Wondering what a French drain is? Is building one necessary? How exactly do they work? Find out in this article. We answer some of the most asked questions and help you get a better idea of what a French drain is and what it can do to prevent damp and keep surface water away from your home.

We strongly recommend seeking professional advice before embarking on installing a French drain. Due to the nature of a French drain and digging close to property foundations, it is recommended that you first get a site visit from a French drain specialist in order to ensure that no damage is caused to your foundations, or neighbouring building’s foundations. 

What is a French drain?

A French drain is a trench, filled with aggregate, that directs subsoil and water away from your property in areas that are prone to flooding or surface water pooling. Installing a French drain does not require any specialist tools or equipment, making it a relatively simple and cost effective way of draining land. French drains are generally installed near the building edge.

Before installing a French drain you will need expert advice, as it is important to make sure you avoid any pipes, cables or any other installations.

Why French drains are important?

The usual reason for installing a French drain is to relieve the build up of water against the walls of a house. If this water is left and not drained away, it can end up causing damp internally. This can in turn lead to the decay of brickwork and the crumbling of mortar joints. If you have an older property, this is particularly prevalent. 

How does a French drain work?

A French drain works to redirect any surface water away from buildings. 

How do you install a French drain?

Planning a French Drain

There are some considerations that you should think about when planning your French drain. Without these considerations your French drain may become blocked, or not drain the water as you wish. 

Water will always find its way to the lowest point in a landscape, therefore a French drain should be installed at a low point within the affected area to allow it to drain the surface water away into the storm drain. A French drain should not be used in replacement of existing surface water drains.

If you live in an old building with shallow foundations, you should position the French drain no less than a metre away from the buildings walls so as to avoid causing any structural damage.

To minimise clogging of the drain, the aggregate size should vary from coarse in the centre to a smaller grain towards the outside.

French Drain Diagram

Where to end a French drain?

When planning where your French drain is going to end, you need to ensure that it will end at the lowest point of the trench. This is so that the water will be able to flow away from the water-logged area. You should make it long enough that it directs the water into a drier area or an area that is able to remove the water. Some suitable options for areas to end your French drain pipe are; an existing gutter, dry well, gravel pit, soakaway system.

Installing a French Drain

There are two ways that a French drain can be built. The traditional way to build one uses aggregate within the trench to allow the water to drain away naturally through the aggregate to a suitable area. A newer method now used in the installation of French drains is the use of a landscape fabric, perforated pipe and aggregate. The use of the fabric and perforated pipe makes the drain less likely to get blocked up with sediment and instead allows the water to flow freely through the pipe and to the end of the drain. The use of pipe work also allows you to maintain the drain with inspection ports and drainage outlets which is essential for the continued life of the French drain.

Below we will walk you through the steps of installing a French drain in two different ways: with and without the perforated pipe and landscape fabric.  

French drain with perforated pipe:

  1. Work out where you would like the water to drain off to.

  2. Plan out the route for your trench - make sure that the route avoids any obstructions, such as trees, pipes and cables. You can mark out your planned route with a piece of string or line marker spray. 

  3. Decide where the waste soil will go - you could use it to make a garden feature or have it taken away in a skip. 

  4. Check the depth of the foundations of the building - the drain should be no deeper than the foundations of the building 

  5. Plan the width of the drain - usually a width of 200-300mm wide is suitable, however this can vary depending on the property and how much water needs to drain. 

  6. Dig your trench - you can either do this by hand or hire a mini digger. The sides of the trench should slope away from the building and be at no less than a 45 degree angle so as to ensure that the surrounding ground remains stable. You will also need to ensure that the trench slopes at no less than a 1% gradient, if the trench does not slope the water will not be able to flow and will instead just sit and stagnate. 

  7. Fill the French drain -

  • To prevent the drain getting clogged, you should line the trench with a water permeable fabric. This fabric will prevent sediment build up and alleviate any water pressure that builds up. Make sure there is enough fabric left at the sides so that the fabric can be folded over the top of the drain. The French drain should be built like a large burrito. Imagine the fabric is the wrap and the pipe and aggregate are the filling.
  • You should now fill the bottom of the trench with a layer of aggregate. You should fill the trench to about a third of the way up. The larger the aggregate, the less likely blockages are to occur and the water will be able to flow through more quickly. The ideal size of aggregate used is around 10-20mm wide.
  • Next, insert the perforated land drain pipe into the bottom of the trench. The pipe should be 100mm wide. Make sure that the perforated holes lay face down, if you lay the pipe any other way, the water will be unable to leave the pipe until it is full of water.
  • Half fill the trench with a layer of coarse gravel to just below the top of the trench. Fold over the layer of fabric to prevent any soil getting into the drain.

       8.  If you would like, you can cover your French drain and conceal the fact that it is there. You can do this by covering the area with grass or decorative aggregate.

Without perforated pipe:

For a French drain without the landscape fabric and perforated pipe follow steps 1 to 6, as explained above.

      7. Fill the French drain -

  • Fill the French drain with aggregate. The water will flow through the gaps in between the individual pieces of aggregate. 

      8. Once the drain is filled to the top with aggregate, you can cover with grass, soil or something else of your choosing to conceal the French drain. You should however consider that there is nothing to stop things falling into the drain and causing a blockage. It is therefore important to take care with what is placed on top of the drain. Your drain will probably need to be maintained regularly to ensure that no blockages occur and that the drain continues to redirect water away from your property. 

Cost of installing a French drain

There are many factors that go into calculating the cost of installing a French drain. It depends on the materials used, the size of the French drain and whether you install it yourself or pay someone else to do it. There will also be a cost of calling out a contractor to inspect the site where you wish to build it.

Below is a rough cost guide for materials to create a French drain of 250mm by 250mm by 25 metres.

1 x Geotextile fabric (also known as Terram) Budget grade 1.25m x 25 m roll @ £50 ex VAT

1 x 100mm x 25m Perforated Pipe @ £50 ex Vat

7 x 20mm Graded Aggregate 1 tonne bulk bag £60 ex VAT (each bag will cover up to 4 linear metres of trench)

Total cost for 25m French drain 250mm x 250mm £520 ex VAT which equates to approx £20.80 ex VAT per linear metre for materials only.

Permagard – French Drain Products at Low Trade Prices

If you have any unanswered questions around French drains or their role in external waterproofing then call our technical team for free advice on 0117 9823282. We are more than happy to share our 30 years of waterproofing knowledge.

Read more on installing French drains and external waterproofing in our guide.