What the Deck? Wood that is ‘Noncombustible’
Friday, July 23rd, 2010It sounds like a great idea- use the top of a building for outdoor living in an urban setting where you can sit outside in the summertime without the benefit of having a patio or balcony. Amidst the glass, steel, and masonry of the metropolitan skyline, these decks form an oasis of outdoor living space in any city, transforming simple roofs to a backyard-feel country club setting. The perfect solution for urban dwellers- or is it? Anyone who rides the elevated trains in Chicago has seen hundreds of examples of rooftop decks on both residential and commercial buildings. Many decks have been built over the years, some following building code regulations, some not.
Rooftop decks can raise some serious concerns when trying to meet regulatory requirements. For example, you may need to have the rafter system of the roof re-engineered to carry the excess loads of the deck and the anticipated number of people on it. Also, building a deck over a roof never takes into account just how you will replace the roof in 20 years without deconstructing the deck to do it. At that point, will the deck be a like-for-like replacement (like a repair) or a reconfigured alteration to an existing building? What about the deck material itself?
Chicago Building Code Section 15-8-510, Roof Structures, indicates that except for certain other limitations, all roof structures placed above the roof of any building within the Fire Limits (essentially the downtown Central Business District) or above the roof of any building exceeding fifty-five feet in height, must be constructed of non -combustible materials and must be supported by construction of non-combustible materials.
Recent experience has shown that conventional flamespread treatments and intumescent coatings are NOT an acceptable alternative to the requirement that the deck be of non-combustible material. We have tried these both at the Building Board of Appeals and the Committee on Standards and Tests and have not been successful in either case. I do not believe the City will be changing its stance any time soon.
As a substitute product, composite/synthetic decking (manufactured from wood fiber and plastic to form a deck profile) is really no different than wood in terms of its combustibility. In addition, there have been problems with discolorations, de-lamination and mold when used in exterior applications. Many of these products are not chemically designed to withstand exterior exposure for an extended period of time.
However, the Chicago Department of Buildings has in the recent past, recommended and approved at least two substitute materials for high-rise decks the we are aware of: Ipe Wood and wood-glass composites. Although both still qualify as combustible, their physical properties are considered to be an acceptable alternative.
Ipe decking has been used in several large residential and commercial buildings, including LEED certified projects. Examples include the Brooklyn Bridge pedestrian walkway, the Treasure Island Casino in Las Vegas, and the boardwalks in Ocean City, NJ and Miami, Flordia.
Ipe is a Brazilian Walnut and has a Janka hardness of 3640.
Note: The Janka hardness test measures the hardness of wood. It involves measuring the force required to embed a 0.444 inch steel ball into wood to half its diameter. This method is used so that the result would leave an indentation 100 square millimeters in size. It is one of the best measures of the ability of a wood species to withstand denting and wear.
Ipe is also rated the same as concrete for flammability testing. This hardwood is so dense that it is resistant to mold, fungus even insects. No chemicals are required and no treatments are needed unless you want to to keep the tones of the wood from fading. If no treatment is used it will patina a silverish grey tone, otherwise a UV oil inhibitor can be used. The lifespan for Ipe decking is over 30 years old without treatment and over 100 years with treatment. Ipe is also available FSC certified with select approved companies. This certification guarantees the hardwood is harvested from a responsibly managed forest. Ipe is a renewable, recyclable, durable, biodegradable, energy efficient and versatile product.
Products that infuse glass into wood decking are another alternative. A chemical bonding of sodium silicate (a mixture of sand and soda ash used since the 19th Century in detergents and as an egg preservative) and wood soaked in this solution, then (literally) baked, becomes an insoluble matrix of amorphous glass, which hardens and essentially “shrink-wraps” the wood fibers throughout. Due to this glass infusion, the wood is inert and will not rot. Because conventional fungicides like copper are not required to pressure-treat the wood, the resulting product is non-corrosive to nails and other fasteners. The fusion of wood and glass creates a product that is approximately twice as hard as conventional wood. As measured by the Janka Scale, the hardness of southern yellow pine, for example, increases from 690-870 lbs-force (range depends on species) to 1560 lbs-force, approximately double. Since the deck material is many times stronger that composite products due to the glass portion fusing parallel to the grain of the wood, this insures that nails, screws and fasteners hold more firmly.
The best part, however, from a regulatory standpoint, is that the wood-glass matrix is not flammable. Years ago we would laugh when a note would appear on construction drawings referring to the “noncombustible wood blocking”; since the only noncombustible wood in existence was petrified rock. Now that is no longer the case. Wood-glass composites carry a Class A classification and also for reduced levels of smoke, in accordance with the 30-minute extended ASTM E84 (Steiner Tunnel) flamespread test.
Both of these alternatives offer the look of wood with the added benefit of being safer and more durable than a conventional wood deck. Now if you could just do something about that bothersome requirement for a second exit off the deck (but that’s another story).
About the Author:
Christopher E. Chwedyk, CSI, AIA is a licensed architect, Director and Chief Code Consultant of The Code Group at Burnham Nationwide in Chicago. He was previously the principal of Gage-Babcock and Associates; a firm specialized in fire protection engineering. With more than 32 years of experience in the architectural field, Mr. Chwedyk has performed numerous code compliance plan reviews for the City of Chicago and other municipalities. he has a BArch degree from UIC and a Masters of Project Management (MPM) from the Keller Graduate School of Management. An adjunct faculty member of Harper College since 1998, Chris teaches courses on building codes and construction drawings.