Flat roof is leaking - Find the cause and get it fixed to limit damage

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If you're flat roof is leaking and you're not sure why, we've covered off the reasons it could be. Best of all, we have solutions and instructions on how to prevent more damage or repair it completely.

This guide will help you to find the cause of the leak, limit any further damage and get it fixed.

Having a roof (a flat roof in this case) over your head is one of the most important things when it comes to keeping you and your loved ones safe and dry.

But what happens when that roof starts leaking? Flat roofs can be fantastic, but they’re also susceptible to damage and leaks.

Here's what could be causing your flat roof to leak and how to get it sorted.

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Why your flat roof leaks in heavy rain

When heavy rain falls, you might notice damp, dark brown patches on the ceiling. Or, you may even see your flat roof sagging and dripping.

Here are some reasons why your flat roof might be leaking.

Natural damage

Your flat roof may have been damaged, for instance by high winds or a falling tree.

Age

Traditional flat roofs made from mineral felt have a maximum lifespan of around 10 to 15 years. However, more modern flat rubber roof could last over 50.

Pooling water

A flat roof shouldn’t technically be totally flat. Instead, it should have a pitch of at least 1:40. However, you may still get water pooling in areas of your flat roof.

When water pools, it can seep into the roof space below and lead to leaks.

Damaged flashing

Flashing exists to cover and seal angles, seams and joints that could otherwise let in water. Over time, lead flashing can get damaged, which can lead to leaks.

Weak seams or overlaps

In areas where different pieces of flat roofing material come together, leaks can be common. This is especially the case if the overlap or seam hasn’t been sealed correctly.

Poor detailing

Detailing around raised sections like roofing vents and pipes can be a common area for leaks to start. Again, this is often because it hasn’t been correctly sealed.

Blistering

If your flat roof has a blistered appearance, this can either be because rainwater has found its way into your roof, or because of condensation coming from below. Either way, when blisters burst, they can cause major roof leaks.

Delamination

Delamination is when layers of flat roofing membrane separate from one another. It’s usually caused by poor installation.

Delamination can cause lots of different problems such as cracks, splits and blistering. Eventually, this can lead to leaks.

Thermal movement

Older roofing systems that were designed before expansion joints became widely used – or that used oxidised bitumen as an adhesive – might struggle to expand or contract with temperature changes. This can lead to cracks, splits and blistering.

Poor installation

Flat roofs are vulnerable to leaks because of their almost non-existent pitch and various weak points. So, it’s important they’re installed to a high standard to ensure that they perform correctly.

Structural issues

There might be an issue with your flat roof’s structure or design that’s leading to leaks. For example, there could be too much of a distance between rafters, the roof deck could be overspanned or there might not be enough of a pitch to prevent water from pooling.

Not sure what to do next?

Why not find a local roofer working near you, get in touch, request a quote, and get your flat roof fixed.

And when you book a job through Checkatrade, it's also covered by our Guarantee. Eligibility and T&Cs apply.

Flat roof leak detection

If your flat roof is leaking, the first thing that you’ll need to do is find the source of the leak. Unfortunately, this can be a little tricky!

On a flat roof, water can travel in any direction between the layers in your roof space before it makes its way through your ceiling. That means the source of your roof leak won’t necessarily be directly above the damp or dripping patch on your ceiling, as it would be if you had a pitched roof.

A professional roofer will be able to help you get to the cause of the leak, but you can also get up and inspect the roof yourself if you’re keen to try a little DIY.

Here’s a step-by-step guide for how to detect the source of your leak:

1. Wait until it’s been dry for at least 72 hours

2. Head up to your roof in the evening when the temperature is dropping. Be safe doing this! (If, for whatever reason, you don't feel confident or comfortable doing this, call a professional roofer

3. Keep an eye out for areas with steam drifting upwards (heat is released from underneath)

4. Look for spots surrounded by a ring of dirt (showing a sign of pooling water)

5. Gently spray the area you suspect is responsible for the leak

6. Check inside to see if the leak happens again (you might have to wait a bit)

7. If no outcome, try again - head back up to your roof and try a different spot

While you’re up on the roof, it’s also worth checking the whole thing over. You want to make sure there’s no other damage that could cause more roof leaks in the future.

How to fix a leaking flat roof

How to fix your leaking flat roof will depend on a few factors, including:

• The cause of the leak

• The material of your roof

• Whether you need a short-term or long-term fix

• The condition of your roof

In particular, if your flat roofing membrane is full of patches, it might be time to replace it. However, there are some fixes you can try first.

Flat roof sealant for leaks

The quickest and easiest way is often to seal over the area using waterproofing paint. This isn’t a long-term solution, but it might just protect your belongings from dripping water long enough to sort out a longer-term fix.

• Clean the area so you don’t seal in dirt and debris, and then apply your waterproofing

• Wait until conditions are dry, although you can apply acrylic-based waterproofing in wet weather if necessary

• Acrylic-based waterproofing is that it won’t crack like the bitumen-based stuff

Just make sure not to use waterproofing paint if there are many sources that are leaking across a large area. This won’t be very efficient and could also risk making things more difficult later on.

Felt roof leaking

Felt roofs are some of the most common flat roofs in the UK. Roofing felt is cheap and easy to install, although it’s also highly susceptible to leaks.

Depending on the type of felt roof you have, this can be due to issues such as:

• Split felt

• Lifting joints

• Broken flashings

• Cracks in the roof's surface

Acrylic-based waterproofing paint is often very effective at treating split felt, punctures and small gaps in joints, so it can be a good short-term solution.

For longer-term fixes, a good solution is often to just replace the felt roof, as opposed to continually repairing it. This is because felt roofing won’t often last beyond 10 to 15 years, and is relatively cheap to install.

Why not check with a roofer what the best solution would be?

Fibreglass (GRP) flat roof leaking

If you have a fibreglass flat roof, you might be dealing with leaks caused by broken flashings or cracks and splits near joints and raised areas.

However, one of the most common causes of leaks in fibreglass roofs is simply from pooling water.

For a short-term fix, you can often just apply a small amount of acrylic sealant with a paintbrush to the affected area.

However, for a more long-term fix, you’ll want to clean the area with acetone, sand it and laminate a chopped strand mat over the hole using resin. Then, once it’s cured, apply a topcoat that matches the rest.

Rubber flat roof leaking

Rubber flat roofs are easy to install and equally easy to repair!

A few common issues that might cause your rubber flat roof to leak include:

• Tears

• Holes

• Shrinkage (if the material wasn’t correctly applied)

However, because rubber flat roofs are applied as a ‘single sheet,’ they’re far less susceptible to roof leaks than felt roofing. After all, there aren’t any seams or joins for water to get in through.

You can use lap sealant as a quick fix when it comes to small rubber roof leaks.

But the most common longer-term fix is to do a ‘patch repair’ where you use self-adhesive flashing and self-adhesive rubber tape to cover the hole.

Concrete flat roof leaking

Concrete flat roofs aren’t very common nowadays, but you can still find them in the UK.

They’re very durable but can still experience leaks if cracks or splits in the concrete form.

To fix a leaking concrete flat roof, clean the surface and then apply a thin layer of bitumen primer, along with bitumen-based waterproofing paint.

If this doesn’t do the job, you can instead get a professional to patch over the crack with a piece of torch-on felt.

Whether you're looking for replacement costs for your existing flat roof, comparing possibilities for changing to a pitched roof, or simply looking for the right roofer, we're here to help.

When to get a professional to fix a leaking flat roof

Although you can certainly try some of the steps we outlined above to fix your leaking flat roof, it’s generally quicker, easier and safer to get professional help with your roof repair.

• It can be dangerous working at height without proper training

• It ensures a leak can be properly diagnosed

• Fixes and repairs can usually be guaranteed

• Jobs are carried out to a higher standard than simple DIY

• Having a professional roofer provides extra peace of mind

• You'll likely avoid more costly repairs in the long run

To find a reliable roofer to fix your leaking roof, simply request a quote. We’ll then send details of your job to some of the best roofers in your area, who can contact you directly.

Alternatively, just search with your postcode to view a selection of approved, reviewed and trusted roofers near you.

Frequently asked flat roof questions

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