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Table of Contents
Introduction
The slope of your roof affects how well it 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 clay and concrete tile.
Note: Pitch and slope are not technically the same thing, but I’ll be using the terms interchangeably here. The building code uses “slope.”
Clay and concrete tiles are part of a roof assembly that includes a solid deck, one or more layers of underlayment, wooden batten boards, flashing, fasteners, and the tiles themselves. Unlike low-slope roofing systems that are designed to be completely-waterproof barriers, tile roofs are water-shedding systems. They rely on gravity and the overlap between 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 tiles, leading to leaks, rotten 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 Clay and Concrete Tile
Clay and concrete tiles can only be installed on roofs with a slope of 2½-in-12 or greater. This means the roof must rise at least 2½ inches for every 12 inches of horizontal run. Anything less is a code violation and a very bad idea. On slopes between 2½:12 and 4:12, the building code requires a double-layer of underlayment to provide extra protection.
Installing clay or concrete tile on a roof with a slope below 2½:12 can lead to serious problems. Because tiles are not watertight and rely on gravity to shed water, low-slope installations increase risks from ponding water, ice dam formation, and water intrusion, especially during rain accompanied by high winds. This can lead to saturated underlayment and roof deck, as well as rot, mold, and interior water damage.
Manufacturers explicitly prohibit such installations, so any tile roof application below the minimum slope will void the 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 tile roofing can technically meet code at slopes as low as 2½:12, it’s really not ideal for anything under 4:12. The fact that the building code requires a double layer of underlayment below that threshold is a clear sign that tile roof performance can be expected to drop off below 4:12. This is common sense and confirmed by my professional experience.
Even though it’s allowed by code, as a roof consultant I would not recommend tile roofing for a 2½:12 or 3:12 roof. Even when installed correctly, tiles on 2½:12 to 4:12 roofs are more likely to leak and suffer from ice dams. If your roof pitch is below 4:12, it’s worth considering whether another material might be a better choice.
A 6-in-12 roof slope for comparison. 6-in-12 is a much more common and appropriate slope for roof tiles.
Better Roofing Options for 2:12 to 4:12 Roof Slopes
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. Standing seam metal roofing usually costs more than concrete tile, but it’s cheaper than clay tile.
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 clay and concrete tile:
Clay and concrete roof tile shall be installed on roof slopes of 2½ units vertical in 12 units horizontal (25-percent slope) or greater. For roof slopes from 2½:12 to 4:12, double underlayment application is required in accordance with Section R905.3.3.
R905.3.3 Underlayment
Underlayment shall comply with Section R905.1.1.
And here’s the equivalent section from the 2024 International Building Code:
Clay and concrete roof tile shall be installed on roof slopes of 2½ units vertical in 12 units horizontal (21-percent slope) or greater. For roof slopes from 2½ units vertical in 12 units horizontal (21-percent slope) to 4 units vertical in 12 units horizontal (33-percent slope), double underlayment application is required in accordance with Section 1507.1.1.
Underlayment Requirements
Here are the underlayment requirements for clay and concrete roof tile just mentioned in section R905.3.3, including a detailed description of how the underlayment should be installed for slopes between 2½-in-12 and 4-in12:
Underlayment Application – Clay and Concrete Tile (Table R905.1.1(2))
Areas Where Wind Design Is Not Required In Accordance With Figure R301.2.1.1:
Underlayment shall be one of the following:
1. For roof slopes from 2½ units vertical in 12 units horizontal (2½:12), up to 4 units vertical in 12 units horizontal (4:12), underlayment shall be two layers 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. For roof slopes of 4 units vertical in 12 units horizontal (4:12) or greater, underlayment shall be one layer applied in the following manner: underlayment shall be applied shingle fashion, parallel to and starting from the eave and lapped 2 inches. End laps shall be 4 inches and shall be offset by 6 feet.
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.
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 felt 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. Distortions in the underlayment shall not interfere with the ability of the shingles to seal. 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.
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. Increased roofing fastener requirements will normally also apply in these areas.
About the Author
Jack Gray spent 20 years as a principal roof consultant with the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. Mr. Gray has worked in the roofing industry for over 25 years, with training and practical experience in roof installation, roof inspection, roof safety, roof condition assessment, construction estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management. He was awarded the Registered Roof Observer (RRO) professional credential in 2009. He also served as an infantry paratrooper in the 82nd Airborne Division and has a B.A. from Cornell University.