Easy Guide to Make Your Cantilever Balcony Waterproof

small wooden cantilever balcony

A balcony is one of the most attractive features of an apartment. However, a leaky balcony in disrepair can completely negate the nice features of a deck space. To ensure your balcony stays as an attractive outdoor oasis, you’ll need to waterproof it.

Waterproofing a cantilever balcony is a process which includes cleaning and drying the substrate, or the topmost material of your balcony’s structure. Next, you lay a waterproof membrane, cutting in strips for the edges. Lastly, you lay your tile and apply your grout.

Waterproofing your balcony can be a complicated process. In this article, we will go through the step by step process of waterproofing your balcony.

Prepare Your Subfloor

The first step in waterproofing your balcony is to prepare your subfloor. This is the material that you will be applying the waterproof membrane onto.

Subfloors are usually made of plywood or concrete sheeting. These are okay to use as long as they have been properly sealed and are watertight. Even though we will be adding a waterproof membrane and tile, these things will eventually crack and allow moisture through. 

When they do, the subfloor is the last line of defense between the important structural parts of the balcony floor and moisture. When water gets into these important parts, it can cause mold, rot, and dangerous disintegration.

To prepare your subfloor:

  1. Make sure it is properly water-sealed. Also, make sure it is not made of chip or particleboard. 
  2. Once this is done, the floor needs to be clean and dry before you can apply the waterproof membrane.
  3. Wipe the floor clean with a cloth and water. Make sure the deck is free from dust, dirt, and all debris. 
  4. Once the water has dried, you are ready to apply the membrane or the primer required by some brands.

Fix Any Cracks in the Substrate

If there are any cracks or chips in the substrate, now is the time to fill them. For concrete sheets, you can use a polyurethane joint sealant, like this Sikaflex Polyurethane Sealant. Make sure to smooth the sealant out so that when it dries, you have an even substrate surface.

For any large damaged pieces of the substrate, especially concrete, use a concrete patcher with a trowel to apply and smooth the new concrete. The Henry Pre-Mixed Concrete Patch is perfect for the job.

Next, make sure your repaired areas are clean, smooth, and dry, like the rest of the substrate.

Apply the Primer

As mentioned, some types of membranes require primers before you can apply the membrane. Once your substrate is clean and repaired, you can apply your sealant. This is often done by pouring your primer into a tray and using a new paint roller to roll on the primer.

Leave the sealant to dry as long as the manufacturer’s instructions indicate, and be sure that no one walks on the primer until it is fully set and dried.

Prepare Your Waterproofer

There are two main types of membrane waterproof systems. The first is a wet or liquid membrane that you spread around to cover your subfloor, and layer as needed. 

The second is a rolled or sheet membrane. These membranes are made of various materials but come in long, adhesive sheets that you lay across your subfloor until it is covered. These sheets can be cut to fit your cantilever balcony.

Which type of membrane you choose is up to you. Liquid membranes were first on the market, and are most cost-effective. However, they are also more work, and you are not guaranteed to create an even surface with a liquid membrane.

Sheet membranes do guarantee you an even surface, and they even act as a layer of reinforcement for your subfloor. They are much faster to install, but you do pay a higher price for sheet membranes.

You might use a combination of the two membranes, or just one or the other.

Seal Your Joints and Junctions

After the primer is applied, you’ll want to seal the junctions on your cantilever balcony. This is where the balcony floor meets the wall and other areas like the rails. The sealing of the corners of your balcony is extremely important because this is one of the main areas where water may pool and seep its way into the subfloor.

To seal the corners, you will apply any materials along the corner, and up the wall about three inches. 

  1. First, apply the primer along this area, if required. 
  2. Then, apply your chosen membrane. 
  3. If you’ve gone with liquid, you apply this using a paintbrush, but if you’ve chosen a sheet membrane, you will cut the pieces to fit the corners.
  4. If you’re using a liquid membrane, you will apply joint bands to the corners. As homes age, they shift, and the corners are where water is most likely to seep into the floor. These joint bands help stop that from happening. 
  5. After applying your joint bands along all of the edges of the balcony, apply another layer of liquid membrane.

Lay Your Membrane

The next step in waterproofing your balcony is to lay your membrane. This is the main line of defense in waterproofing your balcony. It is important to follow the manufacturer’s instructions when applying the membrane.

Whether you’ve chosen liquid or sheet membrane will determine what this step looks like.

Laying Liquid Membrane

After you seal your corners, you can immediately apply your membrane. Start at the edges of your balcony, and paint back towards the door in vertical lines, making sure not to paint yourself into a corner. 

This stage requires the most skill, as you want to ensure that your layer is not too thick or uneven. Professionals with experience painting will know how to do this. It requires a delicate balance between using enough product to fully cover the surface, and not applying so much that the product will lump.

Once your subfloor is completely and evenly covered, let the product dry for as long as necessary, usually between 8-24 hours. Make sure no one walks on the balcony at this time. Once it is dry, you will apply your next coat.

You can also find quick curing membranes which allow you to apply your second coat within the same day. 

Laying Sheet Membrane

If you’ve decided to go with sheet membrane, you will now lay your sheet on top of your primer.

  1. Cut the lengths of sheet membrane to your needed length for your balcony.
  2. Remove the backing of the membrane, and lay it down. 
  3. Be sure to go over each strip of membrane as you work, and remove any air bubbles with a clean paint roller or squeegee. 
  4. Apply the membrane across the whole surface of your cantilever balcony, cutting the edge pieces to the necessary size before laying them.

READ ALSO: How to Extend a Cantilever Balcony: Step-by-Step Guide

Ready for Tile

Your balcony is now fully waterproofed and ready for your chosen flooring to be laid. If you choose to lay tile, be sure that you lay them evenly, and let the grout dry before walking on the new tiles. You can also treat this top layer of flooring with a sealant that will help ensure your balcony stays waterproofed for a very long time.


Waterproofing a cantilever balcony is no easy task. It should not be a home improvement project that is taken lightly. Be sure that if you do attempt to waterproof your own balcony, wear all of the proper personal protective equipment, and do your research into the products you want to use.

LaShawn Toyoda

I love home decor, DIY, and gardening. During my free time I enjoy looking for new ways to improve my home so that it's more organized, comfortable, and relaxing. My apartment in Japan has a small balcony with a great view. I created this site to share and get ideas for how to decorate and better utilize my balcony space.

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