Transcription of Detailing masonry parapets
1 By Walter LaskaS t a ff Arc h i t e cM a s o n ry Advisory CouncilPark Ridge, IllinoisAparapet wall is the portion of awall that extends above theroof. Its purpose might be aes-thetic, hiding mechanical equipmentand providing a distinct roof line. Orit can be functional, helping to preventflames from spreading from rooftop torooftop. Codes often require parapetsfor this parapet walls have two ex-posed surfaces, they can suffer seriousproblems. Both sides of the wall aresubjected to temperature swings andrain. In northern regions, snow driftsup against the walls, further increasingthe possibility of moisture conditions are more severe thanthose that building walls in moderateclimates are subjected to.
2 As a result,parapet walls are more susceptible todisplacement, cracking, efflorescence,and spalling. Avoiding these problemsrequires careful Detailing and vigilantmaintenance. Two good ways of de-tailing parapets are illustrated top of a parapet wall is the mostvulnerable area. Developing an eff e c-tive detail here helps eliminate or min-imize most moisture-related an appropriate cap is thefirst step in doing a variety of copings areavailable to cap off the wall, a copingof similar material to the parapet isstrongly recommended. This would in-clude: limestone, precast concrete,hard-fired clay, or terra-cotta.
3 TheseDetailing masonry parapetsExposed on both sides, parapets are vulnerable to rain and temperaturechangesmaterials have thermal properties sim-ilar to those of brick and concreteblock. Metal caps, on the other hand,have a thermal expansion coeff i c i e n tthree to four times greater than causes a significant amount ofd i fferential movement between theparapet cap and parapet wall. Al-though slotted movement joints usual-ly are provided, leakage in metal capsis type of coping is a brickrowlock cap. Though brick rowlock isattractive, it should be used only inmoderate climates.
4 Rowlock copingscontain many exposed mortar jointsthat can retain moisture. Avoid using itin areas where severe freeze-thaw cy-cles can be cast and formed in-to a variety of shapes, but they must allpossess adequate pitch, sufficient pro-jections (at least 1 1/2 inches), andcontinuous drips. These elements pro-tect the parapet from ponding snowand water and prevent the water fromrunning down the outer wall copings are susceptible tomoisture penetration where the indi-vidual sections butt (at head joints). Toprevent moisture from entering thewall from the top, install a continuousthrough-wall flashing membrane with-in the mortar bed immediately beneaththe coping .
5 This flashing membranecan be made from stainless steel, cop-p e r, asphaltic glass fiber, vinyl, plastic,or a combination of these completely through the wall, itcreates a slippage plane between thecoping and the parapet. To prevent thecoping from sliding or blowing off, itmust be anchored to the wall below. Avariety of stainless steel anchoringsystems can be used. Use a generousamount of mastic wherever the an-chorage penetrates the flashing protects theparapet from moisture permeance atthe top of the wall, but the inner ex-posed side of the parapet remains vul-nerable, especially in areas wheresnowfall is common.
6 To protect the in-ner wall surface, tuck the roof flash-ing under counterflashing or coat thewall with a waterproofing certain conditions are overlooked,displacement of parapet walls, particu-larly at the coping , is common. As aresult, parapet walls can crack, heave,or bow. To allow movement due tothermal expansion, at one end of everycoping section install a soft joint (voidof any mortar) and at the other endplace a mortar joint. Also, a verticalexpansion joint should pass complete-ly through the parapet wall at 30-footintervals. These joints should coincidewith any other expansion joints in theroof and walls below.
7 Constructingboth wythes of a parapet with the samematerial (brick and brick or under cer-tain conditions brick and block) en-ables the separate wythes to react toweather changes as a single limitationsKeep the height of a parapet to aminimum. The American StandardBuilding Code Requirements for Ma-sonry (see reference) does not allowthe height of a parapet wall to exceedthree times its nominal thickness. Ifthe height exceeds this limit, thenmore lateral support is needed. Ve r t i-cal rebars and grout placed within thewall at appropriate intervals can pro-vide lateral support. Piers and pilastersalso can be used.
8 A parapet wall of theproper height and width, however,might still lack the weight necessary tomaintain its integrity. Anchoring thewall to the roof structure with verticalreinforcing dowels (where applicable)should solve this Standard Building Code Re-quirements for masonry , ANSI , American National Standards In-stitute, 1430 Broadway, New York,New York s noteThis article illustrates two good detailsfor masonry parapet walls. But they renot the only proper details. If you de-tail or build masonry parapet walls dif-ferently, share your design with us andour readers. Send us a drawing ofyour parapet detail.
9 Be sure to pointout its main attributes. Mail to ParapetEditor, The Magazine of masonry Con-struction, 426 South Westgate, Addi-son, Illinois #M880132 Copyright 1988, The Aberdeen Gro u pAll rights re s e rv e
