How to Deal with Flat Roof Leaks

Roof Care & Maintenance
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How to Deal with Flat Roof Leaks

Flat roofs are a common sight right across the country, often found atop garages in residential areas, or large commercial and industrial structures. Choosing a flat roof has many benefits, especially if you’re looking to save time and money both during construction and maintenance. However, one issue that often plagues flat roofs is the dreaded roof leak.

Flat roof leaks can happen for a multitude of reasons, that may not be immediately obvious. If your flat roof is leaking, you’ll need to carry out a thorough inspection to identify the root of the problem and get it fixed. Failing to do so quickly could lead to all sorts of issues, including flooding, rotting timbers, condensation, mould, and more.

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Flat Roof Leak Warning Signs

Long before a flat roof leak occurs, there are numerous warning signs you should keep an eye out for.

  • Damp, dark brown patches on the ceiling indicate that moisture is seeping through your roof, which could become a major issue if not dealt with swiftly.
  • If your flat roofed garage or extension is quite old and exposed to the elements, you may notice your flat roof sagging or dipping. This is an indication that the timbers have begun to rot and will eventually give way, meaning a full flat roof replacement may be in order.
  • Be on the lookout for low spots atop your flat roof, as water can pool here, placing undue stress on the structure below and slowly disintegrating the chipboard decking through exposure to excess moisture.

What Causes Flat Roof Leaks?

There are many common causes of roof leaks. Some of these can be controlled, such as choosing the right material, or installing your roof correctly. However, as time goes on, exposure to the elements, such as harsh weather or extreme temperatures, will place stress upon and eventually break down even the most well-built roof. Here are some of the most common causes of flat roof leaks:

1. Age

No matter how well they were made, or how perfectly they were installed, all roofing materials will deteriorate over time. Harsh weather conditions, extreme temperatures, sudden impacts, all of these things will eventually wear away at your flat roof until it is damaged and begins to leak. However, in recent decades, manufacturers have made great strides to advance the strength and longevity of their flat roofing products, which today far outdo those of decades past. Simply put, your flat roof may be leaking because it has reached the end of its useful life.

2. Damaged or Deteriorated Flashing

Flashings are a key part of any flat roofing system. Their job is simple, to cover and seal any angles, seams, or joints, that would otherwise provide the perfect place for water to enter. This could be along an adjoining wall, around roof vents, and more. However, despite its impressive lifespan, lead flashing can falter, particularly when exposed to extreme temperature fluctuations. Over time, as the metal heats up and cools down, it will expand and contract, eventually revealing the edge it was meant to protect. This then, in turn, causes flat roof leaks.

3. Thermal Movement

In the second half of the twentieth century, oxidised bitumen was used as an adhesive to stick rubber roofing membranes onto flat roofs. This had many advantages, however, due to its rigidity, oxidised bitumen has very little movement. Therefore, when temperatures change, it cannot expand or contract as necessary, leading to cracks, blisters, and splits in the membrane above, and eventually causing leaks. Many older flat roofing systems were also designed before expansion joints became widely used, which facilitate effective thermal movement to prevent membrane failure.

4. Poor Installation

Even the most well-crafted flat roofing materials will not perform correctly if they are installed poorly. Flat roofs are highly vulnerable to leaks due to their near non-existent pitch, and many weak points. Therefore, it’s vitally important that any flat roof is installed according to the manufacturer’s guidelines. If you aren’t comfortable doing the job yourself, we’d recommend finding a qualified professional roofer to install your flat roofing system. While it is more expensive, this prevents future maintenance costs and gives you peace of mind that your flat roof is nice and secure.

5. Ponding Water

While your flat roof should feature a pitch of at least 1:40, this may not be enough to completely shed water or snowfall after heavy precipitation. It can also be the case in newer homes that the structure will settle over time, causing a flat roof to shift and lose its pitch. All of this may cause water to pool in areas on your flat roof. While most flat roofing materials are designed to handle some pooling water, if the water is allowed to remain on your roof for more than 48 hours, it can begin to seep into the roof space below, deteriorating its structural integrity and eventually leading to roof leaks. Therefore, it’s imperative to drain pooling water off of your flat roof to prevent long-term damage.

6. Seams and Overlaps

One of the most common areas where flat roofs fail and spring leaks are the seams or overlaps between different pieces of flat roofing material. This is especially common where the overlap has not been sealed correctly, as water will quickly find its way between the two layers. Seams and other joins such as overlaps are natural weak points that water can attack, and should be closely inspected at least twice per year to ensure no future problems are on the horizon. This is why we recommend EPDM membranes so highly. Installed as a single layer, there are no seams or overlaps that could cause potential leaks in your flat roof in future.

7. Blistering

Blistering is a common issue with flat roofs. If you spot raised blisters along the surface of your flat roof, it’s a strong sign that water has gotten underneath your flat roofing material. As the water seeps down, it becomes vapourised by the higher temperatures below, this water vapour then rises, but with nowhere to escape, the pressure causes blisters. Blisters are generally caused by water ingress, where rainwater or other precipitation finds its way into a weak point of your roof. Alternatively, blisters could be caused by damp timbers or insulation, as well as condensation. Blisters are a weak point in your flat roof, and when they burst can cause significant roof leaks.

8. Delamination

Delamination occurs when the different layers of flat roofing membrane separate, becoming “delaminated” from one another. This can lead to all sorts of problems, including cracks, splits, and even blistering as water seeps into and between the two layers of material. Delamination is generally caused by poor workmanship during installation, when the layers are not heated enough to seal correctly, or when water gets in between the layers and builds up inside.

9. Detailing Around Vents

Some of the most common areas where you’ll find a flat roof leaking is from poor detailing around or damage to raised sections such as the area around roofing vents, gas lines, pipes, and more. Just like seams and joins, these present yet another weak spot for water to attack your flat roof should they not be correctly sealed. Once again, this stresses the paramount importance of finding a qualified roofer to install your flat roof if you don’t think you’re up to the task.

10. Structural Issues or Poor Design

Along with the more easily avoided issues above, two problems which can cause flat roof leaks are structural issues, and poor structure design. For example, where the roof deck is overspanned, or where the distance between rafters is too great. This means that your flat roof will be more susceptible to thermal movement, and the structure will not be as strong. Another aspect of flat roof design which is often overlooked is ensuring that it features an adequate pitch. Failing to do so will lead to pooling water, and leaks. Finally, another common error is shallow upstands. The upstands in a flat roof should be at least 150mm to prevent rainwater from overflowing after a heavy shower.

How to Find a Leak in a Flat Roof

Finding the source of a leaking flat roof can be tricky. On a pitched roof, it can be as simple as tracing the leak to materials directly above, but on a flat roof, water can travel in all directions between the layers in your roof space before leaking into the rooms below. If your ceiling is damp, stained with water, or dripping, it does not necessarily mean that the roof leak is directly above. To find the source of a leak in a flat roofing system, you’ll need to get up and inspect the roof.

The best time to look for a leak in a flat roof is when it has been dry for at least 72 hours, ideally in the evening. This is because as the temperature drops, the source of your flat roof leak will gently steam as heat is released from underneath. You should also be on the lookout for low spots surrounded by a ring of dirt, which suggests pooling water was there previously.

Once you’ve found your suspected source of the roof leak, use a hosepipe to gently spray the area, and then go back down and see if the leak reoccurs. You may have to wait a bit, as the water will have to navigate its way through the many layers in your flat roofing system. This method is highly effective, but takes time, and you may not find the right spot on your first try. While you’re up on your roof, you should give the whole thing a thorough once-over, to ensure there is no damage that could cause roof leaks or other issues in future.

How to Prevent Flat Roof Leaks

There are many things you can do to prevent your flat roof leaking in the first place, saving you plenty of time, money, and worry in the long-run!

  • Inspect your flat roof regularly. If you check your roof at least twice a year, in the spring and in the autumn, you’ll be able to spot most issues before they become nightmare leaks.
  • Take a look at your roof after any harsh weather. This includes storms, heavy rainfall, strong winds, and especially snowfall. Ensure that any debris or ponding water is swiftly removed.
  • Inspect your guttering system, ensuring that there are no clogs or other issues that might prevent it from working. Learn how to clean gutters in our handy guide.
  • Replace your flat roof. If your flat roofing membrane has more patches than a boy scout, it’s time to replace it. We’d recommend ClassicBond’s cut-to-size EPDM membrane.
  • Install flashing. Adding flashings to your flat roofing system will prevent the most common sources of roof leaks. Flashing seals the seams around chimneys, guttering, vents, and more to prevent water ingress.

Flat Roof Leak Repair

Repairing a flat roof leak can range from a simple DIY job, to a large-scale project that requires expert help. The first step in finding out which you’re dealing with, is to closely inspect your ceiling and roof to spot where the leak may be coming from. To do this, you may have to clear away a bit of water and debris from your roof’s surface.

Once you’re satisfied that you’ve identified the source of your leaky flat roof, it’s time to choose your course of action. Sometimes, a temporary fix may be your best option, as it will give you time to consider your next steps, save for a full flat roof replacement, or get quotes from professional roofers. However, if the damage to your flat roof is more extensive, the only option left to you may be a more permanent fix such as a full flat roof replacement.

What to Do When You Spot a Leak During Heavy Rainfall

Spotting a leak whilst it is raining is a common occurrence, and whilst you may feel the need to jump on a repair straight away it’s unlikely that you’ll be able to address the problem whilst it is still raining.

That being the case, we would suggest containing the leak for the time being, with a bucket or something similar, and then contacting a roofer immediately. They will be able to book in a time to inspect the problem when it is dry.

Of course, if you feel up to the task you can look into the problem yourself – we have a number of potential solutions you could employ below. Even a temporary fix whilst waiting for your roofer may be the best course of action. Though we suggest consulting with them before taking action.

How to Temporarily Fix a Leaking Flat Roof

Fixing the leak in your flat roof temporarily is often a great option to buy yourself time, if only to get your valuables out of the way of the dripping water for a few days. Generally, the best stopgap method for repairing a flat roof leak is to use waterproofing paint to seal over the area. Do note however, that you shouldn’t use waterproofing paint if there are multiple sources of your roof leak across a large area, as this is inefficient and can make things more difficult down the road.

Flat Roof Sealant for Leaks

The advantage of acrylic-based waterproofing is it will not crack like bitumen-based waterproofers, and can even be applied during wet weather, although dry conditions are best. You will of course, need to clean the area first, as there’s no point sealing in moss, dirt, and debris!

How to Fix a Fibreglass (GRP) Roof Leak

Repairing a leaking GRP or fibreglass roof is relatively simple, as long as you only use products designed for fibreglass roofing. Some of the most common causes of a leaking GRP roof include broken flashings, and cracks or splits near joints and raised areas. Fibreglass roofs are also known to be susceptible to ponding water.

An excellent choice for repairing a fibreglass roof leak temporarily is to use a small amount of Acrylic sealant around the affected area. The best way to apply this is using a brush, but only once the area has been thoroughly cleaned of any dirt, moss, or algae. If you’re looking to perform a more long-term repair however, you should first scour the area with acetone and sand it using 60-grade grit paper. You should then laminate a 600g chopped strand mat over the hole using resin, and apply a topcoat to match your roof once this has cured.

How to Fix a Rubber Roof Leak

Rubber roofing has been a revolution for DIYers looking for an effective flat roofing system. Thankfully, rubber roof membranes are as easy to repair as they are to install. Common problems include tears, splits, holes, and shrinkage if the material was not applied correctly. Thanks to their “single sheet” installation, EPDM membranes rarely suffer from roof leaks when compared to felt roofing, as there are no seams or joins for water to attack.

If you’re looking for a quick fix for a rubber roof leak, and the issue you’re dealing with is small, we’d suggest using lap sealant around the affected area. This should give you a solid, albeit, temporary fix to the rubber roof leak you’re dealing with. For a more permanent fix, the most common way is to perform a “patch repair”. This involves using self-adhesive flashing, along with self-adhesive rubber tape to cover the hole. You then need to use an EPDM primer to stick the patch to your roof. We’d also recommend sealing the edges with lap sealant, especially if it’s in an area where water pools.

Felt Roof Leak Repair

Felt roofs are perhaps the most common form of flat roof across the country. Roofing felt has many advantages, including being cheap, and easy to install. However, felt roofs are highly susceptible to damage and leaks. Depending on the type of felt roof you have, the leak could be caused by splitting felt, impact damage, lifting joints, broken flashings, or crevices in the roof’s surface. Due to their low initial cost, it’s often best to simply replace a felt roof rather than continually repair it, however here are some of the ways to fix a felt roof leak.

The simplest way to repair a felt roof leak is once again to use acrylic-based waterproofing paint. This is highly effective at treating split felt, small gaps in joints, and punctures. If your flashing is coming away, we’d recommend repointing or redressing the old flashings to restore their waterproof seal. For a more long-term fix, you’d generally use a patch of torch-on roofing felt, or add a full, additional layer over your felt roof. If repairing your felt roof requires the use of hot flames, always leave this to a qualified professional, as it is too hazardous for an inexperienced hand.

Concrete Flat Roof Leak Repair

Although they are not too common these days, concrete flat roofs are still around in British homes and commercial premises. As you may have guessed, concrete flat roofs are incredibly hard-wearing, but can still suffer from leaks if there are cracks or splits in the concrete. The best way to handle a leaking concrete roof is first to try applying a thin layer of bitumen primer onto a cleaned surface, followed by bitumen-based waterproofing paint. If this is not enough to fix the leak, you could try patching over the crack with a small piece of torch-on felt.

How to Get Rid of Water Stains on a Ceiling

Now that you’ve found and fixed the source of the leak, it’s time to address some of the knock-on effects. Namely, any stains that the leak has left behind on your ceiling. These will be very obvious to spot and may well be what led you to the leak in the first place.

A water stain will present on your ceiling in a couple of ways

  • Brown, murky patch
  • Damp smell

The first step to getting back to pristine white ceilings is to clean the affected areas with a bleach solution. We recommend 1 part bleach with 3 parts warm water. Make sure to wear gloves, cover the floor below and keep the room ventilated.

Dry with a tea towel and then leave to dry overnight. If the stain is still visible in the morning, it’s time to repaint. First, scrape the stain to remove any dry flakes next you can apply your undercoat and finally your paint. We would suggest using a paint specifically designed to cover stains if possible.

How Much Does It Cost to Repair a Flat Roof Leak

The exact cost of repairing a leaking flat roof will depend on the scale of your issue. A tin of Cromapol Acrylic-based sealant will cost you less than £40 including delivery, and felt used for patch-repairs can cost up to £20 per square metre. However, if you’re looking to undertake a full flat roof replacement to repair the leak, that could cost you anywhere from £200 to over £1000 depending on the size of your roof. If you don’t do the job yourself, you should also budget in around £200 per day for a competent roofer.

Can I Claim On House Insurance for a Leaking Flat Roof?

Claiming on your home insurance for a roof leak is a tricky matter. Often, if the leak was not caused by an unexpected event such as a heavy storm or a falling tree, this damage would fall under general “wear and tear” and you’d have to pay for any repairs yourself. Your insurer will look for obvious signs of external damage that may have caused the roof leak, such as damage to your roofing membrane. If no such damage exists, you are likely out of luck.


We hope this guide has been helpful to anyone currently dealing with a leaking flat roof! If you’ve still got any more questions, please get in touch with our award-winning customer service team. They’ll be able to talk you through the exact products that are right for your needs. Call them on 01295 565 565, email [email protected], or use the handy livechat below.

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