If you are planning on building a sleeper wall then you will have come across protecting the wood with a membrane, but is this needed? Let's have a look.
Why use a membrane?
The idea behind using a membrane behind a sleeper wall is that it helps keep water and moisture away from the sleeper.
This will slow down the rotting of the wood and make the sleeper wall last longer.
You do not want your sleeper to be always sat in damp soil. Using a proper membrane can add 10-20 years to your sleeper's overall life, with many caveats.
A membrane can also help the wall to act as a water barrier.
If you get a sudden deluge of water, it can often come flooding out of walls. With a fully waterproof membrane, the idea is that the water would flow down the backside and then under the wall rather than through it.
This can cause issues of its own though, which we will discuss now.
Issues With Membranes
So what potential problems can be caused by using a membrane behind a sleeper wall? Well, one obvious one is a build-up of water.
If you go with a fully waterproof membrane but don't provide adequate drainage in other areas, you could end up with a lot of water just sitting behind the wall.
This will put a lot of strain on the wall and could eventually lead to collapse.
It will also make the ground very soggy, so it won't be particularly productive if this is a flower or veg bed.
If your sleeper wall surrounds a lawn, then before long, it will turn into a mudbath, so tread carefully here.
You must also consider how the water drains away under the sleepers.
If you just run the membrane down to the bottom of the sleepers and don't add extra drainage, then all that water will flow into your sleeper foundations.
Depending on how you made these foundations, this could lead to them getting washed away. And if your foundations get washed away, you can be sure your wall will soon follow.
Membrane Then Gravel
A good system to use if you are going to use a membrane is to then backfill behind the membrane with gravel. This essentially creates a drainage channel, like a mini french drain.
Semi-Permeable or Fully Waterproof
This is a question that often comes up. If you use a membrane that lets a bit of water through, this could ease the pressure behind the wall and prevent water build-up.
Or should you use a fully waterproof membrane that stops all water, like a thick plastic barrier?
In my opinion, if you are going to the effort of adding a membrane to your sleeper wall, then I would say go the whole hog.
Use a fully waterproof membrane and backfill with gravel.
I would also be tempted to paint the rear and bottom of the sleepers with bitumen paint to add even more protection.
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