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What Is the Minimum Slope for a Metal Roof?

Metal roofs are ideal in applications where durability and speed of installation is important. However, inappropriately installed metal roofs can cause as much damage as they prevent.

Metal roofs are unique in that when installed according to the manufacturer’s directions and well maintained, they can last virtually forever. Metal roofs however, are subject to deterioration like rust prevention, rubber seal maintenance, and minimum slope requirements. 

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The most important rule to follow when installing a metal roof is to respect and follow the minimum slope allowed for the material. All roofing materials are judged on their ability to shed water effectively, including metal roofing.

Today we will discuss what a minimum roof slope means and why it is important when installing a metal roof.

What Is a Metal Roof?

Traditionally, metal roofing refers to molded sheets of steel installed in an overlapping pattern to form a roof that sheds rainwater. Metal roofs are one of the oldest roofing technologies still in use in the United States. Metal roofs are traditionally used in country style residential architecture, outdoor structures like barns, and industrial buildings that value function over form. 

To be clear, from a roofer’s perspective, metal roofing and metal shingles are completely different. Metal shingles are actual shingles, just made from metal. Metal roofing uses a different technology to construct a roof system. We will be discussing metal roofing, not metal shingles.

A metal roof profile known as “5V” has long been the most common metal roofing design. Steel metal roofing panels are often 24” wide x 96”-120” long, and have two parallel “V” shaped bends along each long edge and one in the center. The V shaped bends add rigidity to each panel and prevent it from folding onto itself under stress.

What Does Minimum Slope For a Metal Roof Mean?

When a manufacturer requires a minimum slope for their roofing material installation, it means the material needs to be installed on a minimum angle, or pitch, for the product to work as it should. The slope is a ratio of rise to run, which is another way of saying how much the surface rises (or falls) over a given distance. 

Slopes are important for your yard, your driveway, and even your stairs. The vast majority of residential homes have a roof pitch of 6:12, but many older homes will have a 4:12 roof pitch. The higher the first number, the steeper the roof. Shallow roofs with a low slope, typically less than 3:12, take too long to shed the water, which will likely cause a metal roof to leak. 

A glass of water slowly tipped over will allow the water to run down the outside of the glass, but a quickly tipped glass stays dry. The same happens to a metal roof if the water is allowed to linger too long. A minimum slope is required for this reason, because metal roofing relies on gravity pulling the water down towards the ground to protect the structure.

Roofs are measured by how tall they get compared to how long they are. The slope is measured from the highest point on the roof, known as the ridge, to the lowest point, which is usually the gutter or fascia board. Professional roofers use this slope to calculate how many squares (or 100 square feet) of metal roofing your home needs.

How Do I Calculate the Slope For My Metal Roof?

Calculating a slope for a metal roof is simple and only requires two numbers. As mentioned previously, the slope is calculated as a ratio of the rise to the run. The easiest way to make this measurement is with a level and a 12” carpenter’s square. You’ll need to face the edge of the roof so that one side is lower than the other. In most cases, this will be on the end, or side.

To measure your roof slope (or pitch), hold the carpenter’s square in front of you so that the long side is horizontal and the short side is vertical. Now rest the edge of the carpenter’s square on the edge of your level. You should be holding the level horizontally at eye level, with the carpenter’s square on top, pointing up.

Your level will have measurements on one or both edges. For example, if you have a 2’ level, 24” will be marked along the edge. A 4’ level will have 48” marked, and so on. Now align the end of the long side of your 12” square with the end of your level, so that the scales line up. 

Let’s assume you are on the left side (the right side will be a mirror image of the left side) and it is the low side. Find the left corner of the carpenter’s square and place it anywhere along the roof’s edge. Now raise the level (and square) until the bubble on the level is between the two lines. When it is, both the level and the carpenter’s square are perfectly level.

The last step is the easiest. Simply look to the vertical edge of the carpenter’s square and note where it intersects with the roof. In most cases, the number will be 4 or 6, which indicates you have a 4:12 or 6:12 roof slope. In contrast, if the roof is flat, the carpenter’s square will read zero. If the square reads 8, you have an 8:12 slope, and so on.

Why Is the Slope On a Metal Roof Important?

Metal roofing panels work much like a shingle, except these panels are much larger. Any shingle must shed water fast enough to prevent gravity or wind from pushing the water under it as the water exits the roof. Metal roofs are usually installed without sealants, so metal roofs rely entirely on the quality of the installation and the slope to work effectively.

What Happens If the Slope Of My Metal Roof Is Too Low?

A low slope roof is generally considered any slope less than 3:12, which is the minimum for most metal roof products. Most roof materials, not just metal roofing, adhere to a similar requirement, like fiberglass/asphalt shingles. However, we reached out to Landmark Roofing and Siding about this to get more clarification. They mentioned, “Roof slopes lower than 3:12 usually require some form of bitumen, which is a petroleum based roofing sealant”.

Metal roofing panels require a minimum 3:12 roof pitch, or the water will move so slowly it can puddle. Since metal roofing traditionally uses no sealants, if puddling occurs at an overlap joint the water can seep between the panels, causing a leak. The same happens with unsealed nail holes, which is why most installers use grommet screws.

Do All Metal Roofs Have a Minimum Slope?

Normally, any metal roofing panel (or fiberglass roofing panel) will require a minimum slope to function. Even if panels are used that have some form of seal between the panels, the seal will eventually fail. If puddling continues, inevitably the roof panels will leak. Slopes are designed to take mechanical failure out of the equation, so they work whether they are sealed or not.

Metal roofing has evolved over the years and will probably continue to. In fact, some modern roofing products have replaced the metal component entirely, in favor of fiberglass and polycarbonate plastic. One characteristic that most of these materials share is the way they are installed, which is with grommeted, self-tapping, galvanized screws.

Originally, metal roofing panels were simply nailed onto the roof framing. However, these nails then needed to be sealed and maintained over the life of the roof. In modern construction, these nails have been replaced with grommeted, galvanized, self-tapping screws to eliminate both the source of common leaks, while avoiding constant maintenance.

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