SPECIAL REPORT 95-19

Roof Blisters

Cause and Cure

Charles Korhonen and Brian Charest July 1995
Abstract

Blisters are a major problem of built-up roof membranes. They are caused by

voids built into the roof during construction. They develop into the characteristic dome-shaped humps by a breathing action driven by thermal cycling. A

small pressure relief vent was patented by CRREL as a cost-effective way to

repair blisters. Though these vents cannot prevent blisters from forming, they

can lengthen a roof’s service life by repairing the blisters before they break.

Two demonstration projects were conducted to transfer the blister vent technology to the military community. Most participants in the demonstration

projects found the vent easy to use and that it performed as designed. The

main objection to the vent was its price.

For conversion of SI units to non-SI units of measurement consult Standard

Practice for Use of the International System of Units (SI), ASTM Standard E380-

93, published by the American Society for Testing and Materials, 1916 Race St.,

Philadelphia, Pa. 19103.
US Army Corps

of Engineers®

Cold Regions Research &

Engineering Laboratory

Special Report 95-19

Roof Blisters

Cause and Cure

Charles Korhonen and Brian Charest July 1995

Prepared for

OFFICE OF THE CHIEF OF ENGINEERS

Approved for public release; distribution is unlimited.
ii

PREFACE

This report was prepared by Charles Korhonen and Brian Charest, Research

Civil Engineers, Civil and Geotechnical Engineering Research Division, Research

and Engineering Directorate, U.S. Army Cold Regions Research and Engineering

Laboratory (CRREL), Hanover, New Hampshire.

Funding for this work was provided by the Office of the Chief of Engineers

under FY92 Facilities Engineering Applications Program, Roof Blister Repair.

This report was technically reviewed by Alan Greatorex of CRREL. The authors

wish to thank all the personnel from the various DoD test sites that were involved

with this study and who provided the excellent feedback that was received.

The contents of this report are not to be used for advertising or promotional

purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.
INTRODUCTION

Since 1974, when the National Roofing Contractors Association began surveying its members

to identify recurring roofing problems, blistering

has been the most commonly reported problem

on bituminous built-up membrane roofs. In 1993,

bituminous built-up roof systems accounted for

nearly 31% of the $12.4 billion commercial roofing market. Of this market $9.6 billion, or 77.5%,

was attributable to roof failures (LaValley 1994).

Since blistering is a large problem, accounting for

24% of all deficiencies reported for bituminous

roofs (Cullen 1993), it is safe to assume that blistering is a large cause of roof failures and associated reroofing expenses each year.

Blisters are spongy, dome-shaped areas caused

by gases expanding beneath or within roof membrane plies. Their effect is to shorten membrane

life by dramatically increasing its vulnerability to

physical damage and to weathering. The sloped

sides of a blister will cause aggregate surfacing to

roll down hill, exposing the remaining flood coat

and felt to increased embrittlement by ultraviolet

rays. Dropping tools on blisters or stepping on

them can easily burst them, allowing direct access

for water entry into the roof system.

In 1988, after measuring pressures within some

blisters and physically examining many others,

the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) developed and patented a miniature pressure relief vent designed to

be inserted into the top of blisters to deflate them

and prevent them from returning. This vent was

seen by many as the perfect way to extend the life

of a blistered roof and to minimize any risks associated with blisters. In 1989, Cor-A-Vent, Inc.

(16250 Petro Drive, Mishawaka, Indiana 46544)

obtained the rights to manufacture the vent and

in 1990 they introduced the blister vent to the

roofing market.

Acceptance and use of the vent by building

owners and roofers has been slow. In a potentially huge U.S. market containing thousands of

built-up roofs, most of which are expected to

have at least some blistering, vent sales have not

been impressive, amounting to only several

thousand per year. (Cor-A-Vent reportedly gives

more samples away as part of their advertising

than they sell.) Part of the problem of poor sales,

at least initially, can be attributed to the newness

of the vent—few people were aware of it, and

there can be a natural resistance to try anything

new. A high selling price also presents a barrier

to widespread use. In addition, the notion that a

reactive maintenance approach (repair broken

blisters) as opposed to a proactive one (repair

them before they break) was the appropriate

solution to blisters further dampened potential

sales.

In an attempt to transfer this new technology

to the military community, CRREL, as part of the

Corps of Engineers Facilities Engineering Application Program (FEAP), conducted two demonstration projects, one that compared contractor to

in-house vent installation in 1990 and another

that supplied vents directly to various DoD installations for their evaluation and use in 1992.

Valuable information was developed from both

demonstrations.

This paper traces the development of the blister vent by reviewing the cause of roof membrane blisters, details the findings from the two

demonstration projects, and recommends how to

overcome some of the barriers to its use.

Roof Blisters

Cause and Cure

CHARLES KORHONEN AND BRIAN CHAREST
likely leaks through microscopic cracks in the bitumen moppings and along the felts themselves

since felts can be microporous. This leads to the

conclusion that blisters inhale and exhale daily.

It is this cyclic breathing that explains why

blisters continually grow larger. However,

growth is possible only when the volume of air

sucked into the blister at night exceeds the volume of air forced out of the blister the following

day; the internal positive pressure that is created

is great enough to break the peripheral bond at

the edge of the blister.

Contrary to popular belief, blisters do not

grow day-by-day during the hottest part of the

year. That is because blister growth is very much

dependent upon relatively cool night air as well

as on high daytime temperatures. During the

cooling phase, the raised membrane must stiffen

so as not to subside when a vacuum forms. If the

membrane stays warm and pliable and is unable

to maintain its shape, it deforms, reducing the

vacuum and thus the volume of air drawn into

the blister. With less inhaled air, the blister is less

likely to expand the following day, no matter

how hot the day may get (Korhonen 1989). The

optimum growing conditions occur when the

days are hot and the nights are cool, which corresponds to spring and fall.

WHAT TO DO

Very few options are available for dealing

with existing blisters. The Roofing Industry Educational Institute’s “Roof Maintenance” manual

(RIEI 1980) provides guidance for repairing blisters. The number one repair procedure is to do

2

THE CAUSE

Blisters develop from voids built into the roof

either between the plies or between the bottom of

the membrane and an impermeable substrate

(Korhonen and Bayer 1986, Korhonen 1989).

Voids can result from skips in the bitumen mopping, entrapped debris, bitumen bubbling, or

curled felt edges. Tight quality control on the job

can minimize many of these problems, but

regardless of how carefully work is done, a perfect, void-free membrane is difficult to fabricate

even under the best of conditions. Thus, some

blistering is likely in any bituminous built-up

roof system.

Blisters grow from the expansion of the gas (air

or water vapor) within a void under the intense

heat of the sun. The mere expansion of this entrapped gas does not account for large blisters,

however. What makes small voids grow into the

characteristic bloated humps several inches high

to a few feet across is a breathing action driven by

thermal cycling. Pressure changes within membrane voids that cause blister growth are documented elsewhere (Korhonen 1989), but they are

summarized here to explain why blisters grow in

size.

Blisters cycle from positive to negative pressures on a daily basis. As shown in Figure 1, a

blister typically goes from over-pressure during

the heat of the day to under-pressure during the

cool of the night. What’s more, the pressures that

do develop are considerably below what might

be calculated from theoretical considerations,

suggesting that a blister is not air-tight. Indeed,

instead of fully containing pressures, air most

Figure 1. The top graph shows the air and blister surface temperature

and the bottom graph shows the pressure difference between the inside

of a blister and the atmosphere. Note how small the pressures are.
3

poking a hole in a blister might work, but the hole

must be left open until the blister collapses unto

the roof, which may take several days or weeks,

and, therefore, is not a recommended approach.

The blister vent serves both to relieve pressures

and prevent leaks. As shown in Figure 3, the vent

consists of a hollow, threaded shaft covered by a

special membrane enclosed in a plastic housing. It

allows gases to escape but inhibits water from

leaking into the roof.

Blister vents are easy to install. The installer

merely cleans the area of gravel and dust, adds a

primer to the surface, pokes a hole into the blister,

applies sealant to the underside of the vent, and

threads the vent into the hole. Once installed, the

vent causes the blister to deflate and the raised

portion of the blister eventually to collapse onto

the roof. In this position the roof membrane is less

susceptible to damage.

The question has arisen as to whether a deflated

and collapsed blister might reseal itself to the roof.

The answer is no, at least not of its own volition.

Several blisters were cut open the year after they

had been repaired with the vent and, though

found to be watertight and flat, they were not well

bonded to the roof. The original, once-raised portion could be easily cut away from the roof. In conjunction with this test, several other collapsed blisters were coated with a resaturant to determine

if it would penetrate the blister wall and reseal it to

the roof. A year later the blisters were sliced with

a knife but could not be delaminated from the

underlying roof membrane without causing damage. What long-term effect adding such a coating

has on roof maintenance is unclear at this point.

More study should be done with this interesting

finding.

nothing. If a blister is intact and unbroken, the

manual suggests leaving it alone, because blisters

that hold pressure do not leak. In addition, blisters should not be intentionally punctured but

rather should be monitored over time so they can

be repaired if individual blisters continue to get

worse (and they will). Eroded bare spots on

blisters should be coated with a cold-process

recoating/resaturant and sprinkled with gravel if

needed.

When a blister bursts, the usual repair process

is to remove the entire raised portion of the blister

and patch the remaining void with alternate layers of bitumen and successively larger diameters

of felt. Alternately, an x can be cut into the blister,

the corners of the cut peeled back, the resulting

cavity filled with bitumen and the corners

pressed back into the bitumen. The blister would

then be patched as before.

Roof breather vents have at times been promoted as a means of relieving internal roof pressures

that lead to blisters. Unfortunately, as Figure 2

shows, such vents only communicate with the insulated space of a roof and, as discussed earlier,

blisters only occur at or above the top surface of

the insulation. Furthermore, pressures large

enough to lift up the roof membrane do not occur

within the insulated space of a roof (Korhonen

1989). Consequently, roof breather vents do not

prevent membrane blistering.

The only way to stop an existing blister from

growing larger and becoming a maintenance

headache is to depressurize it. The cut-and-patch

approach achieves this goal but ends up being

costly in time and money. An average of one hour

is required to repair a single blister, which for

material and labor can easily exceed $30. Simply

Figure 2. Cutaway view of roof breather

vent. Note its size and that it vents only the

insulated space of a roof.

Figure 3. Cutaway view of the blister vent.

Note its small size.
4

TECHNOLOGY TRANSFER

CRREL conducted two demonstration projects

under authority of the Corps of Engineers Facilities Engineering Application Program, one in 1990

and the other in 1992, to increase awareness of the

new blister vent technology, to show its benefits,

and to learn of barriers to its use.

The 1990 project, at Fort Devens, Massachusetts, was conducted to demonstrate how easily

vents could be installed by an outside contractor

and by in-house maintenance workers. In both

cases, no prior training or on-site assistance was

provided to installers. An 8-minute CRREL videotape was all that was used to self-train the contractor and in-house maintenance staff, and both

found the vent easy to use, averaging about two

minutes per vent installation. Each blister vent

was installed for $12 by the contractor and $9.55

by the in-house staff workers, which included $8

for the vent in both situations. Had the blisters

been repaired by the usual cut-and-patch method,

the cost per blister would have jumped to an estimated $35. The savings add up rapidly as blister

numbers increase. The alternative of doing nothing until problems arise can be even more costly,

as reroofing a failed, leaking roof can be prohibitively expensive.

Word of the Fort Devens success was disseminated through technical reports, conference attendance, and magazine articles and via avenues previously established by FEAP. Despite these efforts,

it became clear that few people within the Army

had been reached with the technology. Thus, a

second demonstration, different from the first,

was planned.

In 1992, a program was devised to reach more

building owners in hope that others would learn

from the individual experiences thus gained. The

program made vents available free of charge to

any interested DoD installation. In exchange, the

installation was to provide an evaluation of the

vent by answering a short CRREL questionnaire

(Appendix A). The vent manufacturer participated in the program by matching any vent purchase

with an equal number of free vents. In all, 5000

vents were made available to the program: 2500

purchased by CRREL and 2500 donated by the

manufacturer.

Eighteen installations volunteered to test and

evaluate the vent (Appendix B). Of these, seven

installations returned the CRREL questionnaire,

providing an assessment of the vent. Though

evaluation response was low, the information

gained by conducting the demonstration added to

that provided by the seven installations answering the questionnaire has proven useful in assessing the potential market in the Army and determining the strengths and weaknesses of the vent.

On the question of the market, we had originally

hoped for five installations at which to test the

vent. We anticipated having to contact many locations before finding five that would participate in

the program. To our amazement, nearly all installations contacted were interested. As word of the

program circulated, unsolicited requests began to

come in. Therefore, instead of distributing a lot of

vents to a few installations, we decided to send

fewer vents to many installations. The need for

and the interest in the blister vent appeared

strong.

The results from this demonstration were quite

encouraging. At the seven installations answering

the questionnaire, 13 roofs were treated with anywhere from 3 to 300 vents, depending on the

number of blisters present. Approximately 600

vents were used and installed. Respondents described their experiences with the vent as one of

pleasant surprise. On average, each vent was installed in two minutes, which included preparing

the roof surface and installing the vent. The vents

were installed by in-house maintenance staff in all

but one case, where the roof was maintained by

permanent contract personnel. The only training

anyone received was to watch the CRREL videotape. All installers agreed that the installation process was easy and that blister breakage was not a

problem. The only time blisters were reported to

have broken was when the roof membrane was

dry and brittle.

To date all vents appear to be performing flawlessly. The consensus is that the vents are most effective when the roof has not deteriorated to the

point where it has become brittle. The vents

should be applied when blisters are small. Other

comments suggested that a simple installation

tool could be devised to speed up installation

even more, even though all respondents were

pleased with the already simple installation process.

When the respondents were asked if additional

vents might be purchased or if the vent would be

recommended to others, the answer came back

very clearly that the selling price of the vent was

an issue. Technically, the concept and performance of the vent was not questioned. However,

though blisters may be a costly maintenance item,

even when nothing is done to repair them, the

usual response indicated little desire to purchase
additional vents unless they sold for significantly less than their current $8 selling price. (Incidentally, feedback from potential vent users in

the private sector indicated the same feeling

about price.) A roof with 100 blisters would require $800 in vents. Projected to an installation

level, with numerous roofs involved, the large

number of vents required represents a significant

investment. Thus, the largest barrier to more use

of the vent seems not to be with the technology

but with its current price.

CONCLUSION

Blisters are a leading cause of problems on bituminous built-up roof membranes (LaValley

1994). They weaken the roof membrane, accelerate its aging, and increase its vulnerability to

puncture. They shorten roof life.

Blister vents are the only proactive alternative

to cutting out and patching blisters. Once installed, the vents stop blister growth by deflating

the blister and causing its raised portion to collapse onto the roof where it is less likely to be

damaged. Blister vents can postpone the cost of

reroofing when used early. Although the vents

cannot prevent blisters from happening, they can

lengthen the service life of a roof.

Despite the positive impact that wide use of

the vent could have on built-up roof maintenance costs, the vent is underused, primarily because it is viewed as being too expensive, but

also because of a lack of awareness of the vent in

the roofing community and a reactive attitude

towards repair.

5

RECOMMENDATION

More effective use should be made of the roof

blister vent. In the DoD community, and probably

elsewhere, too few are aware of the vent, and those

that are choose not to use it, primarily due to cost,

even though economics suggests that blister vents

are cost-effective. To increase use, an extensive

education project informing potential users of the

true cost of blisters must be undertaken, or the

price of the vent must be reduced, or both. Since

most people who are aware of the vent agree that it

is technically sound, reducing its selling price significantly seems to be the logical first step toward

overcoming current resistance to use of the blister

vent. However, selling price alone is not expected

to be enough, so concurrent efforts to advertise

and demonstrate the vent are also needed.

LITERATURE CITED

LaValley, A.L. (1994) NRCA’s market survey shows

industry rebound is on the horizon. Professional

Roofing, April.

Cullen, W.C. (1993) Project Pinpoint Analysis: TenYear Performance Experience of Commercial Roofing

1983–1992. National Roofing Contractors Association, 10255 W. Higgins Road, Suite 600, Rosemont, Illinois 60018-5607.

Korhonen, C. (1986) Blistering of built-up roof

membranes: Pressure measurements. USA Cold

Regions Research and Engineering Laboratory,

Special Report 86-29.

Korhonen, C. and J. Bayer (1986) ROOF blister:

Physical Fitness Building, Fort Lee, Virginia. USA

Cold Regions Research and Engineering Laboratory, Special Report 86-35.

RIEI (1980) Roof Maintenance Manual. Roofing Industry Educational Institute, 6851 S. Holly Circle,

Suite 250, Englewood, Colorado 80112.
7

APPENDIX A: QUESTIONNAIRE

The following questions were in the questionnaire that was sent to all Army

installations receiving the free blister vents for evaluation and use.

1. How many roofs were the vents installed on?

2. How many vents on each roof?

3. Who installed the vents? How many people?

4. What were the impressions of the installers regarding ease of installation?

5. How long did installation take?

6. How many (or what percent) didn’t seemed to vent properly?

7. How many blisters broke?

8. What do you think the vents are worth? What do you think a reasonable

selling price would be? Less than $5? $5–10? $10–15? More than $15?

9. Any comments, tips, or ideas to improve the vent or the installation method?

10. What is your overall impression of the vents and would you recommend

them?
9

APPENDIX B: RESPONDENTS

The facilities that received vents, their point of contact, and related information are provided

in Table B1.

Table B1. Roof blister vent disposition.

Facility POC Date sent No. Location

*1. HQ 83D Arcom Barney Kempter 9 Sept 92 500 Ohio

2. Ft. Bliss George Lambert 15 June 92 100 Texas

3. Ft. Campbell Mr. Donald Terrell 8 Aug 93 100 Kentucky

4. Ft. Carson Robert Tucker 5 May 92 500 Colorado

5. DCSC-WIC Robert White 10 Sept 92 1000 Ohio

*6. Ft. Devens Joseph Tammaro 5 May 92 200 Massachusetts

*7. Ft. Gordon Pat Arthur 16 June 92 100 Georgia

8. Ft. Greely James Mellott 26 Aug 93 100 Alaska

9. District Japan Mr. Taira 6 Oct 92 100 Japan

10. Yokohama, Japan John M. Senna 17 May 93 100† Japan

10A. Yokohama, Japan John M. Senna 23 June 93 100 Japan

11. Korea Virgil Beers 17 May 92 100 Korea

*12. Ft. Leonard Wood Stanley Martin 16 June 92 100 Missouri

*13. Loring AFB James Page 2 Nov 92 100 Maine

*14. Ft. Snelling Lonnie Voter 27 Apr 92 200 Wisconsin

*14A. Brainerd AFB Lonnie Voter 7 Dec 92 500 Wisconsin

15. Ft. Mead Donald Jones 11 Aug 93 100 Maryland

16. Ft. Richardson Jeffrey Garnes 28 Aug 93 500 Alaska

17. Ft. Rucker Ron Leatherwood 15 June 92 100 Alabama

18. Ft. Wainwright Charles Ruerup 26 Aug 92 100 Alaska

Remaining CRREL <300

Total: 5000

* Facilities that responded to the questionnaire.

† Vents were never received.

Barney Kemter For the Commander

HQ 83D ARCOM Directorate of Install. Support

P.O. Box 16515 Attn: ATZC-ISE-J, Bldg 777

Bldg 306 DCSC (George Lambert)

Columbus, OH 43216-5004 Fort Bliss, TX 79916-5058

(614) 692-4117 (915) 568-6110

Robert Tucker Robert White

HQ FT Carson DCSC-WIC

Building 304 Facilities Engineering Division

Fort Carson, CO 80913-5023 Building 308

(719) 579-2856 P.O. Box 3990

Columbus, OH 43216-5000

Facilities Division (614) 692-2774

Directorate of Install. Support

Attn: Pat Arthur James Mellott

Fort Gordon, GA 30905-5060 Directorate of Public Works

(706) 791-3483 Fort Greely, AK 96508-5500

(907) 873-3259

The complete mailing addresses and telephone numbers for the people and installations

involved in this study are listed below.
10

Mr. Taira Vergil A. Beers

US Army COE District Japan PSC 450 Box 548

Attn: Unit 45010 APO AP APO AP 96206

Japan 96343 Korea

1-81-(311) 733-8851

Lonnie Voter

DEH Fort McCoy

Building 2200A Sparta, WI 54656-5000

Attn: Stanley Martin (608) 388-3601

Ft Leonard Wood, MO 65473-5000

(314) 596-1817 U.S. Army Aviation Center

ATTN: Ronald Leatherwood

Jeff Garnes Fort Rucker, AL 36362

Directorate of Public Works (205) 255-2988

Fort Richardson, AK 99503

(907) 384-0104 SGT James Page

42nd CES/CEES

Donald Terrell Loring AFB, ME 04751

U.S. Army Garrison (207) 999-2171

DEH Utilities Division

Projects Branch, Bldg 867 Directorate of Engineering

16th & Ohio Streets & Housing

Fort Campbell, KY 42223-1291 Attn: Donald Jones

(502) 798-3685 AFKA-ZI-EH-P

Fort Mead, MD 20755-5115

Joseph A. Tammaro (410) 677-6141)

Deputy Director

Fort Devens, MA 01433-5100 Charles Ruerup

(508) 796-2224 6th Infantry Division Light

Directorate of Public Works

John M. Senna Attn: AFVR-FW-PW-O

Chief, Yokohama Sub-Facility Fort Wainwright, AK 99703-5500

 Engineer (907) 353-9148

Department of the Army

1316th Medium Port Command

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ASTM Standard E380-93, published by the American Society for Testing and Materials, 1916 Race St., Philadelphia, Pa. 19103.

July 1995

Roof Blisters: Cause and Cure

Charles Korhonen and Brian Charest

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road Special Report 95-19

Hanover, New Hampshire 03755-1290

Office of the Chief of Engineers

Washington, D.C. 20314-1000

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

Blisters are a major problem of built-up roof membranes. They are caused by voids built into the roof during

construction. They develop into the characteristic dome-shaped humps by a breathing action driven by thermal cycling. A small pressure relief vent was patented by CRREL as a cost-effective way to repair blisters.

Though these vents cannot prevent blisters from forming, they can lengthen a roof’s service life by repairing

the blisters before they break. Two demonstration projects were conducted to transfer the blister vent technology to the military community. Most participants in the demonstration projects found the vent easy to

use and that it performed as designed. The main objection to the vent was its price.

Blister Maintenance

Built-up roof Repair

13

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL