French drain – design detail guide, including pipe, gravel and cost

  • Originally designed by American Henry Flagg French, French drains were used on waterlogged agricultural land, leading to greatly increased productivity
  • A French drain is one of the most efficient means of redirecting water flow away from structures or sodden land
  • In a similar fashion to a riverbed, French drains use gravity as a means of pulling water away from surrounding waterlogged land
  • Backfilling the drain trench with fine to coarse stones creates a natural water filtration system
french drain design

A typical scheme for a French Drain

Since the days of the caveman we have seen the emergence of simple drainage systems. Used as means to reduce flood dangers or improve irrigation the basic design and structure has remained remarkably constant. We will now take a look at the French drain which has been extremely effective for nearly 200 years.

What is a French drain?

A French drain is a simple but very efficient type of drainage system. You are likely to see this style of drain next to a wall or building as a means of reducing the build-up of water. Over the years we have seen many adjustments and improvements but the basic principles behind French drain remain the same.

While the name “French drain” has led many to believe it originated from France, this is not the case. This specific type of drain was perfected by Henry Flagg French, then assistant secretary of the Treasury of the United States, back in the 1850s/60s. Henry French was particularly knowledgeable in the area of agriculture and with waterlogged land a major problem at the time, this was the perfect solution. This allowed many farmers to reuse land which would otherwise have been rendered useless. There were also concerns about the vapour rising from waterlogged land and the potential to encourage disease.

French drain – design characteristics

While many different materials have been used over the years, a French drain is basically a trench with a land drain installed. The trench is backfilled with an array of different materials most commonly course to relatively fine stones. The finer stones are positioned towards the top of the trench. Larger more course stones are situated towards the bottom of the trench. This creates a natural filtration system which allows water to seep through the fine stones while filtering out soil and other materials. The idea behind a French drain is to quite literally change the pattern of drainage (flow of water) in a particular area.

The vast majority of French drains contain a perforated pipe at the bottom of the trench. As the water seeps through the natural filtration system it will enter the pipe and be channelled off to a different area. This may be an alternative drainage system, water collection point or a man-made soakaway.

French drain design overview

The vast majority of French drains are used to channel away excess water around walls, buildings and basements. A build up of water can compromise foundations and lead to rising damp. It is more common for a French drain to be situated beside a wall where water is known to accumulate. The size of the trench will depend upon the expected accumulation of water. Some larger French drains incorporate numerous perforated pipes to redirect excessive water.

The side of the trench nearest the wall must not encroach under the foundations. The excavation should be within a 45° angle of the structure thereby redirecting any water in the vicinity. The other side of the trench should be dug to a relatively steep inclination with a natural one in 100 or one in 200 slope towards the trench. This encourages as much water as possible to flow into the French drain, through the natural filtration system, enter the perforated pipe to be channelled away from the area. Older structures may have relatively shallow foundations. As a consequence, a French drain may need to be withdrawn at least 1 m from the structure. This helps to avoid any erosion or interference with the foundations.

Depth of a French drain

The depth of a French drain is determined by a number of factors such as:

• The amount of water to be redirected
• The location of the French drain trench
• The depth of the excessive water in the sodden ground

There is no hard and fast rule regarding the depth of a French drain. Traditionally the most common are up to 6 inches wide and 12 inches in depth. However, some situations may require trenches several feet deep and several feet wide. When installing a French Drain next to a structure the trench must go no lower than the lowest foundations. It should also be dug to a 45° angle from the bottom of the structure. This ensures that as much water as possible is redirected away from the structure by simple gravity.

french drain perforated pipe

Perforations along the pipe at regular intervals are essential for French drains

Best pipe to use for a French drain

There are many different types of pipe to choose from for a French drain. Corrugated and solid PVC pipes are by far and away the more popular with many experts advising against flexible corrugated piping. Whatever type of pipe used it is imperative that there are perforations along the pipe at regular intervals. This ensures that water can drain into the pipe and away from the structure or sodden land. The preference for solid plastic materials avoids eventual rusting of metal pipes and potentially costly replacement.

Selecting the perfect French drain gravel

Due to the structure of a French drain, the backfill will include smaller gravel size stones towards the top of the trench. These should be similar in size, shape and density to river gravel. As you move towards the bottom of the trench, larger course stones should be used to compliment the natural filtration of water. The larger stones should be tough long-lasting material so as to avoid constant and potentially expensive replacement going forward. All layers of stone/gravel should be tightly packed. The trench can also be covered with soil/turf but there must be a small gap either side of the trench.

how much does a french drain cost?How much does a French drain cost?

If you check the Internet for the cost of installing a French drain, you will see it varies dramatically depending upon the situation. Even a relatively simple garden drain, redirecting water from sodden land, can cost upwards of £1000. This would include the cost of materials as well as the excavation of the trench. Even small French drains take a minimum of one day to complete. Some of the larger more complicated French drain systems can cost well into the tens of thousands of pounds.

When considering the economics of installing any drain, installed correctly with the right materials, they will save a lot more than their cost. Water flowing around walls and foundations can cause significant damage. Even rising damp, caused by water accumulating around walls, is costly to rectify. Before undertaking the installation of a French drain it is certainly worthwhile taking professional advice.

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