SOME OF THE ADVANTAGES RELATED TO THE GREENING OF ROOFS—BOTH EXISTING AND NEW:

•Becausetheroofsareprotectedfromtheextremeandharsheffectsoftheelements,particularlyultra-violetradiationandveryhighaswellaslowtemperatures,thewaterproofinglastssubstantiallylonger.

•Temperatureextremesatrooflevelaredramaticallyreduced,thusdecreasingtheneedforexpensiveinsulationagainstbothheatandcold.Asaresult,heatingandcoolingcostsarevastlyreduced.

•Water runoff after rain is greatly reduced, especially peakrunoff.Muchofthewaterisretainedontherooftoprovidefor the growth of the plants, where it is returned to theatmosphere without overloading the already badly alterednatural drainage systems. In addition, fewer pollutants reachourwatercoursesandoceans.

•Thegreeningof largeexpansesofroofingalsohelpsreducetheheatsinkeffectexperiencedinlargecities,thushelpingtoreduce the local temperatures.Again, thishas thebenefitofsignificantlyreducingtheairconditioningcosts,particularlyinwarmclimates.

•Thegreeningofurbanroofscanprovidevaluablehabitatforawholerangeofplantsandwildlife,frominsects,reptilesandamphibians, to birds and even small mammals that do notnormallyoccurincitycentres.Forinstance,inpartsofcoastalGermanyandtheNetherlands,thepreferrednestingplacefortheendangeredoystercatcherandanumberofotherground-nestingbirdscanbefoundongreenroofshighuponcityroofs.

IMPORTANT CONSIDERATIONS BEFORE UNDERTAKING THE GREENING OF A ROOF:

Beforeconstructingagreenrooforgreeninganexistingroof,itiswisetofirstdiscusstheproposedgreenroofwithaspecialistingreenrooftechnology.

STRUCTURAL LOADINGIt is essential that the roof to be greened be designed by orcheckedbyasuitablyqualifiedstructuralengineertoensureitiscapableofsafelycarryingthemaximumexpecteddesignloadofthefullysaturatedgrowingmedium,pathwaysandplants,aswellaspeoplewhoareexpectedtobeontheroofatanygiventime.

Water and Root Proof Your Green Roof

Roofs are increasingly being greened in the industrial countries of the northern hemisphere—in particular in Germany—for a number of reasons, mostly related to the environment and to climate change. In this feature, green roof technology specialist Michael Hickman shares with us some critical considerations to

keep in mind when designing and constructing a green roof.

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The completed layup with geofabric, both under and over the 1000 micron HDPE. The bottom layer is for protection to the roof as well as the roof barrier. The top layer of geofabric is to protect the root barrier from physical damage due to planting and maintenance. The waterproofing and root barrier layup is now ready for the soil.

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WATER AND ROOT PROOFINGItisessentialthattherooftobegreenedhasadurableandveryreliablewaterproofingandrootprotectionlayerthatcannotbepenetratedbytherootsoftheplantstobegrownonit,asitisaveryexpensiveundertaking to lift anestablished green roof tohavealeakingroofrepairedandthentoreplaceit.Inparticular,the roof barrier must extend well above the surface of thegrowingmedium andmust bewell-attached or bonded to thestructuretopreventwaterandroots frombeingable toenterbehindit.

BITUMEN-BASED PRODUCTSIn SouthAfrica, the majority of roofs are waterproofed usingbitumen-based products, which do a very good job ofwaterproofing where no vegetation is allowed or required togrowon them. InGermany, a number of roofs exist that havesuccessfullybeenwaterproofedandrootproofedwithbitumen-based products and which have not experienced problems.However, theyhavemostlyplants likeSedumgrowingonthem,whichdonothaveexcessively invasiverootsystems.ForSouthAfrica,Idonotrecommendgrowingplantsdirectlyonroofsthathave beenwaterproofedwith bitumen-based productswithouttheadditionofaverystrongroot-resistingbarrierconstructedoverthebitumenwaterproofing—suchas1mmorthickerLDPEplasticsheetingfittedandweldedtogethersoastopreventrootsfromdamagingthewaterproofing.

MOISTURE TRIGGERED POLYURETHANE MEMBRANE Thisismypreferredsystem,whichishighlyeffectiveandsimpleto install. Furthermore, it has been perfected in Germany andother parts of Europe over many years, specifically wheremillionsofsquaremetreshavebeen installedongreenroofs.Amoisture-triggered polyurethane membrane consists of a glassfibrereinforcedliquid.Appliedpolyurethanemembraneismadeupof a seamless, cold applied, fully bonded, highly elastic, one-component, moisture-triggered polyurethane water and rootbarrier.The glass fibre matting (chop strand) reinforcement isbeddedinthefirstcoatoftheliquidmoisture-curedpolyurethanemembrane, providing a continuous and seamlesswaterproofingandroofbarrier.Thissystemhastheaddedgreatadvantagethatithastheabilitytobemouldedtothemostcomplicatedshapesthatarecreatedbythevariousdetailsonroofsinasimpleandvery effective one-operation waterproofing and roof proofingprocedure.

PROTECTION OF THE ROOT BARRIERThe root barrier may need to be protected from mechanicaldamage.Dependingonthewaterproofedsurfaceoftheroofthatistobegreened,inparticularwhereLDPEplasticsheetingistobeused,itmaybenecessarytofirstplacealayerofgeofabricontheroofasprotectionfortherootbarrier.Itmayalsobenecessarytocovertherootbarrierwithalayerofgeofabricforprotectionagainst mechanical damage caused during planting andmaintenance,orwhenspeciallydesignedgrowingcontainersaretobeplacedontherooffortheplantstogrowin.Hereagainitisessentialtogettheadviceofaspecialistingreenrooftechnology.

ABOUT THE AUTHOR

MichaelHickmanisadirectorwithEcosystemsManagement.HeworkedfortheDurbanParksDepartmentfrom1971to1998 as a Certified Turf Management Horticulturist,responsibleforlandscapeplanninganddesignaswellasthehorticulturalmaintenanceofseveralsuburbs,parks,roadsideplantings, stormwater and wetland projects, recreationalamenities and sports fields.He alsoheldoversight for there-establishment of several kilometres of sand dunes. Inaddition toprivateconsultingworkover theyears,hehasbeenmonitoringGermangreenroof installations forcloseto 19 years. Since 2009, he has been working on theinstallationoftheeThekwiniMunicipalityGreenRoofPilotProject.Mr.HickmanholdsmembershipwiththeInstituteofParksandRecreationAdministration.

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Michael Hickman on a completed roof.

A geofabric layer has been laid over the bitumen-based waterproofing on the roof for protection to the waterproofing as well as the 1000 micron HDPE root barrier being laid over it.

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