spray foam over a leaking commercial roof

Can You Spray Foam Over an Existing Leaky Roof?

Greg PalyaSpray Polyurethane Foam Roofing

Yes, you can install a spray foam roofing system over an existing commercial roof.

But did you know that some commercial roofs are ineligible for a spray foam application?

And for some roofs, it’s illegal to add another roofing system on top of what’s already there?

Here are some common questions we’ll answer below:

  • Why would a building owner want to install spray foam over their existing roof?
  • Are there existing roofs that spray foam cannot be installed over?
  • What preparation is done before spray foam is installed?

At West Roofing Systems, informing a building owner that it’s unnecessary to completely tear off the existing roof is something they don’t hear often.

Since 1979, we’ve been installing spray foam roofs over existing roofs…only when it’s legal to do so…and only when it’s the most cost-effective solution to a building owner’s roofing issue.

Let’s get started.


Why would a building owner want to install spray foam?

A spray foam roof has many benefits, including:

  • Energy cost savings
  • The roof is renewable after its warranty expires (instead of having to tear everything off)
  • There are no seams, where most leaks occur
  • All the penetrations are fully adhered and seamless

And much more.

One of the best benefits is that installing spray foam over your existing roof requires the least amount of tear-off possible.

The only parts of your roof that will be torn off are areas that are saturated with water. These areas are identified with an infrared camera and core samples.

It’s important to understand that after an infrared inspection, if more than 25% of your roof is saturated, it’s sometimes more cost-effective to tear everything off the roof and install a new roofing system.

The reason is that a large part of your roof is already damaged, meaning the rest of the roof may not be too far behind.

If your roof has less than 25% saturation, it’s cost-effective to remove and replace only the saturated areas, then install a new roofing system over the top of the existing roof.

NOTE: Doing the least amount of tear-off not only reduces the cost of your roofing project, but it also reduces the amount of materials that are sent to landfills, which is good for the environment!

Are there existing roofs that spray foam cannot be installed over?

Spray foam can go over almost any type of existing roof, as long as the roof is dry, clean, and sound.

A dry roof means that all water has been removed off the roof before a spray foam installation. Even the slightest amount of water will cause an improper mix of the A and B chemicals used to create spray foam.

A clean roof means that all contaminants must be removed off the roof before installation. This is usually done with power washing.

It’s very, very important to have a clean roof. Our sales guys always say that if you aren’t comfortable eating your sandwich after you dropped it on your roof…then the roof isn’t clean enough.

A sound roof means that the existing membrane (the outermost roofing layer), is fully adhered or adequately fastened to the insulation below.

A bad example of this is a TPO roof that’s mechanically-fastened (using plates and screws) at the seams.

example of seams on a TPO roof

Since the TPO is only fully-adhered at the seams, a cover board would have to be installed over the TPO to get a 100% stable area to apply spray foam on.

Applying spray foam to a non-secure substrate will throw off the slope of the roof. Since the TPO isn’t secured, it can move up or down.

This means that the slope isn’t consistent, which can cause water to stay on your roof instead of moving towards the drains like the system is designed to do.

NOTE: TPO roofs can also be fully-adhered. This means that the TPO has been installed over the insulation with an adhesive. Since this TPO roof is fully-adhered, spray foam may be applied with no cover board, if certain conditions allow, such as a passed adhesion test. A primer/cleaner may also be used before spray foam is installed depending on the condition of the existing roof.

Spray foam cannot be installed over a roof that already has two or more roofing systems.

Per building codes, the maximum number of roofing systems that a commercial roof can have is two.

If you already have two roofing systems, at least one system must be removed to legally install a spray foam roofing system.

NOTE: a good alternative for a scenario where there are already two roofing systems is a silicone restoration system.

If your roof only has one system, congratulations! You can install a spray foam roof with no legal worries.

If you aren’t sure how many roofing systems you have, during a roof inspection, a roofing contractor will pull core samples of your roof:

A core sample of a commercial roofing system

The core sample will determine how many roofing systems you already have.

Is your existing roof ready for spray foam?

In this article, you learned that any existing roof could have a spray foam roofing system installed on top, as long as:

  • There are less than two roofing systems already installed
  • There’s no saturation
  • The existing roof fully adheres to the insulation below
  • The existing roof is dry, clean, and sound

What’s your next step in your journey to get your commercial roof fixed?

Since West Roofing Systems has been installing spray foam roofs over existing roofs since 1979, we’ve developed hundreds of articles designed to help building owners with their roofing problems.

You’re probably wondering about these popular topics:

What Is the Cost of a Spray Foam Roofing System?

8 Commercial Roofing Mistakes Building Owners Make

Good vs Bad Spray Foam Roofing Contractors: 5 Differences


About the Author

Greg Palya

Greg Palya is the Digital Content Manager of West Roofing Systems, Inc. He has a B.S. in Marketing from the University of Akron and an MBA in Marketing from Walsh University. When he's not trying to teach others about spray foam roofing and silicone roof coatings, you can find him on the basketball court or golf course.