finished spray foam roof

What’s the Recommended Thickness of a Spray Foam Roof?

Greg PalyaSpray Polyurethane Foam Roofing

You may be wondering, spray foam roofs get spray applied, meaning they can be a ½ inch, 2 inches, or 10 inches, or really any thickness a spray foam applicator feels like doing.

But what thickness is most beneficial to my roof?

I don’t want the costs of applying additional foam to outweigh the benefits it will provide.

This is very similar to buying a new driver in golf. Yes, I can spend $600 on a professional driver equipped with a high-speed shaft.

But how fast do you swing? If you can’t swing as fast as the pros, this will cause more harm to your golf game than you would benefit.

Maybe you’d be better off buying a $250 driver that’s meant for amateur swing speeds?

Going back to your roof, maybe the “sweet spot” is 2 inches of foam. If you requested 10 inches of foam, the added R-value (which reduces your energy costs) will not provide enough savings to pay for the application of 8 more inches of foam.

At West Roofing Systems, if there’s one thing we know, it’s that every commercial roof is different. Since 1979, we’ve been developing custom spray foam designed roofs that provide the greatest R-value, with the lowest installation cost possible.

Let’s get started…

In this article, there are two variables that will determine the best thickness of spray foam for your roof, they are:

  • What kind of existing roof system do you have? Gravel BUR, Smooth BUR, Modified?
  • Are you required to hit a minimum R-value per building code?

NOTE: At the end of the article, there are 3 bonus questions about spray foam thickness.

What’s the best spray foam thickness for a gravel-built-up roof

If you have a gravel built-up roof, a rule of thumb is that you’ll need an average of 1.5 inches of spray foam in order to obtain the proper surface profile, have it cure correctly, and for the foam to be smooth.

This is because with any gravel built-up roof, you’ll need to use a wet-vac system to remove the loose dirt and gravel from the roof. What’s left behind is a rough surface because of the embedded gravel.

But then you need to determine the slope of the roof.

Is the slope built into the deck?

Is the slope built into the roof?

Or is the slope flat?

If the slope is built into the deck, a consistent application of 1.5 inches of spray foam will keep the slope working as is.

If the slope is built into the roof, you can install a minimum of 1.5 inches of spray foam, and then build drainage saddles in-between drains to help with water flow.

If the slope roof is fairly flat, you’ll have to go a minimum 1.5 inches, but then determine where your roof was previously holding water and how deep that water was.

To eliminate those low areas, you’ll need to install additional foam in order to create slope to the drains.

This is different on every roof because the drains are located in different places.

What’s the best spray foam thickness over a smooth built-up or modified roof?

On these types of roofs, you can install a minimum of 1 inch of spray foam, as long as the roof maintains its proper slope.

If not, you need to install another inch of foam over the areas furthest away from the drains.

The most beneficial installation is 1 inch near the drains, 2 inches away from the drains, which brings your roof to an average of 1.5 inches of spray foam.

The most important point to take away is; the best spray foam thickness is all based on the type of roof that you’re looking at.

What’s the best spray foam thickness over a torn-off roof?

There are times when there’s too much saturation in your current roof, that it’s best to completely remove everything down to the deck and start over.

Usually, this is at the 25% saturation level and is discovered during an infrared inspection.

If your roof is less than 25% saturated, you can re-roof (if you currently only have 1 roofing layer) or you can restore your roof if you have more than 1 roofing layer. In either case, there is no minimum R-value you need to hit to stay compliant with building codes.

If you’re more than 25% saturated, you’ll need to tear off your roof and start over, which according to most building codes is considered new construction.

To stay compliant, your new roofing system will have to exceed a minimum R-value. In Ohio, which uses the 2017 Ohio Building Energy Code, will need to meet, or exceed an R-25.

Spray polyurethane foam has an R-value of 6.6 per square inch of thickness.

To get to an R-25, you’ll need to install an average of around 4 inches of spray foam.

4 in. x 6.6 R-value = R-26.4

Bonus #1 – What if a building owner wants to install 10 inches of spray foam?

A building owner who wants to install 10 inches of spray foam would be wasting money. There’s a time where the cost of more spray foam will not equal the cost savings by reducing your energy bills.

The exact installation thickness can be debated, but since West Roofing Systems has been installing all different thickness levels of foam for more than 40 years, we believe this number to be around 2.5 inches.

How much does more spray foam cost?

For an average 20,000 sq. ft. roof, it costs approximately $0.75 – $1 per sq. ft. for each additional ½ inch of installed spray foam.

To go from 2.5 inches to 10 inches, is a difference of 7.5 inches.

For easy math, let’s say each additional ½ inch of foam thickness is $1 per sq. ft.

15 (half-inches) x ($1) x (20,000) = $300,000

Bonus #2 – Does the initial installed thickness of spray foam reduce over time?

Spray foam does not shrink or degrade over time.

This is because spray foam is protected with a thin layer (usually 20-30 mils) of silicone coating. The coating is installed in two separate coats, with granules embedded into the topcoat.

Here’s a video of a spray foam roofing installation from start to finish:

The granules and coating will wear away over time due to wind, rain, debris, foot traffic, and other natural occurring events, but as long as your roof is maintained, you’ll never damage the thickness of the foam.

Maintenance is important because most building owners never go on the roof. It’s important to recognize small repairs and fix them, before they potentially become a bigger issue.

For a 10-year warranty on a spray foam roof, 20 mils of coating will be installed on top. After 10 years, the 20 mils of coating may be down to 7-8 mils of coating.

All you need to do is make minor repairs (if any), clean the roof and perform a re-coat, which is adding coating back to the level of the initial installation.

As long as you have scheduled maintenance, and a recoat program in place, the thickness of spray foam will never shrink or degrade.

Bonus #3 – Does the R-value for spray foam reduce over time?

Spray foam’s R-value does not decrease over time. Spray polyurethane foam will have an R-value of 6.6 per sq. in. of thickness today, tomorrow, and 10 years from now.

More spray foam isn’t always better

Hopefully today you learned that more isn’t better in every situation.

In the case of spray foam roofing, the goal is to maximize R-value (so you can reduce your energy bills) but to recognize when additional foam will not save you any money.

This break-even point is around 2.5 inches of foam.

Your energy bills will be very similar whether you had 2.5 inches of foam installed or if you had 10 inches of foam installed.

The other main point of the article is that every roof is different. Drains are located in different areas, the number of penetrations can differ, and drainage systems are different.

One of the greatest advantages with spray foam is that a contractor can customize the slope to raise lower levels where water used to be ponding.

Eliminating ponding water is a great way to extend the life of your roofing system.

At West Roofing Systems, we’ve been customizing spray foam roofs for more than 40 years! If you have ponding water on your roof, energy efficiency issues, or a roof leak, contact us and we’ll customize the best “bang for the buck” solution for you.

the ultimate guide to spray foam roofing

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.