Why Is My Shallow Slope Roof Leaking? 

Are you concerned that your shallow slope roof is leaking and causing damage to your roof without you knowing? Are you wondering why shallow slope roofs leak so much and what you can do to prevent extensive damage?

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.

In this article, you’ll learn:

• What a shallow slope roof is
• What problems they cause and why they leak
• Which roof types are best at preventing leaks on shallow slope roofs

By the end of this article, you’ll understand the risk of having a shallow slope roof as a Greater Seattle area homeowner and be able to decide which roof type is best for you.

What is a shallow slope roof?

What does that mean? A shallow slope roof is a roof that isn’t as steep as you’d normally see. It’s more like a ramp than a roof. It’s what you’d commonly see on a double-wide mobile home or a manufactured home. Anything less than a 4:12 pitch is considered a shallow slope.

The pitch of the roof is a way of calculating how steep it is. So, for example, a 4:12 pitch means that for every 12 inches across, your roof goes up 4 inches. You can see an example of what a 4:12 pitch looks like (along with other examples) in the image below:

Image from InspectAPedia

What problems do low-slope shingle roofs cause in rainy areas like Seattle?

The main issue that homeowners face in this area is the constant rain. This can cause problems on any roof type, but on a shallow slope roof that cannot get rid of water as easily, the rain can have dire consequences.

This is especially true when asphalt shingles are installed on a shallow slope roof because they do not hold out well under standing water.

Here are the main issues that we’ve noticed throughout the years on shallow slope roofs with asphalt shingles:

Constant Rain Causes Leaks

Leaks are a big problem in the Greater Seattle area because it is consistently rainy here. This is a big problem on a shallow slope roof because it means that the shingles are constantly wet and don’t have a chance to dry out.

In these conditions, the holes where the nails were shot into the roof become leak points in the roof. These leaks can be hard to spot because they are very small and are usually located in the attic. They will take a long time to show signs of water damage in the ceiling or walls. And they can also cause mold in your attic.

Roof Underlayment Can Cause Rotting Wood

If your roof pitch is less than 4:12 or higher than 2.5:12 you can choose two types of precautionary measures using roof underlayment:

1. A layer of ice and water shield underlayment (also known as self-adhesive underlayment) -or-

    

2. 2 layers of felt underlayment

The cheaper option is to do 2 layers of felt underlayment, so most people choose that. However, the roof will still start to leak after about 1 year.

Ice and water shield is a somewhat better option because it seals around nail holes, however even this type of underlayment leaks after 3 years on a shallow slope roof with asphalt shingles.

No matter which type of underlayment you choose, the water will get underneath the shingles. Eventually, the underlayment soaks through and is unable to dry out because of the constant rainfall.

As a result, the underlayment acts like a big wet blanket that covers the roof, allowing moisture to seep into the roof deck. This causes it to rot, leak, and grow mold. And because the leaks aren’t reaching the ceiling in your living spaces, you probably won’t notice this issue for a very long time.

What do these problems look like for real Seattle area homeowners?

We recently helped a customer who had a low-slope shingle roof that wouldn’t drain all the way during the winter months. As a result, the shingles soaked up a bunch of water and the water soaked through the gaps around the nails.

Essentially, the shingles were acting like a big wet blanket for the roof. This caused the plywood boards underneath to mold, delaminate, and rot. Here’s an example of what they looked like:

To fix this issue we replaced the plywood, added ventilation, and used ice and water shield underlayment all over the roof. New asphalt shingles were installed.

In this scenario, metal would’ve been ideal for added protection, but we understand that this is not practical for everyone’s budget. Even so, with the other precautions we took the new roof is expected to last around 20 years.

Which roof types are best at preventing leaks on shallow slope roofs?

Asphalt shingles are a classic go-to option for many homeowners in the Seattle area, but if you have a shallow slope roof, then this may not be the best option for you.

If you want to take precautions against potential leaks, consider choosing these roof types instead:

1.) Standing seam metal

This option is going to last the longest because it is the most durable. Metal has fewer crevices and gaps where the roof material comes together than an asphalt roof, making it less susceptible to leaking. It is also the most aesthetically appealing out of all the roof material options for shallow slope roofs.

2.) Torch down

Torch down looks similar to asphalt roofs and even has a layer of granules on top. However, it works better than a traditional asphalt shingle because it is sealed together with a blowtorch. There are no holes from nails or gaps between the shingles where water can get through.

It can be risky to hire someone to work on your roof with a blowtorch. Understandably, some people are hesitant to choose torch down for this reason. But, if you choose an experienced roofer who has good insurance coverage, you’ll be in good hands.

3.) TPO and PVC

Just like torch down, these roof materials are ‘glued’ onto the roof using a heat gun. There are no holes from nails on the top layer (the bottom layer is stapled down, but this is covered by the sealed top layer), making the roof water-resistant.

PVC is made mostly of plastic and TPO is made mostly of rubber, but they look and function the same. However, PVC does last longer and has better water resistance than TPO.

They are not very aesthetically appealing. They look like a pool liner and usually only come in white. So, if you are concerned about the curb appeal of your home, this is something to consider. They are also very slippery when wet, which can make them dangerous to maintain.

Protect Your Shallow Slope Roof from Leaks in Seattle’s Rain

Your roof is your home’s first line of defense against the elements. By understanding the limitations of asphalt shingles and considering more durable alternatives, you can avoid costly damage and maintain your home’s value for years to come.

Materials like standing seam metal, torch down, PVC, and TPO provide more reliable long-term protection than asphalt shingles, saving you money on repairs down the line.

Want to find out if your shallow slope roof is leaking before it’s too late but don’t know what to look for? Check out our article “Signs That Your Roof Is Leaking During Seattle’s Rainy Season” to learn how to catch a leak.