The Truth About Slate Flooring: Understanding Its Properties, Grades, and Durability

Frederick M Hueston  StoneForensics.com

Slate is often marketed as a high-end, rustic flooring material, praised for its natural beauty, unique textures, and range of colors. However, when it comes to selecting slate for flooring—especially in high-moisture areas like showers—many professionals, including myself, have encountered serious issues with its performance over time.

While the internet is flooded with promotional content touting slate’s benefits, there’s a noticeable lack of objective, technical information about its properties, hardness, and suitability for different applications. This article will take a neutral, technical approach to slate, discussing its different grades, sources, scratch resistance, and why some types perform poorly in wet environments.

What Is Slate?

Slate is a metamorphic rock derived from shale, composed primarily of clay minerals and fine grains of quartz. It forms under low-grade metamorphic conditions, where heat and pressure compress the sedimentary layers, creating the characteristic cleavage planes that allow it to split into thin, flat sheets.

While slate is often advertised as a “hard” stone, its true durability varies significantly depending on its composition and origin. Some slates are highly durable and used in roofing, while others can be soft and flaky, leading to serious performance issues when exposed to moisture.

Types of Slate and Regional Variations

Not all slate is created equal. The origin of the slate plays a significant role in its hardness, durability, and overall performance. Let’s break down some of the major types:

1. Vermont Slate (USA)

• Hardness & Durability: Generally higher quality than many imported slates. Vermont slates tend to be denser and less porous, making them more resistant to water absorption and spalling.
• Texture & Appearance: Comes in a variety of natural colors, including green, purple, gray, and red. Some Vermont slates have a more undulating surface, creating a naturally textured, slip-resistant finish.
• Best Uses: Roofing, exterior flooring, and some interior flooring applications.

2. Indian Slate

• Hardness & Durability: Highly variable. Some Indian slates are relatively dense, but others are softer and more prone to delamination (flaking apart in layers) when exposed to moisture.
• Texture & Appearance: Often available in multicolor variations like rust, gold, and deep browns. Some have a split-face texture that enhances grip but can also be sharp underfoot.
• Common Issues: Many lower-grade Indian slates absorb water easily, which leads to breakdown over time in wet environments like showers.

3. Chinese Slate

• Hardness & Durability: Typically lower quality, with a high tendency to flake and break down over time. This is due to a weaker metamorphic process that left it less compact than Vermont or Brazilian slates.
• Texture & Appearance: Usually found in black, gray, or greenish hues, often with a rough texture.
• Common Issues: Absorbs water quickly, leading to spalling and surface breakdown.

4. Brazilian Slate

• Hardness & Durability: Generally one of the hardest slates available, making it more scratch-resistant and better suited for flooring.
• Texture & Appearance: Smooth to moderately textured surfaces, often in darker shades.
• Best Uses: Flooring, countertops, and outdoor applications where durability is a priority.

Grades of Slate

Just as with marble and granite, slate is sold in different grades, which impact its performance:

• Grade A (High-Quality, Dense, Low-Absorption)
  • Typically sourced from Vermont, Brazil, and high-end Indian quarries.
  • Low water absorption (<0.5%), making it less prone to spalling.
  • Stronger, with less tendency to flake.
• Grade B (Moderate Durability)
  • Slightly higher water absorption (0.5%–2%).
  • Some risk of flaking over time, especially in wet environments.
  • Often found in mid-range imported slates.
• Grade C (Lower Quality, High Porosity)
  • Highly absorbent (>2% water absorption).
  • Prone to surface deterioration and flaking.
  • Frequently found in low-cost slate tiles from China and India.

Scratch Resistance & Hardness Ratings

Many assume that slate is scratch-resistant, but in reality, its hardness varies significantly. On the Mohs hardness scale, which measures a mineral’s resistance to scratching (1 being talc, 10 being diamond), slate falls between 2.5 and 5.5, depending on its composition.

• Soft Slates (2.5–4) → More prone to scratches, often from Indian and Chinese quarries.
• Hard Slates (4.5–5.5) → More resistant, often from Vermont and Brazil.

A key issue with softer slates is their tendency to scratch from everyday foot traffic, dragging furniture, or even pet claws. This makes some slates a poor choice for high-traffic areas.

Why Slate Fails in Showers & Wet Areas

One of the most common failures of slate flooring is in showers, patios, and other wet areas. Here’s why:

1. Water Absorption & Spalling

• Many lower-grade slates are highly porous and absorb water over time.
• As moisture penetrates the layers, it causes the slate to delaminate and flake off.
• Example: A homeowner installs Indian slate in a shower, only to find it peeling apart within a year due to constant exposure to moisture.

2. Mineral Content

• Some slates contain iron oxide minerals, which rust and cause staining when exposed to water.
• Example: A black slate shower floor begins to show reddish-brown stains as the embedded iron oxidizes over time.

3. Chemical Sensitivity

• Certain slates are sensitive to acidic cleaners, which can accelerate breakdown and discoloration.
• Example: A customer uses a vinegar-based cleaner on their slate shower, only to find the surface turning dull and flaky.

Where Slate Works Best (And Where It Doesn’t)

Good Applications

✅ Exterior patios (with the right sealer) ✅ Interior flooring in dry, low-traffic areas ✅ Fireplace surrounds ✅ Roofing (high-grade slates like Vermont or Brazilian)

Poor Applications

❌ Showers & wet areas (unless extremely dense slate is used) ❌ High-traffic commercial floors (scratches easily) ❌ Kitchen countertops (stains, scratches, and absorbs liquids)

Is Slate a Good Flooring Choice?

Slate can be a good choice, but only when the right type and grade are selected for the right application. Higher-quality, denser slates like Vermont and Brazilian slates hold up well, while softer, more porous varieties from India and China often fail when exposed to moisture or heavy foot traffic.

For shower surrounds, patios, or high-traffic floors, a more suitable alternative like granite, porcelain tile, or sealed limestone may be a better option.

When choosing slate, always consider:

• Its origin
• Its hardness
• Its porosity
• Its long-term maintenance requirements

Not all slates are equal, and as many professionals have learned the hard way, choosing the wrong slate can lead to costly repairs and disappointment down the road.