What Exactly is Flashing on a Flat Roof?

Think of flashing as the waterproof seal between your flat roof and anything that interrupts it. This is usually where the roof meets a vertical wall, but it also applies to skylights, pipes, or vents. It’s the single most critical detail on any flat roof. Get it wrong, and it doesn’t matter how good the main roof covering is – water will get in. I see failed flashing every single week on jobs in Brighton, Hove, and Worthing. It’s the number one cause of leaks on an otherwise decent-looking roof.

Why Does Flat Roof Flashing Fail So Often?

It’s almost never the material that fails; it’s the installation. Here are the main culprits I come across:

  • Poor Installation: This is the big one. The flashing hasn’t been chased into the brickwork correctly, the laps are wrong, or it’s just been stuck on with some sort of black mastic that bakes hard in the Sussex sun and cracks within a year.
  • Wrong Materials: Someone’s used a cheap, non-roofing grade sealant, or worse, just painted some bitumen up the wall. This is a bodge, not a repair, and it will fail.
  • Building Movement: Houses move. If the flashing isn’t installed with enough flexibility to account for minor shifts, it will tear or pull away from the wall.
  • Age: Even the best-installed flashing has a lifespan. Over decades, exposure to coastal weather can take its toll.

“Can I Just DIY Repair My Leaking Flashing?”

This is a question I get a lot. The blunt answer is: yes, you *can* go to a DIY shop, buy a tin of goop like Acropol or some flashing tape, and smear it over the leak. And for a little while, it might even work. But I need to be brutally honest with you: this is a temporary patch, not a fix.

Here’s the problem with that approach:

  • You trap moisture: You’re sealing water *in* as well as out. This lets the timber structure underneath rot away silently.
  • You hide the real issue: The original failure is still there, getting worse under your patch.
  • It makes a professional repair harder: When you eventually call me, I have to spend time scraping all that sticky mess off just to find the source of the problem, which can add to the labour cost.

If you have water pouring in and need a quick fix before I can get there, then a small patch is better than nothing. But please understand it is not a solution. If you’re dealing with a sudden problem, your best bet is to book an urgent leak repair so I can assess it properly.

How I Fix Flashing Correctly

Doing it right means doing it once. When I’m working on a garage flat roof or a rear extension, the flashing is my absolute focus. For a typical brick wall abutment, the proper method is to use lead flashing.

My process is straightforward and thorough:

  1. I cut a new channel (a ‘chase’) into the mortar joint of the brickwork, about 25mm deep.
  2. I cut and shape the new lead (usually Code 4 lead) to the correct profile, ensuring it overlaps the flat roof covering by at least 150mm.
  3. I fix the lead back into the chase with lead wedges.
  4. I seal the chase with a proper lead sealant or a sand/cement mortar mix. This creates a permanent, mechanical waterproof seal.

This is a skilled job. The angles have to be right, the folds need to be neat, and the fixings have to be secure. This is why I offer a specialised expert lead flashing repair service – because it’s a craft that makes or breaks a roof’s longevity.

Don’t Guess, Get a Proper Inspection

Flashing is too important to leave to chance or cover with a patch. If you have any doubts about the edges of your roof, especially if you can see cracked sealant or pieces of felt peeling away from a wall, it’s time for a professional look. A properly installed flat roof should have perfect, seamless flashing details.

I don’t do guesswork. I’ll get up on your roof, find the exact cause of the failure, and give you a fixed-price quote to sort it out for good. You can even see the progress for yourself with daily photo logs in my client portal. If you’re in the Brighton area, you can book a free, no-obligation roof inspection directly on my website, and I’ll tell you exactly what’s going on up there.