I S S U E 2/2018  VOLUME 35  NUMBER 2

Journal of architectural technology published by Hoffmann Architects, Inc., specialists in the rehabilitation of building exteriors. Roof Drainage Systems

Richard W. Off, AIA and Steffany Malarik, Assoc. AIA

A mong the most fundamental of perimeter and into a clay, wood or architectural elements is the roof. Its metal gutter system, or through a principal purpose is to shelter a build- drainage outlet in the exterior walls. ing’s occupants, contents and structure Responding to the local climate of dif- from the elements, most critically from ferent regions, modern roof systems precipitation. While the various layers evolved to include not just steep of the roof assembly play a key role slope assemblies, but also low slope in keeping moisture from infiltrating roofs, often (mistakenly) referred to into the interior, adequate weather as “flat roofs.” Originally found in arid protection also locations with little annual rainfall, low depends on the slope roofs have a subtle pitch, allow- roof’s ability to ing the roof assembly to be hidden drain moisture behind a parapet wall, or portion of away from the the facade that extends above the building. If rain roof plane. water, snow, and ice accumulate, In regions with substantial precipita- the moisture tion, steep slope roofs were more can degrade and common. The pitch can be quite permeate the prominent and play a significant role roofing assembly. in the building’s overall design. Steep Not only does pitches are achieved by a variety of standing water roof shapes, but gable, hipped and negatively impact mansard roofs are among the most Different roof types demand different drainage strategies. the performance, prevalent. integrity and lon- As mechanical and plumbing system gevity of the building enclosure, but it technologies and programmatic re- also can deteriorate the structural and quirements evolved with the modern interior elements the building enclo- world, roof types began to respond sure is meant to protect. less to climactic conditions, and more Historically, roof drainage was ac- to interior layout and contemporary complished by simply sloping the roof styles. Low slope roofs are now com- downward. Water could then pour monly used for high-rise and long-span off overhanging roof eaves onto the commercial and industrial buildings, ground below, or, in more sophisti- as well as urban residential buildings. cated assemblies, flow toward the Steep slope roofs are more typically

Richard W. Off, AIA, Staff Architect, specializes in considerations of urban environments, developing roofing solutions for historic, landmark, and modern structures. Steffany Malarik, Assoc. AIA, Project Coordinator, applies sustainable design strategies to challenging vegetative and traditional roof projects. JOURNAL

(continued from page 1) found on suburban and rural residen- Typical PRMA Low Slope Roof Assemblies tial buildings, as well as many cultural, civic and institutional buildings of di- Pavers on Pedestals verse scales and locations. PAVER To promote the efficient function of PEDESTAL roof drainage assemblies in these dif- FILTER FABRIC ferent roof types, it is important to INSULATION recognize typical deficiencies and ap- DRAINAGE MAT ply preventive maintenance strategies

WATERPROOFING that keep the system working effec- SLOPE OF ROOF DECK MEMBRANE tively. In some cases, persistent prob- ROOF DRAIN lems cannot be addressed through ROOF DRAIN LEADER PIPE maintenance alone, and a design pro- fessional should evaluate the capacity, STRUCTURAL ROOF DECK configuration, and detailing of drainage assemblies to determine whether they are adequate for the climate and roof Pavers in Setting Bed area. Appropriate slope to drains, as PAVER well as flashing details and drain out-

SETTING BED lets, should also be considered as part of a thorough investigation. DRAINAGE MAT Often an afterthought in a reroof- INSULATION ing or new construction project, WATERPROOFING MEMBRANE roof drainage systems tend to be SLOPE OF ROOF DECK BI-LEVEL ROOF given little attention – until they fail. DRAIN Considering the damage water can do ROOF DRAIN LEADER PIPE when allowed to pond on roofs and overflow from gutters, it is clear that STRUCTURAL ROOF DECK ignoring roof drainage issues can lead to urgent problems. Better to design Intensive Green Roof appropriate drainage assemblies and address incipient concerns now, than PLANTINGS to chase after water damage later.

SOIL Low Slope Roofs SAND Low slope roofs are not actually flat, FILTER FABRIC as they may appear, but instead have GRAVEL a subtle pitch (slope) of between one

DRAINAGE MAT quarter inch per foot (2%) and two inches per foot (17%). The pitch of the INSULATION roof is created by varying the height of PLANTER ROOF DRAIN building columns to slope the struc- tural roof deck, by installing tapered ROOT BARRIER insulation, or a combination of both. SLOPE OF ROOF DECK WATERPROOFING MEMBRANE Positive Drainage

ROOF DRAIN LEADER PIPE Achieving expeditious drainage is critical to maximizing the lifespan of STRUCTURAL ROOF DECK a roof. The goal is to direct all water towards roof drains and create positive ILLUSTRATIONS: STEFFANY MALARIK

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roof drainage: “the drainage condition in which consideration has been made for all loading deflections of the roof deck, and additional slope has been provided to ensure drainage of the roof within 48 hours of precipitation” (Chapter 2, 2015 International Building Code). The number and location of primary In this vegetative roof above a parking garage, insufficient drainage caused ponded water(left). interior roof drains is determined by The new assembly (right) incorporates waterproofing membrane, drainage mat, and gravel. structural requirements limiting deflec- tion, and by plumbing requirements, could entrap water if the primary a polystyrene core with an array of based on vertical drain leader diame- drains fail. raised buttons that provide space for ter, roof slope, roof area per drain, and It is common to add such secondary water to move. On the top side, wo- rainfall rate. Typically, a low slope roof drains during a roof replacement proj- ven filter fabric allows water to pass is designed with one primary roof ect. Secondary / emergency overflow through, while trapping soil particles drain and one secondary roof drain -- roof drains look similar to primary that could impede the flow of water. or overflow scupper (drainage outlet) drains, but a standpipe or dam blocks This is an essential layer in the PRMA. at the perimeter of the building -- per water from entering the drain until It is important for the ballast to allow structural bay. it reaches a certain depth. Overflow water to seep down to the drainage Typical Problems and Design scuppers can also act as secondary mat layer. Large river gravel was the Considerations drains at the perimeter of the roof. typical choice, but some areas, such as City, restrict use of stone Clogged Drains. Standing water can Scuppers are typically easier to install during a roof replacement project ballast, and instead pavers on pedes- cause myriad problems, and, if left tals are standard. The pedestals elevate unaddressed, can damage the water- when additional secondary drains are required; however, the visibility of the the pavers and allow water to flow proofing membrane, void the water- through the joints and beneath pavers. proofing warranty, and cause dirt and scuppers may be undesirable. debris to collect, resulting in unwanted Roof Assembly Design Deficiencies. Another form of PRMA sets pavers in vegetation. Eventually, long-term water Other causes of standing water that a sand or mortar setting bed, rather accumulation could cause detrimen- require more extensive work to diag- than on pedestals. Below the setting tal structural deflection. Any sign of nose and fix are related to the roofing bed are drainage mat, insulation, and ponding water should be promptly assembly. a waterproofing membrane adhered addressed. directly to the structural deck. In this Protected Roof Membrane Assemblies type of assembly, most of the water One common cause of standing water, (PRMA) are one of the most common does not flow down through the pav- which can be quickly remedied, is and efficient waterproofing systems ing surface; rather, it flows along the clogged drains. It is recommended that for low slope roofs. Originally referred top of the pavers. Some water will roof drains be cleaned out regularly, to as an Insulated Roof Membrane inevitably penetrate the sand or mor- and drain bodies replaced as part of Assembly (IRMA), this type of roof tar joints, so it is important to include roof replacement projects. system consists of a waterproofing bi-level roof drains, which receive water Inadequate Number of Drains. membrane adhered directly to the through a grate at the paving surface Drainage problems may also be at- structural deck, atop which sit drain- level, as well as through a perforated tributed to an insufficient quantity of age mat, insulation, filter fabric, and screen at the roof membrane level, drains for the size of the roof area or some type of ballast. Water flows for water that seeps below the pav- for the amount of precipitation. The through open joints in the ballast, ing surface. This type of assembly is 2015 International Plumbing Code re- down to the drainage mat layer, where especially susceptible to unwanted quires that emergency overflow roof the slope of the structural deck directs plant growth, as water soaks through drains or scuppers be provided wher- the water into drains. the joints and is not able to freely flow ever the roof perimeter construction Drainage mat is typically composed of through the setting bed to the roof

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drains. Trapped water can also cause tapered insulation are the “two-way” Steep Slope Roofs the pavers to spall, crack or develop and “four-way” systems. Two-way Principal Design Considerations efflorescence. tapered insulation functions by split- As with low slope roofs, the first and Vegetative roofs, commonly referred to ting the roof drain catchment area most essential step in achieving a as “green roofs,” are another form of into two parts and creating a primary functioning drainage system for steep PRMA. Vegetated roofs typically con- slope at each part, forming a V-shaped sist of either a roof assembly and addi- linear valley that directs water toward slope roofs is to maintain a minimum tional landscape material components, the center of the roof. Another layer roof slope. Typically, a steep slope or pre-vegetated trays assembled on of tapered insulation, referred to as roof slope is equal to or greater than top of a roof covering. a saddle, is then used to divert water 3 inches of rise per 12 inches of run into the appropriate roof drain. (25%). In addition to elevated green roofs atop buildings, vegetative roofs at A four-way system functions by creat- Establishing a minimum slope is critical, grade, consisting of lawn areas and pla- ing square roof drain catchment areas because most steep slope roofing as- zas, may conceal underground parking with tapered insulation panels sloped semblies include claddings of asphalt, garages or other usable space. Such equally in four different directions with slate, clay tile or wood shingles, or green roof assemblies usually include the high point at the perimeter of the seamed metal. Although the shingled gravel and sand in addition to the roof and the low point at the roof elements are lapped to provide drainage mat. These additional layers drain. The intersection of the panels positive downward drainage over the of drainage media help prevent plant- creates mitered valleys that direct wa- assembly, the lower and side seams ings from becoming too saturated for ter into the roof drain. between lapped shingles are gener- proper growth. When undertaking Crickets are additional layers of ta- ally not uniformly sealed. Apart from green roof replacement projects on an pered insulation installed on the high ice and water barriers required at underground structure, it is important side of large penetrations to direct eaves, typical felt underlayments do to consider the area of existing pervi- water around the obstacle. Crickets not perform as full moisture barriers. ous (permeable) surfaces. If the area and saddles are typically twice the Therefore, if the roof slope is too low, of impervious surfaces is increased slope of the adjacent roof. it can allow for standing water trapped behind melting snow and ice (known beyond the capacity of the existing Low Slope Roof Solutions storm water management system, a as ice damming), or resulting from oth- new system must be designed and in- Standing water due to clogged or er blockages, to infiltrate beneath the stalled to accommodate the increased inadequate drains, or because of insuf- shingles and into the roof structure. volume of water shed by the expand- ficient slope to drains, is, unfortunately, Of equally great importance to slope ed impermeable area. commonplace for low slope roof is a continuous and controlled path assemblies. The first step to address for water to drain once it has reached Traditional roofing assemblies consist persistent ponding is to determine the lower perimeter of the steep of rigid insulation adhesively or me- the cause of poor drainage, which slope roof. This is ordinarily achieved chanically attached to the structural may involve investigative probes into with either prefabricated or custom- deck, with waterproofing membrane the roof assembly, as well as evalua- designed pitched gutters attached to installed on top. In this assembly, all tion of existing drainage components. downspouts or leader pipes placed at water flows along the waterproof- In addition to regular maintenance certain intervals or intersections along ing membrane and is directed into to prevent clogged drains, the roof a building’s facades. Conductor heads, roof drains. When an existing struc- assembly may need modification to enlarged catch basins that improve tural roof deck does not provide the improve water flow to drains and to drainage capacity, are sometimes used minimum required slope, or other provide sufficient drainage capacity as connector pieces between gutters components of the roof, such as sky- during heavy rainfall. Providing a clear and downspouts. lights or curbs, disrupt the slope of the path for water movement across the structural deck, tapered insulation, as roof assembly, as well as adequate roof Although perimeter drainage com- part of a traditional roofing assembly, slope and flow around penetrations, ponents can be concealed, as with can be utilized to establish an appro- can improve the longevity of the roof built-in gutter linings hidden behind priate pitch. and deliver reliable water management parapets, or leader pipes placed inside The two most common layouts for for the life of the system. walls, they can also be prominent

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design features, especially on Incorrect Termination. historic structures. Gutters Typical Steep Slope Drainage Assembly Properly terminating the and conductor heads may roof drainage system at the double as a decorative cor- base of the facade is also an nice or fascia, and exterior important consideration. At downspouts may comple- a minimum, downspout bas- ment a facade’s masonry or- es should be located above namentation. These ornate precast or stone splash drainage elements are a blocks or drainage trenches major consideration in res- to divert water in a con- toration and roof replace- trolled manner. However, it ment projects for landmark might be preferable to tie structures, as the relevant in the downspouts to the local and/or state regulating UNDERLAYMENT below-grade storm drain agencies, such as the State SHINGLES system. This may require consultation with a plumbing Historic Preservation Office ICE & WATER (SHPO), often require BARRIER engineer to determine if the matching replacement parts storm system can support CLEAT with historic profiles and the rainwater load from the original material type, finish, SHEET METAL roofs. GUTTER and/or color. Material Deterioration.

GUTTER BRACE Distress and deficiencies in Primary Problems, GUTTER Solutions, and System EXPANSION CAP the materials of steep slope SHEET METAL drainage system compo- Maintenance DOWNSPOUT GUTTER BRACKET nents are another signifi- Perhaps the most criti- CONDUCTOR SHEET METAL cant concern. It is essential DOWNSPOUT cal issues with steep slope HEAD for materials to be care- DOWNSPOUT drainage systems are roofs BRACKET fully selected and treated to with inadequately sized, promote durability, provide poorly placed, or even miss- SPLASH BLOCK protection against corrosion ing drainage components. from long-term moisture Per International Building exposure (rust), and avoid Code (IBC) requirements galvanic corrosion, which and design standards pub- results from direct contact lished by Sheet Metal and between dissimilar metals. Air Conditioning National Corroded metal ultimately ILLUSTRATION: RICHARD OFF / ARI LEVINE Association (SMACNA), becomes weak and brittle, gutters and downspouts roofs, and at facades below. If no gut- causing it to lose mass and must be designed to accommodate ters or downspouts exist, rainwater form holes. This increased porosity the hourly rainfall intensity of the will be dumped directly onto adjacent precipitates leaks by increasing the geographic area in which the building roofs or ground, which can overcharge potential for moisture infiltration and is located. the neighboring roofs with additional eventual deterioration at adjacent sur- Insufficient Drainage Capacity. If gut- rainwater, and/or oversaturate the faces. Therefore, significantly corroded ters and downspouts do not provide ground. This may lead to structural components should be partially if not an adequate cross-section for rainwa- damage at those adjacent roofs and fully replaced. ter draining from a given roof catch- moisture infiltration and deteriora- Valued for their high workability, ment area and pitch, the water will tion in ground floors, basements, and longevity and natural resistance to become backed-up and spill over, po- foundations, and could create hazard- rust, tin, lead, and especially cop- tentially leading to moisture infiltration ous ice sheets at paved areas in winter per sheet metal was favored histori- and eventual deterioration at adjacent months. cally for steep slope drainage system

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materials, such as copper or even the more immediate concern of fall- wood. Cast iron is sometimes used ing material. It is essential for gutters for downspouts and, because of its and downspouts to have adequate rigidity, commonly employed for securement and frequency of support, downspout boots and other below to preserve their structural integrity, grade pipes. resistance to wind uplift, and minimum Restrained Movement. Connections pitch for drainage flow. between materials are an equally Clogs and standing water can also important consideration in drainage occur at built-in gutters when roof Undersized gutters led to facade systems, to allow for thermal ex- penetrations create obstructions in deterioration at eaves and downspouts. pansion and further protect against the gutter’s drainage path. Where galvanic corrosion. Expansion joints, possible, such obstructions should be regularly spaced openings filled with avoided by placing penetrations high flexible, elastic material, or detailed to along the roof slope, and by installing allow for movement, should be placed crickets, saddle-shaped flashing used at a maximum of every 50 feet along to divert water around penetrations, sheet metal gutters, depending on similar to those used on low slope the gauge of the metal, the depth- roof assemblies. It is also essential to to-width ratio, and the edge distance provide robust waterproofing and from downspouts or other fixed flashing around built-in gutters, as they Clogged with debris, these inadequate points. To avoid leaking and corroded pose a high risk for water intrusion gutters are filled with standing water. connections, joints between metals into the building. components. Over time, with expo- should be sealed with rubber gaskets, Providing conductor heads (also sure to the elements, copper gradually silicone sealant, or lead and oakum, or known as leader heads) at gutter/ forms a patina, an outer layer with a they may be fully soldered or welded, downspout connections is one way distinctive green color that insulates depending on the type of connection to mitigate some of these drain- the copper from deterioration and and metals used. age issues. Not only do they act as corrosion. Copper may also be lead- Standing Water. Arguably the most cleanout points for downspouts, but coated to protect against galvanic ac- commonplace issue with steep slope they also improve flow rates and even tion when tied in to other metals, and drainage, as with low slope drainage, reduce noise by introducing air into to avoid patina staining of neighboring is standing water from clogged gutters the system. During heavy storms, they surfaces. and downspout drains. Clogging can also operate as an overflow drain, Although less workable, stainless and result from excess dirt, debris, and preventing gutters from becoming galvanized steel sheet metal are ac- vegetative growth accumulating in the overcharged by allowing water to be ceptable alternatives. Compared to drainage system. Roofs located be- released at a controlled point, rather galvanized, stainless steel provides the neath trees, especially deciduous trees, than uniformly along the length of the more long-term resistance to rust, as it are most susceptible to clogging from gutter and onto the facade. The use relies on the properties of its chemical leaves and small branches that fall into of snow guards and/or fences along composition, and not just a thin zinc gutters and block the drainage path. the perimeter of steep slope roofs is coating, to provide protection. While These accumulations, combined with also beneficial for winter months, as structurally weaker than steel, alu- the standing water they engender, it mitigates snow and ice build-up in minum and polyvinyl chloride (PVC create spillover in downpours and gutters. plastic) are options that have natural generate excess weight in gutters, po- While other design measures can be resistance to corrosion. Aluminum tentially causing them to sag or break taken to reduce debris accumulation, can be paint finish coated, and PVC loose from the support brackets and such as installing removable gutter can be injection-molded with dyes, connections. Therefore, clogged gut- screen covers and wire strainers at allowing these materials to achieve a ters not only increase the potential downspout drains, it is still impera- wide range of colors which can mimic for leaks and moisture infiltration at tive for building owners or managers other more expensive construction facades and roofs, but they also pose to conduct routine cleanings of the

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Roof Drainage Federal Reserve Bank of New York New York, New York Persistent drainage issues can lead Roof Replacement, Rehabilitation of to big roof problems. Ponded water, Drains, Scuppers, and Drainage Layer vegetative growth, overflow, ice damming, and deterioration are just Farmingdale State College (SUNY) some of the many concerns that can Lupton Hall arise from incorrectly designed or Farmingdale, New York installed drainage systems. Roof Replacement and New Drainage National Air and Space Museum, Udvar- System for Complex Roof Geometry Hazy Center in Chantilly, Virginia. Roof Whether for traditional low-slope Replacement, with Design for Complex Drainage roofs, protected roof membrane First Presbyterian Church Conditions at Aviation Hangar Barrel Roof. assemblies, green roofs, historic slate, New York, New York tile, or copper, or other systems, Roof Replacement, Rehabilitation of removing water from the roof as Built-in Gutters, Masonry Restoration efficiently as possible is critical to roof Washington Irving Library lifespan and maintainability. Brooklyn, New York Hoffmann Architects has developed Slate Roof Replacement, Gutter System solutions to drainage issues at diverse Redesign to Original Historic Proportions facilities, including: Saint Fidelis Church and Rectory Kingsbridge Armory in Bronx, New York. Morgan Stanley College Point, New York Roof Investigation and Drainage Design Analysis. Purchase, New York Building Enclosure Assessment, Replacement of Green Roof, Stormwater Evaluation of Roof Drainage Systems Management System, and Roof Drainage New York, New York Queensborough Community College Roof, Drainage, and Waterproofing Vassar College Medical Arts, Science, and Consultation Sanders Classroom Building Administration Buildings Choate Rosemary Hall Poughkeepsie, New York Bayside, New York Brownell Building Copper Roof Replacement, Redesign of Replacement of Roofs, Multiple Drain Wallingford, Connecticut Built-in Gutters Types, Scuppers, Gutters, Downspouts Roof Replacement, including Slope and Solow Building, 9 W 57th Street Drainage Improvements New York, New York Whitman Requardt & Associates Redesign of Canopy Drainage Channel Headquarters Saint Mark’s Episcopal Church Baltimore, Maryland New Canaan, Connecticut Water Infiltration Investigation, and Parish Hall Gutter Reconstruction Evaluation of Roof Drainage Issues Countee Cullen Library United States Capitol Worcester Polytechnic Institute, Boynton Hall in Worcester, Massachusetts. Slate Roof New York, New York Washington, District of Columbia Replacement, Correction of Deficiencies at Facade, Window, and Roof Replacement, Dome Internal Gutter System Historic Water Diverters. Drain and Scupper Replacements Restoration / Reconstruction

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Hoffmann Architects, Inc. 2321 Whitney Avenue Hamden, CT 06518

ADDRESS SERVICE REQUESTED

(continued from page 6) damaged, or undersized gutters and leaders can compromise roof lifespan JOURNAL is a publication of Hoffmann and pose a danger to building users. Architects, Inc., specialists in the rehabilitation of building exteriors. The By identifying and correcting steep firm’s work focuses on existing struc- slope drainage issues as they arise, tures, diagnosing and resolving prob- building owners and managers can lems within roofs, facades, windows, avoid the costly and disruptive reha- waterproofing materials, structural sys- bilitation of compounded problems. tems, plazas/terraces, parking garages, and historic and landmark structures. We also provide consulting services for At the surface, these pavers seem fine. But Weathering the Storm new building construction, as well as trapped water below may be causing leaks. Effective roof drainage means reliable litigation and claim support. protection from the elements. Regular entire drainage system. This should be For address changes, free subscriptions, inspection of roof water management or information on the topics discussed done by insured professionals at least components, from drains and gut- in this issue, contact our Marketing twice per year, as much as three or ters to crickets and flashings, enables Department at 800-239-6665, four times during especially wet years, proactive remediation of emerging [email protected], or: and even more frequently for steep problems. Sometimes, the fix may be 2321 Whitney Avenue slope roofs located beneath overhang- as simple as clearing a clogged drain. Hamden, CT 06518 ing trees. Other times, the intervention may be 203-239-6660 Key Tips for Steep Slope Roof more involved. In either case, waiting 1040 Avenue of the Americas, Ste. 14C Drainage until problems are severe and perva- New York, NY 10018 sive only serves to make finding the 212-789-9915 Pitched rooflines mean that steep source of the problem, and repairing 2711 Jefferson Davis Highway, Ste. 333 slope roofs have the advantage of it, more complicated and expensive. Arlington, VA 22202 shedding water readily. However, once Whether for steep or low slope roofs, 703-253-9800 that water reaches the edge of the it pays to spare some time and atten- roof, it needs to be channeled in a tion on a regular basis for the decided- www.hoffarch.com controlled manner to the ground or ly un-glamorous, but all too important, Editor/Production: Alison Hoffmann storm water system. Missing, clogged, job of attending to roof drainage.

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