Building Design and Construction

Effects of Roof Size, Shape, and Slope

Roof size and shape often dictate material and system selection. For example, if the roof is only 2 or 3 ft wide and several feet long, a system other than built-up roofing (BUR) would probably be easier to install than BUR.
Roof slope is a major factor in determining the rate of flow of rainwater over a roof to a drainage outlet, and good drainage is essential to good performance of a roof. Hence, regardless of the type of materials or system specified, adequate slope should be provided for drainage.
For low-slope roofs, NRCA recommends that the roof be designed and built to ensure positive drainage. NRCA defines positive drainage as ‘‘the drainage condition in which consideration has been made during design for all loading deflections of the deck, and additional roof slope has been provided to ensure drainage of the roof area within 48 hours of rainfall, during ambient drying conditions.’’ Ponding water can be detrimental to roof systems and can result in:
• Deterioration of the roof surface and membrane
• Debris accumulation, vegetation, fungal growth, and resulting membrane damage
• Deck deflections (sometimes resulting in structural problems and other complications)
• Ice formation and resulting membrane degradation or damage
• Tensile splitting of water-weakened organic or asbestos felts
• Difficulties in repair, should leaks occur
• Water entry into the building if the roof membrane is punctured or fails in a ponding area
• Voiding of manufacturers’ warranties
Every roof has its own specific set of drainage criteria. Simply specifying a standard 1⁄4 in / ft (degrees) slope or 1⁄8 in / ft (degrees) slope will not ensure adequate drainage of the roof system. In order to achieve the necessary slope throughout the  entire roof area, many things should be considered, including: the structural framing system, deck type and characteristics, deck deflections between spans, roof insulation, roof membrane type, rooftop penetrations (and additional support or blocking for the deck), and the building and roof layout.
Other items which can assist in positive drainage are: tapered insulation (crickets and saddles) at key points, such as between drain locations and at the up-slope side of penetrations; recessing primary drains and scuppers slightly below the roof membrane surface; and performing maintenance semiannually to help prevent clogged drains.
The use of secondary or overflow drainage devices (e.g., through-wall scuppers) is recommended and, in most cases, required by building codes.
For steep-slope roofs, the minimum recommended slope is a function of the type of roof-covering material used and the type of underlayment. The minimum recommended slope for most steep-slope roof coverings is 412 (degrees), unless special provisions are made. Depending on the material, some manufacturers and building codes allow lower slopes. Since most steep-slope materials are water-shedding, rather than waterproofing, a steeper slope typically decreases the likelihood of damaging or other undesirable conditions.


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