Roofers know a low-slope roof when they see it, but homeowners may not know what this term means. While asphalt shingles are a popular and cost-effective choice for many roofs, their suitability for low-slope applications depends on several factors. This article discusses low-slope roofs and when you can and can’t use shingles on them.
What Defines a Low-Slope Roof?
Slope, or pitch, refers to the degree of incline of your roof. To determine the slope of your roof, you need to measure the vertical rise over the horizontal run. The slope of the roof and materials used allow water to drain effectively off your roof.
Asphalt shingles usually have a 4” or more rise in the vertical for every 12″ horizontal run along the roof This is a 4/12 pitch. A slope of 3” vertically per foot (3/12 pitch) is low-slope but can be acceptable for asphalt shingles using a double layer of roofing underlayment. Some building codes let you use asphalt shingles on roofs with as low as a 2/12 pitch. However, for any pitch less than 3/12, you should use roofing material made for low-slope roofs.
What Are the Minimum Roof Slope Requirements for Asphalt Shingles?
Manufacturers and building codes provide guidelines for their asphalt shingles in their installation instructions. Most specify a 4/12 pitch as the minimum, as this slope allows for ideal drainage. However, if the builder uses underlayment, some building codes allow asphalt shingles in pitches as low as 2/12.
Building Code Requirements
Building codes may allow for using asphalt shingles on slopes as low as 2/12 but with strict stipulations:
- 4/12 or greater: Standard installation methods are acceptable.
- 3/12 to 4/12: Use a double layer of underlayment.
- 2/12 to 3/12: You need special low-slope application techniques and materials.
Building codes may let you use asphalt shingles on very low slopes, but this isn’t the best choice for long-term performance and durability. Adhering to these codes is key to the safety and effectiveness of the roofing system.
Most manufacturers require a slope of 4/12 or greater to honor product warranty coverage. So, while you may save money initially by opting for the more affordable asphalt shingles, you’ll cover those repair costs if an issue occurs.
Using Extra Underlayment on Low-Slope Roofs
For slopes between 3/12 and 4/12, additional underlayment mitigates some of the risks associated with using asphalt shingles on low-slope roofs. This extra layer of protection improves water resistance and extends the roof’s life.
Building codes often require a double layer of underlayment for low-slope applications. The double layer of underlayment typically involves a base layer of #15 felt or synthetic underlayment and a second layer overlapped by at least 19 inches.
Ice and Water Shield
In addition to the standard underlayment, your roofer should install an ice and water shield membrane along eaves, valleys, and other vulnerable areas. This self-adhering membrane provides extra protection against ice dams and wind-driven rain.
Risks of Using Asphalt Shingles on Low-Slope Roofs
For roofs with less than a 3/12 pitch, we recommend specialized low-slope roofing materials rather than asphalt shingles.
While it may be tempting to use asphalt shingles on a low-slope roof because they are affordable and familiar, there are several risks associated with this approach. Understanding these potential issues can help you make more informed decisions about their roofing materials.
Low-slope roofs are more susceptible to water pooling and wind-driven rain, which can force water underneath shingles. Water under your shingles increases the risk of leaks, especially if the underlayment and flashing details aren’t perfect.
Asphalt shingles on low-slope roofs tend to wear out faster due to increased exposure to standing water and potential ice damming, potentially leading to premature failure and the need for more frequent roof replacements.
Consult a roofing professional in your area to advise you on your situation.
Alternative Roofing Materials for Low-Slope Roofs
For roofs with a slope less than 3/12, it’s generally advisable to use roofing materials specifically designed for low-slope applications. These materials offer better water resistance and durability in low slope conditions. Alternatives to asphalt shingles for roofs with a slope of 3/12 or less include single-ply membrane systems, modified bitumen roofing, and metal roofing systems.
Single-ply membrane systems are popular because they are water-resistant and durable. You can find these systems made from materials including TPO (thermoplastic polyolefin), EPDM (ethylene propylene diene terpolymer), and PVC (polyvinyl chloride).
Modified bitumen is an asphalt-based product reinforced with polymers for added strength and flexibility. It’s available in rolls, and you can apply it using torch-down, self-adhered, or cold-applied methods. This material offers robust protection and longevity for low-slope roofing systems.
Certain metal roofing systems, such as standing seam metal roofs, can be effective on low-slope roofs. They offer excellent water-shedding capabilities and long-term durability. Metal roofs can also enhance the style of a home while providing reliable performance.
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