Lead Valley Repair

The Ultimate Step-by-Step Guide to Lead Valley Repair

Including traditional methods and modern alternatives

Understanding a Lead Valley

A roof valley is the internal angle where two sloped roof sections meet, directing water down to the guttering. These valleys are often lined with lead flashing to create a watertight channel. Over time, movement, corrosion, or poor workmanship can cause leaks—requiring a full repair or replacement.

Step-by-Step Repair of a Traditional Lead Valley

1. Preparation & Safety

• Assess the damage: Look for cracks, splits, or signs of water ingress.
• Scaffold or access platform: Ensure safe access to the valley.
• Protect surrounding roof: Lay down sheets to catch debris or displaced tiles.

2. Remove Surrounding Roofing Material

• Carefully lift and remove slates, pantiles, plain tiles, or interlocking tiles adjacent to the valley.
• Remove any nails or clips securing them in place.

Material-Specific Tips:

• Slates: Often fixed with nails; use a ripper to remove without breaking adjacent pieces.
• Pantiles: Usually hooked or nailed—remove gently to avoid cracking clay edges.
• Plain tiles: Double-lapped; remove in courses to avoid disturbing underlying tiles.
• Concrete interlocking tiles: May clip together—unclip without breaking nibs.

3. Remove Old Lead & Inspect Valley Boards

• Strip out the old lead flashing, taking care not to damage the underlying valley boards.
• Check boards for rot or delamination—replace with treated timber if necessary.

4. Prepare New Lead Flashing

• Use Code 4 or Code 5 lead depending on valley width and roof pitch.
• Lengths should not exceed 1.5 metres to allow for thermal expansion—lap joints should be at least 150mm.

Tip: For very wide valleys or exposed conditions, consider welted edges for extra durability.

5. Bedding Mortar on Fibre Cement Strips

Place fibre cement support strips (such as valley boards or pre-formed fillets) along each side of the valley. These are not needed for slates or plain tiles.

Bed any mortar onto these not directly on lead when suing a material such as pantiles or concrete interlocking rod tiles, which helps:

• Avoid cracking from thermal movement
• Prolong life of both mortar and lead

• Maintain structural support

6. Install the New Lead Valley

• Dress the lead neatly into the valley, allowing adequate overlap into tile lines.
• Secure lead using lead clips or copper nails at the edges—never fix through the middle.
• Ensure lead is free to expand and contract.

7. Reinstall Roofing Materials

Carefully replace slates or tiles over the new valley:

• Maintain minimum side laps (typically 100mm) to direct water into the valley.
• Keep tile edges slightly raised off the lead to allow water flow beneath.

For pantiles or deep-profile tiles, use mortar bedded onto a fibre cement strip where necessary to avoid gaps.

8. Mortar Bedding at Edges (if applicable)

If bedding is required (commonly with plain tiles or slates):

• Use a strong mix (e.g., 3:1 sharp sand to cement)
• Bed onto fibre cement understrip, not the lead
• Avoid blocking water run-off paths

Modern Alternatives: Dry Valley Trough Systems

In many new-build homes, traditional lead is being replaced with dry valley troughs, typically made from GRP (Glass Reinforced Plastic) or polypropylene.

Lead Valley Vs Modern Valley

Key product differences

Verdict:

• Dry valleys are ideal for modern homes, offering ease and speed.
• Lead valleys are superior for aesthetics and longevity, especially on heritage buildings or premium builds.

Final Tips

• Always use patination oil on lead to prevent staining.
• Ensure adequate water flow—avoid tight overlaps that may trap debris.
• Maintain ventilation beneath tiles to avoid condensation build-up.

When considering a lead valley repair, the project should be considered in context.

Importantly, the cost benefit should be examined based upon how much other work there is to do around the valley in order to facilitate repair.

In some instances, a lead valley that needs a repair, could be patched or lead welded to offer a temporary fix without disturbing the cut slates or tiles to the margins.

However, if the lead lining requires replacement, then costs should be carefully devised to ensure the solution provides sufficient value over time to make it viable.

When considering whether to repair or replace a valley, it is important to consult an expert and listen to what they have to say or you may waste time and money.

Replacing a lead valley can be time consuming, owing to the amount of work to the interfaces using either slates or tiles.

Further, if there has been water ingress through the lead over time, the substrate supporting the lead, may have perished and also require repair or replacement.

It is difficult to consider this until the lead lining has been removed, but it is prudent to consider a contingency sum for external timber work when looking at a lead valley repair.

The Costs Involved 

Whether replacing or repairing a lead valley, the costs involved may be as follows:

• Initial inspection and diagnosis.
• Access scaffolding.
• Labour, a pre defined period of time should be allotted for repair.
• Materials, lead, fixings, mortar if appropriate.
• Sourcing and providing matching slates or tiles, often reclaimed or salvaged.
• Relevant plant and equipment.
• Contingency for timber repairs or replacement substrates.

Further, a roofing contractor must make provision for overheads and profit.

Although the material element may be inexpensive, labour costs may make the process of carrying out a repair or valley lining replacement costly. These costs should be weighed up against the repair, which is likely only to provide an isolated fix, and not address any wider problems.

Materials Needed 

A roofing contractor will consider carefully the materials needed to replace a lead valley lining.

If replacing the lead, commonly used sizes are 450mm wide lead or 600mm wide lead, usually in code 5. 

Further, replacement slates or tiles to match the existing covering will also be needed.

For example, if you have a roof covering with Welsh slates, it is important to source replacements that are or the same dimension as the existing, The slates will need to be cut and extra wide slates may also be needed.

If using pantiles or concrete roof tiles, then replacement tiles will be needed, along with a fibre cement strip, to provide an inert separation between the lead and the mortar, sand and cement to mix mortar. PVA and mortar plasticiser are also advantageous.

Further, the lead valley will need to be fixed, this can be done using copper leadwork nails, and the roof materials will also require fixing using appropriate nails.

Importantly, fixings used should comply with BS5534 and be fit for purpose.

Signs a Repair is Needed

A defective valley lining will usually cause water ingress towards the base of the valley, i.e the lowest point.

Water stains may be seen inside the property but also on the external walls.

As water penetrates the defective valley lining, it will track down between the substrate and the lead lining.

Common signs are: 

• Stains at an intersect, for example on a terraced house where the rear elevation meets the rear offset.
• Dampness on an external wall in a similar location.
• Drips above the area of intersect, if the lowest point is not the bottom of the valley but created by a joint in the substrate.

A leaking valley will usually cause obvious damage to a property, and if left unresolved can cause wider issues with decay beneath the surface of the lead facilitating a need for a more comprehensive repair. .