Flat Roof Drainage

Alan Carson July 26, 2017

Roofs should not be dead flat. They should slope to the perimeter or interior drainage. Any roof that still has water on it 48 hours after a rain is defined as a ponding roof. If there has been no rain for a considerable amount of time, there may be no water, but you may be able to determine that there has been ponding. Circles that are particularly dirty on the roof, or show algae or vegetation growth, are indications that water ponds on the roof.

Ponding may be caused by:

  1. Poor design with no slope.
  2. Sagging of the structure, disrupting the slope.
  3. No drains or an inadequate number of drains.
  4. Drains that are blocked or are at high points on the roof.

Ponding on flat roofs has a number of implications including reduced life expectancy of the roof, structural damage, leakage and possible vegetation growth.

Water on the roof is your first clue that there is a ponding problem. Dirt, algae and vegetation are other indications. Look at the roof slope and evaluate the drainage opportunities for water to escape. Have emergency secondary drains, where an interior drainage system is used. This is often in the form of scuppers through parapet walls. The scupper drains are 2 inches above the low point of the roof so that they do not act as primary drains. They will allow roughly 2 inches of water to accumulate on the roof before discharging. Watch for and be critical of vegetation growing on the roof.

Drainage from roofs can include:

  1. Gutters and downspouts.
  2. Internal drains.
  3. Scuppers.

Gutters and Downspouts

Gutters and downspouts carry water from the roof away from the building, protecting the cladding system and the foundation. Gutters are also called troughs or eavestroughs. Downspouts are also called rain water leaders, leaders or conductor pipes. These are handled exactly the same way for flat roofs as they are for sloped roofs. Gutters and downspouts can be made with several materials, the most common of which are aluminum, galvanized steel, vinyl and copper.

Internal Drains

Internal drains are typically connected to pipes that go down through the house and discharge into a storm sewer. These are often laid into a roof similar to how a stack vent flashing is installed, except that the flange is at the top, and the sleeve extends down below the membrane inside the roof drain in the building. The roof membrane is laid up to the edge of the drain opening. The drain, which includes a 3-inch flange around the top, is set into the membrane and then stripped in.

Drains are typically provided with a strainer to catch debris. This very simple roof drain can become more complex with the addition of such things as a flexible sleeve at the top of the drain pipe. This allows the roof to move up and down and sideways slightly, relative to the drain without disturbing the connection. In some cases, a sediment collar is provided that will catch the sediment in the drain, allowing water to drain down through holes or spill over the top of the pipe, but holds the sediment around the outside of the pipe. In some cases, the strainer has a hinged top to allow for easy cleaning. Many people maintain that metal roof drains are better than plastic, which may be prone to breakage.

Scuppers

Scuppers are simply openings through parapet walls or perimeter curbs that allow water to drain into a downspout running down the exterior of the building. Scuppers are essentially a continuation of the roof membrane. The scupper itself is usually surrounded with a metal box that acts as a flashing and allows water to run through the wall. Good roofing practice includes having primary scuppers at the elevation of the roofing membrane, and secondary scuppers about 2 inches above the low point, to act as emergency drains in case the primary scuppers are obstructed.

Flat Roofs That Drain Onto Steep Roofs Below

Where a built-up flat roof is located above a steep roof, the proper detail is to provide a gravel stop at the top and a counter flashing that extends down over the steep roof for several inches. If the slope of the steep roof is relatively low and wind-driven rain is a risk, a base flashing can be provided that extends from the flat roof membrane down across the sloped roof under the counter flashing. Ice and water shield is commonly used for such a transition.

Steep Roofs That Drain Onto Flat Roofs Below

Where there is a steep roof discharging onto a flat roof, the flat roof membrane should extend about 3 feet up under the steep roofing shingles or tiles. Depending on the material, the shingles may extend down to the flat membrane, but if rigid tiles are used, they should not contact the membrane for fear of piercing it during differential building movement.

Some roofers like to use a metal flashing extending up the steep roof a few inches. This recognizes the possibility that water and/or snow may accumulate at the junction between the flat and steep roof, and many steep roofing materials are not intended to be wet over the long term. Where this detail is used, the top of the metal is overlaid by the shingles or tiles.

 

We have briefly introduced the topic of drainage for a flat roof system and discussed issues with ponding when flat roofs are not properly drained. More information is in the

ASHI@HOME training program, which details conditions related specifically to all flat roof systems and their associated implications and strategies for inspection.

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