How to waterproof a flat roof? Our advice!

Waterproofing a flat roof is a key element in the construction of abuilding: it serves to limit the risk of leaks, as well as improving the thermal insulation of the structure. There are several types of waterproofing for flat roofs, each with its own advantages and disadvantages. Whether vegetated, with photovoltaic panels, in bitumen, accessible or not: roof waterproofing is essential.

In this article, you'll find a list of the different materials forwaterproofing your flat roof, their prices, and information on the steps involved in installing a waterproofing system.

Contents

• Different materials and techniques for waterproofing flat roofs
• Special roofs: how to waterproof them?
• How do you waterproof a flat roof?

Different materials and techniques for waterproofing a flat roof

There are different materials for waterproofing a roof:

Bitumen

EPDM (ethylene propylene diene monomer)

PVC (polyvinyl chloride)

SEL (liquid sealing system)

For the installation of each of these materials, we recommend you call in a professional to optimize the benefits of a well-made waterproofing system.

Waterproofing a roof using bitumen

Bitumen is the most widely used waterproofing method. Made from a mixture of elastomers or plastomers, it waterproofs the roof and protects it from weather and dust.

It comes in two forms:

Rolls

Liquid

Installation can be single- or two-layer, depending on the model chosen:

For bitumen rolls, installation is by cold gluing, nailing to the roof or hot gluing with a welding system.

For liquid bitumen, application is by roller or brush.

The advantages and disadvantages of bitumen:

Its advantages: it's an inexpensive, popular material, whose installation is offered and mastered by many professionals. Bitumen is easy to lay, and easy to repair if damaged.

Disadvantages: It' s heavy and not very durable.

The price, without roll laying, is estimated at between 10 and 35 euros per m2, making it one of the least expensive waterproofing materials. Installation costs range from 25 to 70 euros per m2.

PVC to protect your roof terrace

PVC, or polyvinyl chloride, like EPDM, is a membrane that is bonded to the surface of the roof. It is also resistant, durable and requires no underlayment.

There are three installation systems for PVC:

Ballasting

Gluing

Mechanical fastening

The advantages of PVC are :

UV resistance

Installation: PVC is an elastic, easy-to-install material that is cold-glued.

On the other hand, it's a material that requires a great deal of skill, so be sure to choose a serious professional.

PVC costs between 10 and 20 euros per m2.

So PVC is a good choice when you're looking for a roof decking material that's light, hard-wearing and relatively easy to install, especially on large, flat surfaces.

Choosing EPDM membrane for flat roof waterproofing

EPDM sheeting (for ethylene propylene diene monomer) is a synthetic rubbermembrane that will waterproof flat roofs for many years. In fact, it's one of the most durable materials available, since EPDM membranes can be effective for up to 50 years if the terrace roof is properly maintained. The material is resistant to the elements: UV, acid rain and gases.

♻️ EPDM is said to be environmentally friendly, as it is long-lasting and does not contain any materials that are toxic to the environment. It is laid by cold gluing with two adhesives: an acrylic adhesive, then a marouflaged adhesive.

In terms of advantages and disadvantages, there are strong similarities with PVC:

✔️ EPDMis an ecological, durable, UV-resistant and easy-to-install material.

❌ However, it can be damaged during application and requires installation by experienced professionals with a good knowledge of this type of material.

The price of EPDM membrane ranges from 7 to 15 euros per m2. Installation costs between 50 and 60 euros per m2.

Cover the building with asphalt to ensure watertightness

A mixture of bitumen and aggregates, asphalt is also used to cover roadways. It can also be used to waterproof flat roofs. To lay asphalt, the material must be heated to a high temperature. It can be laid in one or several layers.

✔️ The mainadvantages of asphalt are its price and durability.

❌ The disadvantages are in the laying : it may require several layers and is done at high temperatures. Asphalt cannot be used for roofs with a slope greater than 3% (as a reminder: to be considered a flat roof, the roof must have a slope of less than 5%).

Prices without installation range from 10 to 40 euros per square metre. Installation costs between 45 and 55 euros per square metre.

Opt for a liquid waterproofing system

Theliquid waterproofing system is an alternative to tarpaulin systems. Simply applied with a roller or brush, it creates an effective waterproofing system , and is recommended for amenity roofs.

The advantages and disadvantages of this system are mainly related to its installation.

✔️ For SEL, installationis cold. It's ideal for a variety of roof types, even complicated ones. What's more, if small cracks appear during the life of the roof, this does not affect the material's effectiveness.

❌ It's an expensive system that requires special preparation of the roof before installation. The price per square meter ranges from 25 to 65 euros per m2 without installation.

💡 Need help with a waterproofing project? Consult our complete file

Special roofs: how to waterproof them?

Waterproofing a green roof

This type of roof has a number of advantages, including good thermal insulation in summer and winter, as well as contributing to the greening of urban spaces.

In general, they are not used as a waterproofing system, but rather as a complement to one of the above-mentioned methods. Rooftop vegetation is seen as a means of adding an extra layer of protection to the roof.

🌳 In theory, if vegetation is natural, plants absorb rain. However, it's advisable to install a drainage system in addition, to avoid the accumulation of water that can weigh heavily on and damage the roof.

Waterproofing a roof with photovoltaic panels

Photovoltaic panels are not waterproof. They are installed on roofs that already have a resistant waterproofing system.

There are several options for waterproofing a roof with photovoltaic panels:

A single-layer or two-layermembrane, made from synthetic materials specially created for photovoltaic panels.

A complete system with welded studs for attaching rigid photovoltaic panels to the flat roof's waterproofing system .

The installation of solar panels is particularly sensitive, as the technical nature of this type of installation means that the existing waterproofing must be renewed.

How is a flat roof sealed?

Calling in a professional to handle the waterproofing of a flat roof?

You can install a waterproofing system in-house, but this is no trivial task. If the installation is poorly carried out and the building suffers water damage, you could be held liable by the new owner if you sell the property before the end of the 10-year guarantee period.

To avoid potential long-term inconvenience and save money, we recommend that you call in a qualified professional.

What are the steps involved in installing a waterproofing system?

There are several stages in the installation process:

1. First, the flat roof is cleaned to removeany undesirable elements that might interfere with installation.

2. If necessary, cracks are repaired. In some cases, levelling (a floor coating used in renovation work to obtain the flattest possible surface) can be done to get the roof as flat as possible before applying the material.

3. Before installation, a thermal insulation and a vapour barrier can be installed. An underlay is then laid.

4. Next comes the most important stage: installation of the roof's waterproofing system. As mentioned above, this can be EPDM, PVC, SEL or bitumen.

5. Finally, the installation of a covering according to the use of the roof terrace: for a roof without accessibility, this covering can be made of bitumen or vegetation. For an accessible terrace, you'll need to add a dedicated covering before placing a foot or a piece of furniture on the roof.

Why we strongly advise you not to neglect the waterproofing of a flat roof

A flat roof is directly exposed to the elements. Neglecting to waterproof the roof means running the risk of water seeping inside the building, and of thermal insulation problems on hot or very cold days.

A flat roof is directly exposed to the elements. In condominiums, for example, maintenance of roof terraces is essential to prevent water infiltration, which can affect several parts of the building. Neglecting waterproofing means risking water infiltration and thermal insulation problems. The same applies to the waterproofing of a steel deck, where poor waterproofing can lead to serious damage such as cracks or water blisters. Waterproofing is essential to preserve the structure and guarantee good insulation.

Here's what should convince you to take care of your roof's waterproofing: infiltration of a flat roof, caused by faulty or aging waterproofing, can lead to structural damage, mold and mildew, and rapid deterioration of the building.

This damage ranges from simple cracks to the creation of water blisters in the building, not forgetting the overall deterioration of the roof. Eventually, the entire structure could be damaged.

Waterproofing a flat roof is therefore essential to maintain a healthy structure, limit the risk of leaks and provide thermal insulation. Different waterproofing solutions are available to suit different roof types, budgets and needs. In all cases, we advise you to call on a roof insulation professional and to be accompanied in your choice of the best solution. Last but not least, don't neglect roof maintenance to preserve its lifespan.