Are you frustrated because the roof you replaced only a few years ago is already leaking into your residents’ units? Feeling confused about what the right solution is for your low-slope roof when some roofers say shingles are okay, but others say not to use them?
At RoofSmart, we’ve inspected many low-slope roofs around the Seattle area where asphalt shingles struggled with slow-draining or pooling water and eventually leaked. In this case, the problem was made worse by blocked intake vents, improper ventilation, and shingles installed on roof sections as low as a 2:12 pitch.
In this case study, I’ll show you what I found when we inspected these apartment buildings, why the low-slope roof and ventilation system contributed to leaks, mold, and rotten decking, and how we corrected the problems.
Recently, I was scheduled to inspect the roofs of a few buildings in an apartment complex. The owner reached out because some of her residents were concerned about leaks in their units. The HOA board had hired another roofing company to redo the roofs about four or five years ago, so they were not sure why they were already leaking.
Once I got there and inspected the roof and attic, the issue was immediately obvious: they had shingles on a low-slope roof and blocked intake vents, which was causing the leaks and mold.
The buildings were using bird blocks as their intake, but they weren’t working properly because they were blocked with insulation. Some spots also had soffit vents, which is not good because you are not supposed to have multiple forms of intake ventilation.
Basically, the takeaway here is that the attic ventilation on these apartment buildings was messed up. The leaks were causing excess moisture to enter the attic and cause mold to grow because there wasn’t enough airflow to dry it out.
Here's the short version: we won't put shingles on anything under 3:12, even where code says you can. Below 3:12, standing seam metal or flat roof materials are our pick. If you're not sure where your roof lands, a SmartMatch Needs Analysis will sort it out.
On each of these buildings, one side of the roof has a 3:12 slope and the other side has a 2:12 slope.
On that low of a slope, water does not run off the roof very well and can start to pool on top of the shingles. Asphalt shingles are not designed to withstand standing water, so they will absorb the water. As a result, the water will wick back into the shingles, causing leaks.
That’s why it’s so common on a low-slope roof that the shingles are overwhelmed by the water and the roof begins to leak as a result. At RoofSmart, we’ve seen it happen tons of times on roofs all over the Seattle area.
Eventually, the water seeps through the roof decking or makes its way into the attic through a gap or hole. This moisture helps create an ideal environment for mold to grow. When the roof is left to leak for a few years, the rot and mold have plenty of time to spread all over and become an expensive headache to deal with.
This is what the attic and roof decking looked like:
Unfortunately, the last roofer that our customer hired either did not know or did not care that you’re not supposed to put shingles on a low-slope roof. Some manufacturers claim that their shingles can be installed on as low as a 3:12 slope, but in our experience, this pretty much always causes leaks.
The last roofer went the cheap route with this installation and the customer did end up saving money up front, but it ended up costing the HOA board several hundreds of thousands of dollars in repairs after about five years.
Almost every time we’ve been called to repair or replace a low-slope roof, it has been leaking because the roofer put shingles on it and took no other steps to prevent leaks (like installing ice and water shield). If you don’t at least do that, then your low-slope roof is going to leak.
Even though we discourage installing shingles on a low-slope roof, this customer wanted asphalt shingles to help keep their costs lower. So, here’s a low-cost way to install shingles on a low-slope roof with better protection:
Ideally, this roof would be entirely metal, PVC, or another flat roof material. However, the HOA board wanted to maintain the aesthetics of the buildings and property, so we went with half shingles and half PVC.
The shingles are still at a high risk of premature failure caused by standing water, but the ice and water shield will help to prevent leaks for as long as possible. The PVC side of the roof will hold up well against pooling water.
The HOA board hired us to do the roof replacement, mold remediation, and insulation. So, before we installed the new PVC, underlayment, and shingles on the roof, our crews went in and treated all of the attics for mold and installed new insulation in the attic. They also replaced the rotten, moldy roof decking boards with new ones.
On this roof, the rot and mold were so extensive that most of the roof decking had to be replaced, which increased the total project cost.
The HOA board was very pleased with the work that we did, and now they always call us if they need roof maintenance or a repair. Here’s how the roof turned out:
| Roof Problem | Why It Happens | Potential Damage | Repair Approach Used on This Project |
|---|---|---|---|
| Asphalt shingles on a 2:12 slope | Water doesn't drain quickly enough for shingles to perform reliably | Water intrusion, leaks, mold, and rotten decking | Remove shingles and install a PVC membrane roofing system |
| Asphalt shingles on a 3:12 slope | Slow drainage increases the risk of water working beneath the shingles | Leaks and moisture damage | If shingles must be used, install ice and water shield across the entire 3:12 section for additional protection |
| Standard synthetic underlayment on a low slope | Doesn't provide the same level of protection against pooling or slow-moving water | Water can reach vulnerable penetrations and fastener locations | Upgrade to self-adhering ice and water shield |
| Blocked bird block vents | Attic insulation covers the intake openings | Reduces airflow into the attic | Clear the intake ventilation path and correct the ventilation system |
| Multiple forms of intake ventilation | Bird blocks and soffit vents are combined without a properly designed ventilation plan | Ventilation may not function as intended | Redesign the intake system rather than simply adding more vents |
| Moisture entering the attic | Roof leaks introduce water while poor ventilation prevents adequate drying | Creates conditions where mold can develop | Stop the leaks and correct attic airflow |
| Moldy attic | Long-term moisture problems remain unresolved | Mold can spread throughout the attic | Complete mold remediation before rebuilding the roof assembly |
| Rotten roof decking | Decking remains wet from repeated or long-term leaks | Plywood becomes soft, rotten, and structurally compromised | Remove and replace damaged decking |
| Condensation concerns beneath PVC | Low-slope assemblies need to account for condensation as well as exterior water | Moisture can affect the roof assembly | Install foam insulation boards beneath the PVC system |
| Mixed 3:12 and 2:12 roof | Different slopes have different water-management needs | One roofing approach may not be appropriate for the entire building | Use shingles with enhanced underlayment on 3:12 sections and PVC on 2:12 sections |
A low-slope roof that starts leaking only a few years after replacement can be especially frustrating, but this project shows why the age of the roof isn’t always the real problem.
In this case, asphalt shingles had been installed on 2:12 and 3:12 slopes without any protection against pooling water. Blocked intake ventilation made the situation worse by preventing moisture in the attic from drying effectively.
By the time I inspected these apartment buildings, the problem had progressed beyond repairing a few leaks. Mold had spread through the attics, much of the roof decking needed replacement, and the HOA was facing hundreds of thousands of dollars in repairs.
If you have a shallow-slope roof that is leaking, understanding why it’s happening is critical before paying for another repair or replacement. Otherwise, you could fix the visible leak without correcting the underlying problem that allowed water into the roof in the first place.
Read our article “Why Is My Shallow-Slope Roof Leaking?” to learn the common reasons these roofs develop leaks and what you should look for in the attic to understand what kind of fix your roof needs.