Are Clay Bricks Waterproof or Water-Resistant?

The Truth That Specification Writers and Procurement Teams Need to Know

A few months ago, a project manager at a commercial real estate development company sent me a message that went something like this: “The architect has specified clay bricks in the facade design. The rainy season starts in six weeks. Should I be worried about water seeping in?”

This question hits the nail on the head. The answer isn’t a simple yes or no—it depends on how you define “waterproof,” the system in which the material is used, and whether you’re discussing the material itself or the entire structural system it’s part of.

First, a straightforward answer: Clay bricks are not waterproof. They are water-resistant, and in a properly designed system—especially a ventilated curtain wall—they perform exceptionally well in managing moisture. But the distinction between “waterproof” and “water-resistant” isn’t just a play on words. This distinction will be reflected in your test reports, warranty terms, and ultimately in long-term maintenance costs.

This article breaks down the technical relationship between clay bricks and water, focusing on ASTM test data, system-level performance, and the key points B2B buyers need to verify before finalizing specifications.

Table of Contents

Terminology Dilemma: Waterproof vs. Water-Resistant—What’s the Difference?

What the Test Data Says

The Material Itself: Pores Are Not Defects, They Are Functional

System-Level Performance: Curtain Wall Construction Changes the Rules of the Game

Glazed vs. Unglazed: A Line That Cannot Be Ignored

What Procurement Teams Should Verify

Frequently Asked Questions

Conclusion

Terminology Dilemma: Waterproof vs. Water-Resistant—What’s the Difference?

Let’s clarify the concepts first, as this confusion is all too common.

In materials science and civil engineering, “waterproof” refers to the property of a material to remain impermeable under specified hydrostatic pressure conditions. For example, waterproof membranes prevent water from penetrating even when subjected to a certain water head pressure. This is the acceptance standard for roofing membranes, underground waterproofing layers, and certain sealing systems.

“Water-resistant,” on the other hand, describes a material’s ability to resist water penetration to a certain extent, but it will still allow water to seep through under sufficiently high pressure or prolonged exposure. Clay bricks fall into this category.

Clay bricks are a type of sintered clay product. Like all sintered clay products—including the clay bricks discussed in our previous article—they possess a porous microstructure. These pores are a network of capillaries left behind by the evaporation of water during the drying and firing of the green body. The size, distribution, and connectivity of these pores determine how easily the material absorbs water.

So if someone tells you that clay bricks are “waterproof,” it is either a figure of speech or a misunderstanding. What clay bricks offer is controlled, predictable water resistance—and in the right construction system, that is exactly what you want.

What the Test Data Says

ASTM C67 is the standard governing the sampling and testing of bricks and structural clay tiles, and water absorption testing is included in it. According to this standard, water absorption is measured by immersing test specimens in water—either cold water for 24 hours or boiling water for 5 hours—and then calculating the percentage increase in weight.

What are the typical performance levels for clay bricks?

In short, there are variations between products and manufacturers, but the range itself is meaningful. The water absorption rate of clay brick products after 24 hours of cold water immersion is typically less than 8%, and less than 12% after 5 hours of boiling, with a saturation coefficient below 0.69. By comparison, standard clay facing bricks produced in accordance with BS EN 771-1 generally require a water absorption rate of ≤13%, with some grades permitting higher rates under specific exposure conditions. However, due to better raw material control and higher firing temperatures, architectural clay bricks can have a water absorption rate significantly lower than that of ordinary clay bricks.

A low water absorption rate is beneficial. It is directly linked to freeze-thaw durability—the lower the saturation coefficient, the smaller the probability that the material will reach critical saturation during freeze-thaw cycles. It also reduces the risk of efflorescence, as less water passing through the material means fewer soluble salts are carried to the surface.

Initial water absorption (IRA) is also tested in ASTM C67, but it measures something else: the rate at which bricks absorb water from the mortar during installation—you can think of it as “absorption capacity.” Acceptable IRA values generally range from 10 to 30 grams per minute per 30 square inches of masonry surface. As recommended by ASTM C216, bricks with on-site IRA values exceeding a certain threshold should be pre-soaked before installation to prevent the brick body from absorbing water too quickly from the mortar, which could compromise bond strength.

The key takeaway is: Clay bricks do absorb water, but they do so slowly and in a controllable amount. This is the definition of water-resistant, not waterproof.

The Material Itself: Pores Are Not Defects, but Functional Features

At this point, I’ve seen some novice specifiers start to get confused. If clay bricks aren’t waterproof, isn’t that a problem? Why not use something that doesn’t absorb water at all?

This question touches on building physics. A completely impermeable curtain wall material traps water vapor inside the wall, creating conditions for condensation, mold, and material degradation. In contrast, materials with controlled porosity—those that allow for some vapor permeability—enable water vapor to migrate out of the wall rather than accumulate inside.

This is precisely the role played by the pore structure of clay bricks. The material can absorb some moisture during rainfall and release it through evaporation once the weather clears and conditions dry out. This is why clay bricks are often described as “breathable”—a term that may sound like marketing jargon, but is actually grounded in physical principles. Modern glazing and firing techniques make clay bricks water-resistant, eliminating the destructive effects of moisture penetration.

The firing process plays a decisive role in this property. As the firing temperature rises, the silicates in the clay begin to melt, forming a glass phase. This reduces the overall porosity while retaining sufficient open pores to allow water vapor to migrate. Higher firing temperatures and longer holding times—which are more common in clay brick production—result in a denser material with lower water absorption.

Therefore, porosity is not a manufacturing defect. It is a controlled variable in the production process; when properly calibrated, it actually enhances the long-term durability of the wall.

System-Level Performance: Curtain Wall Systems Are a Game-Changer

At this point, we must shift our focus from materials science to building envelope engineering.

When clay bricks are used as part of a ventilated curtain wall system—their most common application in contemporary commercial buildings—the question “Is it waterproof?” becomes less relevant. Curtain wall systems are not designed to be watertight to begin with. Even so-called “sealed” systems may allow a small amount of moisture to penetrate the outer cladding. The real secret lies in the details.

A well-designed clay brick curtain wall system consists of several layers working in concert: the outer clay panels (the visible curtain wall layer), the cavity behind them, the weather-resistant barrier (WRB) attached to the structural wall, and the structural wall itself. The outer clay panels block most wind-driven rain. Any moisture that penetrates the outer panels—whether through open joints or as water vapor—enters the ventilated cavity, where it is either drained downward by gravity or evaporated by airflow within the cavity. The WRB is the true waterproofing layer of the entire system. The clay panels serve to protect it from UV radiation, mechanical damage, and the direct impact of the harshest weather conditions.

For example, Aliva UK’s clay brick curtain wall system has received BBA certification and has been explicitly rated as having “excellent resistance to water penetration.” The system incorporates extruded aluminum profiles and stainless steel concealed fasteners, and the entire construction is designed to manage moisture rather than attempt to keep water out entirely. This is the industry standard practice for ventilated curtain wall systems.

For procurement teams, the implication is clear: do not evaluate the water resistance of clay tiles in isolation. Material specifications are just one piece of the puzzle. System design—cavity depth, ventilation nodes, WRB selection, and installation quality—is the key to successful moisture management.

Glazed vs. Unglazed: A Distinction That Cannot Be Ignored

When it comes to water resistance, not all clay bricks are created equal. The presence or absence of glaze makes a significant difference.

Unglazed clay bricks have an open-pored surface, allowing water to be absorbed directly. This does not mean they are unsuitable for outdoor use—far from it—but it does mean their water absorption rate tends to be higher than average for clay bricks, and over time, surface staining caused by environmental pollutants is more likely to occur.

Glazed clay bricks are coated with a vitreous glaze before the body enters the kiln; during firing, the glaze fuses with the body. This glaze layer is essentially impermeable—it forms a waterproof barrier on the material’s outer surface. The clay bricks on Chicago’s Arrow Building, installed nearly a century ago, are white glazed clay bricks that still look as good as new today.

However, one detail should not be overlooked: the glaze exists only on the surface. The body itself remains porous, meaning water can still penetrate through unglazed edges, cut surfaces, or cracks in the glaze. For this reason, even glazed clay brick systems still rely on the principles of cavity drainage and ventilation—the glaze reduces surface water absorption but does not eliminate the need for proper moisture management.

For specifiers, the practical decision-making criteria are as follows: if you require maximum surface resistance to moisture (such as in coastal locations with high wind and wave exposure), glazed clay tiles are the superior choice. If breathability and natural texture are more important considerations, unglazed tiles may be more suitable.

What Procurement Teams Should Verify

If you are on the buyer’s side, the following are the key points you should focus on when preparing documents and conducting due diligence.

First, request ASTM C67 water absorption test data. Any reputable clay brick manufacturer should be able to provide this. Focus on the 24-hour cold water absorption rate, 5-hour boiling water absorption rate, and saturation coefficient. These figures indicate the baseline for long-term wet performance.

Second, verify system-level certifications. For curtain wall applications, look for BBA certification (for the UK market) or the equivalent third-party certification in your region. These certifications evaluate the water resistance, structural integrity, and durability of the entire system—not just the clay brick material itself.

Third, understand the installation requirements. Even the best clay brick system will leak if installed incorrectly. The specifications must include clear cavity dimensions, drip edge placement, drainage hole placement, and ventilation opening details. The purchase contract should include requirements for installer qualifications or provisions for third-party inspections.

Fourth, consider post-installation surface treatment. Penetrating sealants are available for clay tiles, typically based on silane/siloxane systems. Solvent-based water-repellent protectants, such as FILA ES82, prevent moisture absorption while maintaining the material’s breathability. Other options include water-based alkylalkoxy silane emulsions, suitable for porous substrates like clay tiles and brick. These treatments can be applied after installation to reduce water absorption and the risk of efflorescence, particularly in regions with severe freeze-thaw cycles or high rainfall. Note that sealants require periodic reapplication—they are a maintenance measure, not a one-time solution.

Frequently Asked Questions

Q: Are clay bricks waterproof?

No. Clay bricks are water-resistant, meaning they absorb water slowly and in limited quantities, but they are not completely impermeable. It is system-level moisture management that keeps a building dry, not the “waterproofness” of the material itself.

Q: How much water do clay bricks actually absorb?

Under ASTM C67 testing, the 24-hour cold water absorption rate of clay bricks is typically less than 8%, and the 5-hour boiling water absorption rate is less than 12%. Specific values vary by manufacturer and product grade.

Q: Can clay bricks be used in rainy regions?

Yes. Clay bricks are widely used in tropical and coastal environments. The key lies in proper curtain wall system design, ensuring adequate drainage and ventilation. Glazed clay bricks also provide additional surface protection.

Q: Do clay bricks require sealing?

Not necessarily. Many clay brick curtain wall systems perform well without sealants. In areas with severe freeze-thaw exposure or where efflorescence is a particular concern, a penetrating sealant may be applied. Sealants require regular maintenance and reapplication.

Q: What are the standards for testing the water absorption rate of clay bricks?

For North American projects, refer to ASTM C67, which governs water absorption testing for bricks and structural clay tiles. For European projects, refer to BS EN 771-1 and BS EN 772-21, which cover clay block specifications and water absorption testing methods, respectively.

Q: Does glazing make clay bricks waterproof?

The glaze forms an impermeable layer on the glazed surface, significantly reducing surface water absorption. However, water can still penetrate through unglazed edges, cut surfaces, joints, or glaze defects. Glazed systems also rely on cavity drainage and ventilation.

Conclusion

Clay bricks are not waterproof. They have never been waterproof, nor do they need to be.

What they offer is predictable, verifiable water resistance that, when integrated into a properly designed building envelope system, can perform reliably for decades. The material absorbs some moisture and releases it through evaporation, while protecting the system’s actual waterproofing layer from direct exposure to the external environment.

For B2B buyers and specifiers, the key takeaway is: don’t fixate on whether the material itself is “waterproof.” That’s the wrong question to ask. What you should focus on is whether the system—comprising the material, construction details, and installation quality—can effectively manage moisture throughout the building’s design life. Request test data and certification documents, and ensure every member of the project team understands that the porosity of clay bricks is not a problem to be solved, but rather part of the solution.

If you’re evaluating the use of clay bricks for your next project and need help clarifying specification requirements or supplier qualifications, let’s talk. That’s what we do.

Note: The water absorption ranges cited in this article are based on industry reports and publicly available technical specifications for architectural clay bricks. Specific values vary by manufacturer, product grade, and firing conditions. Be sure to request current test data for the specific product when purchasing.

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