Hands-on

.e

fFollowing are criteria for evaluating blisters, as well ashands-on techniques for repairs

by Jim Carlson

cut out the voided or unbonded pliesand install a patch over the location.

However, some blisters do notneed to be cut out-some simplyshould be reinforced with the appro-priate type and number of reinforc-ing patching plies. Other blisters arebest left undisturbed, and. of course,certain blisters should be cut out andthe appropriate type and number ofpatching plies installed.

Inspect, evaluate firstBefore prescribing remedies, profes-sionals first should inspect and evalu-ate blisters for characteristics thatsignal the need for repair, such as:. Weak membrane in the blistered (void-

ed or unhanded) area. A blister thathas weak membrane walls (suchas one that consists of one or two

plies) or could rupture easilyfrom foot u-affic or heavy snow /icing conditions should be re--paired (see Photo 1).

Weak blisters usually are evidentwith a quick, visual inspection-the blister appears to be vulnera-ble and capable of rupture. To de--termine whether a blister is weak,a professional can apply thumband finger pressure to the blister,simultaneously pressing down onits top and side. The combinedpressure will help an experiencedperson assess whether the blisterwalls are weak and only made ofone or two plies.

When inspecting BUR mem-branes and prioritizing variousareas or locations for repairs,weak blisters should be marked.

By marking blisters with theI priority code numbers of "1"

(for cut out and repair); "2"(for patch with reinforcingplies); or "3" (monitor),appropriate maintenancehas been initiated, and theremedy or repair can bepeformed.. Fatigue cracks or stress lines

around the blisters circumfer-ence. A blister that has ex-isted for some time andhas undergone extensivethermal movement (i.e.,expansion and contrac-tion of the blistered areadue to the membrane

uring the past 15 years orso, advancements in mate-rial technology and appli-cation practices have

helped eliminate most interply blis-tering of built-up roof (BUR) mem-branes. Sufficiently porous ply sheetsand other reinforcement fabrics thatallow air and hot bitumen gases toescape during application have re-duced the potential for buildingin voids during BUR membraneapplication.

Type IV fiberglass ply sheets andpolyester fabrics specifically madefor BUR generally are good rein-forcement materials for hot- andcold-applied asphalt BUR mem-branes, depending on the asphalttype and particular project's designcriteria. In general, these materialspossess the tensile strength,pnrosity and other physicalcharacteristics that someof the previous blister-prone organic felts lacked.However, even with soundmaterial engineering, con-servative roof design andgood craftsmanship duringply sheet application, iso-lated blisters occasionallyoccur.

When blisters are discov-ered, some believe that allof them should be repairedpromptly. In addition,many believe that the onlyrepair procedure is to

Photo 1: Cutting a blistered, organic-reinforced BUR mem-brane with a straight-blade utility knife in x-shaped fashion.

20 Professional Roofing March 1997

membrane embrittlementor cracking. The precedingcharacteristics signal theneed for repair.

heating and cooling)will have stress lines orsmall fatigue cracksaround its base. As theblister thermally cycles,the membrane fatigues

I Repairsat the blister's circum- If the inspection suggestsference. The stress lines that the blister(s) should beeventually lead to small cut out and patched, the fol-fractures or fatigue lowing step-by-step instruc-cracks. The fractures tions are recommended:and cracks weaken the 1. Prepare the sunvund-membrane's integrity ing surface. The area aroundand suggest that the blis- the blister should be pre-ter should be repaired. I pared to ens.ure a~hesion

If thumb and finger Photo ~ The author uses a hook-blade utility knife (held of the patchIng; plies topr:essure detects that.the almost parallel to the roofs surface) to remove all blistered the .memb~ne s surface.bliStered membrane lS plies. 'JYplcally, thiS means remov-finn, has integrity, is not ing debris, aggregate orin jeopardy of rupturing, is not in loose surfacing from the area. Ifa traffic area and is not full of the membrane's surface was flood-water, it does not need to be cut coated and has embedded aggregate,out and may be repaired with a re- the aggregate should be spudded offinforcing patch. Integrity means the surface carefully.that the blister is durable even if In addition, to achieve a successfulleft alone, such as when the blister tie-off and obtain redundant adhe-is composed of three or four well- sion to the existing membrane's sur-laminated plies. Such a condition face, the patching plies should be se-may exist when three or four plies quentially feathered-out well beyondhave blistered off the underlying the blistered location. Therefore, it isinsulation or base sheet. Profes- a good idea to extend the area beingsionals should remember that prepared at least 18 inches (457 mm)once a membrane is cut, all in- beyond the bmter. This gives thetegrity is removed. craftsmen room to work without get-

. Excessive loss of surfacing and evi- ting gravel or other debris back ondent membrane deIeriomtion. The the location being serviced.blister should be repaired when it The author prefers using a blunt':/has led to excessive loss of surfac- nosed spud shovel (rather than a ;-ing, such as where premature weighted, sharp-bladed spud bar) tomembrane deterioration is evi-dent at the blistered location.This is true for an aggregate-surfaced or granule cap sheet-surfaced BUR membrane. Blistersshould be repaired if they havelost aggregate or granules and theexposed membrane has sufferedaccelerated weathering. This blis-ter type does not always needto be cut out but should receiveattention.

. Blisters in side or end laps. Whenblisters occur at critical locations,such as side or end laps, theyshould be repaired. Repair is im-portant especially if dimensionalcycling (swelling and shrinking)

has begun to sheer and separatethe lapped seam. If the sheerforce has reduced the coverageof the seam's adhered portion,repai~ are needed.

. Blisters in high-traffic areas. Blistersthat have developed in a portionof the roof experiencing regularfoot traffic. such as from rooftopequipment maintenance per-sonnel, should be cut out andrepaired.

MonitoringIf inspection and evaluation indicatethat the blistered membrane is ofsufficient integrity and does notneed a patch, it should be moni-tored. When an isolated blister is tobe monitored,especially on roofsystems that re-ceive annual orbiannual inspec-tions, it should bemarked. One wayto do this is to out-line the blisterwith spray paint.

Then,duringperiodic inspec-tions, the blistercan be monitoredfor any changes,such as increasein size, excessiveloss of surfacing,

Photo 3: Setting the first patching ply in solvent-basedasphalt roof cement.

March 1997 Professional Roofing 21

spud around blistered loca- installed all the way backtions. A lightweight spud into the voided edges, voidsshovel is easier to control are built into the repair andat varying angles, while a can result in future blisters.weighted spud bar can hole 3. Prime to promote patchor puncture the membrane adhesion. Many believe thateasily, often making it neces- priming the surfaces of ex-sary to install a larger patch. isting bituminous mem-

After spudding, loose ag- branes is critical to achiev-gregate, dirt, dust and bitu- ing patch adhesion; for themen chips should be swept most part, this is true. How-off the surface. A l2-inch- ever, successful, quality(305-mm-) wide, stiff- patches can be installed overbristled push broom works certain types of asphalt BURwell. For sweeping most sur- membranes without primingfaces (other than large areas Ph 4:]j ell' halt f t the fi t t h the existing membrane'sof spudded aggregate or a . oto.: row 'r1f!asp roo cemen over .rs pa c - surface.

I. . BUR Ing ply In preparation for the larger, second patchIng ply. Wh . h h Ipee mg coaung on a en usmg ot asp a tmembrane), a stiff-bristled house With a straight-blade knife, it is as the adhesive and waterproofingbroom held on edge works fine. easy to get off track and make an in- for the patching plies, it is critical to

2. Cut an Unbonded, blisten!d mem- complete cut around the blister's prime the membrane's surface, par-brant' with a hook-blade knife. Once the circumference. An incomplete cut ticularly when the existing mem-area surrounding the blister has been can leave unbonded material at brane is aggregate-surfaced and hasprepared and swept, the blister can be the edge of the previously blistered been spudded. Priming also is criti-cut. The author prefers using a hook- location. cal for some coated, smooth-surfacedblade utility knife to cut out blisters. A Then, when a patch is applied membranes-especially if an acrylicsharp hook blade can be inserted into over the area, the unbonded materi- or aluminum-pigmented coating wasthe blister at its base circumference. al is trapped in a portion of the used. With some coated surfaces, it isThen, the blade's hook portion can patch. In time, the interlying, un- beneficial to scrub Qr use a propane-be used to guide the knife around the bonded material can cause a portion fired, weed-burner-type torch to heatblister's base (see Photo 2). of the patch to begin to blister away and remove the coating in the loca-

By holding the blade almost paral- from the underlying membrane. tion being patched.lei to the roof's surface and follow- For this reason, the author sug- But for most well-draining, rela-ing the void around its circumfer- gests not slicing a blister with an x- tively clean, cap sheet-surfacedence while employing a pulling shaped cut Usually, after an x-shaped asphalt-based membranes and hotpressure against the blade's hook cut is made, there are four unbond- asphalt-glazed, smooth-surfacedportion, professionals can cut the ed, triangular-shaped "flaps," or tabs, membranes, priming is not necessaryvoided, blistered plies easily from the left attached to the unbonded mem- as long as solvent-based asphalt roofadjacent, well-adhered membrane. brane (see Photo 1). Typically, the cement is used to adhere the patch-

roofing worker ing plies.lifts these portions When primer is necessary forof unbonded ma- patch adhesion, it can be appliedterial, applies bitu- with a medium-nap roller. However,minous roof ce- trying to prime small areas with ament or hot bitu- roller sometimes can be difficult-men under them, the primer tends to be applied tooand then presses heavily as the tradesman tries tothem flat It can be avoid leaving skips or holidays. How-difficult to install ever, thick primer applications canthe bituminous ce- further break down already weakment or hot bitu- membr.mes, such as older, organic-men all the way reinforced BUR membranes. Andback into the far the thicker the primer is applied,edges of the un- the longer it takes to dry.bonded tabs. If Another efficient method forthe cement or priming isolated locations for repairbitumen is not is using a weed sprayer. Primer can

Photo 5: Installing the second patching ply to repair thepreviously blistered location.

22 Professional Roofing March 1997

remaining in the existingroof membrane, manydifferent types of patchingmaterials can be usedsuccessfully.

Various types of poly-ester fabrics, set in solvent-based asphalt adhesive orcut-back asphalt, are usefulfor patching hot asphaltBUR membranes. Also,heavyweight, relativelythick pieces ofSBS-modified asphalt basesheet or interply sheet

Photo 6: Applying the third layer of asphalt roof cement to (when set in roof cement)the repair location. work well for filling a de-

pressed location whereblistered plies have been cut out.

In addition, various grades of APP-modified asphalt membrane materialwork well for patching blisters in dif-ferent situations. Using torch-grade,APP-modified bitumen material also

be sprayed relatively quickly,and, therefore, thinner,more consistent applicationsare possible without skips orholidays. For example, whena light film of primer isdeemed best for adhesion toa clean but older membrane(e.g., cap sheet-surfacedmembrane), a weed sprayermakes "haze" (light, buteven) priming applicationpossible. Haze priming oftenis all that is needed to pre-pare an existing asphaJt-based membrane's surfacefor a patch that will be in-stalled with solvent-basedasphalt roof cement.

4. /nstaUation of patching plies. Mterthe existing membrane has been pre-pared and blistered plies have beenremoved, the patching plies can beinstalled. Typically, patching plies for

a hot asphalt BUR membrane consistof Type IV fiberglass ply sheets set insolvent-based asphalt roof cement orhot asphalt (see Photo 3).

However, depending on the condi-tions and approximate service life

~ 4, ~'t,ft4l:te 2:uata9 ~m~6-K 1/t4~ ~i1e9'.2;

STEELALUMINUMCOPPER

MUL TIPLE PROFILES-Standingseam, Snap seam & Nail strip!

COMPACTv

LIGHT WEIGHT i l.IMPAOVmModeI177OAEASY TO USE &

TRANSPORTPOWERED SKA TE FORBEST RESUL TS

NEW SWIVELING CRADLES &UPRIGHTS

TRANSPORTS EASIL Y WITHOUTMA TERIAL IN MACHINE

CHANGE PROFILES IN LESSTHAN ONE HOUR

VARIPANTMModel KR-21VP KlJudson ~

MANUFACTURING, INC. ~10401 W. 12001 Ave. . enx.T*k1. COOUI USA5m1

T~: :m'4$-21 01 . Fax: ~~

Check 1134 on Reeder Service CBld

March 1997 Professional Roofing 23

fiberglass-reinforced, hot asphaltBUR membrane that otherwise isin good shape has an isolated blisternear a low-lying area (which occa-sionally may hold water), a reinforc-ing patch over a small blister can bebeneficial. Polyester is a good rein-forcement to use when patching overminimal blisters because of its elon-gation characteristics.

For instance, if a membrane hasbeen evaluated to have six to eightyears of service life remaining, it maybe prudent to reinforce the blisterwith two plies of polyester fabric setin solvent-based asphalt adhesive. Asthe two plies are installed and thesecond is extended beyond the first,two separate tie-offs are obtained.Now this blister is reinforced with apatch capable of some elongationthat will accommodate the blister'sthermal cycling and ensure theminor blistered location willlast as long as the surroundingmembrane.

applying asphalt roof cement, butthe author prefers to wear elbow-length, heavy-duty rubber gloves. Anappropriately protected, glovedhand can spread greater quantitiesof asphalt roof cement than a hand-held trowel.

When setting patching plies, theyshould be rubbed in fully to promotethorough adhesion. Also, using atrowel or back edge of a straight-blade utility knife can help embedthe perimeter edges of the top oruppermost patching ply. A straight-blade utility knife held at an anglecan be pressed around the last patch-ing ply's perimeter to set the sheet'souter edge down into the roofcement or bitumen, thus "sealing"the exposed perimeter of thepatch.

5. Apply or reset surfacing. When agranule~urfaced cap sheet mem-brane is being repaired, the lastpatching ply should be a cap sheet ofa similar granule color. If the exist-ing membrane is aggregate-surfaced,the surfacing that previously wasspud off the existing membranemay be re-adhered when the patch-ing materials have set.

Smooth~urfaced existing mem-branes that have liquid-applied coat-ings should receive new coatings perthe coating manufacturer's instruc-tions. These may dictate a specificwaiting or curing period for thefresh bitumen used in or on thepatch to set and oxidize before ap-plying the coating.

Effective inspectionsEffectively conducted, periodic in-spections by experienced profession-als can prolong the life of low-slopemembrane roof assemblies when thecorrect maintenance and repair pro-cedures are prescribed at the appro-priate times.

During the inspections, deficientlocations, such as blisters, should beexamined for possible repair. If theblistered membrane's integrity indi-cates that the blister need not be re-paired, it should be marked and out-lined for future monitoring.

If the inspection indicates that re-pairs might be needed, a close exam-ination is recommended so the ap-propriate repair can be prescribed.Some blisters should be cut out com-pletely, and the appropriate type andnumber of repair plies installed topatch the cut membrane. Other blis-ters, with reasonable integrity in themembrane, can be repaired effec-tively with a simple reinforcingpatch. [!!I

--

Jim Carlson is an NRCA deputy directorof technical services.

precludes the use of primer in manysituations because torching the exist-ing membrane's surface can prepareit for patch adhesion. When using apolyester, mid-reinforced, commodi-ty grade of APP-modified membranematerial for patching, the authorsuggests employing two sequentiallyapplied plies when blisters consist ofmore than two plies of ply sheet thathave been removed from the exist-ing membrane. This is especially im-portant if the existing BUR mem-brane is relatively new, made of threeor more plies and has substantiallymore than 10 years of service liferemaining.

To further describe the basicpatching process, the author Willexplain a typical scenario for a fiber-glass-reinforced, hot asphalt BURmembrane.

In general, the number of patch-ing plies to be installed should bethe same as the number of blisteredplies removed. However, if removalof the voided plies has left a signifi-cant depression in the existingmembrane, it is a good idea to setat least one ply (cut to the void'sshape) in roof cement to help fillthe depression. Then, the actualpatching plies can be installed-they should be set in hot asphalt orsolvent-based asphalt roof cementso they feather out and extend fur-ther onto the prepared membranesurface with each sequentially ap-plied larger ply.

The first patching ply should coverthe previously blistered location andextend at least 6 inches (152 mm)beyond the cut-out location (seePhoto 3). Then, each succeedingpatching ply should extend at least 3inches (76 mm) beyond the previousply (see Photos 4,5 and 6). For mostisolated blister repairs, the authorprefers using solvent-based asphaltadhesive or asphalt roof cement in-stead of hot asphalt to adhere patch-ing plies. However, when there arenumerous blisters to be repaired andthe patching can be performed dur-ing waml weather, hot asphalt expe-dites patching.

A trowel commonly is used for

Reinforcing patchWhen it is deemed appropriate toleave minimal bliste~ intact, buta reinforcing repair is in order, allthe steps mentioned previously areapplicable except step 2 because themembrane is not cut Each overlyingreinforcing ply installed should berubbed in thoroughly. With isolatedblisters that have integrity and aredurable, a knowledgeable roofingprofessional can design an effectivepatch of overlying reinforcementto consist of the necessary ply or

plies.For example, if a four-ply,

24 Professional Roofing March 1997