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Spraying Polyurethane Foam Over an Existing Roof

Spray foam isn’t just used for insulation — it can also be used for roofing

The flat roof of this 1957 Eichler-designed house was a good candidate for a polyurethane spray-foam roof.
Image Credit: Mike Litchfield, Renovation 4th Edition
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The flat roof of this 1957 Eichler-designed house was a good candidate for a polyurethane spray-foam roof.
Image Credit: Mike Litchfield, Renovation 4th Edition
A good deal of prep work must precede the foam-spraying. Here, electrical conduits running above the roof had to be raised so they would be accessible after 4 in. of foam was added.
Image Credit: Mike Litchfield, Renovation 4th Edition
Spray foam adheres aggressively, so it's important to mask everything you don't want foamed.
Image Credit: Mike Litchfield, Renovation 4th Edition
After the spray foam has cured, it is painted with an acrylic coating to protect it from UV exposure.
Image Credit: Mike Litchfield, Renovation 4th Edition
Renovation 4th Edition contains field-tested tips and techniques that hundreds of master builders and tradespeople shared with Mike Litchfield, a founding editor of Fine Homebuilding.
Image Credit: Taunton Press

When Taya and Stephen Shoup’s old tar-and-gravel roof began leaking, the couple hoped to find a replacement roofing that would be energy-conserving, leakproof, cost-competitive, and reasonably green. As the house’s roof sheathing doubled as a finished ceiling, the old insulation was scant. They broiled in the summer and hemorrhaged money during the heating season.

The Shoups live in a 1958 Eichler-designed house in Emeryville, California. Taya is a landscape architect and Stephen is the principal of BuildingLab, a green design/build firm whose reincarnated shipping containers were chronicled in a GBA article titled “Creating an Urban Oasis.”

Ultimately, the Shoups chose to install a closed-cell sprayed polyurethane foam (SPF) roof — sprayed directly over the old tar-and-gravel roof — because:

Insulating inside was out. The exposed underside of the 2×6 T&G roof deck was classic. Besides, the ceilings were just a shade above 8 ft. and the couple didn’t want to lose 4 in. to 6 in. of headroom to insulation.

Closed-cell SPF has an R-value of 6.5 per inch, making it the perfect antidote to the old roof’s dismal energy profile. The old roof assembly, with two layers of 1/2-in. fiberboard, achieved (maybe) R-3 or R-4. Four inches of SPF would bring it up to at least R-26; six inches would bring it to R-39. Spray foam was also particularly well suited to this roof because it had parapets, which defined the edges of foam. (Exterior spray foam only works on flat or low-pitch roofs; for pitches greater than 4/12, rigid foam panels are the over-roof-deck choice.)

A spray-foam roof could be applied directly to the old roofing once it was prepped and cleaned up a bit. Not stripping the old roof and sending the debris to the landfill was also a big green plus. Besides, spraying-over would save the labor costs of stripping the old roof.

Four inches of closed-cell polyurethane is a Class II water retarder, a semi-impermeable membrane that makes leaks all but impossible. As the foam expands and adheres aggressively, it would seal the many pipes and conduits running across the roof of a house with no cavity under the sheathing in which to hide pipes.

Compared to the obvious alternatives, an SPF roof was actually less expensive than installing and then insulating either a new torch-down roof or a PVC membrane. And that didn’t even include the cost of stripping the old roof. There were energy-efficiency rebates available for installing an SPF roof but the couple was so busy that, as Stephen put it, “We couldn’t afford the time to save money.”

Is spray foam roofing green?

So was this spray polyurethane foam a green choice? Yes and no, but mostly yes. Since the early days of sprayed foam insulation (the earliest foams were urea-formaldehyde based), the insulation industry has cleaned up its act a lot. Although the two chemical components of SPF are largely petroleum-based, once the polyurethane mixture foams and cures, it is chemically inert and doesn’t off-gas. (Manufacturing PVC, in comparison, involves some particularly toxic chemicals.)

Not sending old roofing to the dump is a plus, as is the excellent R-value of SPF foam. (Sometimes it takes oil to save oil.) An SPF roof can also be quite durable when the polyurethane is sprayed with a special acrylic top coating that protects the foam against UV degradation. Lastly, the foam’s light color will reflect sunlight so, all in all, it should be a relative cool roof.

This posting was excerpted from Renovation 4th Edition, newly published by Taunton Press. The book contains the collective wisdom of hundreds of master builders and tradespeople whom I began interviewing when I helped launch Fine Homebuilding. The book’s 614 pages contain more than 250 technical illustrations, roughly 1,000 photos (of the 40,000 I’ve taken over the years) and thousands of field-tested tips and techniques.




  1. davidmeiland | | #1

    Far from convinced

    Not stripping the old roof and sending the debris to the landfill was also a big green plus

    Some day, the old roof is going to the dump. Roofing over it doesn't avoid that fact.

    Other questions:

    In the photo of the electrical conduits, there is a scupper in the background, at the level of the existing roof. Will this be left open as a drain, or is the SPF going to cover it? Based on the masking paper in the photo, it looks like it's going to get blown full.

    In the photo of the chimney, is there going to be flashing added to the brick, or a metal cap added to the whole thing, to flash the foam? Or, is the installer assuming that no water can get down between the foam and the brick, even over a period of decades?

    Is there any possibility that this foam will suffer the types of cracks and voids that we've all heard so much about in wall and ceiling installations, when the foam is applied in thick lifts, and/or there are mix issues?

    How durable is this foam under foot traffic?

    And, what is the expected lifetime of the product?

    I understand that Eicher houses are a real problem to retrofit (I have worked on a few), but it is hard to escape the conclusion that this is a product with a high lifetime cost and some serious potential downsides.

  2. Expert Member
    Dana Dorsett | | #2

    Relative greenliness of closed cell polyurethane.
    Greenliness of closed cell polyurethane in this application is probably relative to how thick it was applied, unless it was blown using one of the newer very-low greenhouse impact blowing agents rather than the ubiquitous HFC245fa. At 2" it's almost certainly a net-positive for the environment from a lifecycle greenhouse gas emissions point of view, but maybe not, depending on the projected mix of energy sources for heating & cooling. At 4" maybe not, and at 6" there is almost certainly a heavier GHG footprint than the energy use offset. With the product-release of newer lower-impact HFC blowing agents this should change within the next couple of years, but for now only the very few water-blown closed cell foam products can claim to be low greenhouse impact. (Icynene MD-R-200 and the Aloha Energy product line are two, there may be other water -blown foams out there, but if so they're in stealth mode.)

    It's the acrylic coating, not the foam itself that has the favorable high albedo, and that coating may be doing as much or more for cooling load reduction than the last 3" of cc foam (assuming 6") by cutting the average mid-day difference between roof temperature & conditioned space temperatures by half (cutting peak temp deltas by more than half.) The foam itself while far more reflective than mopped-on tar with stone ballast, is nowhere near as reflective as the acrylic (and would become far LESS reflective in only a matter of months without the coating.) The acrylic coating increases the average heating load by a tiny amount though.

    BTW: With what appears to be less than 12" of masonry and no other insulation between the flue liner and the foam in the picture may constitute a code violation, depending on how the exceptions are interpreted by local code inspectors.

  3. GBA Editor
    Martin Holladay | | #3

    Response to David Meiland
    Q. "Is there going to be flashing added to the brick, or a metal cap added to the whole thing, to flash the foam?"

    A. Spray-foam roofing is usually referred to as a "self-flashing" form of roofing.

    Researchers who have looked into the question of whether spray foam can actually perform as a "self-flashing" roofing have generally reported that the system works.

    I wrote an article on the topic that was published in the April 2005 issue of Energy Design Update. There is more information in that article on the flashing question.

    The link to my article is included in the "Related Articles" box above. I'll include it below as well:

    Roofing With Foam.

  4. shshoup | | #4

    Responding to Mr. Meiland
    first, i will say that i and several of our clients are thrilled with our SPF roofs. in the 8 years since we first spec'd the system we have suffered only one problem and that resulted from a poor skylight installation by a crew who removed some of the vinyl coating. that said, the following comments do not aim do convince. as with most building systems, SPF has its plusses and minuses. you should choose, recommend and spec what you feel confident in. to address some specific concerns:

    lifespan and sending the old roof the dump:
    with some inexpensive but regular maintenance (re-coating) this roof should outlive both me and my 3-1/2 year old daughter. (the vinyl coating should be re-applied at year 5 and then every 15-20yrs thereafter.) heading to landfill may be an inevitability, but i will likely be there too by the time it happens.

    filling the scupper:
    the scupper was not filled, rather the foam was tapered to direct the flow of water out through the scupper

    no flashing at the chimney:
    as Mr Holladay points out, SPF is a self flashing system. i personally have complete faith in this assertion. it is indeed the installer's job to verify the compatibility of the substrate and to prepare (clean) it properly.

    cracks and voids:
    i have heard of but not experienced such a failure. as you suggest, the proportions of the mix are essential to the quality of the application as is the skill of the installer. in this sense, the foam is not different than any number of building systems.

    durability under foot traffic.
    durability is perhaps my greatest concern with this system; if the vinyl coating is compromised the foam becomes quite vulnerable to rapid UV degradation and water infiltration. durability under foot , unless wearing crampons or golf spikes, is not what worries me. a branch, a hammer or other sharp object falling on the roof however, stands the potential to cause serious problems. consequently we recommend close inspections following heavy storms or work being done on or around the roof.

    regarding the question of blowing agents and greenliness brought up by another contributor, this is an interesting component of lifecycle assessment. our SPF contractor claims that their system is devoid of all HFAs, HCFCs, HFCs, and formaldehyde. i am not sure how typical this is. regardless, you have inspired my to find out exactly what the blowing agent is......

    regarding the application at the chimney being a potential code violation:
    in reading the code, i would agree that it appears this may technically represent a violation. if it is, the inspectors in our area choose not to enforce it. so while the academic in me is inclined to pursue the question, the pragmatist insists that i invoke the principle of the sleeping dog.

  5. jinmtvt | | #5

    Didn't even know this type of roofing was possible !!
    Nice write up !

    But i do not understand how this kind of roofing finish is desirable.

    Is it cheap ?? How cheap compared to screwed insulation boards + white EPDM adhered ??

    As mr Stephen pointed out, what about branches falling on the roof or another hard object driven up by wind ??? i would be very scared of the acrylic layer getting cracked ...

    Then, leaving the foam to seal the edges .. is basic flashing too complex to achieve in this situation??

    durability of this system would be my greatest fear ...
    And i don't really see any advantages against EPDM or similar safer systems, unless cost is much lower ...still... roof protects everything that lies underneath and deserves the most $$$ attention.

  6. kevin_in_denver | | #6

    I'm a Fan
    I'll add my 5 cents about polyurethane roofing (PUR). It's far from perfect, but it's still my favorite system for low-slope roofs. This is true even though I have owned a couple of problematic polyurethane roofs in Denver for almost 30 years.

    The bad news first:

    1. Hail.

    Denver is probably the worst place in the country to have a PUR roof because our large hail will crack the acrylic coating. That won't cause the roof to leak. The thick foam doesn't crack. But after a few years of our freeze-thaw cycles, that crack will telegraph down to the the roof deck. Now you have a real mess if you don't catch it in time. Maybe gravel ballast would protect the roof from hail.

    Many of the best commercial roofers in this region advise against PUR because of the messes that hail can cause. PUR is more common in AZ, where I guess they don't have much hail, and benefit much more from the heat rejection qualities.

    But since the roof cost only half as much as the other alternatives, you just need to inspect it for damage every September. That's after the hail season, and before the damage can start. An annual inspection should be performed on ALL types of low-slope roofing.

    2. Aesthetics - It goes on kind of bumpy, and therefore can be ugly. A 4-or-6-inch-thick roof sprayed on that Eichler house could look bad at the edges unless preventative steps are taken, like a metal flashing around the perimeter. See the photo in this good article:
    The bumps can also impair drainage and cause ponds. Ponds can reduce the life expectancy of the topcoat.

    3. I have found it difficult to find a good contractor in hail country. Nothing else matters if you can't hire someone to do it.

    Now for the good news-

    1. If well maintained, it NEVER needs to be torn off and replaced. Every 5 to 10 years, a new acrylic topcoat is applied. That gives you a brand new roof for maybe 5% of the original cost. So a newly coated 50 year old roof is actually better than a brand new roof, because the elastomeric top coat is five times thicker. There is no other roofing material that has an indefinite lifespan.

    2. It's the perfect insulation scenario - it automatically eliminates any and all thermal bridging issues and stops summer heat flow at the best possible spot - before it penetrates into the roof assembly or attic.

  7. jinmtvt | | #7

    Kevin: how does it "stop heat
    Kevin: how does it "stop heat flow " differently than other systems?

    What is the usual "PSI" or "PSF" of this kind of roofind system ?

    In the article you linked, it says that the price starts at around 3$/sq ft.. is that per inch or ?
    missing an important value there ...thickness

  8. Expert Member
    Dana Dorsett | | #8

    Non HFC foam? No kidding! (response to Stephen Shoup )
    " our SPF contractor claims that their system is devoid of all HFAs, HCFCs, HFCs, and formaldehyde. i am not sure how typical this is. "

    It's very ATYPICAL- the vast majority of 2 & 3lb polyurethane foam is blown with HFCs, usually HFC245fa, which was considered a really huge improvement over earlier blowing agents for meeting the Montreal Protocol limits on ozone destruction potential. I only know of two manufacturers in North America who can make an HFC-free claim, and neither has the density appropriate for roofs. Only one of them comes in at a better than R6/inch: Aloha Energy, a small regional player in upstate NY whose densest standard product is 1.8lbs, which is probably NOT going to cut it in a roof application, (Roofs usually call for 3lb density goods for the higher compressive strength and overall durability.)

    The other vendor is Icynene, with a water-blown 2lb. product that runs ~ R5/inch, and would also be marginal for a roofing app. If there are others I'd LOVE to know about them, since the HFC/greenhouse issue is the overriding caveat in the "It's really a great material BUT..." , especially for high-R applications.

    No flashing at chimneys is one thing, but how do you meet code on flue-liner-to-combustible-foam distances through masonry? (The ignition temp of foam is lower than that of wood, and it's a far better insulator, which results in potentially higher temps where it counts on masonry chimneys.)

    Jin: $3 per square foot for a ballasted or acrylic coated 3-lb foam solution would probably be about an inch, or R7 (which IS the R value mentioned in the article). The article did state $3/foot as a minimum number:

    "As stated above, the minimum cost for polyurethane sprayed roof insulation is currently running at around $3.00 per square foot, which comes out to $300 per square. Keep in mind though that this is the “minimum price”, so you’ll be lucky to come away with it."

  9. jinmtvt | | #9

    Thanks Dana for the
    Thanks Dana for the confirmation,

    3$ sq ft + acrylic coating ?

    How is this kind of roofing system competitive with other methods ?
    Does it lessen the involved labor cost because it "masks" everything ?

  10. kevin_in_denver | | #10

    Usually about half the cost

    PUR gets the insulation in the best place, which is outside of everything else. That ensures that it eliminates most of the common thermal bridges in a roof. (Note that a ventilated attic does not benefit as much).
    $3 includes the coating.

    YMMV, of course it depends on the roof. PUR saves a lot of labor and material on flashing the roof penetrations.
    Oftentimes the quote is about half as much as, say, EPDM or metal. And you get R7 for free.

    For someone who plans on owning the building for a long time, remember that well-maintained PUR is the only system that gets better with age, and may never need replacing.

    Do your homework for your roof, climate and location. Most experienced roofers won't recommend it. Don't ask them, try to find a few independent roofing engineering consultants.

  11. Richard Beyer | | #11

    A. Spray-foam roofing is usually referred to as a "self-flashing
    Martin wrote: A. Spray-foam roofing is usually referred to as a "self-flashing" form of roofing.

    This may be true in some cases, but it is a code violation to spray foam in full contact with a chimney used for combustable materials. ie: fireplace and oil fired furnace.

  12. conklinroofer1 | | #12

    Spay Foam Comments
    This is Eddie, I am a spray foam roof contractor. I have been spraying foam for 8 years and will be fully accredited with SPFA in a couple of months. With that, I learn something new everytime I start spraying. All the comments and questions are great. Spray foam is usually considered self-flashing and for roofs you need at least a 3 lb. foam. Foam itself is not UV stable and needs a good coating to cover it. With a little maintanance, as stated in the other comments, the foam roof should never have to come off the buiding. As there is no degregation to the foam when it is coated properly and maintained. Sizeable hail on anything can cause damage. We have done some hail damage repair on some of our foam roofs. With minimal damage, we cleaned, repaired cracks as necessary, with no degredation to the foam and no leaks inside the building. We were able to put the roof back under warranty. I have only seen major cracks and splits on one older foam roof that I didn't spray. It was caused by building movement. The walls and foundations were also cracked. I am a contractor in Middle Tennessee. I service Nashville, Knoxville, Chattanooga, Murfreesboro and all points in between. To learn more check out our website at

  13. lutro | | #13

    What's the frequency of leaks?
    "Four inches of closed-cell polyurethane is a Class II water retarder, a semi-impermeable membrane that makes leaks all but impossible."

    I don't have a lot of experience with foam roofs, but of the ten or so that my friends, family, and two businesses that I am connected with had installed on their buildings, several have had bothersome leaks. Those leaks were hard to find and hard to repair successfully. I don't take my anecdotal experience as definitive, but it does lead me to question "all but impossible" part of the statement quoted above. Are there any national figures for the percentage of foam roofs that have problems with leaks?

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