But which gravel is ideal for a French drain? Hard rocks like washed river rock or granite chips are the ideal gravel for French drains. The gravel pieces should ideally average at least 1′′ wide to provide the best permeability for water to run along.
Bury Drain Pipe in Gravel
In the next step of building a French drain, place the drain pipe on top of a fabric lining. Add gravel until the pipe is completed covered. There should be 5 inches from the top of the gravel to the ground.
crushed stone or gravel, perforated pipe, more gravel, then wrap the fabric around and secure it, then a layer of sand on top to further protect it from intrusion by finer soil. The perforated pipe can be level, but should connect to a solid line sloped down and away from the house, and drained to daylight.
No gravel is necessary beneath the pipe. Perforated pipe, wrapped with geotech fabric can be placed at the bottom of the trench and then covered with gravel to a level at least 4 to 6 inches above the pipe - so assuming a 4 inch pipe you will want about 8 to 10 total inches of gravel at a minimum.
Cory, As you note, the ideal backfill material would be a granular, free-draining material like crushed stone or gravel. If you go that route, it's a good idea to install landscape fabric on top of the gravel to prevent clay particles from clogging the system.
River rock tops the list of the best rocks for installing in French drains. Natural round rocks are ideal because their size and shape allows the water to drain more effectively. It's vital to use washed rock because it has been cleaned of debris and particles, which will help prevent clogging the drainage system.
You would need around 0.72 cubic feet of gravel per foot length of French drain for a typical French drain width of 10 inches and a depth of 12 inches if you're using a 4" drain pipe. On the other hand, you would need around 0.6 cubic feet of gravel if you're using a 6" drain pipe.
While Pea Gravel is an excellent option, other materials can also support effective drainage. Crushed stone, with its angular shape, creates voids that allow water to pass through efficiently. River rock, larger and smooth, provides excellent drainage and adds a natural look to your landscape.
Mobility: Due to its round and smooth shape, pea gravel can easily shift underfoot, which can lead to instability in walkways or patios. This mobility can also make it challenging for outdoor furniture to stay stable. Weed Growth: While pea gravel can deter some weed growth, it's not a foolproof solution.
The minimum cover required when the drainage system is in a traffic area with drainage pipe diameters from 100 mm through 1200 mm (4”-48”) must have at least 0.3 m (12”) of cover over the pipe crown, while 1500 mm (60”) pipes must have at least 0.6 m (24”) of cover.
The French drain system can prevent water from getting near your house's foundation. French drains improve drainage in your yard, but they require the proper size gravel to function properly.
Pea Gravel: Small, smooth, and round, pea gravel is both aesthetically pleasing and effective in allowing water to pass through quickly. River Rock: Larger and more decorative, river rock can be the best choice for a significant drainage project. It offers a balance between functionality and landscape aesthetics.
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.
Versatile Size: The 1/2 inch to 1 1/2 inch size range of #57 gravel suits various construction and landscaping needs. Effective Drainage: Due to its porous nature, # 57 Stone allows water to flow through, preventing pooling and erosion.
Traditionally, you would fill it with gravel, but you may wonder if you have any alternatives. Could you instead fill it with sand? Yes, you can use sand instead of gravel for your French drain when your project calls for it. It is less effective than gravel, but it can get the job done.
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.
You can put your gravel directly onto the firm soil, though this isn't recommended as it won't be a stable base for your gravel driveway. If you are planning to forgo the driveway sub-base, we do suggest using a weed membrane underneath the gravel to keep it clean.
To accurately determine how much gravel is needed, start by calculating the volume of your project area. Begin by measuring the area (length * width) and the depth of the space. For a standard gravel layer, a depth between 2 and 4 inches is typical.
Crushed stone is typically larger and has rough, jagged edges. These edges allow the material to lock together and create a firm surface. Meanwhile, gravel is usually smaller and naturally rounded, providing a softer, more aesthetic appearance.
The stone should be no less than ¾” and dense (we suggest Granite, or River Gravel) so it will allow good water flow. Lime Rock, although inexpensive, is a poor choice for a French Drain because it will begin to deteriorate over time and eventually begin to pack down and restrict flow.
Many contractors prefer to use ½” or ¾” crushed stone as a subbase material before pouring concrete. Since it's durable, it is also regularly used for backfill, drainage solutions, or pipe bedding.
Pour and compact about 3 inches of gravel or landscaping stone along the bottom of the trench. This will act as bedding for the drainage piping.
Choice of Materials
Higher-quality materials often come at a higher price but may offer better durability and performance, potentially reducing long-term maintenance costs. The gravel or rock needed for a French drain can range from $10 to $60 per cubic yard, depending on the quality and type.
A well-designed French drain, with the ideal trench dimensions, pipe size, and slope, can manage significant water flow, averaging about 75 gallons per minute (GPM). However, the drainage capacity varies based on several factors, such as soil drainage, proper installation, and more.