What is the Optimal Roof Pitch?

It’s no exaggeration to say that the roof is the most vital part of the building envelope, and therefore, the most critical investment.  It’s also no overstatement to say that we now enjoy a wider range of roofing materials and roofing system than at any other time in history. However, not every roofing system works in every application.   Finding the right system involves weighing a multitude of variables including cost, weight, lifespan, maintenance requirements, and most importantly, aesthetics. Of all these variables, roof slope (a.k.a. “pitch”) is perhaps the most important.

The roof pitch selected affects drainage, maintenance requirements, and materials used more than any other single factor.  It is considered the primary factor in roof design.  It also has a major impact on the finished style of the building, whether it’s a steep-pitch sloped roof visible from street level, or a low-slope roof design with less visual impact. An understanding of the major commercial roofing systems—and how their performance is affected by roof slope—is critical to maximizing the effectiveness of the covering.

Roof Pitch
The slope, or pitch, of a roof is typically expressed as the amount of vertical rise (in inches) for every foot of horizontal length along the gable.  A roof with a rise of six inches per foot would be called a 6/12 roof.  Conventional slope roofs, with a pitch between 4/12 and 9/12, are the most common in residential work.  Roofs with a pitch exceeding 9/12 (37 degrees) are termed steep slope roofs.

In commercial work, low-slope roofs (with a pitch between 2/12 and 4/12) are most common.  Roofs with a pitch of less than 2/12 are considered flat, even though they technically have some slope.  The minimum allowable slope for drainage is ¼” per foot.

Steeper sloped roofs are generally more visually pleasing and tend to last longer, as the water runs off immediately and ice damming is avoided. However, they also cost more because of the additional materials required to build them, and are impractically tall for larger buildings.

Roof material selection is highly dependent on roof slope.  For instance, single-ply or torch-down roofs are not appropriate for high-slope applications.  On the other hand, visually appealing roofing products such as shingles or tiles do not work well on low-slope roofs.

Additional Factors
Of course, roof pitch is not the only factor in system selection.  Often, the weight of the roof plays a deciding factor.  Vegetated and ballasted roofs, for instance, can put a significant load on structural elements.  Similarly, roof underlayment and insulation can eliminate some roofing materials from consideration.  Hot-applied or torch-down roofing is not compatible with rigid foam insulation.  The Army Corps of Engineers estimates that 75% of the roofing industry consists of re-roofing existing structures, so issues such as construction noise, fire hazards, fumes, and building access can also come into play.

Shingles and Tiles
For steep and conventional pitched roofs (4/12 and above) shingles and tiles are an attractive option that perform well. Asphalt is most economical. Tile provides a long-lasting roof with little maintenance. Synthetic wood and slate are durable, long-lasting, and appear identical to the natural materials they imitate. For conventional and steep-slope roofing, shingles and tiles are the best way to go.  They have been used for roofing for hundreds of years, and are still a great choice for conventional roof pitches.

Clay tiles and natural slate have a proven track record stretching back centuries, and modern products will last a lifetime if properly applied.  Warranties of 75 years are not uncommon.  The primary drawback to tiles is their cost and weight.  Clay tile runs $6 to $10 a square foot in most areas of the country.  Real slate is at least double that.  Concrete roof tiles are available at prices similar to clay, and can imitate both slate and clay tiles.  They’re typically warranted for at least 30 years. All three products, clay, concrete, and slate, weigh between 900 to 1,200 pounds per 100 square feet, so the roof deck and supporting structure must be able to support this additional weight.

Asphalt shingles are the most commonly used roofing material in North America.  They’re economical, versatile, and work well on most residential roof pitches. They’re easy to install, relatively long-lasting, and available in virtually any color and style an architect could desire. Asphalt shingles are likely the most affordable roofing option for moderate and steep sloped roofs, running between 50 cents to $1.50 per square foot.  They weigh at least 250 pounds per 100 square feet, on the light end for roofing materials.

The main drawback to asphalt shingles is related to the service life. Asphalt roofing shingles are available in grades with an expected life of 20-50 years depending on the price. However, durability issues and wear-out or material failures occur earlier than expected in some situations. It should be noted that unlike the other products mentioned so far, asphalt shingles can be used on low-slope roofs with a pitch between 2/12 and 4/12, but they require special underlayments and installation techniques to handle ice damming and other water issues.

Metal Roofing
The two most common metal roofing materials are painted aluminum and steel. Copper and stainless steel are also metal roofing options, but their cost is high enough that they’re seldom used. Aluminum has become a top choice because it does not rust, it muffles the sound of rainfall, and can simulate cedar shakes, tiles, and slate.

Metal can be used as a roofing material on any roof pitch. For low-slope structural roofing, standing seam roofing is generally used. Some low slope metal roofing requires machine seaming during roof installation to ensure a watertight seal. A seaming apparatus is simply rolled along the panels to crimp the panel seams together. Metal roofs have a long service life compared to other low-slope roofing options. A 2005 study conducted by Ducker International found that respondents expected metal roofs to last 40 years –17 years longer than built-up and 20 years longer than single-ply systems.

Typically, metal roof systems weigh from 40 to 135 pounds per 100 square feet, making them among the lightest roofing products.  Because metal roofing comes in large panels, it’s also among the easiest to install. In summary, metal roofs can provide easy installation, a long service life, low maintenance requirements, light weight, and meet sustainability and recyclability concerns.

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