What Are the Risks of Improper Shingle Installation?

Are you worried your newly installed roof might already be failing, but you’re not sure how to tell? Did you hire a roofer you thought you could trust, only to find signs of leaks, rot, or moss creeping in just a few years later?

Many homeowners don’t realize their shingles were installed incorrectly until it’s too late. By then, the damage had already cost them thousands or voided the manufacturer’s warranty entirely.

At RoofSmart, we replace hundreds of roofs every year. As THE meticulous roofing nerds, we pride ourselves on paying careful attention to detail every time so that your roof looks great AND works how it’s supposed to. We even inspect the roof after it’s done to make sure everything is up to our standards. 

In this article, we’ll walk you through the most common mistakes we see on improperly installed asphalt shingle roofs in the Seattle area. You’ll learn exactly what to look for, why it matters, and how these errors can quietly destroy your roof over time.

What happens when shingles are installed wrong in Seattle?

In a humid, rainy climate like we have in the Seattle area, you want to make sure that your roof is 100% watertight. Otherwise, you risk getting a leak somewhere in your roof. 

Leaks are a big problem because they can easily be hidden within your roof and go unnoticed for a very long time. Meanwhile, the leak is wreaking havoc on your roof. 

Leaks can lead to issues like: 

• Mold
• Mildew
• Rotting wood
• Heaving or delaminating plywood

If the leak is big enough or there are multiple leaks across your roof, then with enough time, it can cause structural damage to your roof. But even if it doesn’t get that drastic, roof leaks are still a hassle and can cost thousands of dollars to repair. 

Is improper roof installation common in Seattle?

Unfortunately, the answer to this question is yes. Why? Well, in Washington State, you don’t have to take any tests or prove you have roofing experience to become a roofer. All you need to do to become a roofer is get insurance, be bonded, and have a contractor’s license. 

Yeah… that’s it. Anyone who can afford the insurance and application fees can become a roofer. Sometimes, when you’re deciding between roofers, it can be really hard to tell who knows what they’re doing and who doesn’t. 

If you end up choosing a roofer who is inexperienced (or is rushing through your installation just to get the job done), they might make mistakes and cause some serious issues on your roof. 

What are the ways that shingles can be installed wrong?

There are many, many ways you can mess up an asphalt shingle roof during installation. We’re going to explain 12 common mistakes. 

All of these mistakes can lead to eventual roof leaks, rot, and premature failure, often long after installation and outside the scope of manufacturer warranties.

1. Ignoring Manufacturer Instructions

Every shingle pack includes specific installation, nail, and offset patterns. Most installers never even read them, which leads to widespread mistakes. 

Every single shingle has its own pattern for nailing and its own pattern for how it's supposed to be placed. This is not just for aesthetics: if shingles don’t get installed following the right pattern, they are not set up to properly protect your roof from Seattle rain.  

For example, incorrectly placed cuttings (like reused half-shingles) can shift the pattern. This creates water channels that lead directly to nails, causing hidden leaks that you won’t notice for a long time.

2. Incorrect Nailing Pattern

Different shingles require specific nail placement. Incorrect or haphazard nailing can cause shingles to lift, shift, or leak prematurely. This step also requires reading the installation instructions. 

3. Improper Exposure (Reveal)

Shingles must be installed with a precise vertical overlap. The nail strip on your shingle should be covered by the shingle above it with several inches of overlap. 

Too much or too little exposure impacts your roof’s ability to shed rain properly. The water might also flow into gaps between the shingles instead of off the roof. Excessive moisture can weaken the durability of your asphalt shingles.

This also means that the nails are in the wrong place. 

4. Not Using Real Starter Shingles

Starter shingles are a special type of shingle that gets placed around the edges of the roof, typically at the gables. Sometimes, roofers use pieces they’ve cut off of regular shingles as starter shingles instead of the real thing. 

Using scrap shingles or omitting starter shingles at gables disrupts the pattern and can expose nails or direct water to the roof decking underneath the shingles, causing leaks.

5. Flashing Mistakes at Eaves

Metal flashing is installed along the outer edges of the roof to help direct water off the roof and into the gutter. 

This flashing needs to go beneath the underlayment, not above. When it’s installed above the underlayment, it lets water run under and leak into the roof system.

6. Improper Valley Installation

To properly protect vulnerable areas like valleys, you need to have either metal flashing or ice and water shield installed underneath the shingles. 

Shingles must be cut (dog-eared) at the valley and sealed. Without this, water tracks sideways and leaks. It can also make it harder to shed debris in the valleys, leading to a build-up of leaves and pine needles where the water is supposed to flow. 

We often see rot in valleys because they were not set up to be protected properly. Areas with heavy water flow, like valleys or second-story downspouts, need additional waterproofing. 

7. Underlayment Installation Errors

Improper overlapping of your roof’s underlayment or using staples instead of cap nails causes underlayment failure and potential water infiltration.

8. Improper Bath Fan Venting

 Sometimes, bath fans often vent into the attic instead of being directed all the way through to the outside. Even when ducted, installers may use non-insulated hoses or cheap plastic caps, causing condensation and cracking.

It’s also common for the area on the roof where the bath vent sticks out to not be sealed properly, leading to leaks. 

9. Cheap Pipe Flashings

Low-cost neoprene flashings degrade after about 12 years. Premium options last longer but are more expensive, so often roofers and homeowners choose the cheaper option to cut down on costs.

10. Faulty Ridge Vents

Many homes have ridge vents installed. This is one of the most common types of exhaust ventilation. 

Common ridge vent brands don’t have external baffles, which provide a protective barrier that keeps debris out of the vent. They clog easily and trap moisture, leading to mold in the attic.

Here’s an example of what it looks like when there’s water clogging the vent: 

And here’s an unfortunate case of moss growing in the ridge vent: 

11. Improvised Hip & Ridge Caps

Some installers use cut-up shingles instead of actual ridge caps. These wear out early and are often nailed incorrectly.

This image shows a roof we inspected where the previous roofer used leftover shingles for ridge cap. It looks ridiculous AND won’t protect your roof properly. 

Repurposing scrap shingles might save money in the moment, but it will lead to leaks and damage down the road. 

12. Concentrated Water from Downspouts

Second-story downspouts dumping onto lower roof sections keep areas wet, leading to rot. It’s preventable but often overlooked and not covered by warranty.

How Do You Know If a Roofing Contractor Is Truly Qualified?

Now that you know how improper shingle installation can quietly lead to leaks, rot, and thousands of dollars in damage, it’s clear that who installs your roof matters just as much as what shingles you choose. 

In Seattle’s wet climate, where moisture is a constant threat, there’s no room for error. But with so many contractors calling themselves “roofers,” how can you tell who actually knows what they’re doing?

Your next step is to read “Is Your Seattle Roofer Actually Qualified? Here’s How to Tell Before It’s Too Late”. This article walks you through what credentials, red flags, and interview questions to look for so you can hire with confidence and avoid costly mistakes from day one.