Natural Slate Roofing • A Homeowner’s Guide to Slate Roofs

By Jack Gray, Roof Online Editor • Last updated May 15, 2023

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Introduction

Slate is a natural stone that has been used in construction for centuries. It is prized for its beauty, durability, longevity, and minimal maintenance requirements.

Slate roofing has long been considered the finest roofing material available. It’s often found on historic buildings, but a new slate roof adds a mark of distinction to residential homes as well.

The lifespan of a properly-installed slate roof is in a league of its own. The only roofing materials remotely comparable are the very best clay roofing tiles, and possibly very thick copper panels.

Slate roofs have been known to last for centuries.

Natural slate tiles are an ideal choice for homeowners who want the highest quality roofing materials and the best performance from their roof.

Synthetic Slate

Natural slate should not be confused with what a roofing contractor will often refer to as “synthetic slate”.

Synthetic slate is not slate and nothing in this article applies to it.

While a synthetic slate roof looks a lot like a real slate roof and may have positive qualities as a roofing material, synthetic slate is an entirely different product.

Synthetic slates and tiles are typically made of recycled, molded plastic or rubber. In the roofing industry, these materials are often referred to as synthetic composite shingles, tiles, or slates.

While they can typically last 40 to 50 years, this material is far less durable than natural slate.

However, a synthetic slate roof is also lighter in weight and more affordable than natural slate, so that’s something to consider.

Pros of a Slate Roof

• Appearance – Slate has a timeless, classic look that adds beauty to any home.
• Durability – Natural slate roofs can last centuries.
• Eco-Friendly – Slate roof tiles are an all-natural product and the most environmentally-friendly roofing material. No toxic chemicals are used to make the product and they have virtually zero negative environmental impact. Slate roofing materials are considered to be “green” roofing materials.
• Fire-Resistance – Natural slate is non-combustible and carries a Class A fire rating, the best available.
• Insect-Resistance – Slate is basically impervious to insect infestation.
• Low Maintenance – Unlike other roofing materials such as asphalt shingles, slate tiles seldom require maintenance or repairs.
• Recyclable Material – Natural slate tiles can often be reused after the old roof is replaced. Reclamation yards that sell old slate tiles are pretty easy to find.
• Wind Resistance – Slate is both heavy and rigid. A new slate roof is highly resistant to wind damage.

Cons of a Slate Roof

• Expensive to Install – Natural slate is one of the most expensive roofing materials.
• Expensive to Repair – If you need repairs, material and labor costs will be much higher than for more common roofing materials.
• Weight – Slate is one of the heaviest roofing materials, and may be too heavy for your house.
• Easily Damaged by Foot Traffic – Although slate has good impact resistance when it comes to hail, it can easily crack and break if you walk on it.
• Hail Damage – Standard 1/4-inch roofing slate will typically receive a Class 3 impact resistance rating. Thicker 3/8-inch slate can get a Class 4 impact resistance rating, which is the highest rating for roofing materials. If you live in an area prone to heavy hail, you’ll need thicker (and more expensive) slate for your roof.
• Finding a Qualified Slate Roofer – You do not want just any roofing contractor working on your slate roof. You want a slate specialist, and they can be hard to find.

What is Slate Roofing?

Natural slate is a metamorphic rock that formed over 500 million years ago. It comes from quarries, where blocks of slate are removed from the earth and then split, sawn, and finished into the slate tiles used on roofs.

Slate has been used as a roofing material at least since the Middle Ages, with north Wales being the first notable center of slate production.

As a natural stone, slate makes an excellent roofing material, being naturally waterproof, impervious to decay, and extremely durable.

Standard slate roof shingles are rectangular and ¼-inch (0.64 cm) thick, although thicker slates up to 1-inch thick (2.54 cm) are available. Standard roof slate widths are 6, 8, 10, and 12 inches; standard lengths go from 12 to 24 inches (in increments of 2).

A slate roof is installed using copper or stainless steel nails because they don’t corrode. Anything else and you run the risk of the nails failing long before the rest of the roof. Metal slate roof accessories like ridge coverings, flashings, or gutters are often made of copper, lead-coated copper, lead, or zinc.

Grades of Roofing Slate

ASTM International classifies roofing slate (ASTM C406/C406M-15 Standard Specification for Roofing Slate) into three grades based on physical characteristics which lead to an expected service life for each grade:

• Grade S1 (hard slate) – minimum 75 to 100 years (0.25% water absorption)
• Grade S2 (soft slate) – minimum 40 to 75 years (0.36% water absorption)
• Grade S3 (soft slate) – minimum 20 to 40 years (0.45% water absorption)

Roofing slate in the US today is almost exclusively Grade S1 or S2, with most of it being S1.

Slate Roof Styles

There are several different styles when it comes to installing a slate roof:

• Standard (Uniform) Slate – The slate shingles are 1/4 inch to 3/8 inch thick. They are all the same length and width and have straight edges.
• Patterned Slate – Requires installing slates with different colors or end cuts arranged by course or in some other pattern to produce a decorative effect.
• Random Width Slates – Uses slates that are uniform in length and thickness but have different widths.
• Multicolored (Blended) – Multicolored slate roofs use slates of various colors to create the desired effect.
• Graduated Length Slates – The slates have different lengths and widths, with the longest and widest slates located at the eaves and the shortest and narrowest slates at the top.
• Graduated Length / Graduated Thickness – The slates are arranged in descending order of size and thickness, with the smallest and thinnest slate at the top.
• Textural Slate Roof – Mixing slates with rougher surface textures and varying thicknesses throughout a roof creates a slate roof with a textured appearance.
• Hang-down (Staggered Butt) Slate – Uses longer slates randomly, with the tops of the slates all lined up at the same height, with the extra length extending beyond the ends of the adjacent slate pieces.

You can learn more about each of these styles on the National Slate Association website here.

Key Features of Slate Roofs

Aesthetic Appeal

Slate roofing is famous for its natural beauty. Slate is available in a variety of colors, styles, thicknesses, and widths, allowing homeowners to choose a roof that matches the look and feel of their home.

Many varieties of slate have an attractive sheen due to high levels of mica present during the geological formation of the deposits that the slate is taken from.

The appearance of slate roof tiles is also affected by the way the slates are cut and processed, how they are finished, and how thick they are.

Slate can be cleaved, or split, to produce surfaces with naturally textured grains. Sawing and sanding produce a different look. Slate can be finished in a range of different textures.

Durability

Slate is incredibly dense and hard, making it highly resistant to damage from hail, wind, and other forms of extreme weather. Slate is also naturally fire-resistant, insect-resistant, and water-resistant, and it won’t be damaged by mold, algae, or other decay.

Unlike almost all other roofing materials, slate roofs do not deteriorate due to the effects of ultraviolet radiation from the sun.

The density and inert nature of slate results in longevity unmatched by other roofing materials.

Lifespan of Slate Roofs

A typical slate roof made with Grade S1 hard slate can be expected to last more than 100 years if it’s properly installed and treated well. Roofs made with thicker slate can last up to 200 years and even longer.

One well-known slate roof on a Saxon chapel in Stratford-on-Avon, England was installed in the 8th century and lasted 1,200 years.

With proper installation and maintenance, a slate roof will easily last 4-5 times longer than asphalt shingle or wood shake roofs.

Compare the expected service life of a slate roof to some other roof types:

Roofing Material	Expected Service Life
Architectural Asphalt Shingles	25 – 40 Years
Cedar Shingles	25 – 40 Years
Clay Roof Tile	50 – 150 Years
Concrete Roof Tile	40 – 100 Years
Fiber Cement Shingles	30 – 45 Years
Standing Seam Metal (Steel)	40 – 60 Years
Standing Seam Metal (Aluminum)	50 – 80 Years
Roll Roofing	5 – 15 Years
Slate Roofing: Hard Slate (S-1 Grade)	100 – 200 Years
Synthetic Slate	40 – 60 Years

Sustainability

When it comes to being “eco-friendly”, there’s really nothing that compares to slate.

Unlike asphalt roofing products or synthetic composite slate or tile, natural roofing slate contains no petroleum products or any toxic chemicals at all.

Compared to metal roofing or concrete tile, the slate production process uses minimal energy.

Even wood shingles will typically be treated with chemicals to promote fire and insect resistance, while slate doesn’t need to be.

Roofing slates can also be reused in many cases. There are salvage or reclamation yards that specialize in roof tile and slate.

When you add to that the fact that slate roofs only need to be replaced about a third as often as other roof types, slate roofing is the ultimate example of a sustainable building product.

Weight

The weight of standard 1/4-inch-thick slate roof shingles is around 10 lbs per square foot (asphalt shingles weigh around 3 pounds per square foot). Thicker slate can be much heavier.

Here are the typical weights of some common roofing materials for comparison:

Roofing Material	Pounds per Square Foot
Asphalt Shingles (Architectural)	3.05 lbs per square foot
Asphalt Shingles (Luxury)	4.25 lbs per square foot
Clay Tile Roofing	6 – 15 lbs per square foot
Concrete Tile Roofing	6 – 11 lbs per square foot
Slate Roofing (1/4-Inch Slates)	10 lbs per square foot
Slate Roofing (3/8-Inch Slates)	15 lbs per square foot
Slate Roofing (1/2-Inch Slates)	20 lbs per square foot
Standing Seam Metal (Aluminum)	0.692 lbs per square foot
Standing Seam Metal (Steel)	1.49 lbs per square foot
Wood Shingles	3.2 lbs per square foot

If your house was designed and built with a lighter-weight shingle or metal roof in mind, there’s a very good chance that the supporting roof structure (rafters, trusses, etc.) of your house will not be able to safely handle the extra weight of a slate roof.

It is possible to augment the roof structure so it can carry the extra weight, but this will increase the cost of installing the new roof.

If you’re thinking of installing a slate roof, it’s a good idea to consult a structural engineer to make sure the structure can handle it.

Slate Roofing Costs

A good slate roof is one of the most expensive roofs to install, comparable in price to high-end copper roofing or the finest clay tiles. The high cost should be considered in light of the old saying “You get what you pay for”.

Slate roof costs are high because the production of slate tiles is labor-intensive and the supply of slate is limited by geography.

A slate roof also needs to be installed by roofers who specialize in slate and have special training and experience in installing slate. A slate specialist can and will charge much more than a typical roofing contractor.

Slate Roof Cost – Breakdown

The cost of installing a slate roof usually breaks down to around 60% labor and 40% material.

Labor costs for slate installation or repairs will typically be about 4 times higher than for asphalt or wood shingles, standing seam metal roofing, or other similar alternatives.

Material costs for a slate installation will typically be about 5 times the cost of standard architectural shingles.

Slate Roof Cost – Comparison

The actual cost of a slate roof will depend on several variables, including the size of your roof, your location, the complexity of the roof, and the specifics of your particular project (including any special style or design elements).

That said, a new slate roof constructed with high-quality materials and installed by a qualified slate roofing contractor will cost you anywhere from $20 to $30 per square foot.

This means that it will cost somewhere around $50,000 to install a new slate roof on a house with a roof that has a surface area of 2,000 square feet (which is around 20% larger than the average roof, but people having slate installed seem to have larger houses).

Compare that to around $14,000 for luxury shingles or $35,000 for a standing seam metal roof on the same house with everything else being equal. According to the Department of Energy, the average cost for a roof replacement on the average house in the US was around $10,000 (in 2021).

Working on Slate Roofs

For all of you DIY’ers out there, you should really think twice before trying to install or repair a slate roof yourself. That also goes for roofers who don’t specialize in slate and don’t have at least a few years of experience with it.

One thing we’ve noticed when we inspect slate roofs is the large number of broken slates due to people with no slate experience getting on the roof.

Maintenance workers, workers from other trades (like HVAC mechanics), or even roofers with no slate experience will often damage slate roofs.

While I wouldn’t go so far as to describe slate as “fragile”, it does break easily if it’s stepped on or mishandled. You need to know what you’re doing.

Working on slate roofs also requires specialized tools and the know-how to use them. Slate roofing tools include the slate cutter, slater’s hammer, slate ripper, slater’s stake, and special ladder hooks.

If you are determined to work on your own slate roof, I strongly recommend that you get a copy of Slate Roofs: Design and Installation Manual by the National Slate Association and The Slate Roof Bible by Joseph Jenkins. Read them both. The money you save on installation mistakes will more than cover the cost of the books.

Find a Slate Roofing Contractor

It’s important to hire a roofing contractor whose business is primarily dedicated to installing or repairing slate. (See our article How to Choose a Roofer for general guidelines on selecting a roofer.)

Here’s a quote from the above-mentioned Slate Roof Bible that you should take to heart:

The “ruin of many a good slate roof can be traced directly back to a roofing contractor. Why would a roofer who makes his money stapling shingles to plywood want to repair or restore a stone roof when he can easily talk the owner into destroying it instead?

Many shingle roofers don’t know how to repair genuine slate roofs, even though it’s not difficult for any competent roofer to do. The installation of new slate roofs requires skill and knowledge that is not commonly held by standard shingle roofers.”

It can be difficult to find a qualified slater. I know this from experience.

On one particularly large project in New York that we were involved in, the general contractor had to bring in slate roofers from Europe because he couldn’t find enough of them in the area who were available when the project was scheduled.

You’ll probably need to accommodate a good slate roofer’s schedule, not the other way around. Be patient.

One decent resource for finding a slate roofing contractor is the Slate Roofing Contractors Association. You can see their member directory on their website.

Slate Roofing Warranties

Material warranties for roofing slate are commonly available from the company that owns the quarry and produces the slate. Material warranties are guarantees that the material itself will be as specified and free from defects or performance issues for a stated period after purchase.

Roofing slate material warranty periods will typically be anywhere from 50 to 100 years, the highest of any roofing material.

Contractor workmanship guarantees cover the work performed during the installation (but not the material). They also don’t apply to damage caused by weather events such as hail or other storm damage, which would be covered by homeowners insurance.

A workmanship guarantee is a legally-binding promise made by the roofing contractor that the roof will be free from defects or problems due to a poor installation for a certain period after the project is completed.

Typical workmanship guarantee periods for a slate roof will be anywhere from 5 to 20 years.

Best Natural Slate Roofing Brands

Here’s a list of some of the top slate roofing producers in the world. For more, see our Slate Roofing Manufacturers Directory.

• Cupa Pizarras – “1 out of every 2 pieces of slate installed in the world comes from Cupa Pizarras, with more than 100 years of history, is now the world leader in the production and distribution of natural slate.”
• Greenstone Slate Company – “The Greenstone Slate Company has been a leader in crafting Vermont roofing slate for well over half a century and has shipped slate all over the world for residential and institutional projects.”
• New England Slate – “Located in the heart of Vermont’s slate valley, The New England Slate Company has been serving customers around the world for more than 30 years.”
• Newmont Slate Company – “Located in Vermont, Newmont Slate is the largest roofing slate producer in the United States. Newmont Slate Company produces the most and finest Vermont Black Slate in the United States.”
• North Country Slate – “North Country Slate’s origins began as Newfoundland Slate Inc., a company formed in 1990 to develop and exploit a beautiful clear purple slate deposit on the east coast of Canada at Burgoyne’s Cove, Newfoundland.”
• SSQ Group – “For forty years, SSQ has been bringing the most outstanding slate and natural stone products to customers all around the globe. By combining the finest natural materials, uncompromising quality standards, and decades of passion and expertise, we’ve become one of the world’s leading suppliers of stunning Spanish slate and Argentinian Phyllite.”
• Vermont Slate Company – “Vermont Slate offers the widest range of roofing slates in the market, with more than 40.000 combinations of models, sizes, and shapes.”
• Vermont Structural Slate Company – “Vermont Structural Slate Company was founded in 1859 and incorporated on November 14, 1866, by an act of the Vermont legislature. The Company started with a small quarry and modest production facility in Fair Haven, Vermont. Over many decades of investing in our business, we now own and operate quarries and mills in the 3 primary slate-producing states: Vermont, New York, and Virginia.”
• Welsh Slate – “Welsh quarries have been supplying the world with the highest quality slate for hundreds of years. Welsh Slate owns and operates the famous Penrhyn, Ffestiniog, and Cwt-y-Bugail quarries in North Wales.”

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Pictures of Slate Roofing

1. A Look at a Five-Year-Old Slate Roof

Related Pages

1. Clay Tile Roofing
2. Historic Roofing
3. Roofing Slate Colors and Where They Come From
4. Roofing Slate Manufacturers List
5. Slate Roofs: Sample Roof Condition Report
6. Weight of Roofing Materials

External Sources & References for Slate Roofing Information

1. Book Recommendation: The Slate Roof Bible by Joseph Jenkins is a widely respected, important, and valuable asset for anyone who has any sort of responsibility for a slate roof. We always keep it in our reference library.

2. Book Recommendation: Slate Roofs: Design and Installation Manual by the National Slate Association

3. General: For an introduction to slate roofing, read some of the articles available at Traditional Roofing Magazine.

4. General: Quite a bit of information about slate roofing is available at the website of the National Slate Association.

5. General: An excellent “bullet points” primer on slate roofing, “Slate Roofing – How to Install a Roof that will Last a Century (or Two)“, from 2012, is available at the Slate Roof Experts website.

6. General: See this very good general article about various roof systems on the Whole Building Design Guide site, which is maintained by the National Institute of Building Sciences.

7. Building Codes: “R905.6 Slate Shingles” from the 2018 International Residential Code and “1507.7 Slate Shingles” from the 2018 International Building Code. Both available on the UpCodes website.

8. Historic Roofing: Slate Roofs – National Slate Association – (published in 1926, 83 pages). The original text of the 1926 slate book that remained the standard authority for decades.

9. Historic Roofing: The Story of Slate – Charles H. Davis – Davis Slate & Manufacturing Company – (Published in 1923, 7 Pages)

10: Inspection of Slate Roofs: Mastering Roof Inspections: Slate Roofs, Part 1 at the International Association of Certified Home Inspectors website is a good place to start. There are twelve more parts in this slate roof inspection guide, and they can be accessed from this page.

11. Safety: See this material safety data sheet for health and safety information about roofing slate at the GAF website. This is only an example and other products and brands may be different.

12. Standards: “Roofing Slate Standards: A Critical Review” is an incredibly detailed piece of work and well worth taking a look at. Available on ResearchGate.

13. Technical: See this short installation guide for quick overview of how slate roofs are put together. On the Slate Roofing Contractors Association website.

14. Technical: See this product data sheet for the technical characteristics of roofing slate at the North Country Slate website. This is only an example and other products and brands may be different.

15. Technical: “The Repair, Replacement and Maintenance of Historic Slate Roofs“ is a comprehensive resource available on the US National Park Service’s Technical Preservation Services website.