The mistake I see every week in Brighton and Hove

In my 18 years on the roof, I’ve seen more failed ‘DIY sealant jobs’ than I can count. Usually, a homeowner notices a damp patch, rushes to a local hardware store, buys the biggest tin of black bitumen paint they can find, and slaps it over the problem. A month later, the leak is back, and now the roof is covered in a sticky mess that makes a proper repair twice as difficult.

When customers ask me about using roof sealant for flat roofs, I tell them straight: sealant isn’t a magic wand you wave at a dying roof. However, when used as a surgical tool for specific details, the right high-performance sealant is an absolute lifesaver. Here is how I approach it as a professional.

The ‘Surgical’ approach: Sealant for precision, not coverage

If your entire roof membrane is porous and failing, no amount of sealant will save you. In those cases, you are much better off checking my flat roof repair and replacement costs to see what a real fix looks like. But, if your roof is generally sound and you have a specific leak around a pipe penetration, a corner, or a skylight, that is where high-end sealants shine.

I use sealant for the tricky bits—the areas where traditional torch-on felt is difficult to manipulate. If you have an urgent leak that needs booking, I often use a professional-grade liquid membrane to seal those complex ‘weak points’ that water loves to exploit.

The chemistry: What I actually use (and why it matters)

Forget the cheap acrylics or bitumen-based tins. When I’m working on your home in Sussex, I only use products that stay flexible. Flat roofs move—they expand in the summer sun and contract in the winter frost. If your sealant doesn’t move with the roof, it will crack within months.

  • MS Polymers: These are my go-to. They can often be applied even in damp conditions, which is essential given the weather we get on the coast. They stick to almost anything and remain incredibly flexible.
  • Silicone-Based Sealants: Great for UV resistance, but they can be a nightmare if you ever want to coat the roof again in the future, as nothing sticks to silicone—not even more silicone.
  • Polyurethane: Very tough and durable, but it can be fussy about how clean and dry the surface is during application.

Preparation is 90% of the job

If you ask me, the secret to a leak-free roof isn’t the liquid in the tin; it’s the work done before the lid is even opened. Most DIY attempts fail because they apply sealant over moss, grit, or old, flaking paint. When I take on a repair, I spend the bulk of my time scrubbing the area back to a sound substrate and using specific primers. If the bond isn’t perfect, the water will find a way under it eventually.

Whether I’m fixing a bay window flat roof or a large extension, I document this entire preparation process. My clients get access to a secure online portal where I upload daily photo logs. You can see exactly how I’ve cleaned and prepared the area before the sealant or felt goes down. It’s all about total transparency.

Is sealant a permanent fix?

I’ll give you my honest advice: sealant is a fantastic way to extend the life of a roof or to fix a specific ‘detail’ failure, but it is rarely a substitute for a new roof if the structure is compromised. For example, if you have a garage flat roof where the timber boards are sagging and rotting, slapping sealant on top is just throwing good money after bad.

However, for protecting a parapet wall or firewall capping, a liquid-applied sealant system can be a brilliant, cost-effective long-term solution.

Get a fixed-price quote from a local pro

I know the roofing industry has a reputation for ‘guessing’ prices and adding extras at the end. I don’t work like that. You can use the calculators on my site to get an instant estimate, and if you’re happy with that, I’ll come out for a free site survey. I’ll provide a formal, fixed-price quote so you know exactly what you’re paying before I even pick up a trowel. No jargon, no fluff—just 18 years of experience applied to your roof.