Are you dreaming of a new metal roof but worried it might lead to costly repairs due to your roof’s low slope? Are you wondering which type of metal roofing is actually safe to use in a rainy climate like Seattle?
At RoofSmart, we have repaired and replaced hundreds of low-slope roofs. Not only are we familiar with the problems they bring, but we’ve also learned the best way to install metal without causing costly damage.
So yes, you can absolutely use metal on a low-slope roof as long as you plan ahead and choose a leak-proof panel style.
If you're dealing with a low-slope roof and considering metal, this article will walk you through the risks, the slope limitations, and the only types of panels that are truly safe and watertight, especially in wet climates.
We'll break down exactly what qualifies as low-slope, the dangers of using the wrong panel system, and how mechanical seaming could be the best solution to protect your home from costly water damage.
How low is a low-slope roof?
The average roof in the Seattle area is at a 4:12 slope. Anything lower than that is considered a low-slope roof.
You might be wondering, “Well, what is a 4:12 slope?” Roof slope is described using the rise and the span.
So, for a 4:12 slope, this means that for every 12 inches it goes across the roof (the span), it’s also going up 4 inches (the rise). Here’s an example of what different roof slopes look like:
Image from Kentucky Metal Roofing
What’s the lowest pitch you can use with a metal roof?
This answer will vary depending on the type of metal panel you want to use.
Typically, the cutoff is a 3:12 slope, but the absolute lowest you can go is a slope of one-quarter inch up per every 12 inches across (1/4:12).
This leaves just enough slope to still help move the water off the roof. The problem with flat roofs is that water collects on top of them rather than running off, which leads to a higher risk of leaks. The water drains more slowly and has more time to soak into the roofing material or seep through holes in the roof.
That’s why flat and low-slope roofs require special roofing materials, and you have to be selective about what type of metal panels you choose.
Which metal panels can I use on my low-slope roof?
In a wet, rainy area like Seattle, you need to make sure that your roof is protected from leaks. This is especially important if it has a low slope.
The good news is that if you’re determined to install metal on your low-slope roof, it’s still possible as long as you choose standing seam metal.
Standing seam panels fasten together in a few different ways:
- With a snap lock panel, you can only go down to a 3:12 pitch
- With a clip lock panel, you can only go down to a 2:12 pitch
- With a mechanically seamed panel, you can go all the way down to 1/4:12 (although typically roofs go down to 1:12 pitch)
To avoid leaks in your low-slope roof, it’s important to find a qualified roofing contractor who has experience installing metal on low-slope roofs.
Make sure you can trust their recommendations about which panel can go on a certain pitch, otherwise, you might end up paying thousands of dollars in repairs down the road.
Need help finding a qualified roofer? Download our guide to quickly and confidently evaluate your roofing contractor options.
What is mechanical seaming, and why is it important?
Mechanical seaming is a process your roofer can use to seal the seams of your metal roof to make sure that it has a stronger barrier to keep water out. This is crucial on a low-slope roof because you’ll need extra protection to prevent pooling water from seeping in.
To mechanically seam a roof, your roofer will snap or clip in the panels as normal and then use a roof seamer to seal each seam by folding them together.
For added protection, your roofer should add butyl (which is basically a rubberized asphalt) onto the seam so that when you fuse it together, there's also a sealant inside to help keep water out.
There are two options for mechanical seaming: single lock and double lock.
- Single lock: This means the seam is folded once. It’s good for extra strength and waterproofing, but it isn’t as weather-tight as the double lock.
- Double lock: The seam is folded twice and is now strong enough to keep out water in extreme conditions. If you have a really low slope roof, double locking is the best option to protect it from the constant Seattle rain and pooling water.
Your roofer can help you decide which is better for you based on the slope of your roof and the environment you live in.
Taking the extra steps to add butyl and mechanically seam the roof can increase the cost of the job by about 8% to 10% (compared to the cost of installing a metal roof on a standard slope).
Metal roofs are already expensive, and adding to the cost can be tough, but keep in mind that the cost of getting your metal roof mechanically seamed is probably way less than what you could end up paying for water damage repairs in a few years if you skip that step.
Which metal panels should I avoid on my low-slope roof?
Even though metal roofs are long-lasting and fairly waterproof, you’ll only see those benefits if you choose the right type of panel, especially if you have a low-slope roof.
In a rainy area like Seattle, your number one priority for your roof should be preventing leaks and water damage. To do that, here are some metal panels you should AVOID if you have a low-slope roof:
- Stamped metal panels: When they’re installed, the panels are interlocked together. This means that they’re held securely in place, but they are NOT suitable on anything lower than a 4:12 pitch because they will leak. Interlocking panels are waterproof on regular or steep roof slopes, but they aren’t designed for low-slope roofs.
- Exposed fastener panels: These panels are installed with screws that remain visible and open to the elements. This can quickly become a problem because there are hundreds of small holes all over your roof where water can get in. Typically, the screw heads are covered to prevent leaks, but the caps can wear out over time. We do not recommend using exposed fastener panels on any residential roof, but the risk of leaking increases exponentially on a low-slope roof that can easily collect standing water.
Still worried about roof leaks? Here’s why they happen on low-slope roofs
At the end of the day, anyone considering metal for their low-slope roof is doing it to avoid one thing: leaks. But even with the right materials, if your roof is shallow enough — and especially if it's not installed correctly — water intrusion can still happen.
Now that you know which metal panels can work on low-slope roofs and what mistakes to avoid, it’s time to go a step further.
Read our article “Why Is My Shallow Slope Roof Leaking?” to understand why low-slope and shallow-slope roofs are so prone to leaking in the first place.
Want an expert opinion? Contact us today so we can help you protect your home and make the smartest roofing decision for your slope, your climate, and your budget.
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