TL;DR: Summary
Why Asphalt Shingles Fail on Low-Slope Roofs
Installing asphalt shingles on a low-slope or flat roof is a common but costly mistake that leads to premature roof failure. Because asphalt shingles rely on gravity to shed water, they cannot effectively drain on shallow pitches, causing water to pool and seep into the underlayment. This results in shingle curling, rotted roof deck trusses, and severe moss or mildew growth. For long-term durability on low-slope homes, materials like standing seam metal, PVC, or TPO are recommended, as they provide a seamless, water-tight barrier that prevents leaks and structural damage.
Have you ever wondered why some people end up replacing their roof way more often than others? Or how poor installation choices by past contractors can cost homeowners thousands in the long run?
At RoofSmart, we have repaired and replaced hundreds of shallow-slope roofs. Over the years, we have seen the damage that they can cause to your roof and have learned which roof types work best on a shallow slope roof.
This article shows what happens when your asphalt shingles leak and damage your roof, what needs to be fixed, and what materials you can use on a low-slope roof.
We’ll walk you through exactly what went wrong and the steps to fix it so that you can avoid making the same expensive mistake.
Think you might have water damage and need a roof replacement? Get a ballpark estimate of the cost to replace your roof using our Instant Estimator tool.
Why Did Asphalt Shingles Fail on a Low-Slope Roof?
What makes Greg’s roof uncommon is that it’s low-sloped. Greg was having such massive problems because roofers kept installing asphalt architectural shingles on his home.
Most people reroof their homes once in their lives—MAYBE twice. But Renton homeowner Greg? He was going on his third roof replacement… in the last 20 years.
Why? Because the two previous roofers put the wrong TYPE of roof on his home. As a result, Greg’s home had experienced several big problems.
Here’s what happened and how we solved Greg’s roofing woes for good.
When we arrived at Greg’s home for his free roof inspection, we didn’t even need to get out of the truck to determine his problem. We saw it as soon as we pulled into the driveway.
Greg’s home and old roof.
That’s a problem because asphalt shingles and flat/low-sloped roofing go together about as well as peanut butter and garlic.
In other words: They don’t.
What Happens When You Use the Wrong Roof Material on a Low-Slope Roof?
Greg’s main problem was water damage. When you put asphalt shingles on a flat or low-sloped roof, water will eventually get under those shingles. After all, it’s just sitting there with nowhere else to go.
Greg’s moisture issue ran DEEP and caused several problems:
1. Curling shingles
This wavy, curly look in the shingles happens when too much water is absorbed. This can also happen when water gets into the underlayment underneath the shingles and is absorbed there.
Curling shingles are always a sign that something is wrong with your roof. And in a rainy area like Seattle, they leave your roof vulnerable to leaks and further water damage.
2. The trusses were broken and rotted
In cases like this one with extreme water damage, this means that the water damage extended into the wood all over the roof. The trusses were too rotted to bear the weight of the roof, so some of them broke.
This is why the roof deck was sagging two inches from end to end. And, as a result, the ceiling was also drooping inside the house.
3. Mildew and moss grew on the shingles.
Even though the Seattle area is constantly rainy and wet, most roofs don’t have mildew AND moss. This only happens in cases with extreme amounts of moisture.
Moss grows in damp, shady areas, and mildew is the same. A low-slope roof with asphalt shingles is the perfect place for this to happen. Moss and mildew can be removed, but you’ll likely have to pay a roofer to clean your roof.
How Do You Fix Damage on a Low-Slope Roof?
Greg’s roof faced extensive structural damage due to the constant leaking. To fix this, basically every part of the roof had to be removed and replaced.
The first step was to tear off the shingles. There’s nothing that can fix curling shingles, so you just have to get rid of them. Tearing off the old shingles meant that the mildew and moss also got removed.
Next, all the damage underneath the shingles had to go. Here are the steps we took to fix all of these issues:
- We tore out the rotted wood and removed the roof deck
- We replaced the broken or sagging trusses
- We reframed the ENTIRE roof.
- We installed new plywood boards for the roof decking
Once we had all the underlying issues taken care of, we had one mission left: Make certain this is the LAST time Greg needed to replace his roof by choosing a roofing material that will prevent the same damage from happening.
Greg decided on standing seam metal for his new roof.
Why Is Metal the Best Choice for Low-Slope Roofs?
To ensure Greg never had moisture issues with his roof again, we installed the roof material his home actually required: metal.
Because it has no seams or gaps, water can’t penetrate standing seam metal roofing. And since it has a smooth finish, it easily sheds snow and rain. This makes metal the appropriate material for low-sloped roofs like Greg’s.
Metal roofing is a good choice for your low-slope roof if you want great protection from water damage and the issues that come with it, but you still want a modern, nice-looking roof.
Other low-slope roofing materials include:
- PVC
- TPO
- Torch down
- EPDM
These will provide you with good leak protection, but they aren’t as aesthetically appealing as metal. Also, these flat (or low-slope) roofing materials don’t last as long as metal roofing.
What Should You Do If You Think Your Low-Slope Roof Was Installed Incorrectly?
After watching Greg go through three roof replacements in 20 years, it’s clear how costly the wrong roofing material can be—especially on low-slope homes. If your roof is leaking, sagging, or showing signs of wear long before its time, there’s a good chance the real issue isn’t age… it’s installation.
At RoofSmart, we’re not just here to fix problems—we find the root cause and solve it right the first time.
Read our article “Why Is My Shallow Slope Roof Leaking?” to learn what might be causing your shallow-slope roof to leak and how to spot the warning signs in your own home.
Worried your low-slope roof is leaking and needs to be repaired or replaced? Reach out to us today to schedule an inspection:
Frequently Asked Questions
Can I install asphalt shingles on a flat roof?
No, asphalt shingles are designed for sloped surfaces where gravity can pull water away. On a flat or low-slope roof, water sits in the seams, eventually penetrating the shingles and causing deep structural rot.
What are the signs of water damage on a low-slope roof?
Common indicators include curling or wavy shingles, sagging sections of the roof deck, interior ceiling drooping, and the presence of both moss and mildew, which thrive in excessive moisture.
Why does a low-slope roof cause shingles to curl?
Shingles curl when they absorb too much water or when moisture gets trapped in the underlayment beneath them. This is a primary sign that the roofing material is failing to shed water correctly.
What is the best roofing material for a low-slope home?
Standing seam metal is often considered the best choice because it has no horizontal seams for water to penetrate and a smooth finish that easily sheds rain and snow.
How many times should a roof be replaced in 20 years?
Ideally, only once. If a homeowner like Greg in the case study is on their third roof in 20 years, it is usually a sign that the wrong materials were used or the installation was handled incorrectly.
What happens if roof trusses become rotted?
Rotted trusses lose their structural integrity and can no longer support the weight of the roof. This leads to a sagging roof deck and can eventually cause the interior ceilings of the home to droop or collapse.
Are there cheaper alternatives to metal for flat roofs?
Yes, materials such as PVC, TPO, Torch Down, and EPDM are effective for low-slope protection. While they are often more affordable than metal, they generally have a shorter lifespan and a more industrial aesthetic.
Can moss and mildew be removed from shingles?
While moss and mildew can be cleaned off, their presence on a low-slope roof is usually a symptom of a deeper moisture problem. Cleaning them is a temporary fix if the underlying drainage issue isn't addressed.
How do I fix a sagging roof deck?
Fixing a sag involves a full "tear-off" where shingles and the old deck are removed. Contractors must then replace the rotted wood, repair or reframe the trusses, and install new plywood decking.
Does a low-slope roof require a special installation team?
Yes. Because low-slope roofs are prone to leaking, they require "EggHead" level technical precision and materials (like metal or membranes) that differ from standard residential steep-slope applications.
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