Are you worried your flat roof might be leaking, but you can’t see any obvious signs? Have you noticed mysterious moisture, rotting trim, or even mold forming around the edges of your ceiling?
While PVC is an excellent flat roofing material, it’s only as good as the contractor who installs it. Without proper insulation, drainage tapering, and detailed sealing at scuppers, skylights, and parapet walls, your flat roof could become a water trap, leading to costly repairs down the road.
At RoofSmart, we’ve repaired and replaced hundreds of flat roofs throughout the Greater Seattle area. We’ve seen everything from small leaks to extensive rot that covered the entire roof, allowing us to learn the best ways to avoid leaks during installation.
In this article, we’ll walk you through the most common mistakes we see on poorly installed PVC roofs in the Seattle area, and how you can spot them before they lead to major issues.
What common mistakes do roofers make when installing PVC in Seattle?
PVC is a great flat roofing material, so if your PVC roof is having issues, it could easily be due to poor installation. Here are some common installation mistakes we see on PVC roofs:
1. Condensation due to poor insulation
The most common thing that roofers get wrong on flat roofs is not properly insulating them to prevent condensation.
If you don’t properly insulate your flat or low-slope roof with layers of different types of insulation around the roof decking, then you leave your roof at risk of condensation build-up. The condensation will then soak into the insulation, become condensation in cold weather, and spread to the roof framing.
This causes issues like:
- Rotting wood
- Water damage
- Mold
We see damage from condensation on flat and low-sloped roofs all the time. The bummer with condensation is that it is happening within the roof. So, much like most roof leaks, you don’t know that it’s happening and causing damage until it’s really bad.
Here’s an example of extensive roof rot caused by condensation on a recent customer’s roof:
We had to entirely replace the roof decking because it was rotting and crumbling everywhere. Unfortunately, this is a pretty common consequence of condensation.
2. Not properly sealing the roof at vulnerable leak points
PVC is a good flat roofing material because it is (in my opinion) the best one for sealing around potential leak points. However, an unskilled or sloppy roofer can easily get it wrong.
Often, we see leaks in these vulnerable areas:
- Scuppers (flat roof drains)
- Skylights
- Where parapet walls meet the roof (short walls surrounding a roof deck or flat roof)
- Door frames
For example, here’s the damage and rot that we found underneath a leaking scupper on that same customer’s roof:
These areas need to be sealed because they naturally have a gap where water can get into the roof (this is a problem with any flat roofing material).
Flat roofs must be able to hold up under standing water, especially in a constantly rainy area like Seattle. Having gaps or poorly sealed areas is a guarantee that your Seattle roof will leak.
3. Not tapering the roof to help with drainage
Although flat roofs should be able to withstand pooling water, there shouldn’t constantly be water collecting on your roof. You need to make sure that it drains properly.
One way to help the roof drain is to taper it so that the roof slopes down slightly towards the scuppers. However, this is a step that many roofers skip.
Flat roofs need to be framed to hold extra weight (especially if you want a rooftop deck), but they also need to be structured to taper so that your roof can drain. Proper framing makes your roof safe and functional.
How do I avoid water damage on my PVC roof?
Knowing about potential issues ahead of time can help you avoid problems on your PVC roof. Make sure that you consider these factors to ensure that your PVC roof is installed properly:
1.) Condensation control:
Having proper ventilation and insulation in your flat roof is key. Without it, you risk letting condensation rot out your roof (no matter how great your roof works up top). This means that you need a warm roof design, which is when you have a flat roof that has insulation placed directly on top of it.
The most important step (that most forget) is to put a waterproof membrane underneath the roof decking to act as a barrier for water. This prevents condensation from forming underneath the roof.
2.) Choose the right material:
Choosing a quality material is especially important on a flat roof because it is likely to end up with standing water pooling on top of it. There are different PVC systems made by different manufacturers. They all have their own requirements.
And our favorite is Duro-Last, although we've installed everything. They'll all work, but one of the reasons we like Duro-Last is that they make flashing components for corners, scuppers, and penetrations with a reinforced membrane.
It's standard for pretty much every other manufacturer to have an unreinforced membrane at those joints. But this is risky because unreinforced membranes are more likely to crack. And if it cracks, you have a leak that’s pretty hard to find.
And so, if you're making all these components that are unreinforced (specifically in places that are more vulnerable to leaks), that's obviously an Achilles heel.
We also like Duro-Last because it has polypropylene-dipped fibers, which makes it non-wicking. It doesn't need a cut-edge sealant. Basically, this means that the edges of the membrane are protected from absorbing water, so you don’t need to bother with the coating of sealant that wears away after about 10 years anyway.
3.) Choose the right roofer:
If there's any weakness in your flat roof, water will find a way in. That’s why choosing a quality roofer is just as important as choosing a quality roofing material.
Your roofer should act as a trusted guide to help you choose the right roofing material for your needs and budget. You should also be certain that they have lots of experience installing the type of roofing material that you want.
Make sure you do your research by reading reviews and looking for pictures of completed roof projects on the company’s website.
Torchdown vs. PVC: Which is better for your flat roof in Seattle?
Now that you understand how important proper installation is for protecting your flat roof from leaks and rot, the next question is: which roofing material will give you the best long-term performance in Seattle’s rainy, unpredictable climate?
Both torchdown and single-ply PVC are popular options for flat and low-slope roofs, but they perform very differently when it comes to water resistance, maintenance, and lifespan. Choosing the right one can mean the difference between decades of worry-free protection and costly water damage repairs.
Check out our article “Torchdown vs. Single-Ply PVC: Which One Is Best for Your Roof?” To see how these two materials compare head-to-head.
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