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Architect Disciplined for Designing a Flat Deck

by Thomas K. Butt, FAIA
March, 1999

The Uniform Building Code requirements for plaza decks (waterproofed surfaces designed for pedestrian-type occupancy) has always been somewhat ambiguous in the past, inviting experts in construction litigation to testify both in defense of and in criticism of architects who have designed decks without slope. One part of that argument has been whether or not a deck is a "roof." As far as the surface of decks (roof or no roof) are concerned, the California Building Standards Code is now clear. Section 1506.1 states, "Unless designed for water accumulation in accordance with Section 1605.6 and approved by the building official, roof systems shall be sloped a minimum of 1/4 unit vertical in 12 units horizontal (2% slope) for drainage." Section 1402.3, Waterproofing Weather-exposed Areas, states, "Balconies, landings, exterior stairways, occupied roofs and similar surfaces exposed to the weather and sealed underneath shall be waterproofed and sloped a minimum of 1/4 unit vertical in 12 units horizontal (2% slope) for drainage."

What the California Building Standards Code does not address is the substrate where a membrane is applied on a plaza deck with a protected membrane. This type of construction is common where the walking surface is concrete, ceramic tile, stone or other paving material, and a bi-level drain is used that accepts both surface run-off and water at the membrane level. All published industry standards, including those of the National Roofing Contractors Association (NRCA), the Tile Council of America (TCA), and ASTM Standards clearly require or recommend a positive slope to drain at the membrane level, usually between 1 and 2 percent. Similarly, the overwhelming majority of waterproofing membrane manufacturers require or recommend positive drainage at the membrane level. Some defenders of flat decks have maintained that certain manufacturers recommend, authorize, or even "guarantee" their products for use on flat substrates. Our experience has been that, when confronted with a requirement to document such a position, these manufacturers usually waffle or decline to be pinned down. Even when a "guarantee" is extracted, it is usually limited to replacement of any waterproofing material that has failed.

Although the debate over substrate slope in protected membrane roofs may continue, a recent disciplinary action by the California board of Architectural Examiners seems to have ended any debate over surface slope. The board issued an administrative citation that included a $500 civil penalty to Robert N. Thompson, Architect License #C-16572, for a violation of Business and Professions Code Section 5584 (Negligence). The action was taken based on evidence that he prepared drawings for a condominium project that included an open deck over a parking garage. The drawings for the deck provided three floor drains; however the deck was flat without any sloping to allow drainage, causing water to stand on the deck during and after the rains, as well as after washing and cleaning of the deck. Consequently, the surface material deteriorated, and standing water was a problem. The design of a flat, elevated solid surface exposed to the elements without adequate means for water to flow and drain was determined not to be within the standard of care and could be dangerous. Thompson paid the civil penalty, satisfying the citation. (California Architects, a newsletter of the California Board of Architectural Examiners, summer 1998)