Lead valleys vs GRP valleys — what's the right call

A valley is the V-shaped join where two roof slopes meet. They’re the most water-stressed part of a pitched roof — all the rain falling on both slopes channels through the valley and into the gutter at the bottom. The material you use matters.

Lead valleys — the traditional choice

Lead has been the standard valley material in the UK for centuries. On a Bath stone property, it’s almost certainly what’s on the existing roof.

Pros:

• Lasts 60-100+ years on a typical valley
• Hand-dressable to fit any valley shape — straight, tapered, asymmetric
• Weathers gracefully — develops a protective patina
• Compatible with any tile or slate covering
• Repairable in sections if damaged

Cons:

• More expensive than alternatives
• Skilled labour required — not every roofer can dress a proper lead valley
• Heavier than alternatives
• Can be stolen for scrap value (rare in Bath, but happens elsewhere)

When we recommend it:

• All period properties (Georgian, Victorian, Edwardian)
• Listed buildings or conservation area work
• Where the existing valley is lead and the rest of the roof is lead (consistency matters)
• Anywhere long-term reliability is the priority

GRP valleys — the modern alternative

GRP (glass reinforced polyester, fibreglass) is a single-piece valley lining shaped to standard sizes. Used increasingly on new-builds and refurbishments since the 1990s.

Pros:

• Cheaper than lead
• Single piece — no laps to fail
• Lighter than lead
• Can be installed by less skilled labour
• Looks tidy when new

Cons:

• Shorter service life than lead — typical guarantee is 20-25 years
• Rigid once cured — can crack at stress points, particularly where the valley meets a complex roof shape
• Difficult to repair cleanly — patching GRP usually looks obvious
• Not as flexible as lead for non-standard valley shapes
• Failure mode is sudden — see our notes on GRP flat roofs

When we recommend it:

• Modern properties (post-1980) where the rest of the roof is non-period
• Budget-sensitive work
• Outbuildings and garages
• Where the property owner specifically wants a lower upfront cost

Why the choice matters

A valley is hard to access and expensive to redo once it’s failed. The choice between lead and GRP isn’t just about upfront cost — it’s about how many times you want to do the work in the lifetime of the roof.

A lead valley done properly: done once in your lifetime, maybe once in the house’s lifetime. A GRP valley done properly: probably done twice in the house’s lifetime.

If you’re planning to stay in the house for decades, lead usually wins on lifetime cost. If you’re selling in five years, GRP may be the cheaper answer.

What a proper lead valley looks like

1. Code 5 or code 6 lead (depending on the valley size)
2. Individual pieces lapped by at least 150mm (more on exposed sites)
3. Wooden valley boards underneath — the lead sits on a smooth timber substrate, not directly on the rafters
4. The lead is dressed up under the tiles or slates on each side, with each course of tile/slate bedded into the lead with a small welt
5. The bottom of the valley discharges into the gutter properly — usually with a separate “splash strip” or similar

A machine-rolled lead valley can be OK but a hand-dressed one is better, particularly on non-standard valley shapes.

What a proper GRP valley looks like

1. Single piece of GRP valley trough
2. Bedded on a smooth substrate
3. Tiles or slates bedded into the edges of the trough with mortar
4. The bottom discharges into the gutter

Less to go wrong but the service life is shorter.

Mixing the two — what to watch for

Sometimes the right answer is “use both”:

• Lead for the main valley runs
• GRP for short sections or non-standard shapes

More often, mixing happens by accident — old lead replaced with GRP in a previous repair, then the GRP fails and needs replacing. The smart move at that point is usually to put lead back, even if it costs more.

What to ask if you’re getting a quote

• Is the valley lead or GRP? (most roofers should be able to tell you)
• What code of lead? (4, 5, or 6 — for lead only)
• Is it hand-dressed or machine-rolled? (hand-dressed is better)
• Is the valley substrate (the wood underneath) being replaced? (often the right time to do it)
• How is the valley discharging into the gutter? (should be properly detailed, not just dumped)

If the answers are vague, walk away. Valleys are not the place to save money with shortcuts.

If you’ve got a valley question, get in touch. We can usually tell from a photo whether what you’ve got is sound or whether it needs work.