What is Your Roof Pitch, and Why is it Important?

Written by Trevor Leeds of Chandler's Roofing
Updated July 9, 2013
home with a flat roof
A low-slope or flat roof is being seen more often in modern residential architecture. There are special considerations for roofs installed with this type of pitch, such as using the right roofing materials.
Photo by Frank Espich

A flat roof, with its lower pitch, presents different challenges and requires different roofing materials.

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Roof pitch? No, we’re not talking about your sales pitch or your favorite throw in baseball. Your roof’s pitch is an important calculation in determining the proper roofing system for your home or business. In fact, it is so important that certain manufacturers will void warranties if specific protocols are not met when installing a new roof on certain slopes.

Why is roof pitch important?

Your roof’s pitch determines the type of roofing system and materials that are suitable for your building’s architecture. More than aesthetics, roof pitch determines how a roofing system will perform long term, and it determines how quickly water and debris will shed from the roof.

Low-slope or flat roofs are primarily seen in commercial roofing applications, although they are being found more and more in today’s modern residential architectural designs. As modern architecture and modular home building continue to advance with the integration of solar and green roofing systems, low-slope roofs may become even more prevalent in the United States.

A low-slope roof is determined as anything that rises 3 inches or less per horizontal foot. Traditional roofing systems for flat roofs consisted of hot-mopped bitumen and cap sheet layers, coated with gravel to help with water evaporation and UV protection. These roofing systems, though still installed by many roofers today, are becoming a thing of the past, because of the more technologically advanced flat roofing now coming to the market.

Roofing materials and installation options abound

Leading the pack of the newer low-slope roofing materials are single-ply roofing systems, manufactured with waterproof rubber or thermoplastic compounds, such as PVC and TPO. These systems come in large rolls. They are Energy Star rated and cool-roof compliant, come in several colors and are hot-air welded, eliminating the smelly, hazardous tar kettles seen in hot-mop roofing.

Additionally, these single-ply systems are class A fire-rated and provide outstanding, no-dollar-limit warranties, which cover all the roofing system components and the contractor’s workmanship for up to 25 years.

Another leading low-slope product is the modified bitumen (torch) roofing system, which advances the hot-mop process by impregnating the bitumen into a fiberglass and polyester mat that is then rolled out and torched directly to your roof deck. This is a terrific, mid-range flat roof product and comes in several different colors including a Title 24-compliant, class A fire-rated sheet.

Pitch determines the roofing product

Certain products however, are not designed to be installed on roofs with pitches below 3:12. For example, composition shingles and tile roofs are not intended to be on a roof slope below 3:12 and typically require a double underlayment, or modified peel-and-stick protection on slopes from 3:12 - 4:12. The primary reason for this is the design of the system and the inability of the water to shed properly from the roof, causing water to back up under the tile and pool.

Single-ply roofs and modified cap sheets are designed to withstand some form of ponding water, be it hours or days, with single-ply systems able to withstand ponding water for far longer. Composition shingle and tile roofs however, are not designed to withstand this ponding water and in fact are designed for roofs with greater than 3:12 slope.

Talking roof pitch with roofing contractors

Do not let a contractor talk you into a new roof that is not intended for your low-slope roof. We have witnessed the damage this causes first hand and it’s a very expensive mistake. Roofing manufacturers will void warranties of their product if their material is installed on improperly sloped roofs and you could be left footing the bill for expensive roof leak repairs and new roof replacement.

About this Experts Contributor: Trevor Leeds is president of Chandler's Roofing, providing roofing services in San Pedro, Calif. Originally established in 1942, Chandler's Roofing has earned industry awards such as 2011 Roofing Contractor of the Year from Roofing Contractor Magazine, consecutive Angie's List Super Service Awards from 2010 to 2012, and Master Elite Roofing status with GAF, America's largest roofing product manufacturer. Follow them on Twitter @ChandlersRoof or Google Plus

As of July 9, 2013, this service provider was highly rated on Angie's List. Ratings are subject to change based on consumer feedback, so check Angie's List for the most up-to-date reviews. The views expressed by this author do not necessarily reflect those of Angie's List.

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