Does a garden wall need a DPC?

A DPC is an important part of any property, it stops water rising from the ground below. This is known as rising damp, and it can result in lots of problems if it is not controlled properly.

In modern properties, a DPC is a legal requirement and must be installed in all new builds. This has been the case since the late 1800’s. In older properties built before the 1880’s, a DPC was not installed. However, more breathable building materials were used. Also, in modern times many of these older properties have been renovated and had DPC installed.

So, we have established that building regulations require a DPC for all properties. However, does a garden wall need a DPC? After all, this will often be free standing and not in contact with the main property.

In general, a garden wall will not need a DPC. Rising damp is far less of an issue with an external wall, this is because you are not protecting the internal of your property. Also, a DPC can weaken the strength of your garden wall.

Will you need a foundation for your garden wall?

A new garden wall should always be built on a solid foundation. This is quite easy to achieve and will require you to dig a small trench at the base of your new wall. The trench should be around 75mm below ground level.

Once you have dug the trench, you can refill back to ground level with a semi dry concrete mix. This should be tamped level and left to dry overnight.

Once your shallow concrete foundation is completely dry, you will have a solid base to work off. At no point during this process will you need to add a DPC.

What if you are building a retaining wall?

If you are building a retaining wall, there could be an argument for some level of protection against water penetration. The way you achieve this will depend on many different factors. Including:

• What type of masonry will the wall be using?
• Will you be using a render to finish the wall?
• What is the wall retaining?

Some masonry will have much greater absorption than others. If there is potential for a lot of moisture building up behind your new wall, it may make sense to opt for a brick with lower absorption. For example, engineering bricks have absorption between 4.5-7%.

Also, stone tends to be less absorbent than standard clay bricks. Obviously, this will vary across different types of stone.

In modern clay bricks absorption can be as high as 20% and in older handmade bricks it can be even higher, up to 35%.

If you do have a lot of moisture that can penetrate through, the main issue you are likely to face, is salts been drawn through the brickwork. This can cause discolouration to the finished wall.

As well as the type of masonry you use, you can also reduce damp penetrating through the back of your wall, by waterproofing the retaining surface. This could be achieved by using a tanking slurry, or even an exterior damp proofing cream.

Would a garden wall ever need a vertical DPC?

The one time you may need to take extra precautions with damp, are when your new garden wall will be in contact with your existing property wall.

Many times, a garden wall will be free standing and not connected to your house. However, if you plan to connect the wall to your home, you will need to take more steps to avoid penetrating damp from the garden into the home.

Rising damp may not be an issue in an external garden wall, but that doesn’t mean it isn’t happening. If your new garden wall is in contact with the house, water could bridge across into your home.

For this to be possible, the water would simply need to rise above your homes DPC in the garden wall. If this happens, you are essentially creating another route for water to travel, that completely bypasses the properties existing DPC.

The way to solve this issue is quite simple. You will need to install a vertical DPC. This is quite common when you have two walls meeting each other and there is a risk of damp passing from one surface to another.

Conclusion

In most cases you will not require a DPC in a garden wall, as it doesn’t really serve any purpose. What’s more, it could actually end up weakening your wall.

Certain types of tanking can be used in retaining walls. Also, a vertical DPC may be used to stop moisture passing from one surface to another. However, there are very few reasons you would ever need a standard DPC in a garden wall.