French drains are a cost-effective way to add drainage to your yard but they need the correct size gravel in the pipe in order to drain correctly. In this guide, our team will review the different factors that impact the best size gravel for your french drains.
In this guide we’ll review the best gravel and sand for your french drain:
Gravel size and rock type is something that homeowners do not think about until it is too late. Picking the right size is important to facilitate easy drainage and water flow in the trench. French Drains are powered by gravity and work by using a slight 1 or 2 degree downslope.
The layers of filler rock and sand help prevent blockages and protect the piping. We recommend putting a layer of sod and larger pieces of rock around the entrance and exit for extra protection as well as the best size gravel for drainage.
Recommended French Drain Gravel Size
The best gravel size for french drains is small rocks and pebbles .5″ to 1″. Gravel this size is size enough to fit in the drain lines and big enough to properly filter the water.
We like to use river pebbles or crush stone for this layer since it has already been exposed to running water and is difficult to erode – meaning it will last for at least 20 years. Be careful if it feels like the stone you are putting into the bottom layer feels like it can crumble in your hands – it will likely break up in the drain line. River rock on the other hand will be smooth to the touch and unbreakable!
Recommended French Drain Sand Size
Sand and the upper layer of french drain gravel also need to have the correct size. Not all sand and filter particles are created equal – some grains are much larger than others. If the sand is too fine it will be washed away in the rainwater or become compacted at the bottom of the lines.
Common Gravel Sizing Problem
If the gravel in the drain lines is too big or too small you may experience drainage problems. If your fill rocks and pebbles are too big water will drain too quickly from the line and there won’t be enough surface area for the top layer of sand to sit on top. Keep in mind larger gravel is not always the answer!
If the gravel you choose is too small then it may compact in the drain lines and cause a clog. The bottom of the trench needs to have enough room to let water flow – without being packed in too tight! Remember, a french drain system is basically an underground river that lets you drain water into an area of your choosing – as long as you can slope the drain downhill.
Experiencing Clogged Drains Every Season?
If your french drains clog every Spring you likely need to loose the bottom layer of gravel in your drainage system or double check the perforated pipe.