You’d think winter’s the real troublemaker for roofs – snow, storms, freezing rain. And yet, the summer sun? It’s quietly doing its own damage, too. Flat roofs, in particular, tend to soak up the worst of it. When it gets too hot, especially over several weeks, things start to shift, crack, and in some cases, bubble up.
This article takes a closer look at what the heat actually does up there – the bit you can’t always see from ground level. From UV damage and thermal movement to subtle signs like soft patches or ponding water, we’ll explain what to watch out for. And if your flat roof’s hot in summer and causing concern, we’ll talk through how to deal with it – including whether a liquid flat roofing solution might be the right fix.
UV Degradation
It’s not just sunburn you’ve got to worry about – your flat roof’s feeling it, too. Long spells of sunshine might look lovely, but those UV rays are slowly wearing away the surface above your head. And it’s not always obvious until the damage has already started.
What’s happening, more or less, is that the sun breaks down the protective top layer of many flat roofing materials. Over time, this leads to brittleness, fading, and a loss of flexibility. The roof stops flexing as it should, especially during warmer spells, which can cause it to crack or shrink in places. That’s when leaks tend to creep in.
Older felt systems tend to be more vulnerable to UV damage. Modern materials – especially those used in a liquid flat roofing solution & Modern PVC systems – usually stand up better, as they often include UV-resistant componants. Still, even the best systems can degrade eventually if they’re left unchecked for too long.
Thermal Expansion and Contraction
Roofs don’t just sit still – they stretch and shrink a little with every temperature swing. Sounds harmless, but over time, this constant back-and-forth puts strain on the materials. And flat roofs, by design, bear more of that stress across wider, uninterrupted surfaces.
On hot days, roofing materials expand slightly. Then, as the sun dips, they cool down and tighten back up. This daily cycle of movement, especially during a heatwave, creates pressure on seals, joints, and flashings. If these points were already ageing or poorly installed, you might spot splits, cracks, or loose edges forming before long.These issues are part of a broader problem known as thermal expansion roofing fatigue. It’s often subtle, yet it can make a big difference to how watertight the roof stays. If gaps appear or seams weaken, rainwater finds its way in – usually without much warning. So, keeping an eye on signs of movement-related wear can save a fair bit of trouble later on.
Membrane Drying and Blistering
Now, if your flat roof’s hot in summer for days on end, it doesn’t just expand – it starts drying out. Older systems, can lose their flexibility and begin to pull away from the surface beneath. The result? Blistering. And while it might seem minor at first, it’s often a sign of deeper trouble.
Blisters happen when trapped air or moisture beneath the membrane heats up and expands. With nowhere to escape, it creates raised bubbles. These weak spots are usually softer underfoot and, if left alone, can crack or split, leading to leaks.
Here are a few things to look out for:
- Small bubbles forming under the surface
- Spongey or springy patches when walked on
- Fine cracks or crazing across the membrane
- Discoloured or faded areas
This kind of flat roof blistering is more common during hotter months – and it’s often linked to poor ventilation, ageing materials, or minor water ingress that’s been baked from below.
Water Pooling Risks
Oddly enough, it’s not just rain that causes water trouble – extreme heat can play a part too. Over time, the structure beneath a flat roof can shift ever so slightly due to repeated heat cycles. That subtle movement or softening of certain materials can lead to minor depressions across the surface.
These dips might not look like much, but during summer storms, they stop water from draining properly. Instead of running off, it gathers, which is what we mean by water pooling on a flat roof. The longer it sits, the more likely it is to cause problems.
Here are a few telltale signs:
- Water that sticks around for more than 48 hours after rain
- Slight sagging or shallow dips across the roof
- Algae or staining that won’t scrub off
- Loose or lifting edges around the puddled areas
Left unchecked, these ponds can stress the waterproofing layer, invite plant growth, or worse, start seeping inside. And all that usually begins with just a little heat.
Do You Need a New Flat Roof?
It’s easy to assume the worst – that one crack or soft spot means the whole roof needs ripping off and starting again. But actually, most summer flat roof problems can be sorted out with targeted repairs, if they’re caught early enough.
You’re usually looking at repairs when the damage is localised: small blisters, surface cracks, or early signs of pooling. If, on the other hand, large sections are lifting, water’s leaking inside, or the surface feels unstable underfoot – that might be the time to consider a bigger upgrade.
One option growing in popularity, especially in extreme heat, is liquid roofing. Applied directly over the existing surface, this kind of flat roofing creates a seamless, watertight layer that’s designed to handle thermal movement and UV exposure far better than older systems. It’s quick to install, too, usually without needing to strip off the old roof, subject to the condition of the underlying roof structure.
So, is your flat roof hot in summer? Probably. And while that’s not unusual, it’s still worth keeping an eye on what the heat’s doing up there. From UV damage and blistering to subtle movement and pooling, summer tends to quietly test the strength of your roofing system.
Spotting the early signs of roof damage from heat means you’re far less likely to face bigger repairs later. And if things are looking a bit worse for wear, the team at AAC Flat Roofing are here to help with long-lasting fixes, expert advice, and smart solutions like liquid flat roofing.
