WATERPROOFING MEMBRANES

A waterproofing membrane is a thin layer of water-tight material that is laid over a surface. This layer is continuous and does not allow water to pass through it. For example, on a flat terrace, a waterproofing membrane could be laid above the structural slab and below the finish tiles. This will ensure that water does not seep into the structural slab. The tiles and membrane must be laid over a filler material that is sloped to ensure that water flows into sumps and drains. Any water that remains as puddles over the tiles is likely to seep into the slab over time, so puddles are to be avoided at all costs.

These membranes are composed of thin layers of waterproof material. Most are about 2 to 4mm thick. There are essentially 2 types of membranes, sheet based membranes and liquid applied membranes.

Ideally, a waterproofing membrane should be strong, flexible, tear-resistant and elastic so that it can stretch to cover cracks and also move with the building. If the membrane is to be exposed to the sun, then it should be UV stable. The membrane should be flexible enough to take any shape it is laid over, and be capable of turning up and over walls and other construction features.

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A bituminous membrane being installed.

SHEET BASED WATERPROOFING MEMBRANES

A blowtorch is used to heat the bottom of the membrane.

As the name implies, these are membranes that arrive at the site in the form of rolls. These are then unfurled and laid on a firm surface. The most common type of sheet based membrane is a bituminous waterproofing membrane. This type of membrane is stuck to the substrate with a hot tar based adhesive using blowtorches. 

Joints between adjacent membranes are also made with the same hot adhesive. The sheets are overlapped by about 100mm (4") to form a waterproof joint. Some membranes are even joined by melting them with a hot air gun and then overlapping them on the previously laid sheet.

With this type of membrane, joints between sheets are critical, and must be done perfectly to avoid leakage.

Other types of sheet based membranes are PVC membranes and composite membranes. The latter have a fabric base that provides strength and tear resistance, and a chemical that coats the fabric to provide resistance.

Since these membranes are factory-produced excepting the joints, they are consistent in quality.

Spray-on membrane being applied. Image courtesy bridgepreservation.com

LIQUID APPLIED WATERPROOFING MEMBRANES

Liquid applied membranes come to the site in liquid form, which are then either sprayed or brush-applied on the surface. The liquid cures in the air to form a seamless, joint-free membrane. The thickness can be controlled by applying more of the liquid chemical per unit area.

Since the application procedure is very quick, a contractor will try and finish the entire area to be waterproofed in a single day to avoid cold joints. However, if a very large area is to be done on successive days, cold joints can easily be done by overlapping the new membrane over the old - the chemical will stick to itself readily.

These are generally considered to be superior to sheet based membranes as they are joint-free. However care must be taken in application to provide just the right thickness. The membrane can tear or break if it is too thin.

HOW TO SELECT A WATERPROOFING MEMBRANE = =>

Check for the following properties of the membrane:

• UV Stability - if the membrane is to be exposed to the sun, than it must be UV stable or UV resistant, else it will degrade over time.
• Elongation - this is the ability of the membrane to stretch. It is measured in percentages. An elongation of 150% means that the membrane can stretch to 1.5 times its length when pulled. Elongation is a must in buildings that will move, such as high-rise buildings, or buildings made with steel, which is flexible. This property will allow the membrane to stretch over cracks that may develop in the future. Membranes with elongation properties of over 200% are available.
• Tear Resistance - this is an important property, as many membranes that have good elongation also can tear easily. Take a small sample of the material in your hand, and try and tear it into two pieces. This gives a fair idea of its tear resistance. You are looking for a membrane that will not tear even if a reasonable force is exerted on it.
• Chemical stability - check that the membrane is chemically inert with respect to its environment in the building. Some membranes, especially outside basement walls, are exposed to the soil and rainwater outside.
• Case Studies - ask the manufacturer or contractor to give you case studies where the membrane has been used. Ideally, it should have been in place for over eight years. Check with the building owners to see if any leakage or problems have occurred.