Waterproofing flat roof – instructions, material & prevention

Although flat roofs are strong and durable, the sun’s harsh ultraviolet rays and extreme weather conditions take their toll over time. Hiring a professional roofing crew to recoat your flat roof will go a long way in saving your bank account. You can quickly breathe new life into the roof, and it’s not as difficult as you might think. Use a paint roller and a squeegee to get the roof back in shape and keep your money in your own pocket.

With winter just around the corner, a leaky roof is the last thing you want to deal with. Unfortunately, flat roofs are among the most susceptible to leaks. To help you avoid disaster when the snow comes, we’ve put together a quick guide to spotting potential leaks and sealing flat roofs to avoid problems in the future.

  1. Table of contents
    Warning signs of a leaking flat roof
  2. Locate the problem
  3. How to waterproof your roof
  4. Waterproofing membranes – installation instructions

Warning signs of a leaking flat roof

Flat roofs are very popular thanks to their good value for money, ease of installation and maintenance. They are most often seen on extensions or outbuildings such as garages and garden sheds. Unfortunately, this also means that they are not checked as often, giving water more time to seep in and damage your roof structure. Here, we take a look at some of the early warning signs that let you know your flat roof needs repair before it can be sealed.

Dark brown spots

Before the water begins to trickle through, excess moisture will penetrate your roof structure, resulting in very obvious moisture stains. If you spot these stains, you are almost certainly dealing with a potential leak and need to take the time to investigate.

Safety tip: It is incredibly important that you do not climb on your roof if you have any doubts about its strength. Instead, consult a professional who can advise you on the best course of action for your particular problem.

Cracks, ripples and splits

Where there is moisture, there is a crack, ripple or split. When examining your roof, look for irregularities in the roofline that could allow water to seep in. If your roof has been compromised, it’s not enough to simply seal your flat roof. Take photos of any problems you find and show them to a qualified roofer.

Getting to the root of the problem

Unfortunately, it’s often not as easy to find a roof leak by looking directly over wet spots. This is because water that enters your attic space can run in all sorts of directions before seeping through the roof framing into the space below.

To find the source of the problem, you should first wait until your roof has been dry for three days. This allows the precipitation to drain away and makes it much easier to identify the remaining moisture. It is best to just wait until the evening, because after cooling the leak should release the heat stored during the day.

If that doesn’t help, measure from the outside wall of the room, along the ceiling and across to the location of the moisture or leak. If you highlight this area, you can narrow down your search considerably. Now look on the top of your roof above this marked area for anything that could be the cause of the leak, such as a broken flashing. Once this is repaired, you can now seal your roof. For more help finding and repairing roof leaks, check out our guide.

Waterproofing flat roof – instruction

A very simple solution, suitable for a wide range of roof coverings, is to apply a liquid waterproofing membrane such as Classic Liquid Top Coat. It can be used for the following applications:

  • Metal roofing
  • EPDM membranes
  • GRP membranes
  • Bitumen roof waterproofing
  • Asphalt roofing
  • OSB3 T&G roof formwork

The longevity of this solution depends on the thickness of the applied layer. That is, if you are coating your garage and do not intend to replace the roof, you should apply a thick coat (which can last up to 20 years). However, if you are working with a shed or other smaller garden building, a thinner layer is probably more than suitable. To give you an idea of how thick or thin your coats need to be, check out this chart:

Life expectancy

  • 10 years
  • 15 years
  • 20 years


  • 1.5kg/m2 (15kg per 10m2)
  • 2.1kg / m2 (15kg per 7.2m2)
  • 2.5kg / m2 (15kg per 6m2)


The following things are needed to independently waterproof the flat roof:

  • Top coat
  • Primer
  • Joint sealant (if you need to fill cracks or crevices).
  • Detailer (if your roof has upright areas).
  • Self-adhesive tape (for joints and seams)
  • Broom or stiff brush
  • Sponge
  • Mastic application gun and sealing tool (if you use joint compound).
  • Long-handled paint roller (for applying the primer)
  • Long-handled brush or broom with soft bristles (for applying the top coat)
  • Stirrer or piece of wood (for mixing the top coat)

Cleaning your roof

An obvious first task is to make sure your roof is clean, dry and free of dust, oil and debris (including twigs and moss). Any cracks, breaks or fissures found during your roof inspection will need to be filled in, as well as any irregularities, joints or other gaps. This can be done with a putty gun with classic liquid joint compound and, if necessary, self-adhesive tape. In the process, you may also notice that your roof has some bubbles. These occur when air gets under the membrane. Blistering does not always need to be treated as long as you are sure that water is not getting in. You should check that the bubbles are not losing granules and that the area around them is worn, cracked or broken. If this is the case, as with the other problems, they must be repaired before you proceed. You should take extra care if your roof is walked on, and replace the entire affected area if multiple blisters are causing problems.

Application of Primer & Detailer

Before using the topcoat, you should first apply a layer of primer. This can be applied with a brush or roller. The exact amount needed depends on the porosity of your roofing material, but typically 0.1 – 0.3 liters per square meter should be used. This means that a 4-liter can covers 20 square meters.

Vertical areas, such as upstands or pipe collars, require the use of a detailer. In contrast to a primer, fibers are incorporated here to prevent the liquid from sagging. It should be applied at a minimum coverage rate of 18 kg/sqm.

Before proceeding, make sure that both the primer and the filler have dried completely. This should take about 2-3 hours.

Applying the top coat

When your roof is fully prepared and dry, you can proceed with the application of the liquid paint. First, mix the topcoat with your stirrer until the mixture is uniform. This can then be easily applied to your roof with a long-handled brush or broom, making sure to cover the entire roof evenly. If a second coat is required, make sure the first coat is completely dry before applying it. After that, just wait for the last coat to dry and your now sealed roof is complete. Clear Liquid usually takes 24 hours to dry completely and be passable, but is shower-safe in as little as 30 minutes. However, some other liquid applied sealers can dry much faster, so it is important that you do not mix more than you can apply in this time frame.

Residues of topcoat or primer that have fallen to the floor can be soaked up by sprinkling some construction sand on top.

Recommendation: Polyurea liquid plastic

However, the most sustainable and long-term solution is jointless spray sealing with polyurea. The liquid plastic was originally developed for heavy industry. A wide variety of surfaces can now be coated and sealed with polyurea. Polyurea is particularly popular for flat roofs due to its high load-bearing capacity and durability. For a non-binding consultation on this, please use the contact form below.


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Initial technical consultations and information as well as quotations are free of charge. We request that all necessary information be provided to allow for accurate bidding.

25 years of innovation in the waterproofing and coating sector with polyurea.

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