The use of single-ply mem-branes to re-cover existingmetal roofs on metal buildingshasbecomeverypopular overthepastseveralyears.It’sfast,it’seasy,youcangetawarran-

ty,andit’srelativelyinexpensive.Therearemillionsofsquarefeetdoneeveryyear.Mostpeopleinvolvedinthisactivityarenotawareof the increasedriskthiscauses for them-selves and the building occupants. Simplysaid,apre-engineeredmetalbuildingisnotdesignedtohaveasingle-plyroofonit.

Abuildingthatisdesignedtohaveasingle- plymembraneroofsystemwillbesupportedby bar joists. Bar joists are not connect-ed to eachother frombay tobay, andactindependentlyofeachother.Moreover,barjoists have built-in camber and create alevelsurfaceonce theyareattached to themain frames and dead loads are applied.This surface works great for a membrane

roof application because the water flowsequallyacrosstheentireroofsurface.

Incontrast,ametalroofonapre-engi-neeredmetalbuildingisattacheddirectlytoC-orZ-shapedpurlins,whichare,inturn,lappedandconnectedtoeachotheroverthetopofeachrafter frame.Rafter framesaretypically25feetapart.Purlinsareproducedcompletelylevelandaredesignedtohaveadipordeflectionbetweenframesoncetheyare installed. Equal loading across eachpurlin is critical to the design of a metalbuilding;andsincethepurlinsareconnect-edtoeachother,whathappensinonebaywill affect theadjacentbays.Furthermore,many pre-engineered metal buildings thatare standing today are designed for bal-anced loading across the entire structure.The assumption is the load on a buildingwouldbeuniform;thus,thepurlinsinonebaywill not get pushed down or deflectedmorethantheadjacentbays.

When there is a ribbed metal roof inplace,waterflowsdownbetweentheribsofindividualpanelstothegutter.Thisensuresequal loading on the purlins and confine-mentof thatwater toan individualbay. Ifonechoosestorecovera low-sloped,metalroof on a metal building using single-plymembranes, the water flow is changed.Since thereareno ribs to confine theflowasthewatermovesdownslopetothegutter,the water will naturally flow towards themid-span of the purlins where maximumdeflectionoccurs. This canbe seen clearlyin Figure 1.

On lower-slopestructureswithametalroof in place, if there are any dips or lowspots caused by awide span bay, a dam-agedoroverloadedpurlin,ordamagedroofpanels, the water may pond locally butstayswithin the confines of the individual panels. With a single-ply membrane inplace,localizedpondingoveraweakpurlin

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Figure 1 – This low-slope metal building with a trapezoidal standing seam was re-covered with a single-ply membrane. Water tends to flow in the deflection mid-span on the purlins.

willgrowover timebecause thewaterflowwithintheentirebaywillbedrawntowardit every time it rains. As the dip accumu-latesmore andmorewater and forces theweak purlin down in the affected bay, thepurlins in the two adjacent bays will rise.Eventually,thiscandivertwaterfromadja-centbays into theaffectedbay.The resultcan be catastrophic, as shown in Figure 2, aphotographofaschoolwithaninternallydrainedtrapezoidalstanding-seamroofthatwasretrofittedwithamechanicallyattachedsingle-plymembrane.

Thereareseveralwaysthiscanhappenwith a single-ply re-cover, such as in thefollowingscenarios:

1. An overloaded or damaged purlin that ponds water to start with. Building occupants hang all kindsof things from the purlins inmetalbuildings with little regard for orunderstandingofthecapacityofthatpurlintohandleadditionalweight.

2. A dip in the panels between the eave strut and the first purlin.

Thisiswhereyouaremostlikelytoseepondingwateronametalbuild-ing roof. It is normally due to poorinstallation practices or too muchfoot traffic, such as maintenancepersonnel accessing the same rooflocationeverytime.

3. A sudden surge of water laterally from one bay across into another. Thiscanbetheresultofthebillow-ingeffectofamechanicallyattachedsingle-ply membrane under highwind loads.Negativewindpressurelifts the area between the seamsof a single-ply roof like a balloon.Therisenpartoneachsheetmovesrapidly across the roof, displacing

water as it goes.Onametalbuild-ing,watercouldeasilygetdisplacedfrom several bays across into onebayinamatterofsecondswiththisbillowingeffect.Watch thevideoonthefollowinglinktoseeasingleplybillowingandpushingwaterduringa strong thunderstorm recently:https://youtu.be/5LUlX9aNr80. Inthe video, this is happening alongtheoutsideedgeofthebuildingthatis exposed to the wind. Can youimaginewhatthiswouldlooklikeonawide-openroofthatisnotprotect-ed by a 20-story building like thisone is?

4. A clogged scupper or drainpipe. During a single-ply retrofit of thebuildinginFigure 3,theinternalgut-ters were filled, and scuppers wereadded to handle the water flow outof the parapet wall. This is a verycommon practice when re-covering existing internally drained metalbuildings, and it creates the worstpossiblescenarioonametalbuildingstructure. A clogged scupper outletcaused the initial backup of waterontothefirstpurlinupfromtheeavein thewidest-span bay. As the firstpurlin was being pushed down ordeflectedwiththeadditionalweightofthewater,thepurlinsintheadjacent

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Figure 2 – This same building roof collapsed due to several factors related to a single-ply recover.

Figure 3 – Interior shot of collapse.

therearemanypositiveargumentsforre-covering anexistingmetalbuildingroofthatisnotperforming. The trick is to do it in such a way as to eliminate the reasonstheroofisnotperformingtostartwith.

bays rose and forced the waterfrom the two adjacent bays into thewider span bay where the low spotoccurred. It also prevented waterfromreachingtheoverflowandadja-centscuppersbecausetheeavestruthad not deflected along the parapetwall.Asaresult, it tookamatterofminutesfortherooftocollapse.

Acoupleofyearsago,wedidsomebasetestingtodeterminewhateffectdoingmetal- over-metalretrofithadonpurlinstrength.Abasetest iswhereyoutaketwo full-lengthpurlins in a single-span condition, with ametalroofattachedtothem,andpushdownon the purlins until they buckle. The firsttestwasdoneoveratypicallow-slopemetalbuilding installation:8-in.-tallby16-gaugeby25-ft.longpurlinswitha24-gauge,3-in.by24-in.trapezoidalstanding-seamrooftoestablish a baseline for the retrofit tests.(SeeFigures 4 and 5.)

Withouttheretrofit,thepurlinbuckledat 23.9 psf. One inch of water weighs 5.2poundspersquarefoot.Thatmeansinthiscase,thepurlinwouldhavefailedwith4.6inchesofwateronit.

It would not be hard to imagine 4 to5 inches of water accumulating on a low-slopedmetalbuildingwithasingle-plyroofonit—especiallyonethatwasdesignedwithagutterthathadbeenalteredwithcricketstodrainoutofscuppers.

There aremanypositive arguments forre-covering an existingmetal building roofthatisnotperforming.Thetrickistodoitinsuchawayastoeliminatethereasonstheroofisnotperformingtostartwith.Thebot-tomlineis:Thebestwaytorecoveranexist-ingmetalroofonametalbuildingistouse

anothermetal roof.A metal-over-metal re-covercanbedone with minimal dis-ruptiontotheoccu-pants and within theguidelinesoftheInternationalBuildingCode (IBC). Insulation may be added andwind-load capacity can be enhancedwith-outalteringthestructuraldiaphragm.

The metal roof system that gives thebest chance for success at re-cover is asymmetrical,site-formed,structuralstand-ing-seam system. These two-piece systemshavethemostwatertightseamdesign,pro-vide unmatchedwind uplift capacitywith-out additional costly corner or edge zoneframing, eliminate leaking end lap joints,andallowforindividualpanelreplacement.

The ability to easily repair or remove andreplace individual panels on a metal roofhasbeenthemissingpieceofthepuzzleformostbuildingowners.

Thereareat leastfiveways to re-covera metal roof on a metal building with asymmetricalstanding-seamsystem.Severalof thesemethodswillactually increasetheload-carrying capacity of the underlyingpurlinsatthesametime.Mostimportantly,allfivemethodshelpmaintainthebalancedconditionforwhichthemetalbuildingwasoriginallydesigned.

Terry Wolfe left MBCI in December 2000 to start Force Engineering and Testing, Inc. As an engineering con-sultant and certi-fied test lab, his firm is a source in the development of metal roofing sys-tems and re-cover applications for the

metal roofing and metal building industry. As the first chairman of the Metal Roofing Systems Technical Committee, he was a pri-mary author of the first edition of the Metal Building Manufacturers Association’s MBMADesignManual for metal roofing systems.

Terrence E. Wolfe, PE

Charlie Smith founded Arch-itectural Building Components in 1989. Over the next 23 years, the company grew into an industry-lead-ing metal roofing and wall sys-tem company. In 2012, the business became a part of

McElroy Metal, enabling Smith to focus on educational and product development efforts. Charlie holds several patents in the re-cover field. As a widely recognized metal roofing authority, Smith helped to write the new RCI Metal Roofing course and is a frequent pre-senter at industry events.

Charlie Smith

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Figure 4 – Roof panels mounted on purlins prior to base testing.

Figure 5 – Purlins after base test failed at 23.5 psf.