Waterproofing of slabs: what is it? What is it for? Understand

In construction and renovation projects, the waterproofing of slabs is one of the most important steps to preserve the integrity of the building. Often, this process is neglected, resulting in leaks, moisture stains, mold, and even structural damage. In addition to the aesthetic aspect, moisture can cause discomfort, unpleasant odors, and even respiratory problems for the residents or users of the property. The slab, being a surface exposed to different climatic conditions — such as sun, rain, winds, and temperature variations — is one of the most vulnerable points to infiltrations. Therefore, the choice of the appropriate type of waterproofing and the proper execution of the service are essential for the durability of the structure.

Why waterproof

The waterproofing of a slab is indispensable to protect the building against the action of water and moisture. When not carried out or poorly executed, infiltration can compromise not only the finishing — with the appearance of bubbles, peeling paint, stains, and mold — but also the structure of the building, affecting beams, pillars, and internal reinforcements.

In addition to structural problems, infiltration reduces the thermal and acoustic comfort of the projects, facilitates the proliferation of fungi and bacteria, and can cause damage to furniture, electronic equipment, and personal items. In the case of buildings, these problems can extend to different floors, creating collective losses.

Waterproofing a slab is important both in external areas — such as roofs, terraces, and exposed slabs — and internally, in slabs that receive flooring and coverings. Even when the slab is not visible, it can accumulate water and moisture if not well protected. Water penetrates through the small cracks and micro-cracks of the concrete, reaching metal reinforcements and accelerating corrosion processes, which weakens the structure over time.

Another relevant factor is the increase in the property's value. Well-executed constructions, with guaranteed waterproofing, tend to appreciate in the market, in addition to reducing the need for corrective maintenance. Therefore, in addition to being a technical and regulatory requirement in many cases, waterproofing is a measure of asset security.

Types of waterproofing

There are different methods to waterproof slabs, and the correct choice depends on the type of work, the use of the area, and the climatic conditions. Here are the main ones:

Asphalt Membrane

It is one of the most traditional systems. Produced from modified asphalt and structured with polymers, it can be applied hot or cold. It is ideal for exposed slabs or under flooring and coverings. Its use guarantees excellent protection against infiltrations but requires qualified labor and proper surface preparation.

Liquid Membrane (Acrylic or Elastomeric Waterproofing)

It is applied with a brush, roller, or sprayer, forming a continuous and flexible film. It adapts to the movements of the slab and withstands climatic variations well. It is recommended for areas with thermal movement and for slabs that will not receive heavy loads or intense traffic. It can be left exposed or protected with paint or a leveling layer.

Polyurethane Membrane

High-performance liquid waterproofing, it is very resistant to weather conditions, cracks, and expansions. It forms an elastic, continuous, and seamless layer, ideal for exposed roofs, terrace slabs, or areas that may eventually receive light traffic. Its application is practical but requires care in preparation and during drying.

Polymeric Mortar

It is a mixture of cement, sand, and waterproofing additives, applied directly onto the slab in layers. It offers good resistance and adhesion and can be used under flooring and coverings. It is more indicated for indoor areas or those that will receive a rigid finish, such as ceramics or porcelain.

Flexible Cementitious Systems

Mixtures of cement with special polymers provide good flexibility and resistance to cracking. They are recommended for internal or semi-exposed slabs. Their application is simple, usually done with a brush or roller, and requires attention during curing and final protection. Each solution has its advantages and limitations. Therefore, the technical analysis of a specialist is essential to define the ideal system for each case.

Steps for proper application

To ensure efficient slab waterproofing, a well-defined process must be followed. Here are the main steps:

1. Surface preparation Before any application, the slab must be clean, dry, free of dust, residues, oils, or loose particles. It is essential to correct cracks, holes, and unevenness, as well as to level the surface with mortar or leveling compound, ensuring a minimum slope of 1% to 2% for water drainage.

2. Application of primer or sealer The primer is a product that prepares the surface, improves adhesion, and enhances the performance of the waterproofing. It must be applied uniformly and according to the manufacturer's recommendation for the chosen system. In the case of asphalt membranes, the primer is essential for proper fixation.

3. Application of waterproofing Each system has its specificities:

• Asphalt Membrane: must be heated and glued to the surface, with overlaps of at least 10 cm.
• Liquid Membranes and Membranes: are applied in crossed layers (perpendicular to each other), respecting the drying time and the indicated thickness.
• Mortars and Cementitious Systems: applied in layers, reinforcing critical areas such as drains and corners with structuring mesh.

4. Reinforcement at critical points Corners, baseboards, expansion joints, and areas near drains should be reinforced with tapes or structuring meshes soaked in the waterproofing agent to avoid failures in more susceptible points.

5. Leak test After completion of the application and respecting the curing time, the slab must be flooded for at least 72 hours. This test checks for leaks or failures in the waterproofing before executing the leveling compound or final finish.

6. Mechanical protection (when necessary) In areas where there will be traffic or finishing on the slab, it is important to protect the waterproofing with leveling compound, protective plates, or appropriate covering. This preserves the system and prolongs its durability.

Common mistakes and how to avoid

Some errors in slab waterproofing are quite common and can compromise the effectiveness of the service. One of the most frequent mistakes is the inadequate choice of the type of waterproofing. Often, the system is selected solely based on price or ease of application, without considering technical factors such as structural movement, intensity of sun and rain exposure, or the final use of the slab.

Another common mistake is application without proper surface preparation. Dirty slabs, with dust, residues, or irregularities compromise adhesion and create weak points in the protection. The lack of slope is also a serious problem, as it prevents water drainage and favors accumulation in certain areas.

The application at an inappropriate time — with very humid, rainy, or excessively hot weather — can harm the drying and adhesion of the materials. Additionally, skipping steps such as primer application, reinforcement at critical points, or leak testing significantly increases the risk of failures.

To avoid these issues, it is crucial to rely on qualified labor, adhere to technical standards, and strictly follow the manufacturer's guidelines for the products used. Investing in a good waterproofing project from the beginning of the work is always more advantageous and economical than making subsequent corrections.

This text was produced with the support of CASACOR Publisher, an agent creating exclusive content, developed by the CASACOR Technology team based on the knowledge base of casacor.com.br. This text was edited by Yeska Coelho.