Flat is never completely flat
A “flat” roof is built with a slight slope, typically at least 1/4 inch per foot, toward edges or drains. Low-slope roofs are a little steeper but still too flat for shingles, so they use membranes (such as modified bitumen, TPO or EPDM), built-up roofing or standing-seam metal. Because water moves slowly across them, where it leaves the roof matters a lot: a blocked outlet means ponding water, extra weight, faster aging of the membrane and leaks.
The four ways water leaves a flat roof
1. Gutters at the edge
Where a low-slope roof ends at an open edge, it can drain into a normal gutter, just like a steep roof. This is common on porches, carports, sheds and additions. The roof edge needs a metal edge or gravel stop that directs water into the gutter, and the gutter must be sized for the roof area.
2. Scuppers
Where a roof is surrounded by a parapet (a low wall above the roof), water exits through scuppers: openings through the parapet lined with metal. Usually a scupper feeds a conductor head (a collector box, sometimes decorative) on the outside wall, which connects to a downspout. Scuppers are common on rowhouses, Southwestern and Spanish-style homes, and modern designs.
3. Internal drains
Larger flat roofs may have drains in the roof surface connected to pipes running inside the building. These are more common on commercial buildings and some modern homes. They need strainers and regular cleaning.
4. Overflow
Every roof enclosed by parapets needs a backup. The International Residential Code (R903.4.1) calls for secondary (emergency overflow) drains or scuppers where roof drains are required and the roof perimeter extends above the roof so that water would be trapped if the primary drains back up. Overflow scuppers must be larger than the primary drains, with a minimum opening height of 4 inches, and set above the low point of the roof so they only carry water when the primary drain is blocked. Water pouring from an overflow scupper is a warning sign that the main drain is clogged.
Common problems
- Ponding water more than a day or two after rain, from sagging decks, poor slope or clogged outlets.
- Clogged scuppers and conductor heads from leaves, seeds and roofing gravel.
- Leaks at scuppers, where metal meets membrane and parapet; these joints need proper flashing.
- Undersized conductor heads and downspouts that back up in heavy rain.
- Porch and addition roofs that drain onto a lower roof or walkway without a gutter.
- Ice in cold climates, when water freezes in scuppers and downspouts.
Sizing
Flat roofs are sized by roof area and local rainfall intensity, like any roof. Because a flat roof holds water rather than shedding it quickly, the consequences of undersized outlets are worse: water rises until it finds a way out, sometimes through the roof. Professional sizing uses rainfall intensity tables and methods like those in SMACNA’s architectural sheet metal manual. When replacing downspouts on a scupper system, choose sizes at least as large as the existing ones, and consider 3x4-inch or larger. See downspout size and placement.
Porches, carports and additions
The most common flat roofs on houses are small: a porch roof, a carport, a dormer or a rear addition. These often drain onto a walkway or right at the foundation. A small gutter and a downspout that carries water away can prevent icy steps, splashback on the siding and a wet crawl space. On porch roofs with built-in (box) gutters, the gutter lining must be sound; see box gutters on old homes.
Conductor heads and downspouts
A conductor head (also called a leader head or collector box) sits on the outside of the wall below a scupper. It catches water that shoots out of the scupper, slows it down and feeds it into the downspout. Without one, water from a scupper can overshoot a downspout or splash against the wall. Conductor heads come in plain rectangular shapes and decorative forms in aluminum, galvanized steel or copper. They should be large enough for the scupper flow, have an overflow opening near the top and be fastened securely, because a full conductor head is heavy.
The downspout below should be at least as large as the outlet of the conductor head and discharge well away from the foundation, just as with any gutter. On rowhouses, rear downspouts often connect to underground lines or the sewer; many cities now encourage disconnection. See disconnecting downspouts.
Canales on Southwestern homes
Pueblo Revival and Territorial homes in the Southwest use canales, projecting channels through the parapet that throw water well away from the wall. They are part of the architectural style but can drop water onto walkways and splash stucco walls. Rain chains, splash basins, gravel beds or small rain gardens under canales help control that water. See rain chains vs. downspouts.
Membrane roofs and gutter edges
Where a membrane roof drains into an edge gutter, the metal edge (gravel stop or drip edge) is flashed into the membrane. The gutter should be installed so its back sits under that metal edge, and hangers must not puncture the membrane. When a membrane roof is replaced, it is a good time to replace edge metal and gutters together so the details match.
Where flat and low-slope roofs are common
- Rowhouses in Philadelphia, Baltimore and Washington, DC, often with rear scuppers
- Mid-century and modern homes in Arizona, Denver and Los Angeles
- Spanish and Pueblo-style homes with parapets and canales in the Southwest
- Porch and addition roofs everywhere
Warning signs inside the house
Flat roof drainage problems often show up indoors first: ceiling stains below the roof edge or near a scupper, peeling paint on the inside of parapet walls, or damp spots on walls under conductor heads. If you notice these, check the roof outlets after the next rain before the leak spreads.
Maintenance
- Check scuppers, conductor heads and drains in spring and fall, and after big storms.
- Clear leaves and debris from outlets; keep strainers in place.
- Look for ponding a day or two after rain.
- Check that overflow scuppers are clear and not sealed shut.
- Inspect the flashing where scuppers pass through parapets.
Flat roof work is often done by a roofer rather than a gutter installer, especially scupper flashing. Gutter companies commonly install conductor heads, downspouts and edge gutters. Ask who handles which part.