Lead Flashings — Everything Farnham Homeowners Need to Know

What Are Flashings?

Flashings are the waterproof seals that cover the junctions between your roof and other elements — chimney stacks, dormer walls, roof abutments, valleys and skylights. Where two surfaces meet, water would otherwise find a way in. Flashings prevent this.

Lead has been used for flashings for centuries because of its exceptional durability, malleability and resistance to corrosion. A correctly installed lead flashing will last 60–100 years. Alternatives exist — aluminium, zinc, and various synthetic flashings — but lead remains the standard for quality residential work.

The Lead Sheet Training Academy specifies that Code 4 lead is the minimum standard for most domestic flashings, with individual pieces limited in length to allow for thermal expansion and prevent fatigue cracking.Source: Lead Sheet Training Academy

Types of Flashings

Step flashings follow the line of a sloped roof against a vertical wall, stepping up with each tile course. Soaker flashings sit under each tile at the junction. Apron flashings are the horizontal lead sheet at the base of a chimney facing down the slope. Saddle flashings cap the top of a chimney. Valley flashings run the length of a valley between two roof slopes.

The most common failure in Farnham is the chimney flashing assembly — the combination of step flashings, soakers and apron that waterproofs the junction between a chimney stack and the roof slope.

Why Flashings Fail

The primary cause of lead flashing failure is thermal fatigue — the lead expands significantly in summer heat and contracts in winter cold. Over decades, this repeated movement causes fatigue cracks, particularly at bays where the lead hasn't been allowed to move freely. Sheets fixed too rigidly, or sized too large for the bay, develop fatigue cracks faster.

The second most common cause is incorrect installation — particularly lead that has been pointed with hard cement mortar into masonry joints rather than properly wedged and sealed. As the mortar cracks (which it always eventually does), the flashing loses its seal.

Why Mastic and Sealant Are Not Solutions

When a lead flashing fails, the short-term temptation is to seal the joint with mastic or bitumen. This works briefly — sometimes for a year or two — but it prevents the lead from working as designed, accelerates the underlying failure, and ultimately makes the proper repair more difficult and expensive. If your roofer is suggesting mastic as a solution for a failed flashing, ask why they aren't proposing to strip and relay the lead.

How Long Should They Last?

Code 4 lead (the standard for most flashings) properly installed with correct expansion provision should last 50–70 years minimum. Code 5 (used for flat roofs and higher-exposure applications) even longer. If your flashings are failing after 15–20 years, either the wrong code was used, the sheet sizes were too large, or the fixings were too rigid. A quality lead installation on a Farnham property should outlast the current owner.

The Code Numbers Explained

Lead sheet is graded by weight per square metre. Code 3 (lightest) is used only for soakers. Code 4 is the standard for flashings and valleys. Code 5 is used for flat roofs and larger exposure applications. Using Code 3 where Code 4 is required is a common cost-cutting measure that significantly reduces lifespan. We always specify and use the correct code for the application.

Last reviewed: 19 July 2026