BUILDING STORM WATER DRAINAGE SYSTEMS

1 STORM WATER

Storm water is the result of rain, snowmelt, sleet, or hail. It is part of the natural hydrologic process. If not properly con trolled, storm water runoff can result in moisture problems.

Most storm water problems in buildings are caused by poor roof and surface drainage. These problems develop when storm water from rain and snowmelt does not adequately drain away from the building, and thus finds its way into the structure.

When storm water is allowed to pond next to the building foundation or when the underground water table rises such that it's near the basement or crawl space floor, water can be driven into the building area below grade. The driving force behind water flow is hydrostatic pressure: the same force that drives water down a stream or through a hole in a leaking bucket. Hydrostatic pressure can drive water into a building through construction joints such as the intersection of a concrete floor slab and foundation wall, through cracks that develop in the floor slab or foundation wall, and through porous materials such as concrete and brick mortar.

When it precipitates (i.e., rains, snows, sleets, or hails), storm water must be controlled. Runoff from roofs, courtyards, and paved areas (such as parking lots) must be carried away from the building and properly disposed. Storm water may be directed to drains in the building roofs, parking areas, court yards and then be directed into:

• A private storm sewer or drywell

• A community storm sewer

• A creek, stream, lake, or pond

If the storm water is directed to a community storm sewer system, the only concern will be that the elevation of the storm sewer line is low enough that the private storm line can run into it. Many communities have storm sewer systems, which are also used to drain storm water from the streets and safely away.

Reoccurrence Interval

Statistical techniques are used to estimate the probability of the occurrence of a given precipitation event (i.e., rain storm, snow storm, and so on). The recurrence interval is based on the probability that a given event will be equaled or exceeded in any given year. For example, assume there is a 1 in 50 chance that 5.0 in of rain will fall in a certain area in a 24-hr period during any given year. Thus, a rainfall total of 5.0 in during a consecutive 24-hr period is said to have a 50-year recurrence interval.

See Table 1. Storm water systems are typically designed to handle storm water from a specific precipitation event. Typically it's based upon a recurrence interval of 100 or 500 years.

Img. 1 A community storm drainage line before placement of backfill.

TBL 1 RECURRENCE INTERVALS AND PROBABILITIES OF OCCURRENCES.

Recurrence Interval, O in Years (Average) G | Probability of Occurrence in Any Given Year | Percentage Chance of Occurrence in Any Given Year

500 1 in 500 0.2 100 1 in 100 1 50 1 in 50 2 25 1 in 25 4 10 1 in 10 10 5 1 in 5 20 2 1 in 2 50

Ill. 1 A downspout (or leader) discharging into a drywell.

2 STORM SEWERS

Private Storm Sewers

If the water is collected into a private storm sewer line, the line can discharge (outflow) into:

• A drywell or drainage field, which allows the water to be absorbed directly into the ground (see Ill. 1 through 3).

• An area of low elevation on the site, which allows the water to percolate into the soil

• A nearby river, creek, or stream

• A public or private lake or pond If the storm sewer line serves a large area, the force of the running water, after a heavy rain, may cause considerable dam age where it runs out the end of the line. An engineer with experience in drainage should carefully consider potential for damage caused by outflow.

Ill. 2 Construction of a drywell.

Ill. 3 Leaders or downspouts discharging into disposal fields.

Ill. 4 Soil stacks and storm leaders discharging into a combined (storm and waste) drain system.

Ill. 5 Building and street drains discharging into a community storm sewer.

Running the storm sewer private line so that it outflows into a creek, stream, or lake may require a permit and may not be al lowed in some areas. Usually, before approval, the design of the system must be analyzed to ascertain that there is no possibility of sewage wastes (chemical, human, or industrial wastes) getting into the storm sewer line and contaminating a lake, stream, or creek. Many times on large projects, the storm water is run into a pond so that the water will be available for non-potable uses, such as for irrigating lawns and gardens or circulating in fountains.

Combined Community Sewers

In some cities, especially in older areas, storm sewers from the city streets and the private buildings, driveways, and parking areas all run into the sanitary sewer line. This system is called a combined sewer. It should be done only if no other solution is available and if the city allows storm lines to be tied into its sewage lines. This storm water creates a tremendous excess load for the city (county or municipality) sewage treatment plant. This unnecessary burden often requires that the sewage treatment plants built be much larger than they would be if only sewage were to be treated. In cities with such systems, the cost to separate the storm and sewage lines now would be prohibitive. (See Ill. 4.)

Separate Community Storm Sewers

In most instances, water pollution regulations require that there be separate storm and sewage lines, and that only storm water runoff be discharged to storm sewers. Such regulations typically require that only the following can go to the storm sewers:

• Storm water from the land, parking lots, and streets

• Roof drainage

• Water from subfloor and footing drains

None of the inlets to the storm sewer system should be used to dispose of any waste materials. Cities with only sewage lines may or may not allow storm water to be introduced into the sewage lines, and this should be carefully verified with the local authorities.

Ill. 5 Building and street drains discharging into a community storm sewer.

Ill. 6 A cross-section of a roof drain.

Ill. 7 A low-sloped (near flat) roof surface is sloped to interior roof drains.

Ill. 8 Roof drains collect and direct water into vertical storm drains (leaders). Collected water then drains into a horizontal storm drain, which discharges storm water into a private or community storm sewer.

Ill. 9 Leaders from roof drains can run through exterior walls and outflow at the building perimeter.

3 ROOF DRAINAGE DESIGN

Storm water from the roof can be removed by any of three methods: a roof drain system, a gutter and downspout/leader system, and allowing the water to run off the building without drains or gutters. The third method is generally considered unacceptable unless provisions are made to ensure adequate drainage away from the building. In most instances, storm water collecting near the building foundation saturates the foundation soils. Most basement moisture problems are tied directly to poor drainage of roof and surface storm water. Foundation problems can also develop as a result of excess soil moisture.

Roof Drain System

A roof drain system consists of roof drains at regular intervals on the roof surface that collect storm water, transport it through pipes, and discharge it at ground level or into a storm sewer. A roof drain is a bowl-shaped collecting sump with a strainer on top that prevents leaves and other debris from entering the drain.

It is designed for draining storm water from low-sloped (essentially flat) roofs. Low-sloped roof surfaces are typically pitched slightly toward the drain to prevent storm water from collecting on the roof surface. (See Ill. 6 through 9.) Drains connect to vertical storm drain pipes, called leaders or conductors, which carry the storm water away from the drain and into a horizontal storm drain or to the exterior of the building.

Leaders may be concealed in the walls or columns if the sight of an exposed pipe is objectionable. However, once enclosed, it's more expensive to make repairs if necessary. A downspout is a vertical storm drain pipe that's secured to the building exterior.

If the leader/downspout runs to the outside of the building and empties, precautions must be taken so that the water will be directed away from the building. Immediately adjacent to the building, a concrete or plastic pad, called a splash-block, or other similar device is required so that the water coming from the end of the pipe will not strike the soil with such force that it will wash it away, causing soil erosion and permitting the possibility of wet foundation walls. Minor problems, such as staining the building, may also occur. However, undermining of the building structure or other elements (e.g., walks, stairs, driveways, and so forth) is possible in extreme cases. (See Photos 2 through 5.) Once the water is directed away from the building, surrounding contours (slopes of grade) must keep the water flowing away from the building. Good drainage must prevent ponding at the building site and flow of excessive storm water to adjacent building sites.

In locations where the leaders/downspouts can be tied into the sewage system, the system is referred to as a combined sewer. Because many locales don't allow the installation of combined sewers, be certain to check with local authorities.

Ill. 4 illustrates how the leader might be tied into the building drain. In this situation, most codes require a trap on the leader (storm sewer systems seldom require traps, except with combined sewers), and they may specify that the trap shall be a minimum of 10 ft (3 m) from any stack.

A roof drain system is commonly placed on low-sloped (flat) roofs. These roofs have at least a 1/4 in/ft slope to ensure that the water will not pond on the roof after rain or snowmelt. It is detrimental to the roofing materials to have water pond on the roof surface, so the roof surface should not be flat.

With roof drains, the roof surface should be sloped to ward the roof drains to be certain that all storm water is drained off the roof. Most new structures are designed to slope toward the roof drains. In older buildings with a flat roof deck, the required slope may be accomplished by using a layer of light weight concrete or asphalt. When the deck is made of poured gypsum or concrete, the slope is cast as the deck is poured. A tapered insulation system may also be installed below the roof membrane to create the necessary slope.

Img. 2 Interior roof drains on a low-sloped commercial roof system.

Img. 3 Conductors (pipes) on the underside of the roof surface carry storm water away from the roof drain.

Img. 4 A cleanout is typically provided at the base of a vertical conductor.

Img. 5 The discharge of storm water at the building exterior.

Gutter and Downspout System

Storm water may be directed off the roof into gutters. A gutter is a horizontal trough or channel that runs along the eaves of a building to capture and divert rainwater or snowmelt from the roof and allow it to drain into a downspout or leader. The roof surface must be sloped to direct the flow of storm water toward the gutter. Gutters can be used on low-sloped roofs (roofs with a slope of less than 4 in 12). They are generally always incorporated in the storm drainage system of steep-sloped roofs (roofs with a slope of 4 in 12 or greater). (See images 6 through 8.) Storm water collected by a gutter drains into a leader or downspout, a vertical pipe that carries storm water away from the roof gutter. The downspout may be tied to a storm sewer line (either community or private) or may empty outside the building onto a splashblock or some other means to disperse the water at the ground level. (See Ill. 10.)

Img. 6 An exterior roof drainage system. Storm water from the low-sloped roof drains through openings in a parapet wall into funnel-like conductor heads at the top of the vertical conductor or downspout. The vertical conductor has an open face to prevent blockage from ice or debris.

Img. 7 A series of exterior roof drains.

Img. 8 A residential roof drainage system consists of an ogee-shaped gutter to capture roof water (at roof overhand), a downspout (running vertically), and a downspout extension (at ground level), which is designed to discharge storm water well away from the building.

Ill. 10 Components of a residential roof drainage system.

Downspout extensions are horizontally sloped pipes at the base of a downspout that extend the outflow of the downspout well beyond the building foundation. They should extend a minimum of 5 ft or more away from the building's foundation so roof drainage is beyond the area of construction excavation and backfill. This is especially important in newer homes where backfill soils are unsettled.

Another possible solution is to run the downspout into a small catch basin or disposal area filled with gravel. This may be effectively used on smaller buildings, such as residences, but it's important that the catch basin be located at least 10 ft (3 m) from the foundation walls to reduce any chance for wet walls from the water.

TBL 2 COMMON GUTTER SIZES AND DIMENSIONS. ACTUAL SIZE VARIES BY MANUFACTURER.

TBL 3 COMMON DOWNSPOUT/LEADER SIZES AND DIMENSIONS. ACTUAL SIZE VARIES BY MANUFACTURER.

Materials

Gutters and downspouts/leaders are made of copper, galvanized steel, aluminum, or vinyl. Vinyl and aluminum gutters and downspouts can be custom-made in one piece without seams and are called seamless. Aluminum and steel gutters and down spouts come in many different colors that are ideal for matching the color of trim and cladding. Gutter colors are baked on at the factory or painted on-site. Vinyl is typically available in only a few colors (e.g., brown, black, ivory, or white). Copper comes unpainted and ultimately oxidizes to a green patina.

Most gutters come in several sizes and shapes called profiles. These include the U-shaped trough (a half round channel shape) and the K- or ogee-shaped configuration (a front that looks like the letter K). Common gutter profiles are available in the sizes shown in Table 2. The 5 in (125 mm) ogee-shape is most common on residences. Downspout choices are shown in Table 3. The 3 in _ 4 in (75 mm _ 100 mm) rectangular size is most common on residential installations. On a commercial project, the interior storm drainage piping typically varies in size from 3 in to 8 in (75 mm to 200 mm). The size of exterior piping and gutters may range to 24 in (600 mm) and larger.

The size of the drain pipe often requires special provisions in wall width or furred-out areas. In unfinished spaces (e.g., ware houses), the drain pipe may run exposed next to a column. In finished spaces (e.g., offices), the drain pipe may be enclosed in fireproofing that protects the column. Storm drains that serve the exterior areas of a building project are larger and require extensive planning because they often run under roads and buildings.

4 SURFACE DRAINAGE

When designing drainage for driveways, parking lots, and surrounding ground, the site plan of the project must be reviewed to determine the effect the existing and revised contours will have on the flow of the surface water. It is important that the flow of water be away from the building and not toward it. In housing developments and other large projects, the ground should be contoured so water will flow toward the storm sewer system, usually to a catch basin. A catch basin is an under ground structure, usually at the curb line, with an open grate cover to collect storm water from streets and pathways and discharge it to a storm sewer system. (See Ill. 11.) The elevation of driveways and parking lots should also be reviewed. When storm water is simply being allowed to run off onto the surrounding ground, the driveway and any curbs should be constructed higher than the surrounding ground so that the water will run off and onto the ground. When a storm sewer system will be used to collect and carry away the water, the driveway and curbs may be set lower than the surrounding ground and should be generally sloped toward the catch basins.

On projects without a city storm sewer system, the water must be directed away from buildings, driveways, and parking lots. This is accomplished with a swale, a depression or ditch designed to move storm water away from the building. It typically contains flowing water only during and after a rainfall or snow melt, the intent being to move storm water away from the building and allow it to percolate through the ground or discharge away from the building.

Regulations may require that storm water flow into detention or retention ponds. A detention pond is a human-made or natural pool or basin area used to detain and control flow of storm water runoff during heavy rainstorms. The pond fills with excess storm water runoff and impedes storm water flow. Detained storm water is released gradually into natural or human made outlets at a rate not greater than the flow before the development of the property. If it drains into a combined sewer system, it must flow at a rate that reduces the chance of sewer surcharge (overflow). A retention pond is a human-made or natural pool or basin area used for permanent storage of storm water runoff. It is designed to collect and hold surface and sub surface water. (See img. 9.) Land development and construction activities can significantly alter natural drainage patterns and pollute storm water runoff. Runoff picks up pollutants as it flows over the ground or paved areas and carries these pollutants into the storm sewer system. Common sources of pollutants from construction sites include the following: sediments from soil erosion; construction materials and waste (e.g., paint, solvents, concrete, and drywall); landscaping runoff containing fertilizers and pesticides; and spilled oil, fuel, and other fluids from construction vehicles and heavy equipment.

Most municipalities are required by federal regulations to develop programs to control the discharge of pollutants to the storm drain system, including the discharge of pollutants from construction sites and areas of new development or significant redevelopment. As a result, development and construction projects may be subject to new requirements designed to improve storm water quality, such as expanded plan check and review, new contract specifications, and increased site inspection.

Ill. 11 A catch basin. An underground structure, usually at the curb line, with an open grate cover to collect storm water from streets and pathways and discharge it to a storm sewer system.

Img. 9 A detention pond designed to temporarily hold overflow storm water.

5 SUB-SURFACE DRAINAGE SYSTEMS

Most groundwater infiltration problems are easily corrected by the roof and surface drainage measures outlined in the previous sections. In extreme cases when the building site slopes severely toward the building or if the water table consistently rises above the basement or crawl space floor line, there is need for a more complex solution to these groundwater infiltration problems Remedies include installation of the following systems.

Sump and Sump Pump

A pit or reservoir called a sump can be located in the basement or crawl space floor. It is used for collecting water that's discharged by a sump pump. Some building contractors find it cost-effective to install a sump in the basement slab in case a problem develops. They add a sump pump system only when a problem with subsurface water exists. A building can have one or more sump systems. (See Imgs 10 - 14.)

Img. 10 A sump in a crawl-space floor.

Img. 11 A view into the sump in the previous photograph. A sump pump is situated in the bottom of the sump.

Img. 13 An exterior perimeter drain placed against a foundation wall during construction of a home.

Img. 14 A below-slab floor drain.

Img. 12 An exterior sump discharge line.

Ill. 12: An interior perimeter drain consists of a drainpipe or drain tile placed in a trench below the basement slab and near the inside perimeter of foundation walls. An exterior perimeter drain is laid in a trench around the outside perimeter of the building foundation and backfilled with gravel.

Ill. 13 Components of an interceptor drainage system.

Interior Perimeter (French) Drain

An interior perimeter drain, commonly called a french drain, is a subsurface drainage system consisting of a drainpipe or drain tile placed in a trench below the basement slab and near the in side perimeter of foundation walls. Pipes from the interior perimeter typically drain into a sump (pit). Water is pumped from the sump and discharged to the exterior by the action of a sump pump. Some building contractors install an interior perimeter drain and sump system in case a problem develops.

(See Ill. 12.)

Exterior Perimeter (Peripheral) Drain

An exterior perimeter drain, commonly called a peripheral drain, is a subsurface drainage system consisting of drainpipe or drain tile laid in a trench around the outside perimeter of the building foundation and backfilled with gravel. It discharges into a ditch, sump, or storm sewer.

Interceptor Drain An interceptor drain is a subsurface drainage system involving a drainpipe or drain tile laid in a trench between the building and an uphill source of water. It intercepts the water and discharges it into a ditch or storm sewer away from the building.

(See Ill. 13.) Drain pipes are 4 in (100 mm) diameter or larger pipes made of hard fibrous materials with holes, perforated thermo plastic pipe, or clay tile spaced about 1/4 in (6 mm) apart with the upper half of the joint covered. The pipes are typically laid in a trench in a layer of gravel. They are covered with fibrous material and gravel and then soil is backfilled over the pipe or tile.

The fibrous (fabric-like) material that covers the pipe is porous enough to allow water to penetrate yet impermeable enough to prevent soil and gravel from clogging the pipe. The drain must be installed at a slope so water flows into a sump, drywell, storm sewer, or low spot on the site. Whenever possible, this drain should not be connected to the sanitary sewer system.

6 STORM WATER DRAINAGE SYSTEM INSTALLATION

Roof Drainage Installation: Considerations

It is most important that the gutters be installed with a definite slope toward the leaders/downspouts. This reduces the potential for collection of water that may freeze or cause corrosion. It also minimizes the collection of debris (e.g., roofing aggregate, leaves, and so forth) in the gutter. In roof areas exposed to collecting leaves, snow, or ice, larger downspout diameters than those required minimize clogs.

Poured concrete slabs will require that the interior storm drainage layout be carefully considered. The pipes need to be placed in the ground before the slab is poured, so their accurate placement is crucial. All piping must be carefully located and the system checked for leaks before the concrete is poured because any relocation or repairs of pipes would be costly.

The open spaces provided in truss-type construction make it easier to run piping through to the desired location. The only points of difficulty would be where it needs to pass by ductwork or some other large pipe that's going in the opposite direction. This will require coordination with the contractor in stalling any heating, air conditioning, or ventilating ductwork.

In wood frame construction, there are times when the width of a wall must be increased to allow for pipes running horizontally to pass by drainage pipes (or other pipes) running vertically.

Storm Drainage Installation: Considerations

Pipe material for storm sewers may be the same as that used for the sanitary drainage system. Storm sewer systems, however, may include pipe of much larger sizes than are needed for sanitary sewers. Plain or reinforced concrete pipe (rather than clay, cast iron, or asbestos cement) is generally used for the larger lines. Also, it's not so important that the joints be watertight in storm sewer systems. In fact, the mortar is sometimes omitted from a portion of the joint and washed gravel is placed next to the opening; the storm drain thus also serves as an underdrain to pick up subsurface water.

Storm and sanitary systems may differ in the installation of the piping. Building storm drains should generally be graded at least ¼”/ft. whenever feasible. This amount of drop per foot provides an unobstructed and self-scouring flow. However, a greater drop per foot may be given because fixture traps that might lose their seals are not associated with storm sewer systems.

When a change of direction is necessary, long radius fit tings are used and a cleanout need not be installed. This is especially true in and under buildings. But a manhole is used outside of buildings when a change of direction is necessary, or when two or more lines are connected together.

Ill. 14 Sloped roof and gutters and leaders for the design example

7 DESIGN EXAMPLE OF A STORM WATER DRAINAGE SYSTEM

The four-story apartment building (found in Section A) will be used in this design example. The surface roof is sloped to the rear of the structure (See Ill. 14). A gutter that serves the entire 3200 ft² (300 m²) roof area will be installed along the edge of the rear roof overhang to collect storm water. The maxi mum rate of rainfall will be assumed to be 4 in/hr (100 mm/hr). Gutters and downspouts are sized according to the area of roof they serve following the procedure outlined below.

Sizing Roof Gutters and Downspouts

1. The size of each downspout is based on the number of downspouts and the roof area each downspout will serve.

In this design, four downspouts will be used, each serving an area of 800 ft² (3200 ft²>4 downspouts _ 800 ft²). From Table 4a for a maximum rate of rainfall of 4 in/hr, each downspout must be at least of a 3 in diameter. If the down spouts are connected to a horizontal storm drain, the drain is then sized as discussed later in this section.

In metric (SI) units in this design, four downspouts will be used, each serving an area of 75 m² (300 m²>4 downspouts _ 75 m²). From Table 4b, for a maximum rate of rainfall of 100 mm/hr, each downspout would be 75 mm in diameter. If the downspouts are connected to a horizontal storm drain, the drain is then sized as discussed later in this section.

2. The gutter size depends on the area of the roof that each portion of the gutter serves and the slope of the gutter when it's installed.

In this design, each of the four downspouts serves 800 ft².

The layout would be such that the gutter would feed into the downspout from two sides, so that each portion of the gutter would serve 400 ft² of roof area. Assuming a 1/8 in slope per foot, from Tbl 5a for a maximum rate of rainfall of 4 in/hr, a 6 in diameter is selected for the gutters.

In metric (SI) units, each of the four downspouts serves 75 m². The layout would be such that the gutter would feed into the downspout from two sides, so that each portion of the gutter serves 37.5 m² of roof area. Assuming a 10.4 mm per m slope from Table 5b, a 150 mm diameter is selected for the gutters.

Ill. 15 Horizontal storm drain sketches and computations for the design example.

Ill. 16 Horizontal storm drain sketches and sizes for the design example.

Sizing the Horizontal: Storm Drain

Once the roof drains or gutters and downspouts have been selected and sized, the next step is to size the horizontal storm drain (if one is to be used). The horizontal storm drain data applies to any horizontal storm drain location (e.g., under the roof slab or below the floor slab).

1. The horizontal storm drain is sized from Table 5a; its size depends on the area being served and the slope at which the pipe is installed. The pipe is typically increased in size as it collects the downspouts, so the first step will be to make a sketch of the system (Ill. 15). Next, add the area that each downspout serves and the slope selected for the horizontal drain to the sketch.

In this design, three downspouts are used, serving each 1067 ft² , and the slope is 1/4 in per ft.

In metric (SI) units, three downspouts are used serving 100 m² each, and the slope is 20.9 mm per m.

2. With this information, the first length of drain (labeled "A" on Ill. 16) is sized from Tbl 5 as a 3-in pipe. Be certain that the column for a 1/4-in slope is used in this case. The next length of drain ("B") services 2134 ft² and is a 4-in pipe, while the last length ("C") serves 3200 ft 2 and is a 4-in pipe. The pipe sizes on the sketch are as shown in Ill.

In metric (SI) units, the length of drain labeled "A" in Ill. 16 is taken from Table 5. Using a slope of 20.9 mm per meter, a 75-mm drain pipe is selected. The next length of drain, "B," services 200 m² and a 100-mm pipe is required. The last length, "C," serves 300 m² and a 125-mm pipe is required.

Ill. 17 Combined sewer illustration for the design example.

Ill. 18 Combined sewer sketch for the design example.

Sizing a Combined Sewer

If the roof downspouts/leaders are permitted by code to be connected to the building drain (not common) in a combined sewer (Ill. 17), it will be necessary to convert the roof area into an equivalent number of drainage fixture units (DFU) so that the building drain can be sized to reflect the increased load.

(The procedure used to size building drains for sewage waste was covered in section 14). A typical procedure follows:

1. First, a schematic sketch of the stacks, downspouts, and building drain should be made so that the relationship of the stacks and the downspouts to the building drain can be envisioned.

Ill. 19A Rainfall intensity for 15 min of precipitation (in).

Ill. 19B Rainfall intensity for 15 min of precipitation (mm).

In this example, the design will continue with the four story apartment building (Section A). The sanitary waste stacks have been previously sized (section 14), and a schematic of the design would look similar to Ill. 18. The DFU total (from section 14) of each of the waste stacks is added to the sketch, as is the slope of the building drain selected (1/8 in per foot or 10.4 mm per meter slope).

2. Next, the fixture units served by each stack must be converted into equivalent square feet or square meters. The code sets up a ratio of DFU and equivalent square feet or equivalent square meters. ( Ill. 19).

The equivalent area, based on code, for the first 256 DFU is 1000 ft2. Any additional DFU are converted into equivalent area on the basis that 1 DFU equals 3.9 ft2.

In this design, the 64 DFU would be the equivalent of 1000 ft².

In metric (SI) units, the equivalent area for the first 256 DFU is 93 m². Any additional DFU are converted into equivalent square meters on the basis that 1 DFU equals 0.36 m.

In this design in metric (SI) units, the 64 DFU would be the equivalent of 93 m².

3. The total equivalent area being served by the building drain is determined by adding the equivalent area to the roof area being collected.

The total equivalent area is 3200 ft2 _ (1000 ft2 equivalent area · 2 downspouts) _ 5200 ft2. This is noted on the schematic in Ill. 20.

From Table 5, the building drain size is determined.

Based on a 1/8 in slope, the building drain must be 6 in to the main stack. The complete tabulation is shown in Table 8. A review of the sketch in Ill. 20 indicates that there is less than 200 ft developed length.

In metric (SI) units, the total equivalent area is 300 m² = (93 m² equivalent area _ 2 downspouts) _ 486 m². This is noted on the schematic in Ill. 20.

From Tbl 5, the building drain size is determined.

Based on a 10.4 mm per meter slope, the building drain must be 150 mm to the main stack. The complete compilation is shown in Table 8. A review of the sketch in Ill. 18 indicates there is less than 60 m of developed length.

4. The equivalent area is based on a rainfall of 4 in per hr.

A check of the local weather service will indicate whether the rainfall in the proposed building location is more or less. If so, then the equivalent area must be adjusted proportionately.

Assume the rainfall rate in the proposed location for the apartment is 5 in per hr. How many equivalent area would the 64 DFU equal? 64 DFU _ 1000 equivalent ft². At 4 in per hour:

In metric (SI) units, assume the rainfall rate in the pro posed location of the apartment is 125 mm per hour. How many equivalent feet would the 64 DFU equal?

_ 116 m²

equivalent area

63 DFU _ 93 m²

equivalent area _ (125 mm>100 mm) At 125 mm per hour:

64 DFU _ 93 m²

equivalent area At 100 mm per hr:

_ 1250 equivalent ft²

64 DFU _ 1000 equivalent ft² _ (5 in>4 in) 5 in per hour:

QUIZ

16-1. What methods may be used to dispose of the water from roofs, courtyards, and parking lots?

16-2. What is a combined sewer, and under what conditions should this system of storm drainage be used?

16-3. What methods are commonly used for roof drainage?

16-4. When the roof drainage water is run into roof drains or gutters, what solutions may be used to disperse the water at the end of the downspout?

16-5. What methods are commonly used for subsurface drainage?

Design Exercises

16-6. Design the storm drainage system serving a 16 ft by 32 ft roof area with two downspouts. Assume a sloped roof with gutters (1/8 in per foot slope) and downspouts to the ground and a 4 in per hr rainfall.

16-7. Design the storm drainage system serving a 16 ft by 32 ft roof area with two downspouts. Assume a sloped roof with gutters (1/4 in per foot slope) and downspouts to the ground and a 5 in per hr rainfall.

16-8. Design the storm drainage system serving a 64 ft by 84 ft roof area with four downspouts. Assume a sloped roof with gutters (1/8 in per foot slope) and downspouts to the ground and a 5 in per hr rainfall.

16-9. Design the storm drainage system serving a 16 ft by 32 ft roof area with two downspouts. Assume a sloped roof with gutters (20.8 mm per meter slope) and down spouts to the ground and a 100 mm per hr rainfall.

16-10. Design the storm drainage system for the apartment building in Section B. In this design, assume a sloped roof with gutters (1/4 in per foot or 20.8 mm per meter slope) and downspouts to the ground and a 3 in (75 mm) per hr rainfall.

16-11. Design the storm drainage system for the building in Section B. Use three roof drains and size the down spouts and horizontal storm drainage piping (1/2 in per foot or 41.6 mm per meter slope) to a community storm drainage system and a 5 in (125 mm) per hr rainfall.

16-12. Design the storm drainage system for the residence in Section C. Use gutters (1/8 in per foot or 10.4 mm per meter slope) and downspouts to the ground, and assume a 4 in (100 mm) per hr rainfall.

16-13. Design the storm drainage system for the residence. Use gutters (1/4 in per foot or 20.8 mm per meter slope) and downspouts to the ground, and assume a 3 in (75 mm) per hr rainfall.

TBL 4A required size of vertical conductor, leader, or downspout, in customary (us) units. Quantities in main body of tbl are maximum horizontal projected roof areas, in ft².

TBL 4B required size of vertical conductor, leader, or downspout, in metric (si) units. Quantities in main body of tbl are maximum horizontal projected roof areas, in m².

TBL 5A size of horizontal rainwater piping based on maximum rainfall and roof area, in customary (us) units. Quantities in main body of tbl are maximum horizontal projected roof areas, in ft².