How to Install Lead Flashing on a Roof: Step-by-Step Guide

Installing lead flashing on roof structures requires precision, the right materials, and proper techniques to create weatherproof seals that last decades. Whether you're working on chimney flashings, valley installations, or abutment details, this guide walks you through each stage of the installation process to help you achieve professional results.

Before you start any lead work on roof projects, you need the correct grade of lead sheet and the proper tools. Most flashing applications require Lead Code 4 (Lead Code Blue) or Lead Code 5 (Lead Code Red) lead, depending on the exposure and pitch of your roof. For certain standard flashing applications, Lead Code Yellow (1.50mm, with no numerical equivalent) offers a cost-effective alternative worth considering at the specification stage.

Materials and Tools You Need

Gathering everything before you start saves time and ensures you work safely at height. Your material list varies slightly depending on the type of flashing you're installing.

Essential Materials

Code 4 (Code Blue) lead sheet works for most standard flashing applications including step flashings, aprons, and valleys up to 150mm girth. You need Code 5 (Code Red) lead for larger exposed areas, soakers over 175mm girth, and high-exposure locations. Abbey Metals also stocks Code Yellow lead (1.50mm) which is a BSEN 12588:2006 grade with no old numerical equivalent that sits between Code 3 and Code 4 in thickness. It is suitable for abutment and step flashings, soakers, gutters, back aprons, lead slates, and damp proof courses, and can offer a material cost saving compared to Code 4 (Code Blue) in appropriate applications. Install Code Yellow to a maximum flashing length of 1.5 linear metres and a maximum area of 0.35m². Stock your project with lead wedges for securing flashings into masonry joints. You also need compatible sealant for lead-to-lead joints and pointing mortar that matches your existing masonry.

Required Tools

A lead knife or sharp Stanley knife cuts lead sheet cleanly without tearing edges. You need a wooden dresser or bossing stick to shape the lead without damaging the surface. Keep a soft-faced mallet handy for gentle forming work. A tape measure, chalk line, and straight edge ensure accurate measurements and cuts. For chasing out mortar joints, use a bolster chisel and club hammer. Safety equipment includes work gloves, safety glasses, and proper roof access equipment.

Preparing the Roof Surface

Surface preparation determines how well your lead flashing performs over time. Rushed preparation leads to failures that require expensive repairs later.

Clean all surfaces where lead will sit. Remove dirt, moss, loose mortar, and debris from tiles, slates, and masonry. Check that roof coverings are secure and in good condition before adding flashing. Any movement in the underlying structure telegraphs through to the lead and causes cracks.

For chimney and wall abutments, chase out the mortar joints where you'll secure the flashing. Cut to a depth of 25mm and width of 25mm. Clean out all loose material from the chase. The joint must be sound and deep enough to hold lead wedges securely.

Measure your flashing runs carefully. Account for overlaps at joints, corners, and changes in direction. Lead flashing requires 150mm minimum overlap at all horizontal joints and 100mm at vertical laps. Add these measurements to your cut lengths before you measure the lead sheet.

Step-by-Step Installation Process

The installation sequence depends on which type of lead flashing on roof you're fitting. We'll cover the main techniques that apply across different flashing types.

Cutting and Shaping Lead Sheet

Roll out your lead sheet on a flat, clean surface. Mark your cutting lines with chalk or pencil. Use a straight edge and sharp lead knife to score the lead. You need multiple passes with moderate pressure rather than trying to cut through in one go. Once scored deeply enough, fold the lead back and forth along the score line until it separates cleanly.

For shaped pieces around pipes or corners, create a paper template first. Transfer the template to your lead sheet and cut carefully. Small adjustments are easier than trying to patch gaps from cutting too much away.

Installing Step Flashing

Step flashing protects the joint where a pitched roof meets a wall. You install it in stages as the roof covering goes up. Each step piece covers one slate or tile course.

Cut individual step pieces to the correct height and width. Each piece needs to extend at least 150mm up the wall and 150mm across the roof slope. The vertical leg sits in the mortar joint you prepared earlier. The horizontal leg sits between slate or tile courses.

Start at the bottom of the roof. Position the first step flashing so the horizontal leg sits on top of the first full course of slates or tiles. The vertical leg goes up the wall and into the prepared mortar joint. Secure the vertical leg with a lead wedge tapped gently into the joint. Don't drive wedges in too hard as this splits the lead.

Lay the next course of roof covering over the horizontal leg of the first step. Position the next step piece over this course. The upper edge of each step overlaps the lower edge of the step above by at least 75mm. Continue up the roof, alternating roof covering and step flashing pieces.

Once all steps are in place and secured with wedges, point the mortar joints. Use a mortar mix that matches your existing pointing. Press mortar firmly into the joint over the lead wedge. Finish the pointing flush with or slightly recessed from the wall surface.

Installing Apron Flashing

Apron flashing seals the junction where a roof meets a vertical surface head-on. You find this detail at the base of chimneys and where roof slopes meet walls.

Cut your apron to the full width of the area plus 50mm extra at each end for dressing into side walls. The apron needs to extend at least 150mm up the vertical surface and 150mm down over the roof covering.

Chase out the horizontal mortar joint at the correct height. Position the apron so the upper edge sits in this joint with the lower edge extending well over the roof covering. Secure the upper edge with lead wedges at 450mm centres maximum. Dress the lower edge down over the tiles or slates using your wooden dresser. Work from the centre outwards towards each end. The lead should follow the roof profile smoothly without kinks or bulges.

At each end, cut and fold the lead to dress into the side walls. These side dressings need care to create neat, weathertight corners. Point the horizontal joint with mortar once you're satisfied with the fit.

Installing Valley Flashing

Valley installations require wider lead sheet as the flashing sits in the valley channel between two roof slopes. The width depends on the valley angle but typically ranges from 450mm to 600mm total width. For more detailed guidance on valley installations and other specialized applications, see our Complete Guide to Lead Flashing for UK Roofing Projects.

Cut valley pieces to manageable lengths of 1.5m maximum. Longer lengths are harder to handle and more prone to thermal movement issues. Dress the lead into the valley using your bossing tools. The lead must sit flat against the valley boards with no air gaps or bridges.

Secure valley flashing with copper or stainless steel clips at 450mm centres along both edges. The clips hold the lead while allowing thermal movement. Never nail through lead as this creates leak points and prevents natural expansion.

Overlap each valley section by 150mm. The upper piece sits over the lower piece with the water flow. Seal laps with compatible lead sealant. Trim tiles or slates to sit neatly over the valley edges with a 50mm gap between the roof covering and the valley centre.

Common Mistakes to Avoid

Even experienced roofers make errors when installing lead flashing on roof structures. These mistakes compromise the installation and lead to premature failures.

Never use nails or screws through lead sheet. This creates holes that leak and prevents the lead from expanding and contracting naturally. Always use clips, wedges, or the lead's own weight to hold it in place.

Don't stretch or thin the lead when dressing it into position. Over-working hardens the lead and makes it brittle. Use gentle, repeated movements with your dresser rather than forcing the lead into shape.

Avoid insufficient overlaps at joints. The 150mm minimum overlap exists for good reason. Water driven by wind gets under shorter overlaps and penetrates the roof structure.

Don't forget about thermal movement. Lead expands and contracts with temperature changes. Installations that don't allow for this movement develop splits and tears. Bay lengths should not exceed the recommendations in BS 6915:2001 for your specific application.

Finishing and Long-Term Maintenance

Proper finishing protects your installation and extends its working life. Check all joints and laps for secure fixing. Ensure mortar pointing is complete and weather-struck. Clean any flux residue or marks from the lead surface.

Lead develops a protective patina over time that guards against corrosion. Don't paint or coat new lead flashing. The natural weathering process creates the best long-term protection.

Inspect your lead work annually. Look for signs of movement, damage, or deteriorating mortar joints. Clear any debris that accumulates in valleys or behind upstands. Blocked gutters and valleys cause water to back up under flashings and create leaks.

Minor repairs address small issues before they become major problems. Re-point mortar joints as needed. Replace individual damaged sections rather than waiting until the entire installation fails. With proper maintenance, your lead flashing installation lasts 60 to 100 years or more.

Frequently Asked Questions