What a Valley in a Roof Actually Does (and Why It Leaks So Often)

When customers ask me where the most common leak points are on a pitched roof, valleys are always near the top of the list. A valley is simply the line where two roof slopes meet and channel water into one place. On a rainy night in Brighton or Hove, your valley is taking more water than almost any other part of your roof.

If the valley is even slightly wrong, water won’t forgive it. It will find any gap, pinhole or crack and work its way into your loft. My job is to work out exactly how and where that’s happening, then fix it properly so it doesn’t come back.

In this guide I’m not going to give you a vague textbook definition. I’ll walk you through how I look at roof valleys on real Sussex homes, which problems I see over and over again, and what I normally recommend when a valley is at the end of its life.

How I Quickly Spot a Problem Valley From the Ground

You can tell a lot about a valley before I even put a ladder up. When I turn up at a property, here’s what I look for first from the pavement or garden:

  • Staining or moss strip along the valley line – often means water is lingering where it shouldn’t.
  • Tiles sitting too tight to the valley centre – can choke the water channel and push water sideways.
  • Wobbly or uneven tiles near the valley – usually a clue the valley support or battens have started to fail.
  • Patchy cement or obvious repairs along the valley edges – tells me someone has already tried to chase leaks there.
  • Cracked or rusty metal in open valleys – especially on older galvanised steel valleys, which don’t age well near the coast.

If I can already see more than one of these from the ground, I’ll be honest with you: I’m usually thinking in terms of a proper valley refurbishment, not just another patch.

Common Valley Types I See Around Sussex (and Their Typical Issues)

In my 18 years on the roof around Brighton, Hove and Worthing, I’ve come across all sorts of valley constructions. Each has its own failure pattern. When you know which type you’ve got, you’re halfway to understanding what’s going wrong.

1. Traditional Lead Valleys

Lead valleys are common on older terraces and period properties. Done properly, they’re excellent and can last decades. Done cheaply, they’ll give you grief.

Typical problems I find:

  • Lead too thin or in short “patches” instead of full-length sections.
  • Cracks along the centre line where thermal movement has finally split the metal.
  • Blocked laps or debris build-up where the lead laps over lower sections.
  • Cement right up to the centre of the valley instead of neat, stepped cuts in the tiles.

Lead is very repairable, but once I see big splits or signs it was underspecified originally, I usually recommend a full replacement valley rather than chasing tiny repairs every winter.

2. Concrete or “Trough” Valleys

Some houses have pre-formed concrete valley troughs. These can do a job, but the joints and edges are weak points.

Common issues:

  • Hairline cracks that widen in frost and eventually drip into the loft.
  • Joints between trough sections poorly sealed or completely failed.
  • Tiles bedded into hard cement directly on the trough, which then cracks and lets water track under.

With concrete valleys, I often end up recommending upgrading to a modern GRP or metal system if we’re already doing other roof work. It’s usually money well spent.

3. GRP (Fibreglass) Valleys

Newer builds quite often have GRP valley troughs. When installed correctly, they can be very reliable. The problems come from rushed fitting.

What I see most:

  • Tiles cut too tight so water can’t flow freely down the GRP channel.
  • Nails through the GRP edge, which is a major no-no but sadly very common.
  • Incorrect underlay detailing where the felt doesn’t properly turn into the valley.

If you ask me, GRP itself isn’t the enemy. Poor workmanship around it is. A correctly detailed GRP valley should not be a regular leak source.

The Subtle Signs Inside Your House That Point to a Valley Problem

Often, by the time you call me out, the first sign you’ve seen is inside the property. When someone mentions a stain in a particular place, I can often guess it’s a valley before even seeing the roof.

Here are interior signs that often trace back to a faulty valley:

  • Damp patch on the ceiling near where two roof slopes meet – especially in upstairs bedrooms or landings.
  • Brown “trail” running down a wall under the line of a valley, sometimes appearing quite far from the actual leak point.
  • Mould in the top corners of rooms directly under the valley line.
  • Rotten or spongy timber in the loft along the valley rafter, if you’ve ever looked up there.

The important bit is this: water doesn’t always drip straight down. It can run along rafters, insulation and plasterboard before it finally shows. That’s why I always combine what I see inside with a careful inspection outside before deciding on the repair approach.

How I Actually Diagnose a Valley Leak on Site

People are often surprised how methodical I am with valley leaks. I don’t just jump up, slap some sealant on and hope for the best. Here is how I usually approach it:

Step 1: Talk Through the History of the Leak

I’ll always ask you:

  • When did you first notice the damp or staining?
  • Does it only happen in very heavy rain, or even in normal showers?
  • Is it worse in wind from a particular direction?
  • Has anyone “repaired” this valley before? If so, when and how?

Your answers help me rule out a lot of guesswork. Valley leaks that only show in sideways wind, for example, often point me towards tile cutting and side-lap issues, not just the valley trough itself.

Step 2: External Inspection and Photos

On the roof, I’m looking at:

  • The condition of the valley material itself (lead, GRP, concrete, metal).
  • How the tiles are cut, seated and fixed along the valley line.
  • Whether the underlay is intact and correctly dressed into the valley.
  • Any previous patches – sealants, flashband, random bits of metal or felt.

I take clear photos so I can show you exactly what I’ve found. In my experience, when customers see it with their own eyes, the repair options suddenly make much more sense.

Step 3: Check the Surrounding Details

Valleys rarely fail completely on their own. I’ll also check:

  • Nearby flashings (for example where a valley meets a wall or chimney).
  • Condition of the underlay and battens under the valley tiles.
  • Any guttering issues at the bottom of the valley run.

It’s no good fixing the obvious crack in the valley if the real problem is water backing up from a blocked outlet further down.

Repair vs Replacement: My Honest View on Valley Work

Here is my honest advice: tiny, localised valley faults can sometimes be patched sensibly. But once a valley is aged, distorted or clearly badly designed, you are usually better off replacing it properly and being done with it.

When a Local Valley Repair Can Make Sense

I’ll normally consider a smaller repair if:

  • The valley material is generally sound and not corroded or brittle.
  • The issue is clearly limited – for example, one crack, one failed joint, or a single badly cut tile.
  • The roof itself is not near the end of its life.

A sensible small repair example would be re-cutting and re-seating a handful of tiles that are choking a GRP valley, or replacing a short damaged lead section where the rest of the run is in good condition.

When I Usually Recommend a Full Valley Replacement

I tend to advise a complete strip and re-do of the valley when I see:

  • Multiple cracks, splits or rust spots along the valley length.
  • Obvious signs the valley was installed incorrectly from day one.
  • Water damage to the supporting timbers or battens below.
  • An older roof where other elements will soon need attention anyway.

It might sound like a bigger job, but in reality it’s usually a couple of days’ careful work. Afterwards, you’ve got a fresh, correctly detailed valley that shouldn’t give you trouble for many years – instead of living with repeat leaks and plaster repairs.

How a Proper Valley Replacement Works (Step by Step)

Every property is different, but this is roughly how I approach a full valley refurbishment on a typical tiled roof:

1. Strip Back the Tiles Either Side of the Valley

I carefully remove a band of tiles each side of the valley line, storing any reusable ones safely. This lets me see the underlay, battens and timbers supporting the valley.

2. Inspect and Repair the Structure Underneath

If the valley has been leaking for a while, the valley boards and nearby rafters may be damp or rotten. I replace or reinforce anything that has lost strength before installing the new valley system. This bit is hidden once the roof is back together, but it’s crucial.

3. Install the New Valley System

Depending on the property and the surrounding roof, I’ll normally fit a modern GRP trough, a new lead valley, or another appropriate system. The key details are:

  • Correct laps and overlaps.
  • Proper fixing (no random nails through the main channel).
  • Underlay dressed correctly into the valley, not just cut short.

4. Re-cut and Re-lay the Tiles

This is where neatness and experience really show. The tiles need to be:

  • Cut to give enough water clearance but still look tidy.
  • Fixed in a way that doesn’t puncture or compress the valley channel.
  • Stepped and aligned so they shed water into, not across, the valley.

Done right, the finished valley looks clean, straight and uncluttered – and it actually works properly when the next downpour hits.

Why Valleys Often Struggle on Sussex Homes

Roofs around Brighton, Hove and Worthing have a few local challenges that hit valleys hard:

  • Coastal weather – wind-driven rain puts valleys under extra pressure, especially on exposed seafront or hilltop homes.
  • Older mixed roofs – I often see extensions or loft conversions where a new roof meets an old one, with the valley sitting right on that awkward join.
  • Historic repairs – quick fixes over the years (cement, flashband, silicone) can actually make the valley geometry worse.

Because I work locally, I know the typical house types, the usual problem spots, and the weather patterns that tend to expose weak valleys first.

What I’ll Usually Show You After a Valley Inspection

I like to keep things as clear and transparent as possible. After I’ve inspected your valley, I’ll usually:

  • Show you photos of the valley condition and any damage underneath.
  • Explain in plain English what’s happening with the water and why it’s getting in.
  • Lay out two or three realistic options: for example, a short-term repair versus a full replacement.
  • Give you a clear, fixed-price quote so you know exactly what the valley work will cost.

If your valley leak is urgent and causing active dripping or ceiling damage, it might be worth using my emergency leak repair service so I can prioritise you in my schedule.

How Valleys Interact with the Rest of Your Roof

Valleys don’t work in isolation. A weak valley can be a sign that other parts of the roof are tired too. When I’m up there, I’ll also be checking:

  • The general state of the pitched roof tiles or slates.
  • The condition of the underlay and support battens either side of the valley.
  • Nearby chimney or wall flashings that might be contributing to damp.
  • How the valley discharges into the guttering and downpipes.

If I find wider issues, I’ll go through your options – anything from localised pitched roof repairs to a more thorough upgrade if the roof is at the end of its life.

When a Valley Leak Isn’t Just a Valley Problem

Sometimes I’m called to a suspected valley leak and it turns out the main culprit is something else entirely.

Typical examples:

  • Felt and batten failure just beside the valley, where water getting under tiles has nowhere safe to go. In that case, a section of felt and batten replacement may be needed alongside the valley work.
  • Guttering and downpipe issues at the foot of the valley, causing water to back up into the eaves. Then the fix may involve sorting your guttering and downpipes as well as the valley.

This is why I always step back and look at the full water path, not just the visible crack or stain.

My Straightforward Advice If You Suspect a Valley Leak

If you think your valley is leaking, my honest advice is:

  • Don’t ignore it – valley leaks usually get worse, not better, and the hidden timber damage can be much more expensive than the valley itself.
  • Don’t just keep repainting the stain – you’re only hiding the symptom, not solving the cause.
  • Get a proper rooftop inspection – photos and a clear explanation will help you decide what level of work makes sense.

If you want me to have a look, you can use my free roof inspection booking page to arrange a site visit. I’ll talk you through exactly what I see on your valleys, what I’d do if it were my own house, and give you a fixed price so you can make a calm, informed decision – no pressure, no surprises.