Site Drainage

Summary

Site drainage combines grading, swales, subsurface drains, inlets, and storm piping to carry rainwater and groundwater away from buildings and paved areas to a safe outlet. Slopes, soil infiltration, outlet location, and local stormwater rules decide the layout, so drainage is planned with grading, paving, and foundations from the start.

Cols

Image

Swale: shallow vegetated channel that carries and slows runoff
French drain: gravel-filled trench, usually with a perforated pipe, that collects subsurface water
Curtain drain / footing drain: subsurface drain placed to intercept groundwater upslope of or around a building
Area drain / yard inlet: grated inlet in a low spot that sends surface water into a closed system
Catch basin: inlet structure with a sump that collects water and debris; one built to infiltrate is a leaching basin
Trench drain: linear grated channel across a drive or walk
Drywell: gravel-filled pit that holds runoff while it infiltrates
Daylight outlet: pipe that ends on a sloped grade above ground
Subsoil drain: perforated or open-jointed pipe below grade, minimum 4 in. diameter
Backwater valve: valve that stops flow back into a building from a surcharged storm sewer

Standards

Site grading

Slope ground, drives, walks, and patios away from the building
Where the site is flat or falls toward the building, use swales if space allows, or drains if space is limited
Minimum slopes, distances, and floor-elevation setbacks come from the adopted building code and are collected in the site elements code reference

Swales

Cross-section: trapezoidal or parabolic, with side slopes no steeper than 3:1
Longitudinal slope: 2 to 4 percent; below 2 percent water pools, above 4 percent velocity and erosion rise
Sizing: runoff volume from a 6-month, 24-hour storm
Peak flow limit: swales are generally not used above about 5 cfs
Soil: infiltration above about 0.5 in. per hour; avoid compaction; first 8 in. of soil should drain well
Vegetation: fine, close-growing, water-resistant grass; extension offices recommend regional species
Final grading: schedule after the first rainy season, once the soil has settled

French drains

Pipe depth: at least 18 in. below grade with continuous positive slope to the outlet
Trench: at least 22 in. deep and 14 in. wide
Filter fabric: 8 oz landscape fabric, wide enough to wrap the stone
Base stone: 4 in. of clean 3/4 in. to 2 in. stone
Pipe: 4 in. perforated PVC, perforations down; smooth pipe clogs less than corrugated
Cover: 4 to 6 in. of stone over the pipe, fabric folded over, then at least 9 in. of stone, topsoil, or mulch
Planting: keep shrubs and trees away from the line

Drywells

Excavation: about 4 ft by 4 ft and 4 to 6 ft deep
Lining and fill: landscape fabric; clean 3/4 in. to 1-1/2 in. stone with no fines
Pipe entry: at half height or higher
Cover: fabric overlapped 6 in. and at least 8 in. of soil
Premade units: excavate at least 16 in. wider than the unit

Outlets

A drywell, storm sewer, lower part of the site, or a community retention pond
A daylight outlet ends on a slope that moves water away from the building
A splash block, stone, or sprinkler head disperses flow and limits erosion
Many communities require stormwater to be managed on site

Components

Grading and finish surface: slopes and swales
Collection: area drains, catch basins, trench drains, and French drains
Conveyance: storm piping in bedding; see the pipe bedding note
Storage and infiltration: drywells and retention areas
Cleanouts and backwater valves: access and protection from backflow

Materials

Stone and gravel: washed drain stone and free-draining backfill around drains and behind walls
Pipe: PVC, polyethylene, and other approved storm and subsoil pipe materials
Fabric: 8 oz landscape filter fabric
Lightweight aggregate: drainage and groundwater control fill
Metal gutters and downspouts: recommended in fire-prone areas

Detailing

Set finished floors above the finished grade and account for backfill settlement
Keep the pipe slope continuous and the pipe size from decreasing downstream
Provide cleanouts and a backwater valve where the building could be surcharged
Do not bury sticks or leaves in swales; decomposing organic matter fills the channel
Replace some solid driveways and patios with pavers or gravel to allow infiltration
Increase gutter and downspout capacity in heavy-rain and high-wind areas
Coordinate roof leaders, footing drains, and site drains so each has its own outlet or an approved connection

Uses

Specific uses

Moving water away from foundations and walls
Draining low areas, lawns, and planting beds
Draining drives, parking, and walks
Draining behind retaining walls
Returning runoff to its original watershed through swales

Pros and cons

Pros

Protects foundations, slabs, and landscaping from standing water
Swales and infiltration reduce runoff leaving the site
Simple trench drains are inexpensive to build

Cons

Soil compaction, frozen ground, short grass, and steep slopes cut swale performance
Drains clog with sediment and roots and need cleaning
Infiltration is limited by soils and groundwater
Local codes limit discharge to streets and neighboring sites

Manufacturing and cost

Construction

Grading and trenching with piping, inlets, and stone placed in layers; catch basins and drywells are precast or built in place

Main manufacturers and suppliers

Sources

Building America Solution Center: swales, drains, and site grading for stormwater control, https://basc.pnnl.gov/resource-guides/swales-drains-and-site-grading-stormwater-control
UpCodes: 2024 IPC Chapter 11 storm drainage (GSA adoption without amendments), https://up.codes/viewer/general-services-administration/ipc-2024/chapter/11/storm-drainage
UpCodes: 2024 IBC Chapter 18 soils and foundations, https://up.codes/viewer/department-of-defense/ibc-2024/chapter/18/soils-and-foundations