The trench should be about 18 inches deep and 9 to 12 inches wide. least 1 percent, so the force of gravity will work for you. This means that the drain should slope down a total of at least one inch for every 10 feet of pipe.
Taking their name from 19th century judge and farmer, Henry French, rather than the country, a French drain is a type of land drainage, comprising a perforated pipe within a back-filled shingle trench, around 600mm wide. This is generally 100-150mm diameter, and installed at a 1:80-1:200 fall (slope).
Obviously the French Drain does not have to go in a straight line but it does have to slope at least at a 1% grade. A 1% Grade would have a drop of 1 foot for every 100 feet in length.
Usually 1/8" per foot is plenty for drainage, this is the same as the commonly recommended 1% slope.
The trench should be about 18 inches deep and 9 to 12 inches wide. least 1 percent, so the force of gravity will work for you. This means that the drain should slope down a total of at least one inch for every 10 feet of pipe.
The slope should be descending at a 1 percent rate. For every 100 feet of French drain, such rate equates to a 1-foot drop. Your lot may have a naturally steeper slope, which is perfectly OK. It's vital to remember, though, that a steeper slope generates more velocity.
A French drain needs to be surrounded by gravel in order to filter out silt and prevent clogging the drain pipe.
The pipe length required for this French drain is the same as the trench length, which is 20 meters. Depending on the pipe type and layout, you may need various fittings, such as elbows, T-joints, or couplings. In this example, we will assume two 90-degree elbows are needed to navigate around obstacles.
Disadvantages of French Drains
Another disadvantage of French drains is their potential to attract tree roots. The gravel or rock used in the system can provide a water source for trees, leading to the origins invading the drain. This can cause blockages and damage to the system.
Permits Needed to Install a French Drain
Generally, a small French drain on your property that doesn't change the flow of runoff entering and exiting your yard won't need a permit. Major projects that warrant your French drain to discharge into a municipal sewer or roadside ditch, however, do require one.
“Over sloped pipes (greater than ½ in. per ft.) are as likely to clog as under sloped ones because the liquids move too fast and leave the solids behind.” Why it's costly: Without the proper slope, drain lines may clog more frequently.
Clay soil has poor permeability, meaning it doesn't allow water to pass through easily. In such cases, the effectiveness of a French drain may be limited as the water may struggle to drain away from the area efficiently. French drains may not be suitable for areas with high water tables.
If you want to connect the French drainage system to the gutter downpipes from your home, consider using a 4-inch pipe rather than a 3-inch pipe. In heavy rain or bad weather, 3-inch pipes may not be able to cope with the amount of water that comes out of the downpipes flowing through them.
The proper pitch on a house drain is very similar to the pitch on a sewer line for a home. The general rule is 1/4″ of pitch per foot of run.
French drain depth: About 8 inches to 2 feet deep should be sufficient for many water-diverting projects, though related systems, such as those built around foundations and sub-ground living spaces, as well as the bases of retaining walls, may be deeper.
Swales: A Natural Solution
They're a great option if you're looking for a more natural, eco-friendly alternative to French drains. A swale is typically lined with grass or vegetation that helps slow down and absorb water.
Installing a French drain costs $500 to $18,000, with a national average of $9,250. On average, you can expect to pay $10 to $100 per linear foot, or between $500 and $5,000 to have a 50-foot French drain installed.
Use Natural 1-1/2" round rock for best drainage. Washed rock is ideal. Avoid small pea gravel as the spaces in between stone is smaller minimizing flow rates. Avoid Crushed Rock (minus) that compacts and has "fines" that will clog pipe inlets.
Just like other types of drains, they are susceptible to clogging over time. Ideally, clean the French drain once every year.
Contractors also prefer to work with a PVC French drain pipe. Despite its tough surface, it can easily be cut with a conventional handsaw. It's also easier to set the pipes at the proper slope and keep it in place.
Clogging and Blockages
Clogging is one of the leading causes of French drain failure, often due to root intrusion or sediment build-up.
French drains are best suited for properties with moderate to severe water drainage issues, particularly in areas with heavy rain or poor soil drainage.