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Waterproofing Your Flat Roof System


18th April 2018

To ensure you get the most out of your new flat roof, an effective waterproofing system must be in place. This will protect the long-term integrity of the structure. Too much exposure to the outside elements will lead to rot and decay of the decking and insulation, significantly reducing the lifespan of the roof. This can lead to costly repairs, or worse, the need for an entirely new roof.

There are three main methods used to waterproof flat roofing. The use of these approaches will depend on the environment, size of the roof, budget and type of structure being weatherproofed.

Weatherproofing a Flat Roof Using Reinforced Bitumen Membranes

Reinforced Bitumen Membranes are the most commonly used material to waterproof flat roofs on residential properties. The process involves laying down two or three layers of the material across the full surface of the roof. These are then bonded together by hot bitumen.

The bitumen coating is then combined with a polyester carrier to produce a waterproof material on a roll. Once this has been formed it can be laid across the insulation (on a warm flat roof) or across the structural deck itself if the design is for a cold flat roof.


The waterproof membrane can be attached to the roof using several different methods:

  • Pour and Roll: As the layer of RBM is unrolled across the roof, hot bitumen is poured out in front to seal the two together.
  • Torching: A specific type of RBM is heated using a gas torch without the need to use bitumen as adhesive. Precautions must be taken to lower the risk of a fire occurring.
  • Cold Application: A cold adhesive is spread out across the surface of the roof before the RBM is slowly unrolled, making sure the laps are also sealed using the same bonding material.
  • Self-Adhesive: A strong, high-tack adhesive with release paper is applied to the bottom of the RBM and rolled out onto the roof. Once the membrane is in place, the release paper can be removed and the later bonded into position. This method must be supported by a steady rolling action. Use a broom to smooth out kinks or areas that haven't fully stuck into place.

What is Liquid Waterproofing?

Liquid waterproofing uses a special coating to ensure that your roof system is suitably weatherproofed. This approach can be used for all types of roofing but is particularly popular on flat roofs.

For example, Glass Reinforced Plastic (GRP) waterproofing uses rigid polyester resin in conjunction with a glass fibre reinforcement. An additional requirement for this type of system is that it must be fitted with only high-quality plywood or OSB decking to meet UK building regulations.

Liquid Waterproofing - Roof Stores
Liquid Waterproofing

Polyurethane, acrylic and flexible polyester systems are also commonly applied, although their chemical make-up allows for more flexible use.

  • Fibreglass Liquid Membranes: GRP is the most commonly used version that includes a liquid resin with a glass fibre mat reinforcement.
  • Fleece Liquid Membranes: This version also features liquid resin but with a fleece reinforcement.
  • Other Liquid Applied Membranes: The resins present in this option do not always contain reinforced fibres and are typically used as part of a repair system.

How Long Does Liquid Waterproofing Last?

Independent certification of these products estimates the lifespan of liquid waterproofing to be anywhere between 20-40 years. In most cases, you can safely estimate it lasting at least 25 years.

This is dependent on the quality of the product purchased, the installation procedures followed and the maintenance of the roof surface in the following years.

How Much Does Flat Roof Weatherproofing Cost?

For the most up to date prices on flat roof waterproofing products, follow the links below.

Costs for professional services will vary depending on the dimensions of your roof, as well as the materials required. We would estimate the cost to be anywhere between £1000-£2500 depending on the above criteria.

FIX-R Liquid Waterproofing - Roof Stores
FIX-R Liquid Waterproofing

Professional companies will also include labour and disposal costs into their quote. Comparing at least three quotes will enable you to find the best option.

If you're considering waterproofing your own roof, it's important to find a product that can offer high levels of performance.

Which Other Weatherproofing Methods are Available?

Another waterproofing option is the use of single ply membranes. This is one of the most traditional methods with its usage stretching back over four decades in the UK.

Single ply membranes are typically found in larger commercial buildings as well as expansive private residential homes. These large sheets can be anywhere between 1.2-2mm thick. The width can be anything from 1-2m, stretching up to 20m long. To make installation easier for very large surface areas, there are large sheet products available. These can fit the entire roof if required.

To ensure optimal performance, single ply membranes are usually coupled with warm flat roofs. This involves laying the material over the insulation. In this scenario, an additional layer will be required underneath the insulation and above the structural deck. This is to prevent rising condensation from affecting the integrity of the insulation material.

How to Attach Single Ply Membranes

There are three ways in which a single ply membrane can be attached to the roof:

  • Fastening: Plastic fasteners are screwed into place through the membrane and roof deck to ensure the sheets remain in position. The fasteners can be covered by the next layer being placed above it, or by patching over using a strip of membrane. The strength of the wind in the local environment and the type of membrane being used will determine the number of screws and their positioning. The insulation can also be secured using the same method, which makes this an extremely cost-effective and efficient method of completing the job.
  • Ballasting: To keep the membranes and insulation in position - paving, stone ballast or a green roof is placed on top, which will also require fastening to ensure they remain in place.
  • Adhesion: A more expensive method that ensures the insulation and single ply membrane are secured using polyurethane adhesives. This is typically used for complex roof structures or if fasteners cannot be used on the deck.



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