Preventing Cracks in Veneer and Conventional Plaster System

Advancesinconstructiontechnologyhaveproducedimprovedbuildingmaterials,labor-savingtechniques, constructionefficienciesandcommensurateeconomies.Whilelightweightmaterialsandnewconstruction methods,suchasflatplateconcretedesign,enablebuildingstobeconstructedfasterandtallerthanever before,theyalsopresentnewproblems.Perhapsthemostelusiveoftheseispartitioncracking.

Causes of Cracking Crackingoccursinplastersurfaceswhentheforcesexertedonthemexceedthetensile,compressiveorshear strengthofthematerial.Thoseforcescancomefromavarietyofsources,suchas:

• Shrinkageduringdrying • Humidityexpansionorcontraction • Thermalexpansionorcontraction • Changesinpressure • Impactstopartitions • Structuralmovementofsupportingelementsproducedby: (a) foundationsettlement (b) seismicforce (c) windloads (d) volumechangesinsupportingmaterials (e) gravitationalloads;deadandlive

Crack Control Mechanisms UnitedStatesGypsumCompanyhasstudiedthevariousforcesthatcausecrackingandhasdevelopedmaterials andtechniquestocontroltheeffectofthoseforcesongypsumpartitionsandceilings.

Controlmechanismsfallintotwocategories:controljointsandperimeterrelief.Themechanismselected dependsuponhowtheforceaffectsthepartitionorceilingassembly.Iftheforceaffectstheplaneofthe partitionorceilingmembrane,acontroljointisrequired.Iftheforceaffectsthestructuresupportingorabutting thepartitionorceilingassembly,perimeterreliefisrequired.

Stresses Affecting the Plane Basically,therearetwokindsoffactorsthataffecttheplaneofapartitionorceilingassembly.Thefirstand mostcommonisexpansionandcontractionofthesurfacingmaterialsduetovariationsintemperatureor humidity.Thisincludesshrinkageafterinstallationastheassemblydriesandstretchingorcontractingdueto variationsinweather.

Thesecondfactoraffectingtheplaneisappliedstressesontheassemblyfromwindloadingonbuilding exteriors,slammingdoors,pressurechangesorsuddenblowsoninteriorpartitions,anddeflectionofceilings duetoloadsonthefloorabove.

Tocontroltheeffectsofeachofthesefactors,theuseofcontroljointsisrecommended.Thecontroljointsare placedinthefaceofthepartitionormembrane.

Common Conditions for Control Joints Therearefourprincipalconditionsunderwhichcontroljointsarerequired:

1.Wherepartitionsorceilingsofdissimilarconstructionmeetandremaininthesameplane,e.g.,junctionofa masonrywallandaveneerplasterpartition(seefig.1). 2.Wherewingsof“L”,“U”and“T”shapedceilingsarejoined,controljointtorunwithstructuralmembers (seefig.2).

3.Whereexpansionorcontroljointsoccurinthebasewallconstructionand/orbuildingstructure(seefig.3).

Product and Systems Technology

Fig. 1control joint at junction of veneer

plaster partition and masonry wall.

IMPERIAL gypsum base

Fig. 2control joint in "L" shaped ceiling.

reflectedceiling plan

Fig. 3control joint over structural expansion joint.

PM17

TechnicalInformation

4.Wherepartitionsorceilingsspanlongdistances,controljointsmustbeplacedatappropriateintervals:

• Partitions—30ft.maximum

• Interiorceilings(withperimeterrelief)—50ft.maximumineitherdirection

• Interiorceilings(withoutperimeterrelief)—30ft.maximumineitherdirection

• Exteriorwall/ceilingwithportlandcementplaster—10ft.maximumineitherdirection

Placement of Control Joints Controljointsareusedtorelieveexpansion,contractionorflexuralstressesacrosslargeceilingandwall expanses.Effectivecontroljointpositionsareasfollows:

• Fromdoorheader(cornersofdoorframes)toceiling

• Fromfloortoceilinginlongpartitionsandwallfurringruns(full-heightdooropeningsprovideexpansion controlinlargepartitions)

• Fromwalltowallinlargeceilingareas

Types of Control Joints USGhasdevelopedseveraltypesofcontroljointsforuseinveneerplaster,conventionalplasterandexteriorstucco construction.Thetypesofcontroljointandtheirrecommendedusesareidentifiedbelow.

Sheetrock® Zinc Control Joint No. 093 Forinteriorusewithveneerplasterfinish(seefig.4,5).Mayalsobe usedforexteriorsoffitapplications.

Theno.093controljointisconstructedwith3/32"groundstoaccommodateboth1-and2-coatveneeron ImperIal®veneerbasecoat.

Separationbetweenpanelsmustbe1/4"min.,1/2"max.Flangeisdesignedforstapleattachmenttopanel surface.Requiresplastering.Plastictapethatprotects1/4"groovemustberemovedwhenplasteringiscompleted.

Sheetrock® Control Joint No. 50, 75, 100 Forinteriorusewithconventionalplasterassemblies (seefig.6,7).Mayalsobeusedwithexteriorstucco.

Controljointsizesreflectvariousgrounds:no.50,1/2";no.75,3/4";no.100,1"(forexteriorstucco). Flangesareperforatedforwireattachmenttometallathortemporarystapleattachmenttogypsumlath.Inall casesflangesofthecontroljointmustbepositivelyattachedtoframingmembers.Controljointno.100may alsobeback-mountedbehindImperIal®gypsumbaseforusewithradiantheatceiling.

Separationbetweenpanelsmustbe1/4"min.,1/2"max.Plastictapethatprotects1/4"groovemustbe removedwhenplasteringiscompleted.

Fig. 5control joints in face of plaster partition accommodate

lateral expansion or contraction up to 1/4".

steel studSHEETROCK sealant

SHEETROCK control joint no. 093

SHEETROCK control joint no. 093

IMPERIAL gypsum base

IMPERIAL or DIAMONDveneer plaster

IMPERIAL gypsum base

ROCKLATHgypsum base

gypsum plaster

SHEETROCK control joint no. 50

SHEETROCK sealant

Fig. 7SHEETROCK control joint no. 50 in metal-stud and plaster partition.

Fig. 4control joint no.093.

1/4" 3/4"

1/2", 3/4", 1"

Fig. 6SHEETROCK control joint.

Double-V Expansion Joint Forusewithconventionalplasterassembliesorexteriorstucco(seefig.8). Availableingalvanizedsteelorzincwith1/2"or3/4"grounds.Flangesareexpandedmetalforpositivekeying ofplaster.Separationbetweenpanelsmustbe1/4"to1/2".

Casing Bead #66 Forusewithconventionalplasterassemblies(seefig.9).Availablewithgalvanizedshortflange andgalvanizedorzincexpandedflange.Provides3/8",1/2",5/8",3/4",7/8",1"and1-1/4"grounds

Verticalcontroljointsareconstructedbyintentionallyseparatingbeads1/4".Jointopeningshouldbesealed withSheetrock®acousticalsealant(seefig.10).

Casingbeadsplacedafewinchesabovefloor,separatingwallmembranefromalternatematerialbackingbase boards,alsoprovideprotectionfromfloordeflection(seefig.11).

Control Joint Installation Guidelines 1.Applicationofcontroljointrequiresbreakinpanelsurfaceofapproximately1/2".

2.Floorandceilingframingshouldbeinterruptedwitha1/2"gapatcontroljointsinthestructure.

3.Separatesupportsmustbeprovidedforeachcontroljointflange.

4.Adequatesealorsafinginsulationmustbeprovidedbehindcontroljointwhereversoundand/orfireratings areprimeconsiderations.

5.Whereverticalandhorizontalcontroljointsintersect,verticaljointmustbecontinuousandhorizontaljoints abutit.Applysealantatallsplices,intersectionsandterminals.

Fire-Rated Control Joints Fireandsoundcontrolprotectionmustbeprovidedbehindcontroljointstomaintainfirerating.Protectivebarrier maybelayersof5/8"ImperIalgyspumbase,FIrecode®core,or3"ThermafiberSAFBInsulation(seefig.12).

Fire-rated Control Joints (Warnock Hersey International-495-PSV-0824, 0824A)

1/2" or 3/4"

43/8"

Fig. 8double-V expansion joint.

1/4"

1/2", 5/8", 3/4"

31/8"

1/4"

1/4", 3/8", 1/2", 3/4", 7/8"

31/8"

Fig. 9no. 66 square edge

casing beads (expanded or short flange).

ROCKLATHgypsum base

gypsum plaster

casing bead control joint

SHEETROCK sealant

Fig. 10casing bead control joint in metal-stud and plaster partition. Fig. 11

1/2" max. control joint (both sides)

Two-hour rated steel stud partitions

5/8"

17/8" 31/8"

2 layers IMPERIAL gypsum base

47STC SA-860302

2 layers IMPERIAL gypsum base

1/2" max. control joint (both sides)

5/8"

17/8" 31/8"

IMPERIAL gypsum base

One-hour rated steel stud partition

2 layers IMPERIAL gypsum base

Fig. 12

Forces Affecting the Supporting Therearebasicallytwotypesofstressesthataffectstructuressupportingorabuttingpartitionsorceilingor Abutting Structure assemblies,particularlyinhigh-risestructures.Thenatureofthemisquitedifferent.

Thefirst,racking stress,occurswhenstructuralcomponentssuchasexteriorcolumnsorbeamsarealteredand partitionsareforcedoutofsquare.Essentially,rectangular-shapedpartitionframesbecomeparallelogram- shaped(seefig.13).

Rackingiscausedbyanumberoffactors,including:seismicforces(earthquakes),foundationsettlement, excessivewindload,volumechangesinmaterialsduetodryingand,mostimportant,thermallyinduced expansionorcontractionofmaterials.

Rackingismostlikelytooccurinpartitionsandceilingsthatabutexteriorwalls.Thereasonisthatexterior columnsaresubjecttothermalexpansionwithchangesintheweather,whileinteriorcolumnsofanoccupied buildinghavestabilizedwithinteriorclimatecontrol.Partitioncrackingasaresultofrackingtypicallyis evidencedbyseparationatthetopandsideofthepartitionatonepointandcrushingatanother.Thisismost dramaticatthetopfloorsofhigh-risebuildingswheretheeffectofexpansionisthegreatest(seefig.14).

Thesecondstress,flexural tension,occursasthewallpaneltriestofollowthedeflectionofthestructural(slab) floorand/orceiling.Thisdeflection,particularlyinflat-platedesign,canchangedramaticallyasdeadloadsand liveloadsincreasewithbuildingoccupancy,andincreasesnaturallyforsometimeafterconstruction,dueto plasticcreepoftheconcrete.Itmaytotaltwotofivetimestheamountoftheinitialdead-loaddeflection(see fig.15).Evidenceofflexuraltensionisacrackthatiswideatthebaseandnarrowsinthecenterofthefield.

Perimeter Relief Tocontroltheeffectsofeachofthesefactors,theuseofslipjointsandotherperimeterreliefisrecommended. Perimeterreliefshouldbeprovidedforveneerandconventionalplasterconstructionsurfaceswhere:

(a)partitionorfurringabutsastructuralelement(exceptfloor)ordissimilarwallorceiling(fig.16); (b)ceilingabutsastructuralelement,dissimilarpartitionorotherverticalpenetration(fig.17);and (c)ceilingdimensionsexceed30ft.ineitherdirection(fig.18).

Perimeterreliefisplacedattheends,basesortopsofpartitionsandceilingmembranes.

Typicalperimeter-relieforslip-jointdetailsforveneerandconventionalplasterconstructionareillustratedbelow.

Fig. 13type A cracking – frame movement cause racking cracks.

frame shortly afterremoval of forms

temperature increase

corner cracking

WinterSummer T=110˚F∆T=35˚F

T=-25˚F∆T=100˚F

T=75

˚F (t

empe

ratu

re in

side b

uild

ing)

max

. fre

e exp

ansio

n =

.75"

likely

expa

nsio

n =

.375

"

max

. fre

e sho

rteni

ng =

2.1

6"lik

ely sh

orten

ing

= 1.

00"

Note: movement will be cyclic with seasons, but most severe in winter.

Fig. 14

Section of a 30-story building shows effect of differences in indoor and outdoor temperatures. Right side shows how much differential movement could be expected in top floor-slab during winter; on left, movement during summer.Numerical values assume free movement of exposed exterior column due to temperature change. Actually, free movement is restricted by bending stiffness of frame and actual movement will be less.

Fig. 15type B cracking – caused by slab deflection

formed slab (20' span)

initial midspan deflection(∆=0.33" ) after framework is removed

eventual midspan deflection (∆=1" ) crack in partition

plaster partition

TrademarksThe following trademarks used herein are owned by United States Gypsum Company: Firecode, imperial, Sheetrock, USG, USG in stylized letters. Unimast is a trademark of Worthington Industries.

NoteProducts described here may not be available in all geographic markets. Consult your U.S. Gypsum Company sales office or representative for information.

NoticeWe shall not be liable for incidental and consequential damages, directly or indirectly sustained, nor for any loss caused by application of these goods not in accordance with current printed instructions or for other than the intended use. Our liability is expressly limited to replacement of defective goods. Any claim shall be deemed waived unless made in writing to us within thirty (30) days from date it was or reasonably should have been discovered.

Safety First! Follow good safety and industrial hygiene practices during handling and installing products and systems. Take necessary precautions and wear the appropriate personal protective equipment as needed. Read material safety data sheets and related literature on products before specification and/or installation.

Manufactured byUnited States Gypsum Company550 West Adams StreetChicago, IL 60661

800 USG.4YOU (874-4968)usg.com

PM17/rev.10-10 © 2010, United States Gypsum Company. Printed in U.S.A.

Theprincipleofleavingtherunnertrackfreeofpermanentattachmenttothepartitionatceilings(fig.19)and havingthestudattachedtothestructuralwallorcolumnbutnotpermanentlyattachedtothepartition(fig.20) canbeappliedtoveneerplasterassembliesaswelltorelievethestresspointatpartitionintersectionswith structuralceilingsandwallsorcolumns.

Whereasuspendedorfurredveneerorplasterceilingmeetsanexteriororstructuralwallwhichissubject tomovement,thesupportsfortheceilingassemblyshouldbelocatedwithin6in.oftheabuttingsurfaces, butneitherthemainrunnernorfurringchannelsshouldbeallowedtocomeintocontactwiththewall(fig.21). Alsothejunctionoftheceilingmembranewiththewallfacingshouldallowforsomemovementwithouta build-upofstress.

Wherefurringonanexteriorstructuralwallmeetsanotherexteriorstructuralwall,aminimum1/4"clearance shouldbeleftbetweentheacousticaltrimandtheintersectingwallorcolumn.

Wherepartitionsareconstructedacrossthejunctionoftwofloorslabswithexpansionjointsbetween,partition panelbaseattachmentmustbemadetoonlyoneoftheslabs,allowingtheotherpartitionfacingtofloatfree (fig.22).

Fig. 16 perimeter relief at junction of partition and structural wall.

relief

structure

Fig. 17 perimeter relief of ceiling at structural column penetration.

relief

ceiling

structuralcolumn

Fig. 18 perimeter relief of large ceiling surface.

relief control joint

more than 30'(less than 50' )

IMPERIAL gypsum base

Fig. 19perimeter relief of partition at structural ceiling.

runner

type S screw

metal trim

steel stud

SHEETROCK sealant

11/2" min.

IMPERIAL gypsum base

Fig. 20perimeter relief of partition at structural wall or column.

metal trim

type S screw SHEETROCK sealant

column or structural wall

Fig. 21perimeter relief of suspended ceiling at exterior wall.

IMPERIAL gypsum base

metal trim

flexible dust membrane

metal furring channels

Fig. 22construction of partition across

junction of two floor slabs

attached angle

free floating angle

expansion joint