How to Waterproof a Flat Roof

Roof Care & Maintenance
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How to Waterproof a Flat Roof

Making sure that your flat roof is fully waterproof is crucial, particularly as the wet and colder months approach. Flat roofs can be susceptible to a number of issues such as pooling water and nightmare leaks caused by adverse weather. Thanks to the leaps made in roofing technology in the last few decades, homeowners have a number of ways to waterproof a flat roof available to them. These include patch repair kits, waterproof sealants, and rubber membranes. The right solution for you will depend on your exact needs, which we’ll talk more about below.

Inspecting Your Flat Roof

If you’re looking to waterproof your flat roof because of a reoccurring roof leak, the first thing to do is give your roof a thorough inspection. Thankfully, most leaks in flat roofs will not require a full roof replacement, but rather a patch repair or replacing just one part of your roof. It can be difficult to find the source of a roof leak, and you may need to do a bit of investigating inside and outside to track it down. You can do this by finding the drip and measuring the distance from there to your wall. Then, when you’re on top of your roof later, you should be able to find the spot more easily.

There are a few things to remember before you begin inspecting your roof. First and foremost, ensure that your roof is safe to walk on before attempting to do so, and make sure to secure your ladder properly before climbing it. If you’re ever unsure, you always have the option to call a professional. The same goes for any roofing equipment or tools you may be using – never perform work with tools that are unsafe or unfit for the job you’re doing. Be wary if your flat roof hasn’t been coated to be slip-resistant, as without an anti-slip coating you may need to use boards or pads to ensure you’ve got a proper footing.

The best time to inspect your roof to spot issues such as pooling water is immediately after a period of extended rainfall. This will allow you to safely take note of any areas where puddles are forming which could lead to future leaks, as well as let you know about drainage issues. You should then wait until it is completely dry to get onto your roof and undertake any work, as this is the safest time to do so.

Your guttering system is another common cause of flat roof leaks and other issues that homeowners often seek to solve by waterproofing. This is especially the case when your gutters are damaged, blocked or have been incorrectly installed. While they’re often overlooked, your gutters are the lifeline of a safe and healthy property, so it’s paramount that they’re kept in top condition. If your guttering system is broken and in need of repair or replacement, it should be a top priority. Read our guide to fixing leaking gutters for more information.

How to Spot Damage to Your Flat Roof’s Surface

Once you’ve given your guttering system a once-over, and you’re satisfied you know where any leaks are coming from, it’s time to take a look at the surface of your flat roof. You should first look for any obvious holes or cracks, which while uncommon, can cause serious damage to your roof space and property as a whole.


You’ll also need to keep an eye out for blistering. Blistering occurs when pockets of air become trapped beneath a flat roof’s surface. These pockets can be large or small and can expand when the weather is warm. While these blisters are unsightly, they’re best left well alone if they haven’t burst, as they shouldn’t present any barriers to your flat roof’s functionality. If any blisters do burst on your flat roof’s surface, they need to be patch repaired quickly. This will prevent further tearing and any potential water ingress.

Pooling Water

As mentioned earlier, you’ll want to keep an eye out for puddles on your roof. If water has pooled on your roof and won’t go away two days or more after it last rained, that’s a big sign that a flat roof can’t shed water effectively. Also known as “ponding”, this can put serious stress on your flat roofing membrane. This problem is made doubly worse when your guttering system is blocked or damaged, and both issues should be sorted as soon as possible to stave off potentially major expense in future.


Another common sign of damage to a flat roof is known as “alligatoring”. This is where impact damage, water, debris, or UV rays have damaged your roofing membrane and caused cracks on the surface, that look like an alligator hide. To fix a flat roof with this issue, you’ll need to apply a waterproofing solution, as these shallow cracks will soon deepen and potentially become sources for nightmare leaks.

How Do I Repair My Flat Roof and Make It Waterproof?

Once you’ve identified the issues your flat roof has, and performed a thorough inspection throughout, it’s time to look for the solution. Whether you can simply repair your roof or if it requires a full replacement depends on the severity of the damage. If you’re just dealing with a few popped blisters or slight alligatoring, you may be able to salvage your roof with a few simple fixes.


Waterproof sealants are very flexible substances that have a variety of uses. They’re great for less confident DIYers who need a quick solution to waterproof their flat roof. There are many sealants available for you to choose from, each with their own unique qualities. Most sealants can be used on all sorts of flat roofs, from asphalt to bitumen surfaces. They can be used as a quick fix, to help create a non-slip surface or as a thick waterproofing layer for the whole flat roof that will stand the test of time. Take a look at our waterproofing liquid range to find the right sealant for you. To find out how to seal a flat roof, be sure to read our dedicated guide.

Used for: Smaller jobs and quick fixes. Can be used on the entire roof and can be used for the creation of non-slip surfaces. While sealants benefit from excellent durability, longer-lasting alternatives are available.

How to use:

  • Always begin spreading sealant on the edges of the work area before slowly but surely making your way across the entire space.
  • It is most effective when applied to the entire roof, with any cracks or tears covered up with repair kits or patches beforehand.

Liquid Rubber

Liquid rubber is extremely flexible and is perfect for roofs with dormers or abutments thanks to its strength and reliability. It’s a fantastic flat roof waterproofing solution for DIY beginners as it’s incredibly easy to apply. Once applied, liquid rubber forms a highly elastic coating that can stretch due to thermal movement without being damaged or losing performance. This makes it a fantastic option for whole flat roofs, whether they’re covered by concrete, asphalt, or EPDM rubber.

Used for: Can be spread across projects large and small. Provides a thick coat to the roof that is both waterproof and extremely durable. Not the quickest of fixes, but ideal for brand new roofs that require waterproofing. Can be applied quickly and easily, so is excellent for homeowners of all ages and DIY abilities.

How to use:

  • Most liquid rubber mixtures will require a primer to be applied first, though this is not always the case.
  • Once it has been applied to your roof and dried, you can start to add the liquid rubber solution using a brush or roller, spreading it evenly across the workspace.
  • Most tins of liquid rubber will have the coverage area written clearly on it, so ensure that you have enough tins to cover the whole area.
  • Some liquid rubber brands will also require multiple coats in order to achieve maximum efficiency, so be sure to consult any supplied instructions when using a certain make of the substance.

Repair Kits

Patch repair kits are ideal for smaller areas of your flat roof that may need to be repaired to restore their waterproof status. Most flat roofing manufacturers will also produce dedicated repair kits for their products, that can be applied quickly and easily. There are two main ways to apply flat roofing repair kits. The first is a peel-off backing which reveals extremely strong adhesive, allowing the patch to be applied like a sticker. Others need to be brushed on, sometimes requiring primer or additional adhesive. No matter which you choose, both methods are incredibly quick and easy for anyone.

Used for: As the same would suggest, repair kits are to be used on already-existing roofs that require mending or rejuvenating. They are not designed for brand new projects, though patches can sometimes be used to cover up outstanding gaps in the rubber or membrane.

How to use:

  • For liquid solutions: The liquid solutions are used in a very similar way to liquid rubber roof solutions. Ensure the area is clean before adding a primer if necessary. Once this has been added the liquid substance can be applied evenly using a brush or a roller to ensure that the liquid is evenly applied. Then, wait for surface dryness, and consult instructions to check if extra coats are needed.
  • For patches and other adhesive solutions: Before using, make sure that all surfaces are clean. Stick down any material that may be loose to ensure that the patch is being used on a secure surface. Then you can remove the backing from the patch and slowly apply it to the intended area, making sure it is smooth and no creases are present. Finish by using a roller to firmly press the patch down, encouraging stronger adhesion.

How do I prevent leaks in future?

There are a few things you can do to minimise the risk of leaks in future. The first is to check your roof over more frequently. While this may not necessarily prevent leaks from appearing, it will certainly give you a better idea of your roof’s condition and will keep you alert to any possible signs of deterioration that may lead to leakages.

Flat roofs don’t last forever, and even if they appear to be in good condition after many years, their structure may be starting to fail. It is therefore a good idea to replace the rubber on your flat roof when it begins to reach the end of its lifespan, rather than waiting until afterwards. This will save you time and money in the long run, as fixing a damaged roof tends to be much more expensive than repairing an old one.

Cleaning your roof more frequently is another good way to ensure that your roofing material stays intact for longer. Sweeping away debris, applying specially made cleaners and general washing can help to make sure that various harmful substances damage the surface of your roof and prevent the need for premature replacement.


And there you have it – a look into how best to waterproof your flat roof. Keep in mind of course that your situation may well be unique to you. The best solutions will depend heavily on your structure’s size, shape and even what time of year you are trying to waterproof the roof.

Your newly waterproofed roof will hopefully come as big relief, and you can now be safe in the knowledge that your structure is fully functioning and able to control any water that falls onto the roof, resisting any moisture ingress and safely transporting the liquid away from your roof and down into your drainage system.

If you’ve inspected your roof and you’re satisfied that you may need to undertake a full flat roof replacement, take a look at our range of flat roofing materials. We’re proud to offer homeowners and tradespeople a fantastic selection of EPDM and GRP rubber membranes and kits for a huge range of property styles and structures.

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