Terminology

• Primary drain: main roof drain connected to storm piping
• Secondary (emergency overflow) drain or scupper: backup that operates when primary drains are blocked
• Overflow dam: raised standpipe or dam in a drain body that sets the overflow height
• Combination drain: primary and overflow in one body
• Dome strainer: cast iron, aluminum, or plastic dome that keeps debris out
• Clamping ring: ring that clamps the membrane to the drain body
• Sump receiver / underdeck clamp: components that recess and secure the drain at the deck
• Controlled-flow drain: drain with a weir that limits flow and stores water on the roof temporarily
• Siphonic drainage: full-bore system with anti-vortex drains; smaller, flatter piping
• Scupper: opening through a parapet, often with a conductor head and downspout
• Cricket / saddle: sloped area diverting water around curbs and between drains
• Tapered insulation: sloped insulation boards that create drainage slope on a flat deck
• Ponding: water that remains after drainage; adds load and degrades membranes

Standards

Ratings

• Drains sized for design rainfall intensity and the drain's published flow at the maximum anticipated ponding depth
• Scuppers sized by flow and allowable ponding depth
• Gutter and downspout sizing by SMACNA methods for residential and commercial sheet metal

Test standards

• FM approvals for drain wind and membrane securement where FM-insured
• ANSI/SPRI GT-1: gutter securement where gutters secure membrane edges
• SMACNA Architectural Sheet Metal Manual: gutters, scuppers, and conductor heads

Materials

• Cast iron drains: standard commercial body
• Aluminum and plastic drains: retrofit and lightweight applications
• Retrofit drains: insert into existing drain lines during reroofing
• Gutters and leaders: galvanized or painted steel, aluminum, copper
• Piping: cast iron, PVC, or ABS storm drainage, insulated where condensation forms

Detailing

• Place drains at low points away from columns' high points; avoid walls and edges where possible
• Use crickets on the high side of curbs and between drains
• Set overflow drains or scuppers above primary drains so they act only in an overflow
• Insulate horizontal leaders inside buildings to prevent condensation
• Check deflection and ponding on long-span, low-slope roofs

Uses

Specific uses

• Interior drains on large low-slope roofs
• Scuppers and gutters on parapeted or small roofs
• Gutters and downspouts on sloped roofs
• Controlled-flow drainage to reduce storm piping and site detention
• Siphonic systems on large-footprint warehouses and airports

Pros and cons

Pros

• Interior drains stay warm and free of ice in cold climates
• Scuppers are simple and visible
• Controlled-flow and siphonic systems reduce piping cost
• Gutters protect walls and foundations on sloped roofs

Cons

• Clogged drains cause ponding and structural overload
• Interior piping needs space, insulation, and coordination
• Gutters freeze and ice-dam in cold climates
• Retrofit drains reduce pipe capacity

Manufacturing and cost

Construction

• Drain bodies set in the deck before roofing; membrane clamped at the drain
• Storm piping installed by the plumbing contractor; overflow piping run separately
• Tapered insulation and crickets installed by the roofer per the layout

Main manufacturers and suppliers

• Zurn
• J.R. Smith
• MIFAB
• Watts
• OMG Roofing Products (retrofit drains)

Trade associations: American Society of Plumbing Engineers and SMACNA.

Sources

• Professional Roofing: designing roof drains, https://professionalroofing.net/articles/article/5770
• Roofers Coffee Shop: a review of roof drainage procedures, https://www.rooferscoffeeshop.com/post/a-review-ofroof-drainageprocedures
• J.R. Smith: Raintrol controlled-flow roof drain, https://www.jrsmith.com/products/commercial-roof-drains/raintrol-roof-drain-1085
• MIFAB: dome type roof drains, https://mifab.com/products-category/drains/roof-drains/dome-type-roof-drains/
• Construction Specifier: tapered insulation systems and roof drainage, https://www.constructionspecifier.com/roof-drainage-tapered-insulation-systems/
• UpCodes: 2024 IBC Chapter 15 roof drainage, Chapter 15 Roof Assemblies and Rooftop Structures: Department of Defense Building Code 2024 | UpCodes
• Wikimedia Commons: Bregenz-Flat roof rainwater outlet (flat roof drain)-01ASD.jpg, by Asurnipal (CC BY-SA 4.0), https://commons.wikimedia.org/wiki/File:Bregenz-Flat_roof_rainwater_outlet_(flat_roof_drain)-01ASD.jpg