What Is Slate Roofing?

What slate roofing is

Slate is a rock. It forms when fine clay and mud are pressed and heated deep in the earth over a very long time.

The result is a stone that splits into thin, flat sheets along natural lines.

Quarry workers split blocks of it by hand into roofing pieces, then trim and punch them with nail holes.

On a roof, each piece of stone is called a slate. The code calls them slate shingles ¹.

They are laid in rows, called courses, from the eave up to the ridge.

Each row covers the top part of the row below it, so water runs from one slate onto the next and off the roof.

Slate comes in many colors. The National Park Service notes that green, red, purple and mottled slates come from the New York and Vermont district, and that iron compounds make the red, purple and green colors ³.

Some slates change color as they weather. Others, sold as unfading, keep their first color for many years ³.

Slate is different from synthetic slate, which is made from plastic or rubber blends to look like stone. This page is about the real stone.

How a slate roof keeps water out

A slate roof works by overlap. Each slate is hung by two nails near its top. The next row up covers those nails. The row after that covers the top of the first row again.

The part of a slate that is covered by the row two courses above it is called the headlap. Headlap is what stops water from getting past the joints between slates.

The code sets headlap by slope ¹. On flatter roofs water moves more slowly, so it needs more overlap. On a roof between 4:12 and just under 8:12, the headlap must be 4 inches. From 8:12 to just under 20:12, it is 3 inches. At 20:12 and steeper, it is 2 inches.

The nails do not pinch the slate tight. In slate work, the slate hangs on its nails. If a nail is driven too hard, it can crack the stone.

If it sits too high, the slate above can rock on the nail head and break.

That is why the trade says the nail should fit snug in the hole and the top of the slate below should not be hit by the nail ⁴.

Underneath the stone, the code requires underlayment, a sheet laid over the deck before the slate ¹. The slate roofers' trade group calls for at least an asphalt-saturated felt meeting ASTM D226 ⁴. Read more on our roof underlayment page.

The chart shows the rule in one picture: flatter roofs need more overlap because water moves more slowly.

Slate grades under ASTM C406

The code says slate must meet ASTM C406 ¹. That standard sorts slate into three grades. The service lives below are the Slate Roofing Contractors Association's figures, not a code rule or a warranty ⁴.

For S1, the trade group's guide lists a minimum breaking load of 575 pounds-force, water absorption of no more than 0.25%, and a softening depth of no more than 0.002 inches, each from its own ASTM test ⁴.

Sources: [1] [4]

Slate thickness and the nail length that goes with it

Slate comes in thicknesses as well as grades. The trade group's guide ties nail length to slate thickness, when the deck is 1 inch thick or more ⁴.

The rule of thumb behind the table is about twice the slate's thickness plus one inch ⁴. Thicker slate is heavier, which matters for the roof frame.

Worked example: what headlap does each roof need?

Three illustrative roofs, matched to the headlap table in 2021 IRC R905.6.5 ¹. See how slope is written on our roof pitch page.

More headlap means each slate shows less, so more slates are needed to cover the same area. That is one reason a low-slope slate roof uses more stone than a steep one.

Where slate roofing is used

Slate is most common on older homes, churches, schools and public buildings in the Northeast and Mid-Atlantic, close to the old quarries.

The National Park Service notes that production grew fast after the Civil War, as quarries opened in Vermont, New York, Virginia and eastern Pennsylvania ³.

It also notes that slate from China, Africa, Spain and other countries is now imported, mainly into the West Coast ³.

Slate is chosen today for new homes where the owner wants a stone roof that can last for generations, and for historic homes where the old roof is being repaired or matched.

It is used on steep roofs, because the code does not allow it below 4:12 ¹. Turrets, dormers and steep gables are classic slate shapes.

Benefits of slate roofing

• Very long life. The National Park Service says slate roofs can last 60 to 125 years or longer, depending on the slate, the roof and the location, and some have lasted over 200 years ³.
• Fire resistance. Slate on a noncombustible deck is a Class A roof covering under the code, the highest class ².
• Low water uptake. Grade S1 slate absorbs no more than 0.25% water in its test, per the trade group's guide ⁴.
• Repairable. Single broken slates can be taken out and replaced without tearing off the roof ³.
• Salvage. Sound slates from a roof that is being redone can often be reused, which helps match an old roof.
• Looks. Natural color changes and split faces give a look that is hard to copy.

Limitations of slate roofing

• Weight. Stone is heavy. A roof frame built for asphalt shingles may need an engineer's check before slate goes on.
• Steep roofs only. The code does not allow slate below 4:12 ¹.
• Fragile underfoot. The National Park Service says slate roofs should not be walked on if at all possible ³.
• Few skilled crews. The trade group suggests foremen with at least five years of slate experience ⁴. Not every roofer has that. The group also publishes separate repair and restoration guidelines for owners and crews ⁵.
• Higher first cost than most other coverings, from the stone, the copper flashing and the slower install.
• Soft slates wear faster. The NPS gives Pennsylvania soft-vein slate a life of a little over 60 years, far less than Virginia slate ³.

What drives the cost of a slate roof

We do not quote prices here. These are the things that move a slate quote up or down. For numbers on your roof, try the roof cost calculator.

• Grade. S1 slate is tested to tighter limits than S2 or S3.
• Thickness and size. Thicker and larger slates weigh more and need longer nails ⁴.
• Quarry and color. Some colors come from only a few quarries; imported slate is another option.
• Roof slope. A flatter slate roof needs more headlap, so more slates per roofing square ¹.
• Flashing metal. Copper or lead-coated copper flashing costs more than painted steel but lasts closer to the life of the stone ³.
• Frame work. Any reinforcement an engineer calls for adds to the job.
• Repair or replace. Fixing a sound old roof costs far less than a new one.
• Access. Hook ladders, scaffolding and roof brackets are used to keep weight off the slate ⁴.

How a slate roof is installed

This is what you should see happen. It is not a do-it-yourself guide. Roof work is a fall risk, and a licensed roofer who works on slate should do it.

1. An engineer or the roofer confirms the frame can carry the weight of the slate chosen.
2. The crew checks that the slope is at least 4:12 ¹.
3. The deck is made solid; the code does not allow slate on spaced boards ¹.
4. Underlayment goes down, plus an ice barrier at the eaves where the local code calls for one. The trade group says self-adhering underlayment should be covered with felt ⁴.
5. A starter course is set at the eave, then the field slates go on with two nails each ¹.
6. Copper or other long-life flashing goes in at valleys, walls and chimneys. The code asks for valley flashing at least 15 inches wide ¹.
7. Ridge and hip slates finish the roof. The contract should list slate type, thickness, color, nails, underlayment, headlap and flashing metal ⁴.

Caring for a slate roof

• Have a slate roofer look the roof over every five to seven years, the interval the National Park Service suggests for a full check ³.
• Ask them to note cracked, broken, slipped and missing slates, and how far any flaking has spread ³.
• Have flashings checked for pinholes, open seams and loose pieces ³.
• Keep gutters and valleys clear so ice and water do not back up under the slate.
• Trim tree limbs that could fall on the roof.
• Tell chimney sweeps, painters and dish installers to use ladders hooked over the ridge, as the NPS advises ³.

Warning signs and problems to watch

• Flaking or soft, spongy slates; the NPS calls this delamination, where thin layers come apart ³
• Slates sliding out of place, often a sign that the nails have rusted through
• Cracked or missing slates after falling limbs, ice dams or foot traffic ³
• Leaks at valleys and chimneys, where flashing often fails before the stone
• Patches of roof cement or tar smeared over slates, a sign of past quick fixes
• Steel or galvanized nails on an older roof; the NPS says they usually rust out long before the slate wears ³

What the building code says about slate

These rules come from the 2021 International Residential Code, read as adopted in Maryland. Most states adopt the IRC, sometimes with changes, so your building department's edition is the one that applies.

Sources: [1] [2]

Slate vs other steep-slope roof coverings

All slopes and decks come from the same chapter of the 2021 IRC; fire classes from R902.1 ^{1 2}. Compare looks and weight on the roofing materials guide.

Common misconceptions

Myth An old slate roof that leaks needs replacing.

Reality Often it does not. The NPS points out that nails and flashing usually fail first; sound slates can be rehung with new nails and flashing ³.

Myth All slate lasts forever.

Reality Life depends on the slate. The NPS gives Pennsylvania soft-vein slate a little over 60 years, and Buckingham Virginia slate 175 years or more ³.

Myth Any roofer can repair slate.

Reality Slate needs special tools, such as slate rippers and hook ladders, and crews who know the stone ⁴.

Myth Roof cement is a fine fix for a broken slate.

Reality Tar on slate traps water and makes later repair harder. The NPS describes replacing the slate and securing it with a copper strip or hook instead ³.

Myth Synthetic slate is the same thing.

Reality Synthetic slate is a plastic or rubber product made to look like slate. It is not graded under ASTM C406.

Local codes and climate

The rules on this page come from the 2021 International Residential Code. States, counties and cities adopt the code on their own schedule and can change it. Your building department decides which edition applies and whether a permit is needed for a slate repair or a new slate roof.

Climate matters too. In cold places, the code may require an ice barrier at the eaves, and snow guards help stop snow sliding off in one sheet and breaking gutters.

The NPS also warns that one method of securing a replacement slate, a folded copper tab, is not recommended in northern climates, because ice can fold the tab and let the slate slide out ³.

In wildfire areas, slate on a noncombustible deck meets a Class A rating ². If your home is in a historic district, the local review board may also have a say in materials.

Is slate roofing right for you?

• Slate makes sense if your roof is 4:12 or steeper, your frame can carry it, and you plan to keep the home for a long time.
• If you already have slate, get a slate specialist to say whether it can be repaired before you accept a tear-off quote.
• Ask for S1 slate on new work, and ask for the grade in writing ⁴.
• Ask what nails and flashing metal are planned; copper or stainless nails and copper flashing are what the trade guidance names ⁴.
• If the frame cannot carry stone, compare lighter coverings with the roof material selector.

Keep going

• Roof repairFixing broken slates, nails and flashing
• Repair or replaceHow to decide what an old roof needs
• Roof inspectionWhat a full roof check covers
• Choosing a rooferQuestions to ask before you hire

Related terms

Clay Tile Roofing

Clay tile roofing is a roof covering made of tiles shaped from natural clay and fired in a kiln, laid in overlapping rows over a solid deck and a waterproof underlayment.

Synthetic Slate Roofing

Roofing tiles molded from polymers or rubber blends to look like natural slate at a fraction of its weight.

Roof Pitch

Roof pitch is the steepness of a roof, written as the inches it rises for every 12 inches it runs horizontally, so a 6/12 roof climbs 6 inches per foot.

Roof Underlayment

Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on.

Valley Flashing

Valley flashing is the lining installed in a roof valley, the inside corner where two roof slopes meet, so the water both slopes send into that one line runs down to the eave instead of into the deck.

Roof Sheathing

Roof sheathing is the layer of plywood, OSB or wood boards nailed across the rafters or trusses.

Ice and Water Shield

Ice and water shield is a self-adhering, rubberized asphalt membrane laid directly on the roof deck under the roof covering, where it seals around nails and blocks water that ice dams or wind-driven rain push back under the shingles.

Metal Roofing

Metal roofing is a roof covering made of metal sheets, panels or shingles, most often steel, aluminum, copper or zinc, fastened to the roof deck to shed water, snow and sun.

Also referenced from: Clay Tile Roofing, Wood Shakes, Composite Roofing, Roof Life Expectancy