped inside the brickwork. It would have had nowhere to go but into your home, leading to blown plaster, rotting structural timber, and unhealthy black mould inside your living rooms.

• The Trade-off: This surface activity is direct proof that the Diathonite is actively drying out your walls. It is continuously pulling moisture from deep within the structure to the surface, prioritising the long-term stability and health of your home over a purely cosmetic, sealed finish.

For the few buildings affected by this, simply washing the wall every few years removes the cosmetic symptom while providing the structural cure, unlike the more than 90% of homes with plastic paint that suffer from this under the same conditions.