SELECTING THE RIGHT FINISH FOR METAL ROOFING Part I: Metallic Coatings

Transcription

1 SELECTING THE RIGHT FINISH FOR METAL ROOFING Part I: Metallc Coatngs By Derek A. Hodgn, P.E., RRO, RRC Wth so many fnsh optons avalable for metal roof panels, choosng the rght one can be somewhat dffcult. Ths artcle s ntended to gude the reader through the selecton process by provdng basc nformaton on the most popular fnshes. In order to provde suffcent detal, I have elected to devote ths artcle to metallc coatngs on steel base sheets. I ntend to provde smlar nformaton on pant systems, coatng systems and unpanted metals n later artcles. The prmary functon of a metallc coatng s to provde protecton of the base metal. Protecton of the base metal (typcally carbon steel) s typcally accomplshed n one of two ways: (1) the base metal can be protected by provdng a sacrfcal metal (galvanc protecton), andlor (2) the base metal can be protected by a physcal barrer (barrer protecton). These concepts are dscussed n further detal below. A metallc coatng may also be panted to provde Mer protecton andlor to add color to a buldng. However, unpanted metallc fnshes have also ganed popularty on steep slope resdental applcatons, where the roof provdes an archtecturally pleasng feature for the buldng. Galvanc Protecton Galvanc protecton refers to the process by whch the base metal acts as the cathode and a sacrfcal metal serves as the anode. In the presence of an electrolyte (water), the less noble metal (anode) wll be compromsed, leavng the more noble metal (cathode) ntact.' The potental for ths process to occur s proportonal to how far apart the two metals are on the galvanc seres (often referred to as the galvanc scale). Ths seres s the smplest way to llustrate the relatve electrode potentals of varous metals. Znc s the most common anode due to ts relatve poston on the galvanc seres. The dsadvantage of galvanc protecton s that the sacrfcal metal must always be present. If the sacrfcal metal s used up, the base metal wll begn to corrode. Roof contractors are well advsed to be famlar wth the galvanc seres, so as not to ntroduce condtons that wll result n galvanc wmson. A smplstc verson of the galvanc seres s provded below for reference.

2 Galvanc Seres! Magnesum znc, ; Cadmum A& Alumnum 2017 Steel (plan) ; Cast Iron g Lead. Tn!... Brasses Copper! Bronzes Ttanum Monel Nckel (passve) 304 stanless (passve) f 316 stanless (passve) ] Slver H# Graphte 1! The galvanc seres shown above ncludes an approxmate groupng of metals based on ther anodc and cathodc potental. Agan, the further apart two metals are n the galvanc seres, the hgher the potental for galvanc corroson to occur. For ths reason, roofng contractors should try to use components fabrcated from the same metal. If ths opton s not avalable, the dssmlar metals should be as close together on the galvanc seres as possble. As a last resort, when dssmlar metals are used wth moderate or sgnfcant potental for corroson, great we should be taken to provde a physcal separaton between the two metals usng an nert materal (.e. nylon washers or a rubber membrane). Another method of protecton s to smply provde a physcal barrer over the base metal. The barrer protects the base metal from corroson by keepng t free from mosture and oxygen. Pant s the most common barrer protecton that s currently used. Whle barrer protecton can be effectve, they must be mantaned. Once the protecton s compromsed by weatherng, scratches, abrasons, or chemcal reactons, the base metal wll be vulnerable to corroson.

3 Bas& Consderatons For the selecton process to begn, a few basc questons should be consdered. A few of the most common consderatons are dscussed below. Wll the roof be vsble from the mound? Desgners typcally refer to roofs as low slope or archtectural. Low slope roofs typcally cover large one-stoly commercal buldngs and are not vsble from the ground Archtectural roofs have steeper slopes (typcally 3:12 or more) and are vsble from the ground. Many buldngs nclude both types of roofs. A typcal example of ths 1s the modem shoppng centers (otherwse referred to as a strp mall). These buldngs typcally consst of a pre-engneered metal buldng wth a low slope roof The front and sde elevatons are o h clad wth brck or stucco wth a steep slope roof extendng from the front elevahon to cover a walkway. Regardless of the applcaton, the answer to ths queston can often determne the type of fmsh you should consder. The vsblty factor also relates drectly to the ablty of the roof to dran. The steeper the roof slope, the less the metal roof panels are exposed to water. For ths reason, steeper slope roof panels can be expected to retan ther fnsh for a longer perod of tme. As the roof slope decreases, careful consderaton should be gven to proper waterproofng detals and the durablty of the selected fnsh. Wll the roof be excased to anv adverse condtons? Some metal fnshes do not react well to certan envronments. A few examples are provded below.* 1. Contact wth strong acds should be avoded. When usng alumnum or alumnum alloy, strong alkals can be detrmental. For ths reason, exposure to wet cementtous mom should be avoded. If necessary, an acrylc coatng can sometmes be used to protect the metallc coatng from mortar. Sometmes, hghly alkalne cleansers are used for cleanng HVAC equpment. These cleansers should be avoded on rooftops wth metallc coatngs. 2. Znc and alum~num are both anodc metals and should be solated from electrolytc contact wth more noble (cathodc) metals such as lead and copper. Flashng should be constructed wth the same metal as the roof coverng whenever possble. 3. Rooftop equpment that ncludes copper lnes that wll drp onto the roof should be avoded. Runoff from the copper wll act as an electrolyte and cause rapd galvanc corroson of the metallc coatng.

4 4. Although coastal applcatons are common, exposure to salt should be lmted due to corroson potental. For ths reason, metallc coatngs - n coastal envronments can be expected to have a somewhat shorter servce lfe. 5. Alumnum and alumnum coatngs are senstve to graphte. In a wet clmate, a heavy graphte pencl mark can w d e metallc coatngs contanng alumnum n as lttle as two to three years. The process wll take slghtly longer n dryer clmates. Galvanzed Galvanzed fnshes offer galvanc protecton by provdng a sacrfcal anode. The concept s that two dssmlar metals are present: steel and znc. Of the two metals, steel s more noble (hgher on the galvanc seres). Therefore, when corroson occurs, t s the znc that s compromsed, whle the steel remans ntact. Galvanzed f~shes are easly dentfed by the coarseness of ther appearance, refd to as spangle. The dsadvantage of galvanzed fnshes s that ther effectveness s somewhat lmted. Dependng on the thckness of the galvanzng and the severty of the envronmental exposure, galvanzed roof panels can begn to rust n as lttle as 10 to 15 years. In general, the rule of thumb s the thcker the coatng, the longer the protecton For all practcal purposes, ths relatonshp can be consdered as lnear.' For ths mn, unpanted galvanzed panels are typcally dscouraged. However, the applcaton of a newer technology coatng can sgnfcantly enhance the long-term performance of galvanzed roof panels, makng them a vable opton. Addtonally, znc coatngs are not resstant to acds and alkals, and must always be topcoated n chemcally aggressve envronments. The advantages of galvanzed coatngs nclude the fact that znc can actually "heal" a nck n the coatng and provde excellent protecton at cut edges and drlled holes. Addtonally, galvanzed panels are typcally less expensve when compared to other protectve metal fnshes. (nsert close-up vew of galvanzed spangle) The mnmum znc coatng weghts for galvanzed steel are desgnated by ASTM n Specfcaton A924 (formerly A525). Coatng weghts for galvanzed sheets vary, but for roofng applcatons are typcally G90 or 0.90 ounces of znc per square foot, total both sdes. The coatng s about 0.76 mls thck per sde. Barrer Coatngs Alumnzed Steel The use of alumnzed steel was ntroduced n the 1930's. However, use of alumnked steel n roofng applcatons dd not begn untl The applcaton of commercally

5 pure alumnum to steel sheet s a process that was developed by Armco Steel Inc.' The alumnum coatng provdes a barrer to the underlyng steel base metal. Whle two types of alumnzed steel are avalable (Type I and Type II), roofng applcatons are lmted to Type 11. When exposed to weather, the alumnum coatng wll form alumnum oxde. Unlke znc oxde, alumnum oxde s not soluble. For ths reason, the durablty of alumnzed steel s not proportonal to the thckness of the alumnum coatng. The mnmum alumnum coatng weghts for alumnzed steel are desgnated by ASTM n Specfcaton A924 (formerly A463). Coatng weghts for alumnzed steel roofng applcatons are typcally 0.65 ounces of alumnum per square foot (desgnated as T2-65) total both sdes. The coatng s about 1.1 mls thck per sde. Ths coatng carres a lmted 20-year warranty aganst panel perforaton due to normal atmospherc corroson. However, 20-year exposure testng has shown that n most envronments t wll far outlve and possbly even double the warranted lfe. Alumnzed steel can be used n panted or unpanted condtons. Galvalume Galvalume fnshes were ntroduced nearly 30 years ago and provde both galvanc and barrer protecton. The fnsh ncludes znc and alumnum. The znc (approxmately 45% by weght) provdes the sacrfcal protecton, and the alumnum (55% by weght) provdes the barrer protecton. Ths hybrd fnsh has become extremely popular for low slope (less than 3:12) roof applcatons. In 1995, t was reported that over 5 bllon square feet of Galvalume roofng panels had been nstalled snce ts ntroducton.' Due to the combnaton of protecton mechansms, ths fnsh has the ablty to provde a longer servce lfe than a galvanzed fnsh n the same exposure. Current marketng lterature suggests a servce lfe of 30 years or more n most envronments, wthout maor mantenance. (nsert close-up photo of Galvalume spangle) The mnmum coatng weghts for Galvalume are desgnated by ASTM n Specfcaton A792. Coatng weghts for Galvalume roofng applcatons are typcally 0.55 ounces per square foot, total both sdes. The coatng s about 0.9 mls thck per sde. GalvalumeTU s a trademark of BIEC Internatonal Inc. Metallc coatngs of many varetes have demonstrated years of successful performance on a wde varety of applcatons. Advances n technology have resulted n a wde selecton of metallc coatng optons for buldng owners, archtects and roofng contracton. Each successful proect should nclude a conscence effort to match the

6 metal roofng components wth the desred appearance and functon of the buldng. However, as wth most constructon proects, proper materal selecton and attenton to detal reman key to long-term performance. References 1. Archtectural and Low Slope Metal Rootng Systems. The Roofng Industry Educatonal Insttute (REI), February Metal Roofng From A (Alumnum) To Z (Znc). Robert M. Haddock. RSI Magazne., September Galvalurne Standng Seam Roofs: 20+ Years of Proven Performance. GALVALUME Sheet Producers of North Amerca. June 1995.