Minimum Roof Pitch for Slate Roofs (Natural Slate)

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Introduction

The slope of your roof affects how well your roof sheds rain, snow, and debris and determines what types of roofing materials are appropriate and can be legally installed. This article explains what the minimum roof pitch is for natural slate shingles, one of the best-looking and longest-lasting roofing materials available. You can learn more about slate roofing here.

Note: Pitch and slope are not not technically the same thing, but I’ll be using the terms interchangeably here. The building code uses “slope.”

Slate shingles are part of a roof assembly that includes a solid wood deck, underlayment, possibly a batten system, flashing, fasteners, and the slate tiles themselves. Unlike low-slope roofing systems that are designed to be fully-waterproof barriers, slate shingles are water-shedding. They rely on gravity and the overlap between the courses to keep water moving off the roof. If they are installed on a roof that is too flat, water can back up under the slates, leading to leaks, rotten roof deck, and possibly mold. This can void warranties, violate code, and cost a lot of money to fix.

If you aren’t sure what your roof pitch is, you can get a very cheap, very accurate slope finder from Amazon. I have one of these and it works pretty well.

Minimum Allowed Roof Slope for Slate Shingles

Slate shingles can only be installed on roofs with a slope of 4:12 or greater. This means the roof must rise at least 4 inches for every 12 inches of horizontal run. Anything less is a code violation and a very bad idea.

Installing slate shingles on a roof with a slope below 4:12 can lead to serious problems. Because slate is not a watertight material and relies entirely on gravity and overlap, low-slope installations increase risks from ponding water, ice dam formation, and wind-driven water intrusion. This can lead to saturated underlayment, damaged decking, and interior leaks.

Slate manufacturers explicitly prohibit such installations, so any slate roof application below the minimum slope will void your product warranty. It also violates the International Residential Code (IRC), which can lead to failed inspections, insurance claim denials, and liability issues for contractors.

While slate shingles meet code at a minimum of 4:12, they perform best at 6:12 or greater. In my professional experience, steeper slopes extend the life of the roof and reduce the likelihood of failure due to snow, ice, or wind-driven rain.

Headlap and Why It Matters

Headlap is the vertical overlap between a slate shingle and the slate two courses above it. It is one of the most important factors in a slate roof’s ability to shed water. If there is not enough headlap, water will penetrate through the slates into the underlayment and deck, causing leaks and structural damage. It’s one of the most common installation errors on failed slate roofs.

To get a more complete understanding of headlap on slate roofs, see this short article.

The required headlap depends entirely on roof slope. According to the 2024 International Residential Code, Table R905.6.6, headlap requirements are:

• 4:12 ≤ slope < 8:12: 4 inches of headlap required
• 8:12 ≤ slope < 20:12: 3 inches of headlap required
• 20:12 and steeper: 2 inches of headlap allowed

This is what is allowed by code. Slate roofing experts typically recommend increasing the headlap above what is allowed by code.

18-inch slates installed with a 4-inch headlap will have a 7-inch exposure. Here’s how that works: headlap is the part of the slate that gets covered by the slate two rows above. To calculate how much of each slate will remain visible on the finished roof, subtract the headlap from the full length of the slate (18 − 4 = 14 inches). Then divide that by 2, because each slate overlaps the slate in the course directly below it by half of that remaining distance. That leaves an exposure of 7 inches.

More headlap means you need more material. More headlap means smaller exposures, which in turn means more courses and more slates per square. An 8″ × 18″ slate might require around 210 slates per square (100 square feet of roof area) when installed with a 3-inch headlap, but closer to 225 slates per square when using a 4-inch headlap. Increasing the headlap also adds weight to the roof and increases labor costs, since more slates must be handled and installed to cover the same area.

Even though steep slopes allow for less headlap, more headlap is better for roof performance. If you’re installing slate in an area prone to ice dams, snow, or wind-driven rain, extra headlap is highly recommended, especially at eaves and valleys.

Roofing Options for Roof Slopes Below 4:12

Slate shingles cannot be installed on roofs with slopes under 4:12. If your roof has a lower slope, you will need to choose a different roofing system. While the building code allows asphalt shingles to be installed on slopes down to 2-in-12 and clay and concrete tile can be installed on slopes down to 2½-in-12, I wouldn’t recommend them for slopes under 4-in-12. The following roofing materials are code-compliant and better suited for such shallow roof pitches:

Metal Roofing (Standing Seam)
One of the best-performing and best-looking roof systems for lower-sloped residential roofs. Standing seam metal panels can be installed on slopes as low as ¼-in-12 by code (and with manufacturer approval, of course). They shed water effectively, resist ice damming, and have a very long lifespan.

Modified Bitumen
Modified bitumen is a common low-slope roofing material, a lot like roll roofing, but it uses asphalt modified with a small amount of plastic or synthetic rubber to produce a much more durable high-performance product. It’s applied in two layers with overlapping courses; this creates a fully watertight roof membrane. Mod bit is surfaced with granules like asphalt shingles and you can get modified bitumen in different colors.

Single-Ply Membranes (TPO, PVC, or EPDM)
Single-ply membranes are typically used on commercial flat roofs, but they can be used for low-slope residential applications too. TPO and PVC come in white, gray, and tan; EPDM is normally black. One drawback of the white membranes is that they really show the dirt (roofs get very dirty). Appearance isn’t normally an issue on commercial roofs, but you might want to think about it if anyone is going to be able to see your low-slope roof.

Code Sections: Minimum Roof Slope

From the 2024 International Residential Code, here’s the section that dictates acceptable roof slope for slate roofs:

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R905.6.2 Slope

Slate shingles shall be used only on slopes of 4 units vertical in 12 units horizontal (33-percent slope) or greater.

R905.6.3 Underlayment

Underlayment shall comply with Section R905.1.1.

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And here’s the equivalent section from the 2024 International Building Code:

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1507.7.2 Deck Slope

Slate shingles shall only be used on slopes of 4 units vertical in 12 units horizontal (4:12) or greater.

1507.7.3 Underlayment

Underlayment shall comply with Section 1507.1.1.

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Here are the headlap requirements for slate shingles by roof pitch from the IRC:

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R905.6.6 Application

Minimum headlap for slate shingles shall be in accordance with Table R905.6.6. Slate shingles shall be secured to the roof with two fasteners per slate. Slate shingles shall be installed in accordance with this chapter and the manufacturer’s instructions.

TABLE R905.6.6 – Slate Shingle Headlap

SLOPE	HEADLAP (inches)
4:12 ≤ slope < 8:12	4
8:12 ≤ slope < 20:12	3
Slope ≥ 20:12	2

For SI: 1 inch = 25.4 mm.

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Underlayment Requirements

Here are the underlayment requirements for slate shingles just mentioned in section R905.6.3:

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Underlayment Application – Slate Shingles (Table R905.1.1(2))

Areas Where Wind Design Is Not Required In Accordance With Figure R301.2.1.1:

Apply in accordance with the manufacturer’s installation instructions.

Areas Where Wind Design Is Required In Accordance With Figure R301.2.1.1:

Underlayment shall be one of the following:

1. Two layers of mechanically fastened underlayment applied in the following manner: Apply a strip of underlayment that is half the width of a full sheet parallel to and starting at the eaves, fastened sufficiently to hold in place. Starting at the eave, apply full width sheets of underlayment, overlapping successive sheets half the width of a full sheet plus 2 inches. End laps shall be 4 inches and shall be offset by 6 feet.

2. A minimum 4-inch-wide strip of self-adhering polymer modified bitumen underlayment complying with ASTM D1970, installed in accordance with the manufacturer’s installation instructions for the deck material, shall be applied over all joints in the roof decking. An approved underlayment complying with Table R905.1.1(1) for the applicable roof covering shall be applied over the entire roof over the 4-inch-wide membrane strips.

3. A single layer of self-adhering polymer modified bitumen underlayment complying with ASTM D1970, installed in accordance with the underlayment and roof covering manufacturer’s installation instructions for the deck material, roof ventilation configuration and climate exposure of the roof covering.

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Note that the second part of this (Areas Where Wind Design Is Required) applies to geographic regions where buildings must be engineered to withstand higher wind forces due to increased risk of strong winds. These areas include designated hurricane-prone regions, coastal high-wind zones, and areas like mountain foothills and tornado-prone zones.

The language in the code means that the double underlayment requirement applies to all roofs, regardless of slope, in these areas.