My neighbor finally decided to ditch his metal roof over the summer, and I asked him what took him so long since he’d been complaining about his shingles for years. He said he was concerned about a low-sloped addition on the home he’d built awhile back. He wasn’t sure if the minimum slope for a metal roof would fit with that part of his home.
The minimum slope for a metal roof with exposed fasteners and seams without sealant is 3:12, which means 3” of drop for every 12” of horizontal run. Roofing with exposed fasteners and lapped seams with sealant can go lower – ½:12. Standing seam metal roofs, which are either mechanically fastened or click into place, can go as lows as ¼:12.
The key to understanding the minimum slope for metal roofs is getting familiar with various types of metal roofs available to homeowners. With metal roofing becoming more popular, there are exponentially more products on the market every year. Below, we’ll go through your metal roofing options and determine which might be best for your low slope roof application.
Can You Put a Metal Roof on a Low Pitch Roof?
Yes, you can use metal roofing for low-pitched roofing. In fact, many contractors recommend that asphalt shingle roofs go no lower than 4:12 slopes, which doesn’t leave you with too many options if your roof slope is any shallower than that.
Luckily, metal roof works extremely well in low slope applications. The most common type of metal roofing, and the cheapest, is a typical corrugated roofing with exposed fasteners. Metal panels overlap and the screws are placed through the laps to hold the sections together on the roof.
Some homeowners may cringe at the thought of exposed fasteners putting thousands of holes literally into your expensive roofing material, especially on a low slope. However, this type of roofing has been working for decades as leak-proof as shingle roofing and much more durable.
That standard type of metal roofing can go only slightly lower than a shingle roof, as the max depth for corrugated metal roofing sheets without sealant is 3:12. However, you can go lower without compromising your structure using metal roofing with sealant and other products, such as standing-seam click-lock panels.
Whichever product you choose, you are getting a metal roof that performs the same or better than a shingle roof. For low slope applications, your options for roofing materials are limited, and choosing a metal roofing product for these types of scenarios is a long-term, cost-effective choice.
What is the Lowest Slope for a Metal Roof?
One of the confusing parts about roof slope is the terminology used when indicating roof slope. While many of us might remember from high school math class that we measure angles – or slope – in degrees, the roofing industry prefers a different measurement. Ratios – such as 3:12 – is the common manner in which roof slope is notated.
Lapped metal roofs with sealant between the laps can go as low as ½:12, which equals around 2.5 degrees. Standing seam metal roofs have an angle of just over 1 degree. For both of these types of roofing, at their lowest slopes, would be nearly flat.
You can also use geometry to determine the angle if you only have a calculator at your disposal, but you’ll at least need to know the length of your rafter and have an understanding of the SIN, COS, and TAN buttons on your device.
Best Roofing Material For Low Pitched Roof
The best roofing material for a low pitched roof depends on the slope of your roof. Assuming your roof is shallower than 4:12 and you want to use metal roofing, then the determining factor is just how shallow your roof is and the amount of money you are willing to pay.
For a roof that is only slightly shallower than 4:12, then your best bet is lapped, corrugated metal. The term “lapped” means that the sections of the roofing panel “lap” over one another on the edges. The fastener – typically a 2” metal screw with a thick rubber washer – screws from above through the overlapping edges of the panel, into the substrate beneath the metal panels.
The neoprene washer provides an excellent seal, and the self-tapping screws themselves make only a minimal intrusion in the metal, as the threads are fine. The washer seals the top of the hole, which is made through the part of the lap that is raised, not the valley. In that way, water will not pool on top of the screw holes but run down the sides of the lap.
If the sealant is not applied to the lapping metal panels, then the shallowest slope is 3:12. Some metal panels that lap require sealant between the seams, in addition to fasteners. If you opt to apply a metal roof sealant, such as a butyl sealant, between the laps, then your minimum slope is ½:12.
The minimum slope for a standing seam metal roof is ¼:12. But what exactly is a standing seam roof? You’ve probably seen them before, as they’ve been commonly used in industrial applications for a long time and are now finding increasing popularity amongst homeowners.
Standing seam roofs are metal roof panels with edges that are 2” or higher above the lowest point of the metal panel. These raised edges interlock with each other, either mechanically or through some type of proprietary click-lock that tends to vary between metal roof panel brands.
The benefit of standing seam roofing is that there are no exposed fasteners and the seam if far above the area where water would pool, hence the reason why this type of roofing has the shallowest slope rating.
Standing seam is an excellent option for low sloped roofing but tends to run more expensive for the click-lock style standing seam panels. These panels can also have a hidden nailing strip that holds the panel in place, and the next panel clicks over the strip, which permanently covers the fastener from the weather.
Mechanically fastened raised seam metal roofing requires a roofing contractor to install the panels, as they typically use a mechanical seamer to fasten the panels together. You can rent a mechanical seaming tool and do it yourself, although this isn’t recommended as it may void the warranty of your roofing panel if you endeavor to do it yourself.
Recommendations for a Low Slope Metal Roof?
Let’s take a look at some of the precautions and best practices when installing a metal roof of any type on a low-pitched roof. For starters, here are some tips that you should consider before installation:
- Use sealant for lapped metal roofing
- Make sure your seam overlap is enough
- Avoid end laps
- Follow manufacturer installation instructions
Why? Once metal panels are screwed together through the laps, the space between the panels is extraordinarily small. That’s a good thing because you want a watertight seal. However, lacking sealant, there is still a tiny amount of space between the lapped metal sheets.
If there happened to be tons of rain or piles of wet snow on your roof, then you could experience a wicking effect on your metal roofing panels. Essentially water looks for the nearest and easiest outlet. If water pressure builds on your roof due to melting snow, then you could get water siphoning into your home.
Or, if you have an extremely long roof run, you may experience water wicking up and under the end laps of your metal roofing panels. In either instance, water has now infiltrated your metal roof and you must repair the leak with sealant. Why not do it right the first time and apply a lifetime-tack butyl sealant between metal roofing panels? It will save you tons of headaches down the road.
Overlapping Metal Seams and Slope
Ensure your metal roofing sections overlap enough so that there is at least a couple of inches overlap on each side. Lapped metal panels are typically 26” wide so that they overlap the trusses or rafters beneath by at least an inch. This allows for around 4” of horizontal overlap per piece.
Manufacturer instructions will indicate the appropriate amount of overlap per their metal roofing panels. If you are installing used metal panels on an outbuilding, however, then be sure there is enough overlap to properly maintain a seal. And always use butyl sealant to ensure a weathertight lock.
Another part of the metal panel that is critical is the end seam. While metal panels can come in lengths of up to 24’, some may only find roofing up to 16’ long. If so, then you may be faced with end laps. Manufacturers will also have recommendations around the length of an end lap, but it should be at least 12” with sealant for low sloped metal panel roofs.
If possible, avoid end laps, particularly if you live in areas with heavy snowfall. Why? The end laps are often a part of the roof that does not receive any sealant since many consider an overlap of 12” to be more than sufficient to avoid moisture wicking issues.
However, moisture can wick 12” and more quite easily given the opportunity. Utilizing sealant will mitigate most of those problems on end laps for metal panels, but an easier solution is to source longer metal panels that will fit the entire width of your roof.
Paying more upfront for special ordered panels may be painful at first, but leaks on your metal roof at the end laps can damage the roof structure beneath and cause major headaches.
Metal Roof Slope Snow Load Requirements
Slope requirements for metal roofs take into account snow load – theoretically. However, if you live in areas of the world with lots of snow, whether in Canada or Maine, banking on a ¼:12 roof not leaking under 4 feet of snow is not a good idea.
Many contractors in areas where there is heavy snow will not put a metal roof on any slope lower than 1:12. If you typically have snow sitting on your roof for an entire winter, this would be considered an area with heavy snowfall.
If you happen to live in an area with heavy snowfall and want a metal roof, but your roof slope is less than 1:12, there are a few things you can do.
First, you can raise the slope of your roof using a proprietary system of purlins, such as those from RoofHugger or similar manufacturers. Purlins are horizontal supports fastened to an existing roof or sheathing to which metal panels are attached, creating an air gap between the substructure and roofing material.
This system allows you to increase your roof slope without removing your old roofing if you choose. You can even use it over an old metal roof, as there are specialized purlins to fit most metal roofing profiles.
Second, you can simply avoid a metal roof altogether and opt for a different type of roofing system – such as those used for flat roof applications.
One of the most important aspects of metal roofing is following the manufacturers’ directions when installing. With so many different products on the market when it comes to metal roofing, simply assuming that one type of metal panel will do better than others in a lower slope application is just folly.
Finally, think logically about your metal roof. Remember that water will seek gaps and holes to enter. Raised seams with hidden fasteners will be more watertight than exposed fastener metal panels, regardless of how much roof sealant you use.
Thanks so much for taking the time to read this article. I hope it was able to clear up some confusion about different types of metal roofing in low-slope applications. As always, please sent me a note to let me know what else I could add to this article to make it more informative.