What’s the Minimum Slope for Metal Roof on a Shed?


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The minimum slope on your metal shed roof is an important component of your shed building project. You want enough slope to enable rain to run off and keep from forming puddles. You want to keep snow from building up and then sliding off in one big sheet.

Still, you don’t want the roof so steep that you can’t get up there and walk around to inspect it from time to time. It’s also important to keep the slope in line with local building codes.

Most building codes will have a minimum slope along with limitations on the total height of the shed. You can get this information when you check with them for requirements and permit regulations before you start building. In fact, the slope of the roof should be specified on any building plans you submit for a permit.

The ideal minimum slope for metal roof on a shed is 1 to 12 if your using lapped seams and sealant and 3 to 12 for metal roofing that laps and doesn’t use sealant on the seams.

This requirement is usually lifted in cases of replacement of roofs on existing buildings or the recovering of an existing roof. If you’re redoing an existing roof, be sure to tell the building authorities this.

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There are many reasons you may want or need a low slope, such as tree placement, replacing roofing on an existing shed, adding a porch roof or overhang over the door, etc. That being said, keep in mind that the more slope you have up to a point, the fewer problems you will have with moisture standing or seepage, or any type of corrosion.

Does the Minimum Slope for Metal Roof Change with Different Materials Used?

There are differences in materials used for roofing as far as the slope goes. However, different metals, such as steel vs. aluminum, don’t really come into play. The type of construction and connection method of the panels is what determines the minimum slope.

The exception is if you plan to use corrugated metal such as “tin” roofing rather than an actual metal roofing system. If you are using corrugated metal roofing, the minimum roof slope is recommended to be 3:12. This means that this type of corrugated metal roofing is just not suitable for a low slope roof. You have other options for roofing, check out my article How to Choose the Best Roofing Material for a Shed for more information.

What’s the Best Metal Roofing Material for Low Slope Roof?

Actually, the most important consideration for a low slope metal roof is the construction, not particularly the material used. Low slope roofs have a harder time shedding rain and snow. The more time water or moisture can spend on your roof, the better chance it has to find a way into a gap around a fastener or start rust forming in a place where the paint was scratched.

Pros and Cons and Cost of Different Types of Metal Roofing

Standing seam roof: Most metal roofing types can be divided into two groups: those having exposed fasteners and those having hidden fasteners. Standing seam roofing has hidden fasteners consisting of metal clips under the metal that are used to attach the panels. The panels are usually locked together with a snap-lock system. This is the most common type of metal roofing system used. If you need more information read my article What’s the Best Metal Roofing for a Shed?

  • Pros: Because there are no screws going through the whole surface of the metal, there are far fewer places for moisture to get in around the fasteners.
  • Cons: For a low-slope roof, the general consensus is that a double-lock standing seam roof with a mechanically seamed profile gives the best weather protection. This type may not be as attractive as a snap-lock system, but it does give you better performance. With a mechanically seamed roof, the panels are joined not with a locking system, but tools are used to crimp the panels together during installation, which creates a watertight seam. This type is so good that it’s used in many commercial applications where a low slope roof is called for.
  • Cost: For a standing seam roof, it can cost $15 to $25 per square foot. In addition, it should be installed professionally by somebody with experience with zinc. One other problem is that after the patina has developed on the metal, water can sometimes cause white streaks on the roof, making it less than attractive.

Corrugated metal roof: These panels have a wavy appearance, with the waves allowing overlap between the panels. This is the type with exposed fasteners between the panels and attaching the panels to the roof base.

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  • Pros: It’s one of the least expensive metal roofing materials, running between $1 to $2.50 per square foot for materials.
  • Cons: This type requires exposed fasteners, both to fasten the panels together and to fasten the panels to the strapping. This means that your roof will be less watertight. Since a low slope roof will retain water and moisture for longer periods, this means that the water has more time to work its way down around the fasteners. More frequent inspections are called for to watch for this.
  • Cost: For a corrugated roofing system, costs can run from $13 to $20 per square foot.

Natural expansion and contraction of the metal with temperature changes will eventually enlarge the holes around the fasteners. Even with washers or gaskets around the fasteners, water will eventually work its way in. This type of roofing requires more of a slope than the standing seam construction.

Zinc metal roofing: This material is not as commonly used in the United States, although it is popular in Europe. Like copper, it gradually develops an attractive patina.

  • Pros: Zinc’s gradual patina development turns it into an attractive blue-grey color. This patina is composed of zinc carbonate, which also protects the metal from corrosion. This factor makes it great for people living near coastlines, where salty, moist air can affect other types of metal. It has a very long lifespan, often lasting 80 to 100 years.
  • Cons: For a shed roof, this type may not be what you want because it is expensive. It’s also hard to find because most zinc roofing has to be imported.
  • Cost: Can vary and is not recommended due to the expensive shipping costs.

Aluminum: Aluminum is the most lightweight metal roofing material available but is still very durable. It’s comparable in strength to other metals despite its light weight. This makes it easier to work with but also means that it won’t add so much to the stress on your rafters or trusses.

  • Pros: Aluminum is malleable and easy to work with, making it easier to bend and cut when needed. It’s also resistant to corrosion, meaning that moisture won’t affect it as much as other metals.
  • Cons: Aluminum is generally more expensive than steel, you can get it in different thicknesses to add strength, but this may cost you more. It’s also available in fewer colors, so if you have your heart set on your shed roof matching your house roof, you may have a problem. It’s also more prone to dents than steel, so if you live in a hail-prone area, you may not want to use it.
  • Cost: It can cost around $3 to $4 per square foot.

Galvanized steel: This is the most commonly used material for metal roofs.

  • Pros: It’s low maintenance, requiring only an annual inspection for damage and to clean off debris. It’s resistant to fire and rot. The fire resistance may have a good impact on your homeowner’s insurance if your shed is insured. It’s also long-lived, lasting 40 to 60 years or more. Steel also costs about 35% less than aluminum and over 60% less than zinc or copper.
  • Cons: While steel is the least expensive of the metal roofing choices, it does cost more than asphalt shingles or other choices. It is corrosion-resistant but not corrosion-free. A moisture-laden environment, especially when exposed to salt in the air near coastlines, will have a negative impact. With exposed fasteners, your roof will need to be inspected more frequently. You’ll also need to inspect for scratches in the paint or finish, which can start to rust.
  • Cost: The cost for exposed fastener panels runs around $2 to $5 per square foot, with standing seam types costing a bit more. Check out my article How to Install Galvanized Metal Roofing On a Shed for detailed information on installing galvanized metal roofing.

What’s the Best Strapping Spacing for a Low Slope Metal Roof?

Strapping, or batten, spacing on metal roofs is generally recommended to be 24 inches apart. However, the roof system you choose may have other recommendations. Check with the materials manufacturer for the best results.

Actually, of late, battens have fallen out of favor for metal roofing in many places. The use of strapping started with the use of wooden shingles long ago. The latest construction trend is to install metal roofing systems over solid decking. This would facilitate the addition of a waterproof membrane under your roof, as you’d need a solid surface for attaching a membrane.

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That being said, some metal roofing systems are designed to be installed over battens. If this is the case with yours, it’s not hard to install battens over your solid decking and then install the roofing over it. Get more information in my article How to Install Roof Strapping for a Metal Roof.

Waterproofing Membranes for Low Slope Metal Roofing

There are different types of waterproof and water-resistant membranes and materials that can be used to further protect your low slope metal roof. These membranes usually withstand the higher temperatures that a metal roof can subject them to than traditional underlayment’s, and will provide water protection even with the low slope.

One is a PVC membrane that is hot-air welded which actually makes its seams stronger than the membrane material itself. It can also be used for flashings, adding more waterproofing strength. These are also listed as thermoplastic vinyl membranes.

Another is a combination of asphalt and styrene-butadiene-styrene or SBS. SBS is a rubbery polymer a bit like chewing gum. This material soaks up the asphalt. The combination provides the waterproofing of the asphalt with the flexibility of the SBS even at colder temperatures while keeping the asphalt from oozing at high summer temperatures.

The mixture is coated onto a mat of fiberglass, then processed into a membrane. The fiberglass mat strengthens the whole against any tears or rips. It also makes the membrane easier to handle while installing. One of the best features is that the asphalt-SBS mixture is resilient, giving it self-sealing abilities around your fasteners. This alone is a great feature to keep your application watertight.

The combination membrane is available in different thicknesses and is applied as a peel and stick product, meaning that it has a peel-off film on the bottom, revealing a sticky underside. Because the air temperature may affect the tackiness of the underside, sometimes making it hard to apply, some manufacturers offer both a summer and winter grade.

If you’re investigating this type of membrane, be sure to tell your supplier when you plan to roof your shed and what sheathing you will be using. The material you will be attaching your membrane to will also play a part in what you need to get.

The one downside that these membranes have is that they don’t tend to perform well in cold environments. If you live in a cold climate, do some investigation and ask questions before deciding on a membrane. Perhaps another type of waterproofing may be better suited to your environment underneath your metal roof.

Is Sealant on Joints Needed on Minimum Slope Metal Roofing?

This depends on the type of seam fastening you choose for your roof. If you choose the less-expensive snap or snap-lock system, you’ll want to use sealant on the joints between the panels. These panels rely on having a sufficient slope to shed water.

In addition, if using this type of panel, you should really consider professional installation. The sealant is best applied by professionals to limit or eliminate the possibility of the sealant breaking apart later due to incomplete or faulty application.

In addition, the panels are pressed together when being put together but tend to relax afterward, once they are joined. There are all sorts of problems that can crop up when applying the sealant, ranging from temperature to any dirt in the seams to improper flow of the sealant and so on.

When using mechanically seamed panels, the panel edges are folded over or crimped together with special tools, making for a watertight seal. In addition, these seams are usually raised to keep them out of any standing water that may puddle on a low slope roof.

In this case, the sealant may or may not be needed. Your supplier can give you some advice on this. If recommended for your particular roof, this is not the place to try to save money. With a low slope roof, you need all the waterproofing you can get.

You’ll also want to look into sealants to apply around any flashing or any vent pipes on your roof if you will have them. You may need a different type of sealant for this.

What’s the Best Type of Fasteners for Low Slope Metal Roofing?

With the moisture problems that low slope roofs suffer from, your best bet is probably roofing screws with neoprene washers. There are those manufactured with plastic washers, but the plastic degrades after a time when exposed to sunlight.

If you’re installing a standing seam roof, you don’t have to worry about the washers as much. Your roofing panels will be fastened with clips underneath the panels and not with screws that will puncture the panels themselves.

If your roofing panel supplier doesn’t supply fasteners as well, ask which ones are recommended. Fortunately, many roofing screws come with various colored heads meant to match your metal roofing.

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Is Snow Load Going to Affect the Low Slope Metal Roofing?

Common sense will tell you that a roof with a steeper slope will shed more snow more quickly than a roof with a low slope. For this reason, the minimum slope for snow country is recommended to be an inch per foot of roof. In addition, the steeper pitched roof will likely catch more sun.

Even when it’s hidden, a dark-colored metal roof will eventually start to absorb rays from the sun and heat to the point that it starts to melt the snow from underneath, causing it to slide off more quickly than a shallow roof will, which won’t really start to heat up until the sun is more overhead.

Steep roofs can also have problems with snow load. Snow tends to pile up in valleys in the roof or around any vents in the roof. While metal roofs are better than most at shedding snow, it can still pile up depending on the heaviness of the snowfall and wind direction. Insulation, which is usually installed under metal roofs, helps to prevent the formation of ice dams.

In places where snow load is common, installing snow guards or snow cleats along the roof edges can help. They help prevent an “avalanche” of snow from sliding right onto your head when you’re trying to unlock your shed door.

Conclusion

Dealing with a low slope roof may be something unfamiliar to you. However, many metal roofing systems are fine for use on these roofs. See what’s available in your area.

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