Planning a Rear Extension in Sussex? Let’s Talk About the Roof.

You’ve got the plans, the builder is lined up, and you’re excited about your new kitchen or living space. But the one thing that gets overlooked, and causes the most headaches down the line, is the flat roof on top of that new rear extension. I’ve been fixing roofs in Brighton, Hove, and Worthing for nearly two decades, and I’ve seen some real horror shows on brand new extensions. Here are the straight answers to the questions you should be asking.

What’s the Best Material for My New Extension Flat Roof?

Forget everything else for a moment. You have three solid options, and here’s my honest take on them:

  • High-Performance Torch-on Felt: This isn’t the old-school felt your grandad had on his shed. Modern, multi-layer torch-on felt systems are tough, reliable, and offer a great balance of cost and longevity. Installed correctly, you’re looking at a good 20-25 years of service. It’s a workhorse and my go-to for many jobs.
  • GRP (Fibreglass): GRP creates a seamless, hard-wearing plastic shell over your roof. It looks sharp and modern and is incredibly tough. The main drawback? It can be noisy when it rains, and it must be installed in perfect weather conditions – not always easy to find on the Worthing coast. A poorly installed GRP roof will crack and fail quickly.
  • EPDM (Rubber): EPDM is essentially a big rubber sheet glued to your roof deck. It’s fantastic for longevity (we’re talking 30-40+ years) and handles building movement well. It’s lightweight and durable, but the finish relies heavily on the skill of the installer, especially around details like skylights and pipes.

Honestly, for most rear extensions in the Brighton area, a modern felt system is the most reliable and cost-effective choice. You can read more about my approach to building a solid rear extension flat roof that’s built to last.

What Are the Biggest Mistakes Homeowners Make?

1. Not Enough ‘Fall’ (Slope)

This is the number one problem I see. Your builder might create a perfectly level roof deck because it looks neat, but that’s a disaster waiting to happen. A flat roof must have a slight, calculated slope (a ‘fall’) to guide water towards the gutter. Without it, you get ‘ponding’ – standing water that sits on your roof for days, stressing the materials and eventually finding a way in. I always ensure the timber structure is built with the correct fall from day one.

2. Bad Detailing Around Skylights and Walls

A flat sheet of roofing material is easy to get right. The real skill is in the details. The points where the roof meets a brick wall, goes around a soil pipe, or joins a skylight are where 90% of leaks happen. This is where proper lead flashing and upstands are critical. Cutting corners here is asking for trouble.

3. Using the Wrong Insulation

Building regulations require a certain level of insulation (a ‘warm roof’ design). Using cheap or incorrect insulation not only means your new room will be cold, but it can also lead to condensation problems within the roof structure itself, causing timber to rot from the inside out. Don’t let your builder skimp on this.

My Extension Roof is Already Leaking. Can I Just Paint it With Something?

I get this question a lot. You see a small damp patch and head to the DIY shop for a tin of bitumen paint or a waterproofing slurry. Look, you *can* patch a small, obvious hole with a product like Acropol or a fibre-reinforced paint as a temporary, emergency fix. It might stop the drip for a few weeks, maybe even a few months if you’re lucky.

But let me be blunt: you are just painting over the problem. You haven’t found the source of the failure, and water is likely still getting trapped underneath your ‘patch’, silently rotting the decking. If you’ve got a leak, especially on a newer extension, it points to a fundamental flaw in the installation. If you’re in that situation, the best thing to do is book an urgent leak repair so I can find the real source of the problem before it causes major damage.

How Do I Know I’m Getting a Good Job?

It’s all about process and transparency. When I build a new flat roof, I don’t just turn up and start. First, you get a fixed-price quote based on a proper survey or your architectural plans. You can even get an instant online estimate on my website to get a ballpark figure right now. You can see all my flat roofing services and costs clearly laid out.

Once we start, you get access to a client portal where I upload photos *every single day*. You see the old roof coming off, the new timber deck being laid, the insulation going down, and every layer of the waterproofing system being applied. You see the quality for yourself, without having to climb a ladder. That’s how you know you’re not getting corners cut.

If you’re planning an extension or have a roof that’s already giving you problems, the smart move is to get it looked at properly. I offer a free roof inspection across Sussex, where I’ll give you honest advice with no obligation.