What is a Roof Slope and Why Should I Care?

The Pitch of Your Roof Can Make a Difference – in Looks and Longevity

We’re not talking baseball. We’re not talking voice lessons or movie sequels. We’re talking about the most important pitch of all, the pitch of the roof standing over your head. However, there is more to a roof’s pitch (or slope) than architectural aesthetics.

The design of the roof helps protect the complete structure and contents inside. It determines how a roofing system will perform long-term and how quickly water and debris will shed from the roof. Your roof’s slope affects details such as the type of roofing system and materials that are suitable for your building’s architecture, cost, walkability, and design proportions.

Roof Slope vs. Pitch

Before diving in, let’s define two important words. Although the words slope and pitch are often used interchangeably, they actually have two different meanings.

• Pitch is the measurement of the rise of the roof against the span of the roof and is expressed as a fraction. The span of the roof is the distance between two wall’s top plates, stretching the width of the house.

• Slope is the measurement of the rise of the roof against the run of the roof and is expressed a the ratio of how much the roof rises for every 12 inches of the roof’s run. The run is the measurement from one of the wall’s top plates to the point in the width of the house that’s directly in the center of the roof’s ridge.

Generally speaking, the pitch of the roof pertains more to the actual construction of the home and will mean more to the framers, while the slope is more relevant to the design of the home and will appear in the front, left side, right side, and rear elevations of the house plans.

What is Your Slope?

In a set of floor plans, you will commonly see the roof slope in the elevations (straight-on views of the front, rear, and/or sides) of the house structure. Look for a right angle or right triangle somewhere near the roof with one number appearing adjacent to the vertical line and the number 12 on the horizontal line. (The pitch will often be found as a fraction in the construction drawings or specifications.)

In an existing home, finding the slope is the most accurate way to determine a roof’s pitch. Of course, you can call in a professional to get the measurements. But calculating the roof pitch is fairly simple, and can be done from inside your attic or on top of your roof. All you need is an 18- or 24-inch level, a measuring tape, a pencil, and a bit of DIY spirit!

In the attic, place the corner end of the level against the bottom of a roof rafter. Make sure it’s perfectly level. From that corner end, label the level at the 12-inch mark with a pencil. Measure vertically from the 12-inch mark on the level straight up to the underside of the rafter just above it. Say the number is 4 — that is the number of inches your roof rises for every 12 inches of run. So, 4:12, or 4 in 12, is the slope of your roof.

Why Does Slope Matter?

The primary concern when it comes to slope is the climate. The main reason for a roof to be sloped in the first place is to direct water, melting snow, and other debris. If you live in an area known for heavy rain and snowfall, it might be best to consider a steeper slope to account for the effects of the weather. In general, roofs in southern states may – and usually are – shallower than those in more northern states.

But the slope dictates a number of other roof features, such as the roofing material to be used and the architectural properties. For example, a very shallow roof below a slope of 3:12 may require asphalt roll roofing or raised-seam or corrugated metal roofing instead of asphalt shingles to prevent water seeping under the shingles whereas conventional and steep roofs may use asphalt or wood shingles, tiles, or metal. In addition, some architectural styles – such as Prairie or traditional Ranch – require shallow slopes, whereas others – such as A-frame or Cape Cod – demand steeply sloped roofs.

Slope also affects the economic aspects of the installation. Steep slopes lead to more roofing material being used, and in turn, this will result in a higher installation cost.

Last but not least, slope affects the strength of a roof, called snow load. A shallower roof can’t bear the same weight as a steeper roof. That’s why the iconic Swiss chalet has such a steep roof: it can bear the enormous weight of snow that falls in the Alps in addition to being able to shed it more effectively.

Roof Styles

A roof does more than just serve the practical purpose of protecting us from the elements. It also plays a major role in the overall look of the house, adds to its resilience, and has the potential to make your home more energy efficient. There are many styles of roofs – gable, hip, gambrel, mansard, shed, saltbox, shed, flat, etc. – and we won’t go into them here. But not all roofs are created equal. To every pro, you’ll find a con. This list is generalized quite a bit, but we put roof styles into three main categories to compare: low slope, conventional slope, and steep slope.

• Shallow slope (including flat roofs): A shallow, or low, slope is anything under a 3:12 ratio. This roof style is most commonly found in commercial roofing application, but is becoming more popular in today’s contmporary residential designs. These roofs demand high-end materials, are more prone to water damage, and require a regular maintenance visit. However, they may be less susceptible to wind damage, which is why you will often see them (especially in hip roofs) along the Gulf Coast, in Florida and other areas in the South, and even in some Plains states. And as modern architecture and modular homes continue to advance with the integration of solar and green roofing systems, low-sloping roofs are sure to pop up in all parts of the United States, regardless of climate.

• Conventional slope: Between a 4:12 and 6:12 ratio, these roofs are still fairly easy to walk on, and are a good option for those looking to add a shed, a garage, or a general room area. These roofs don’t require additional safety equipment for the initial installation or future maintenance projects.

• Steep slope: The slope ratio for these slick slides is typically a 7:12 up to 12:12 or more. These slopes deter the effects of more of the harsh climates, as the angle lets rain and snow easily fall away and prevent damage your house. They are more expensive to replace in terms of materials and labor (and may require additional safety equipment), but their efficiency and stability give the home a quality roof that lasts for years.

Material and Performance Watch

By knowing the pitch of your roof, you will know what type of materials can be used and the best application methods for them. Because there are so many types of roofing styles and materials available, there is no standard roof slope. In most cases, manufacturers will recommend the minimum roof pitch for their roofing materials, but ultimately, it lies upon the trusted installer to recommend a product for your slope. Every project and every roof is unique and requires different considerations.

Your contractor will also help you with material choices. In general, flat roofs tend to be made of built-up roofing composed of tar paper roll and hot tar that is melted into place by torches to create a watertight seal. Asphalt shingles, wood shakes, and slate are suitable for a steeper slope. Metal roofs are a unique exception that we see on flat industrial building and modern residential designs as wells as in roofs that are practically vertical.

When choosing your dream house plan – and ultimately the slope and look of the roof that’s right for you – you’ll need to consider climate conditions, materials, cost, maintenance, and style. From a roof as flat as a matchbox to one leaning at an aggressive angle, each style has a unique combination of factors that need to balance out.

Footnote: The bottom left photo in the leading montage image is of a 3-bedroom, 2-bath Country Ranch home. for more information, click here (House Plan #191-1018).