ABOUT FOUNDATION DRAINAGE
Run it to daylight
Wet basements are common. Even some new-home owners complain of wet basements.
If the slope of the building site allows, perimeter drains should connect to solid pipe that runs to daylight. The solid pipe should be sloped at a minimum pitch of 1/4 inch per foot, although a steeper slope is better. If there is more than 200 linear feet of foundation, add a second outlet or increase the size of the outlet pipe from 4 inches to 6 inches.
When there isn’t enough pitch on the lot, the exterior drains should be connected to a sump pump in the basement via a 6-inch line that penetrates the footing near the sump location.
MORE ABOUT FOUNDATION DRAINAGE
Some sites are too wet for basements
Water that seeps through a basement foundation or is forced upward by hydrostatic pressure can be collected in an interior drain system and routed to a sump for removal. However, if you know that groundwater is likely to be so big a problem that you have to relieve hydrostatic pressure with a perimeter foundation drain system and sump pump, you should seriously consider something other than a full basement foundation.
INSTALLING AN EXTERIOR DRAIN
Dig a trench as deep as the bottom of the footings.
Lay filter fabric first. Unroll 6-foot-wide filter fabric along the trench, lapping the material up the sidewalls of the foundation. Spread the excess fabric away from the foundation.
Add crushed stone and pipe. Over the filter fabric, lay a 3-inch layer of crushed stone, and then install the 4-inch rigid PVC pipe all the way around the foundation. The perforated pipe can be installed level. Window wells should be tied to the drain with solid 4-inch PVC. Add crushed stone to a level about 8 inches above the top of the footing, and then pull the excess fabric over the top of the stone and lap it against the foundation wall.
Finish with coarse sand. A 6-inch layer of coarse sand spread on top of the fabric will prevent soil from washing into the fabric and clogging its pores.
INSTALLING AN INTERIOR DRAINAGE SYSTEM
Create a sub-slab drainage field.
Put down an 8- to 10-inch-deep layer of crushed stone before the basement floor is poured so that the entire area beneath the slab drains. Above the crushed stone, install a layer of extruded polystyrene insulation topped with a puncture-resistant vapor barrier, such as cross-laminated high-density polyethylene, which will prevent any below-grade moisture from rising into the basement.
Install an plastic interior perimeter drain.
In most cases, this consists of perforated 4-inch pipe. Proprietary drainpipe systems are also available, usually at a higher cost. All are designed to pick up water where the basement wall meets the the floor and drain it to a sump, from which it can be pumped out.
Put in a sump pump. Water that’s collected on the inside of the foundation is piped to a cavity, or sump, set below the level of the floor. Use a pump that’s automatically activated by rising water to move water outside and away from the house. If local codes allow, the sump can be connected to the sewer system. (A sump pump could be connected to exterior footing drains if they run to daylight away from the house.)
Battery-powered backup pump.
In areas where flooded basements are common, a battery backup system for the sump pump ensures that the system will work when the power goes out. A maintenance or inspection schedule for the sump pump should be included in the homeowner’s manual. Installing a sump-pump pit cover that achieves an airtight seal will improve the home’s air tightness and reduce the risk of radon entry.
Footing-drain problems are expensive to fix
Building codes require perimeter drains around the outside of basement footings. They are not difficult to install properly before the foundation has been backfilled, but they are costly and disruptive to put in after the fact.
As long as you’re going to protect the bottom of the foundation of a house from water, it’s worth doing right.
See below for:
Image Credits: Krysta Doerfler/Fine Homebuilding #189
Pick the right drain line
The 4-inch black line that comes in coils is cheaper than rigid PVC pipe, but it’s not as crush-resistant, and the narrow slots in pipe walls are more likely to clog than the holes in rigid pipe.
Integrate water-management strategies
The foundation drain shouldn’t do all the work. If groundwater is managed well, underground drains become just one part of a bigger system. Carefully grading the yard can go a long way toward keeping water away. Surface drains and gutters can catch much of the water that does reach the house.
Drainwater can work for you. If the grade allows drains to reach the surface, a rain garden might be a good option. Then water isn’t just diverted, it also provides valuable irrigation to the landscape.
Image Credits: Dan Morrison
Backfill with quick-draining material
It may be tempting to backfill a basement foundation with excavated soil, but it’s best to place coarse, granular material like crushed stone or bank-run gravel against the foundation to encourage drainage. A cap of soil with a high clay content near the surface will encourage surface water to flow away from the foundation.
Sections 404, 405, 406, and 801 of the International Residential Code (IRC) relate to foundations and below-grade habitable space. All code references are to the IRC unless otherwise specified.
Concrete & CMU foundations that contain habitable or usable space need drains [405.1], unless there is good natural drainage [405.1X]. Use filter fabric over drain fields [405.1] and at least 2 inches of stone under pipes [405.1]. If the soil is expansive or collapsible, extend gutter downspouts 5 feet from the building or to an approved drainage system [801.3].
Water-proofing, Damp-proofing, & Backfilling
Below-grade basement walls need damp-proofing [406.1], but if the water table is high, use water-proofing instead [406.2]. Parge CMUs before damp-proofing [406.1], and lap and seal all joints in water-proofing [406.2].
Don’t backfill until foundation walls are anchored to the floor framing [404.1.7] (except walls supporting less than 4 feet of unbalanced backfill [404.1.7X]).
Illustration: from Code Check Building 2nd Edition. click to buy .
Keeping your basement dry pays many dividends: a dry basement is less likely to have mold, and the house may have better indoor air quality and fewer moisture problems in the attic. If you value these dividends, invest in high-quality, crush-resistant footing drains of an adequate diameter, and backfill the entire foundation with coarse granular material that drains well.
It’s better to run footing drains to daylight than to depend on a sump pump. While that may require more excavation, it’s money well spent.
DRAWING LIBRARY CONSTRUCTION DETAILS
WHAT’S WRONG WITH THIS PICTURE?
A footing drain the hard way
These workers are excavating to install a footing drain on an older house — an awkward and expensive task. Most older houses on stone foundations were built without footing drains.
LEED-H ID2 (Durability Management Process) has prerequisites and 3 points for third-party certification of durability processes/practices.
NGBS Under Ch. 6, Resource Efficiency: 4 points for well-designed foundation perimeter drainage as part of durability measures (602.3).