Drainage
Grading and drainage are two of the most important design concepts in residential construction. Damage from water on a structure is impactful and causes billions of dollars in damage in the United States every year. Drainage problems around site elements, including foundations, are either caused by design defects, construction defects, or the lack of maintenance. Therefore, all homeowners need to know that grading and drainage structures require maintenance and repair.
Proper grading and drainage is important at foundations, retaining walls, pavements, and landscaping features.
Common Drainage Conditions
The International Building Code (IBC) has several sections that specifically address foundation and site grading requirements for drainage at properties and around foundations.
Surface drainage is required to be diverted to a storm sewer or other approved point of collection that does not create a hazard. Individual lots must be graded to allow drainage to flow away from the foundation walls.
GRADING AROUND A FOUNDATION
Flat grading (less than 5% slope) prevents water from properly flowing away from the foundation.
The International Building Code recommends a "minimum" ground slope immediately adjacent to the foundation be sloped away from the building at a slope of not less than 1/2-foot vertical in 10 feet horizontal (5% slope). However, Engineers typically recommend 1-foot vertical in 10 feet horizontal (10% slope).
DRAINAGE LOW POINTS AT FOUNDATION
When the slope around a foundation is insufficient, drainage low points can form over time, allowing the water to seep into the foundation backfill, causing potential settlement of flatwork and foundation damage.
DRAINAGE LOW SPOTS ON PAVEMENT AND FLATWORK
Drainage low points on pavements cause deterioration of pavements. Surface water collects at the drainage low point, freezes, thaws, and causes more and wider cracks. Any water seeping into the ground below the pavement will affect the strength of the soil resulting in possible heave, settlement, or other types of pavement deterioration.
ROOF DRAINAGE
Because they are impervious, roof surfaces discharge a large amount of concentration flows. Therefore, proper control of roof drainage is required, or it can cause damage to landscaping, grading, retaining walls, and foundations.
Maintaining and repairing gutters, downspouts, and gutter extensions on an ongoing basis is critical. It is important to make sure that gutter extensions are properly connected to the downspouts. All gutter extensions on the house need to be extended normally and temporarily pulled up for yard maintenance only.
LANDSCAPING
In rock areas, flat graded areas, at edges of concrete, and non-perforated lawn edging, water tends to create shallow pools of water that facilitate water to infiltrate into the subgrade instead of flowing to its intended destination.
Lawn edging is a common barrier, and perforated lawn edging can be used between rock and lawn transitions to facilitate water flow between the two types of surfaces.
BARRIERS TO FLOW
In general, the type of soil, ground slopes, landscaping rock, shrubs, sidewalks, curbs, and other site infrastructure can impede the proper flow of drainage. These elements either affect the flow velocities or function as an effective dam (a barrier) to impede flow.
Landscape rock and sidewalks are common elements that impede roof drainage from properly flowing away from the foundation.
EROSION
The forces of water can erode soil and, as a result, carry soil/sediment to downstream drainage structures. Rills or gulleys are a result of soil erosion by water.
In areas of flat slopes and erosion, soil/sediment accumulation can change the topography affecting the proper flow of drainage. The forces of water can change the shape of the ground surface and create drainage low points where water accumulates and seeps into the soil.
BUILD-UP OF SEDIMENT
The evidence of sediment build-up is a big clue in identifying drainage low points during periods of no precipitation. Water's energy transports these soil particles and drops them off at drainage low points as it seeps into the soil. The build-up of sediment can also be an indicator of upstream drainage problems.
PAVEMENT
Water that migrates through the soil from drainage sources can travel significant distances and seep under pavements, up through cracks and joints, and transports sediment onto surfaces.
RETAINING WALLS
Not only does water cause staining, efflorescence, and vegetation growth, but it also increases the weight of the soil behind a retaining wall resulting in the weakening of the supporting soil.
STANDING WATER, SEEPAGE, AND STAINING
Drainage low points that are perpetually wet or have standing water are signs of drainage problems and promote the growth of algae and molds.
IRRIGATION
Faulty irrigation system problems, in the form of underground leaks and excessive watering, can cause drainage problems for pavements and structures, contribute to algae and mold growth, and the non-viability of vegetation. In addition, water that seeps into the soil below pavement or foundation backfill results in cracked and diselevated pavement or foundation structural issues.
SWALES
Large cobble or rock at the flowline of a swale can impede the proper flow of drainage by slowing water down and allowing it to infiltrate into the soil instead of properly conveying the water away from the foundation. Infiltration of water into the foundation backfill can result in moisture problems or structural instability of the foundation.
Non-existent drainage diversion swale between homes allows water to flow toward the foundation. Infiltration of water into the foundation backfill can result in moisture problems or structural instability of the foundation.
METERS AND CLEANOUTS
Downspouts should not be directed to areas with water meters or sanitary cleanouts. This is because water can migrate into pipe trenches and flow along the pipe bedding into basements and crawlspaces.
INLETS
Inlets require regular maintenance so that water is not impeded from flowing properly into the inlet. Settlement of pavement, grass, or landscaping around an inlet must be repaired so that water does not have a barrier to entering the inlet.
HUD Drainage Guidelines
The United States Department of Housing and Urban Development (HUD) is one of the executive departments of the U.S. federal government. It administers federal housing and urban development laws.
Key points from the Site Grading and Drainage Guidelines from HUD:
-
Surface water drainage should be provided away from all sides of all buildings and off the property to minimize the possibility of dampness in basements and crawl spaces, weaken the soil, and prevent soil erosion.
-
Walks, driveways, retaining walls, and other improvements should be constructed to not interfere with drainage. Sidewalks should not be used as drainage channels.
-
Site grading and drainage should provide immediate diversion of water away from buildings and off the site
-
Avoid concentrating runoff onto neighboring properties where soil erosion or other damage may be caused.
-
Minimize soil erosion.
-
All walls and foundations of buildings and any water-supply well should be provided with minimum protective slopes to assure immediate drainage and diversion of surface water away from these structures and routed off the site. (Refer to foundation "protective zone" for additional information.)
-
Provide a minimum fall of 6 inches away from the structure in 10 feet, except as restricted by side lot lines or other significant considerations, without regard to soil type or ground frost conditions. (Refer to foundation "protective zone" for additional information.)
Link to HUD Site Grading and Drainage Guidelines Publication
Methods to Control Drainage
STEP 1: DIVERSION
Surface features are used in site design to route water away from structures and in desirable directions. Water is diverted away from structures and site elements like foundations, retaining walls, and pavements by the use of topography intentionally to provide ground slopes that direct water away from structures and elements.
A downward slope away from something is commonly referred to as a "positive slope." (A good thing.)
A downward slope towards something is commonly referred to as a "negative slope." (A bad thing.)
For example, you want to maintain a positive slope away from your house to make sure that water does not collect (or pool) next to your foundation.
STEP 2: COLLECTION
Drainage structures are used to collect diverted water. Structures such as storm sewers, gutters, culverts, and ditches collect water from multiple sources and route it to drainage facilities. In more severe rain storms, even the surface of roadways is used as a structure to convey water to drainage facilities.
STEP 3: DISCHARGE
The collected water is discharged into regional drainage facilities. Water is discharged into detention ponds or waterways such as gulches, streams, rivers, and lakes.
Grading
During the development of a residential site, the first phase of construction includes overlot grading, utilities, roadways, drainage infrastructure, and commonly-owned retaining walls before any residential structures are built.
Upon completion of overlot grading, “pad sites” are constructed for future buildings and must comply with the engineered drainage design. Overlot grading does not include the detailed grading and utilities on the individual pad sites.
Per the engineered design of a site, residential pad sites are specifically constructed to accommodate full, garden-level, and walkout basements. In addition, each basement type has its constraints related to grading and drainage.
Grading Around Foundations
The ground around a foundation must have a negative slope (sloped downward) so that water properly flows away from the foundation. In addition, swales are constructed around residential structures to direct flow to drainage structures.
Grading Around Retaining Walls
Drainage must be routed around a retaining wall. This is because the water flowing behind and through a retaining wall affects the structural stability of the supporting materials and wall structure.
Grading Around Pavements
Swales and drainage infrastructure are necessary around pavements such as sidewalks, driveways, and roadways to divert water away from the pavement edges. Water seepage into the subgrade below pavements causes the supporting soils to become weaker and can lead to the failure of pavement.
More information about controlling drainage around pavement >>
Foundation "Protective Zone"
Grading around the foundation is important.
Engineers refer to the ten feet around a foundation as the "protective zone." The industry standards and criteria dictate that precautions are taken during the design and construction of grading in this protective zone to ensure proper drainage. These precautions are critical to avoid damage from water infiltration into the backfill material around the foundation.
The Residential Building Code (IRC) states the following in Section R401.3-Drainage regarding slope around a foundation:
Surface drainage shall be diverted to a storm sewer conveyance or other approved point of collection that does not create a hazard. Lots shall be graded to drain surface water away from foundation walls. The grade shall fall a minimum of 6 inches within the first 10 feet.
Exception: Where lot lines, walls, slopes, or other physical barriers prohibit 6 inches of fall within 10 feet, drains or swales shall be constructed to ensure drainage away from the structure. Impervious surfaces within 10 feet of the building foundation shall be sloped a minimum of 2 percent away from the building.
The optimum slope away from a foundation is 12 inches of vertical drop for the first ten feet. In cases where ten feet of distance is not available, for example, where houses are very close together, 6 inches vertical drop for the first 5 feet may be used.
Concentrated Flows
