Is Slate Permeable? (ANSWERED)

Slate is not considered a permeable material.

Here we’ll explore the properties of slate, and use those facts to explain why it is not considered permeable.

Is Slate Permeable? (ANSWERED)

What is Slate?

Slate is metamorphic rock originating from sedimentary rock like shale.

Let’s break that down some to get a better understanding.

Overview of formation

Metamorphic rock is formed under great amounts of heat and pressure.

An existing rock situated deep in Earth’s crust is subject to those extreme temperatures and weight.

Over time, the heat and pressure irreversibly change the original rock into something else.

Some rocks melt, but others become more dense and compact.

Regardless, the new material is considered a metamorphic rock.

The following are other metamorphic rocks that might sound familiar:

  • Gneiss
  • Marble
  • Phyllite
  • Quartzite
  • Schist

Forming Slate

Slate starts as sedimentary rock composed of many small particles of mineral or organic material.

Specifically, slate is formed from shale or mudstone.

Shale is the most common sedimentary rock and is formed when mud mixes with flakes of other materials.

Most forms of shale contain about 30% mud and the rest is minerals and organic material.

It’s a soft and brittle rock that gets easily eroded and buried by other materials.

Shale is soft and brittle.

It is fine-grained, and easily eroded.

It forms when a mineral rich silt gets buried in a water rick environment (or even submerged), and gets compacted into a much more dense substance.

Erosion weathers shale into thin layers called plates.

That shale or mudstone gets pressed flat under Earth’s surface causing all of the particles to align.

The particles in shale and mudstone align parallel so the final product gets elongated and striped.

You can tell what direction the pressure was applied from by looking at the direction of those stripes.

Identifying Slate

When you find a piece of slate you’ll immediately notice it is gray-blue.

It’s a flat rock composed of finely-ground well-mixed materials.

It almost appears flat.

Slate is a moderately hard rock.

Granite and quartz are a lot harder than slate, while marble and limestone are only slightly harder.

How Slate is Used

Slate has been used for many centuries for things like building and insulation.

It was even used to create stones for the Japanese game, Go.

Slate is often used as roofing tile because it is durable and attractive.

These roofing tiles require little processing before they can be used.

Additionally, they require little maintenance to maintain their strength and beauty.

Slate roofing tiles are energy efficient and resist dangerous elements like frost and fire.

Slate roofs have been known to last hundreds of years.

Early switchboards were constructed of slate because of that fireproof property. Other notable slate items include:

  • Bench tops for laboratories
  • Personal writing tablets before the advent of paper
  • School blackboards
  • Table tops for billiards tables
  • Whetstones
  • Tombstones

What is Permeability?

Permeability describes how easily liquids or gasses can pass through a material.

If they pass easily then the material is said to be highly permeable.

If it takes a lot of pressure for liquids or gas to get through the material then it is said to have low permeability.

Permeability of a material is a sliding scale rather than being “all or nothing” and is affected by the following characteristics:

  • Porosity or how many pores are in the material
  • Pressure or how well those pores are pushed together

Highly permeable materials

Highly permeable materials let liquids or gasses flow easily through them.

Sandstone has lots of open pores that are well-connected so it is highly permeable.

It’s easy for a liquid to pass from one pore to the next since they are very connected to each other.

Not much pressure is needed to move the liquid from one void to the next; sometimes none at all.

Gravel and sand are examples of highly porous, highly permeable materials.

Highly porous materials, those with many voids, are often very permeable but not always.

It’s possible to have many unconnected voids in a material as well.

This would cause low permeability because the liquid can’t flow from one pore to the next without lots of pressure.

Pumice and shale are examples of rocks with high porosity but low permeability.

Low permeability materials

Liquids and gasses have a difficult time passing through materials with low permeability.

Generally, low permeability materials will not have many voids so they’ll also be of low porosity.

Granite is a low permeable material with low porosity.

On the other hand, rocks like pumice and shale are highly porous but difficult to penetrate because the pores aren’t well-connected.

Is Slate Permeable?

So where does slate fall on the scale of permeability?

Recall that slate originates from shale.

And we found that shale, although highly porous, wasn’t very permeable.

Pressure and heat applied to the shale over an extended period of time makes the final product even less permeable.

Slate is that final product and it’s generally considered impermeable because it’s very difficult for liquids and gasses to pass through.

That’s why it’s used for things like roofing tiles and tombstones that can withstand things like rain, snow, and being cleaned with a hose.

Of course, over a few millennia the rocks will change enough to allow water, Nitrogen, and other substances to mix.

But for practical use, slate is not permeable.


Slate has been used as a building material for several centuries because it’s not very permeable.

This makes it withstand the elements and last many lifetimes.

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