Cracking of Foam Roofing | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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Stephen Ahart Posted: Jul 28, 2009 03:32 PM	Cracking of Foam Roofing I visited a building with SPF over densdeck over tapered iso built over a metal deck roof on bar joists. The sprayed roof has cracked along many of the joints of the densglas (with the widest part of the crack at the bottom of the roofing - first pass). The building is just a couple of years old and is 100'x150' and is in otherwise good condition. What would cause this cracking? (foam too thin, inexperienced applicator, wet substrate, problems with densdeck?) Any thoughts would be helpful.
mason Posted: Jul 28, 2009 04:30 PM	Could be the metal deck moving and attachment issues with the iso board and densdek. But, there could be issues with the foam also. You mentioned one, thin foam. In addition the foam might have lower or higher compressive strength that does not accomodate the movement. (should be between 40 to 60 psi) I would recommend obtaining samples of the foam in the areas that are cracking, checking compressive strength and density. Then, check the fastening pattern of the densdek and the iso board. Also check the fastening pattern of the metal deck itself. I have seen some buildings that just were not structurally secure against building movement. The fix may be as easy as adding additonal fasteners or securing the deck with additional welds or fasteners. Question, was there an expansion joint that the roofers covered up?
Stephen Ahart Posted: Jul 28, 2009 10:50 PM	Thanks for the input. There were no exp jts covered up, and the metal deck welds to the joists and sidelaps screws were found to be intact. With this small building chock full with interior masonry bearing walls in each direction, it is not expected that there would be much deck movement at all. I have read of possible shrinkage issues of polyiso in the past which might cause issues with certain roofing materials. Has anyone run into this? Perhaps we will do the foam tests and look at the underlying materials as well. Are there published values for "coefficent of thermal expansion", "modulus of elasticity", or "allowable strain" for spray foam roofing products? I would have thought that they would be fairly flexing and forgiving of minor movement - and would not have such issues on fairly standard roof construction. I have read a little about "stress relief joints" which seem to be joints in the roofing that do not necessarily extend through the underlying structure. Are these a good idea, are they effective and would these be required a certain spacings? Thanks again.
mason Posted: Jul 29, 2009 07:37 AM	On a metal roof deck the size you suggest, I would not expect have to use "stress relief joints". I would still look at the fastening patterns of the densdek and polysio boards. I am not aware of a recent history of shrinkage with polyiso boards, (although there was a facer delamination issue in the early 90s, during the CFC to HCFC blowing agent transition). Polysio boards can "potato chip" ie curl up at the edges if left out in the weather and allowed to expand and contract. This could allow some additional movement and stresses that might affect the foam. When investigating the fasteners, check to see if they are bent or at a angle. How thick is the board insulation, the thicker the insulation the more likely movement will occur.
Posted: Aug 08, 2009 09:41 AM	There was an issue with shrinkage of foam from a supplier that was never made of issue a couple of years back. Whos foam was sprayed and what kind of coating was used?
Michael Fusco Posted: Aug 23, 2009 01:00 PM	When thermal expansion occurs, it occurs. You can put in enough fasteners, and hold the deck tight enough so they (the fasteners) are not damaged, but that does not mean the thermal expansion does not occur...it sinply takes the path of least resistance. I have seen such situations where the deck buckles, becomes wavy, actually bends, if you will. The fact that you are seeing the cracks along the seams of the board would indicate that you might be seeing expansion over a differing plane. The dec expands in a given direction, causing the board to flex....pop at the seams. I would look very carefully.....modulus of expansion for foam is usually pretty high. The lower the density, the higher the expansion. Since the advent of 245, this problem pops up more often. The reason is that 2.8 and 3# foam, shot in the summertime, can actually have substantially higher density due to blowing agent loss (remember thise football shaped drums?). thuis less expansion ability. If the drums are warmer than they should be, the density WILL be higher. First, easy check....take a sample of the foam and check the density.
Andrew Mickley Posted: Mar 31, 2019 03:01 PM	I have a similar issue. It is an SPF roof, only about a year and a half old, and it has linear cracks in the surface. The cracks appear near the central roof drains and along insulation boards. Curious if density of the material is the problem here as well. I'm in the process of trying to get the material tested. Thanks
zaylen cairn Posted: Jun 03, 2026 03:14 AM	Based on the details you've shared, polyiso shrinkage is definitely worth investigating, especially since the cracking appears to be following the board joints rather than showing a random pattern. Even when the deck and attachments are sound, differential movement between the SPF system and the substrate layers can telegraph through the foam and create cracks directly above panel seams. Another possibility is that the first pass of foam was applied too thin over the board joints, creating localized stress concentrations as the roof cycles through temperature changes. SPF is generally forgiving, but it still has limits when repeated thermal movement occurs across long panel runs. That's why checking foam density, compressive strength, and actual thickness at the crack locations could provide useful clues. Roofing contractors who deal with both repair diagnostics and restoration work, such as Covenant Roofing & Restoration (https://mycovenantroofer.com) in Tennessee, often emphasize looking at the entire roof assembly rather than focusing solely on the foam itself. When cracks consistently mirror substrate joints, the root cause is frequently related to movement, dimensional changes in the underlying materials, or installation details at those transitions. Regarding stress-relief joints, they can be effective on larger roof areas where movement is anticipated, but whether they are necessary depends heavily on the roof design, substrate configuration, and expected thermal expansion. In this case, mapping the crack pattern against the DensDeck layout may be one of the most revealing steps. If the cracks align almost perfectly with board joints throughout the field, that would point more toward substrate-related movement than a foam formulation issue.

Author

Comments

Stephen Ahart

Posted: Jul 28, 2009 03:32 PM

Cracking of Foam Roofing

I visited a building with SPF over densdeck over tapered iso built over a metal deck roof on bar joists. The sprayed roof has cracked along many of the joints of the densglas (with the widest part of the crack at the bottom of the roofing - first pass).

The building is just a couple of years old and is 100'x150' and is in otherwise good condition.

What would cause this cracking?
(foam too thin, inexperienced applicator, wet substrate, problems with densdeck?)

Any thoughts would be helpful.

mason

Posted: Jul 28, 2009 04:30 PM

Could be the metal deck moving and attachment issues with the iso board and densdek. But, there could be issues with the foam also. You mentioned one, thin foam. In addition the foam might have lower or higher compressive strength that does not accomodate the movement. (should be between 40 to 60 psi)

I would recommend obtaining samples of the foam in the areas that are cracking, checking compressive strength and density. Then, check the fastening pattern of the densdek and the iso board. Also check the fastening pattern of the metal deck itself. I have seen some buildings that just were not structurally secure against building movement. The fix may be as easy as adding additonal fasteners or securing the deck with additional welds or fasteners.

Question, was there an expansion joint that the roofers covered up?

Stephen Ahart

Posted: Jul 28, 2009 10:50 PM

Thanks for the input.

There were no exp jts covered up, and the metal deck welds to the joists and sidelaps screws were found to be intact. With this small building chock full with interior masonry bearing walls in each direction, it is not expected that there would be much deck movement at all.

I have read of possible shrinkage issues of polyiso in the past which might cause issues with certain roofing materials. Has anyone run into this? Perhaps we will do the foam tests and look at the underlying materials as well.

Are there published values for "coefficent of thermal expansion", "modulus of elasticity", or "allowable strain" for spray foam roofing products? I would have thought that they would be fairly flexing and forgiving of minor movement - and would not have such issues on fairly standard roof construction.

I have read a little about "stress relief joints" which seem to be joints in the roofing that do not necessarily extend through the underlying structure. Are these a good idea, are they effective and would these be required a certain spacings?

Thanks again.

mason

Posted: Jul 29, 2009 07:37 AM

On a metal roof deck the size you suggest, I would not expect have to use "stress relief joints". I would still look at the fastening patterns of the densdek and polysio boards. I am not aware of a recent history of shrinkage with polyiso boards, (although there was a facer delamination issue in the early 90s, during the CFC to HCFC blowing agent transition). Polysio boards can "potato chip" ie curl up at the edges if left out in the weather and allowed to expand and contract. This could allow some additional movement and stresses that might affect the foam.

When investigating the fasteners, check to see if they are bent or at a angle. How thick is the board insulation, the thicker the insulation the more likely movement will occur.

Posted: Aug 08, 2009 09:41 AM

There was an issue with shrinkage of foam from a supplier that was never made of issue a couple of years back. Whos foam was sprayed and what kind of coating was used?

Michael Fusco

Posted: Aug 23, 2009 01:00 PM

When thermal expansion occurs, it occurs. You can put in enough fasteners, and hold the deck tight enough so they (the fasteners) are not damaged, but that does not mean the thermal expansion does not occur...it sinply takes the path of least resistance.

I have seen such situations where the deck buckles, becomes wavy, actually bends, if you will.

The fact that you are seeing the cracks along the seams of the board would indicate that you might be seeing expansion over a differing plane. The dec expands in a given direction, causing the board to flex....pop at the seams. I would look very carefully.....modulus of expansion for foam is usually pretty high. The lower the density, the higher the expansion.

Since the advent of 245, this problem pops up more often. The reason is that 2.8 and 3# foam, shot in the summertime, can actually have substantially higher density due to blowing agent loss (remember thise football shaped drums?). thuis less expansion ability. If the drums are warmer than they should be, the density WILL be higher.

First, easy check....take a sample of the foam and check the density.

Andrew Mickley

Posted: Mar 31, 2019 03:01 PM

I have a similar issue. It is an SPF roof, only about a year and a half old, and it has linear cracks in the surface. The cracks appear near the central roof drains and along insulation boards. Curious if density of the material is the problem here as well. I'm in the process of trying to get the material tested. Thanks

zaylen cairn

Posted: Jun 03, 2026 03:14 AM

Based on the details you've shared, polyiso shrinkage is definitely worth investigating, especially since the cracking appears to be following the board joints rather than showing a random pattern. Even when the deck and attachments are sound, differential movement between the SPF system and the substrate layers can telegraph through the foam and create cracks directly above panel seams.

Another possibility is that the first pass of foam was applied too thin over the board joints, creating localized stress concentrations as the roof cycles through temperature changes. SPF is generally forgiving, but it still has limits when repeated thermal movement occurs across long panel runs. That's why checking foam density, compressive strength, and actual thickness at the crack locations could provide useful clues.

Roofing contractors who deal with both repair diagnostics and restoration work, such as Covenant Roofing & Restoration (https://mycovenantroofer.com) in Tennessee, often emphasize looking at the entire roof assembly rather than focusing solely on the foam itself. When cracks consistently mirror substrate joints, the root cause is frequently related to movement, dimensional changes in the underlying materials, or installation details at those transitions.

Regarding stress-relief joints, they can be effective on larger roof areas where movement is anticipated, but whether they are necessary depends heavily on the roof design, substrate configuration, and expected thermal expansion. In this case, mapping the crack pattern against the DensDeck layout may be one of the most revealing steps. If the cracks align almost perfectly with board joints throughout the field, that would point more toward substrate-related movement than a foam formulation issue.

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