Proper Drainage for a Dry Basement
Runoff from your roof will usually be the main source of water around your home. Unless you properly look after rainwater and snowmelt, you could find their results in your basement.
Your best defense to stop basement water issues is having a good system of gutters and downspouts. They should collect your roof water and get it away from your foundation. Add a backup system of footing drains around your foundation and your basement should stay water-free.
Added measures are needed if you are building in an area that historically has water issues. Reconsider whether you want a full basement if the water table rises above the level of your basement floor some part of the year. If where you are building happens to be at a point that collects water or is in the path of water from surrounding properties, you might require an engineered drainage strategy.
Roof Water Can Cause More Damage Than You Think
You might be surprised how much water runs off off a typical home: a house that has a 1,600 square foot footprint will release about 1,000 gallons of water per inch of rainfall. When those thousand gallons look for a place to go, they’ll go close to your foundation unless you’ve got a good system that gathers and drains the water somewhere else. Water will seep into ground around your foundation and will exert hydrostatic pressure against the foundation. When under pressure, water seeks its way through any cracks it can find in the concrete or block wall, in addition to joints in the basement wall, the footing and the floor.
Any additional water could reach the foundation from areas in the yard and from places like patios and walkways if they do not slope away from your foundation.
The Importance of Gutters & Downspouts
You don’t have to have a wet basement to begin dealing with roof runoff the right way by getting those hundreds of gallons of water away from your foundation walls.
You need horizontal drainpipe extensions to direct water away from your foundation. It stops you from having basement flooding. It’s an easy and straightforward way to keep roof water separated from your foundation.
Nothing extraordinary is needed beyond gutters with downspouts that fit and are kept clear of leaves and other garbage. What matters most to you is where the water goes when it lands near your foundation.
Professional Tip: To ensure your basement stays dry, your finished grade needs to slope away from your foundation for at least 10 feet.
If you have a property that offers a lot of slope and you have clay soil at the surface, you can use a splash block to get rid of the water. A block that stretches for two feet with a channel will divert water away from your foundation but you can also use paver stones. If, however, you have soil that will soak in the water, you can use an extended downspout a number of feet away from your foundation.
If you wish, you can put the extended downspout above grade, or put it below grade, but if below grade use a solid PVC pipe and drain it to daylight. If you have a longer run, you should go with a six-inch PVC.
The key is to ensure you don’t have the drainpipe bringing water right beside your foundation underground. If that happens you’ve mistakenly delivered water to the exact wrong place and that happens more often than you would realize. Don’t ever tie your downspouts to the footing drains. If you do, you can overload your system and that will bring debris and other detritus from the gutters only magnifying basement water issues.
Slope Around Foundation
To make sure the runoff heads away from your foundation, you’ll need a slope with a drop of at least six inches during the first 10 feet. It amounts to just over one-half inch of a drop per foot and is also known as a five-inch slope.
You’ll want to make sure you limit water movement around your foundation. This is best done as you backfill by using clay near the finish. Above the clay, you can use a layer of topsoil to plant a lawn or other plants.
Flat Building Sites
There is another option for drainage on a flat site. You just have to raise the foundation by a half-foot to a foot by doing a little less excavation. All that is needed at that point is a little bit of fill so you have a slope of one-half-inch per foot for the 10 feet or more you’ll need for drainage. Usually, you’ll have the fill from the excavation that has already been done for your foundation. Just do a little forward thinking and you can manage this little trick.
If you don’t have a slope around your foundation because of too close lot lines or an adjacent house or because of some other physical barriers, swales or hardscapes are recommended by code so you can get water away from your foundation. You may also be able to use subsurface drainage via a storm sewer.
Professional Tip: You should make sure that the slope for a swale and any impervious surface for drainage is at least two percent which equals one-quarter inch per foot for at least 10 feet.
You can also use a swale, a wide, shallow ditch that is an alternative cheap and easy-to-maintain approach to drainage. You’ll often find them on lot lines between homes. They are usually planted with grass and plants that can handle moisture. “Grassy” or “vegetated” swales are common terms heard to describe them.
If properly designed, swales move surface water quickly away from houses and yards and prevent flooding around the house during heavy rains and snowmelt. The grasses and other plants slow the water flow, which reduces erosion and promotes infiltration back into the soil.
A good swale will get the water away from houses and yards rapidly and stop flooding around the home when rains and snowmelt hit. They work by having grasses and plants delay water flow, thereby cutting erosion and pushing water back into the soil.
You can have swales that drain into wooded areas or into streams or ponds. They can also drain into man-made ‘retention’ ponds as well. Another option is to have them drain into storm sewers through subsurface piping. Ideally swales need to slope one-quarter inch each foot to encourage drainage. If you have flat land and it has soil that easily sucks up water, swales may be constructed with flat bottoms which hold water as it is absorbed back into the ground or becomes vapor. If there is a steep slope abutting the swale, you’ll need to have a stone or concrete “check dam” at the swale’s bottom to check out fast the water moves and stop erosion.
You have to keep up grassy swales just like other landscape areas by mowing them, getting rid of invasive species and by taking out litter like dead leaves and other detritus that could cause dead grass in time. You need healthy grass growth in your swale so water flow is slowed, erosion is delayed and drainage returns to the earth.
If you can’t use a swale, you might need to go to a curtain drain which is a gravel-filled trench that gets water away from your house. These drains do double duty by getting rid of surface and groundwater and, if allowed, can be tied to storm sewers. Usually, they are two to four feet deep and can be built a number of feet away uphill from your foundation. Often times they are open-ended at the top side but you can cover the rest of the trench with paving stones or other features.
A confusing term for curtain drains and footing drains you will hear is “French drains.” However, curtain drains to get the water before it hits your foundation, while foundation drains act as the next defense after the water has hit the foundation. Foundation drains are underneath the ground right around the foundation footings and need to be under the basement floor to work.
Where you don’t have the option of draining away water from your home by gravity you might have to put in a drywell. In simple terms, a drywell is a hole in the ground that must be a minimum of 10 feet from the foundation you fill with either gravel or crushed stone. Drywells serve to hold storm runoff so it can soak back into the soil.
Professional Tip: Draining to daylight works best, though drywells work well for modest rainfalls.
Your own drywell’s size and its location from your home will depend on the amount of rainfall you get and the type of soil in your area. Drywells usually measure four feet in width by four feet deep. You don’t want it to clog up with silt so you’ll need to line it with a heavy landscape fabric. Finally, it needs to be filled either with gravel or crushed stone over which you’ll cap it with the landscape fabric and then topsoil.
Professional Tip: A drywell’s performance is dependent on several factors: loading, size and how quickly water soaks into the ground. Water that permeates soil easily is better than a slow-draining clay soil.
If you have a large home, you may need two drywells because in a huge storm – one involving an inch an hour of rain bringing down 1,000 gallons of water that all go into your four-foot-deep drywell – it will back up. A two-drywell system means have one drywell on each side of your house. However, if you have maintained your drywell and it is properly designed and protected by fabric, it should last a long time. Don’t let it fill with detritus and debris or else it will collect water and then overflow. Ensure you’ve designed it with a lid for inspection and keep it maintained.