The importance of proper roof slope

1 Tech today The importance of proper roof slope by Mark S. Graham Providing proper slope to aid in timely slope , particularly for new construction. analysis. ASCE 7, Minimum Design P. drainage is an important consideration when designing and installing quality, long-lasting, low- slope membrane roof assemblies. For example, the International Building Code, 2003 Edition, (IBC) indicates, .. roofs shall have a design slope of a minimum of one-fourth unit vertical in Loads for Building and Other Struc- tures, provides additional guidelines about how to conduct this analysis. 12 units horizontal (2 percent slope ) for The reasons This month, I will review guidelines drainage except for coal-tar built-up and requirements for minimum roof There are several reasons why provid- slope applicable to low- slope mem- roofs that shall have a design slope of a minimum of one-eighth unit vertical in ing proper slope is important.

2 Ponded brane roof assemblies. water is detrimental to most low- slope 12 units horizontal (1 percent slope ).. roof membrane types and likely will NRCA guidelines The reason building codes typically result in premature roof surfacing and have prescriptive definitions for mini- membrane deterioration and debris In The NRCA Roofing and Water- mum roof slope is building code offi- accumulation. In addition, vegetation proofing Manual, Fifth Edition, NRCA cials want to readily identify adequate and fungal growth in ponded areas can recommends all roofs be designed roof slope when reviewing plans before result in membrane deterioration and and built to have positive drainage. actual building construction. deck deflection because ponded water NRCA defines positive drainage as the may cause structural deformations.

3 Drainage condition in which considera- For reroofing operations, some tion has been made during design for building codes allow for performance- Also, ponded water can cause ice all loading deflections of a deck and based measures of positive drainage for formation and resulting membrane additional roof slope has been pro- defining required minimum roof slope . degradation or damage, tensile split- vided to ensure drainage of a roof For example, IBC's Section ting with some membrane types, diffi- area within 48 hours following rainfall Exception indicates, Reroofing shall culties in making repairs and water under conditions conducive to drying. not be required to meet the minimum entry into a building if leaks occur. design slope requirement of one- NRCA has chosen the performance- NRCA's Project Pinpoint, a database quarter unit vertical in 12 units hori- of problem roofing projects submitted based approach to positive drainage and zontal (2 percent slope ) in Section 1507 by NRCA-member roofing contractors, not a prescriptive minimum roof slope for roofs that provide positive roof reveals some compelling data support- value, such as 1 4-in-12 ( degrees), be- drainage.

4 IBC has adopted NRCA's ing the importance of proper roof cause specific project conditions often affect roof assemblies' abilities to drain definition for positive drainage. slope for low- slope membrane roof sys- properly in a timely manner. For exam- One item to be aware of when pro- tems. More than two-thirds of all prob- ple, for buildings constructed with roof viding a roof slope less than 1 4-in-12 lem job reports in Project Pinpoint had decks that truly are flat, installation of a ( degrees) is most codes then re- roof slopes reported to be less than 1 4- 1. 8-in-12 ( ) tapered installation quire a roof structure be designed to in-12 ( degrees). This single at- system may be adequate to provide pos- account for ponding instability. IBC's tribute is by far the largest correlating itive drainage.

5 Section Instability attribute of all the problems reported indicates: .. Roofs with slopes less in Project Pinpoint. On the other hand, roof structures and decks constructed with marginal than one-fourth unit vertical in 12 From this, one reasonably can con- finish tolerances, built-in camber or units horizontal (2 percent slope ) shall clude providing for adequate roof excessive load deflection may require be investigated by structural analysis to drainage is the most important consid- more than 1 4-in-12 ( ) slope ensure they possess adequate stiffness eration in designing and installing to achieve positive drainage. to preclude progressive deflection ( , quality, long-lasting, low- slope mem- instability).. The larger of the snow brane roof assemblies. load or rain load shall be used for this Code requirements analysis.

6 The primary drainage system Mark S. Graham is NRCA's Most building codes contain prescriptive within the area subjected to ponding associate executive director provisions mandating minimum roof shall be considered blocked in this of technical services. 64 Professional Roofing | March 2005.