FAILURES
Deborah Slaton, David S. Patterson, AIA, and Timothy Penich, AIA
A few years after construction, a mid-rise building in a freeze-thaw climate exhibited problems with the cast stone coping units atop parapet walls at balconies and the roof perimeter. The units are typically 610 to 1219 mm (24 to 48 in.) long, 406 mm (16 in.) deep, and approximately 102 mm (4 in.) in height, tapering to the interior. The parapet wall consists of an exterior wythe of masonry veneer with a wood stud backup and a single-ply roof membrane covering the interior side of the parapet—a self-adhering through-wall flashing membrane bridges the gap between the exterior brick and the backup wall and laps stainless steel drip edges located along each side of the parapet. The cast stone coping units are set on a bed of mortar placed atop the through-wall flashing. Sealant is installed at the joints between the coping units.
Photo © Timothy Penich, WJE
During an inspection of the parapets, the mortar bed joints beneath the coping units (some as wide as 50 mm [2 in.]), were found to exhibit widespread transverse cracking and conditions consistent with cyclic freeze-thaw damage. Where mortar remained intact, bond separations were observed between the mortar and stainless steel drip. The sealant between coping units exhibited adhesive failures, and several coping units were found to be displaced. Removal of a coping unit revealed there was no anchorage or dowels securing the coping units to the parapet structure and the mortar beneath the unit was severely deteriorated. Additionally, the self-adhering membrane did not sufficiently lap the drip edges, was torn in some locations, and had sagged into the cavity between the masonry veneer and backup wall where the membrane was unsupported, thus creating a gutter (or reservoir) where water entering the assembly could collect.
The lack of anchorage, overly thick mortar bed, and incorporation of an unsupported membrane through-wall flashing with poor integration with metal drip edges represent a confluence of conditions that culminated in failure of the coping assembly. The thick mortar bed laid on an impervious, non-absorptive flashing created several problems. For example, curing of the mortar was retarded, as the only direction of drying was through the exposed portion of the joint, resulting in the retention of moisture within the mortar and surface cracking related to localized drying-shrinkage—the membrane/metal drips also created a cleavage plane preventing mortar bond with the parapet. The lack of anchorage of the coping units and mortar bond to the membrane flashing allowed for movement of the coping units, which, in turn, contributed to sealant failure at unit joints. This failure permitted more water to enter the assembly and collect in the sagging membrane flashing where tears in the membrane enabled water to enter the wall construction. Retarded curing in conjunction with conditions allowing additional moisture to enter (and be retained) within the assembly rendered the mortar bed more vulnerable to damage from cyclic freezing and thawing and further movement of the coping units.
Recommended repairs included removal and resetting of all the coping units, with new mechanical anchorage to provide lateral securement. Replacement of the existing membrane flashing and stainless steel drip edges with a new sheet metal through-wall flashing was also recommended to provide better long-term protection against water intrusion. In lieu of a mortar bed, coping units were supported by high-density polyethylene shims, and coping unit joints (including joints between coping units and flashing) were sealed with an elastomeric sealant with weeps to allow drainage of water that may enter through breached sealant joints over time.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of The Construction Specifier or CSI.
Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com.
David S. Patterson, AIA, is an architect and senior principal with WJE’s office in Princeton, New Jersey. He specializes in investigation and repair of the building envelope. He can be reached at dpatterson@wje.com.
Timothy Penich, AIA, is an architect and senior associate with WJE in Northbrook, Illinois. He specializes in historic preservation. Penich can be contacted via e-mail at tpenich@wje.com.
