The Property

Lead Flashings to Chimney Stack – Howsham, North Yorkshire

The Old School in Howsham, North Yorkshire, is a Grade II listed building with a rich history. Constructed in 1852, it was rebuilt by Colonel Cholmley, as indicated by the datestone above the central window. The structure is built from rockfaced limestone ashlar and rubble, featuring a pantile roof and a single-storey design. Its architectural elements include an ogee-arched doorway, flanked by similar windows with brick arches.

Although the clay tiles on a roof like this can last for centuries, structural movement and deterioration of supporting materials, can ultimately lead to the roof failing. The underlay, battens, fixings and flashings often fail first. Freeze-thaw action can crack and spall tiles, corroded nails allow tiles to slip, and roof timbers may sag or decay. Failed mortar, ridge pointing and chimney or valley flashings can allow water ingress. Moss, poor repairs and foot traffic can worsen the problem. Typical signs include cracked or missing tiles, sagging roof slopes, damp patches, leaks around chimneys, moss growth and crumbling mortar.

Chimney flashings are important because they create a watertight junction between the chimney and the roof, preventing rainwater from penetrating where the roof covering meets the masonry. Since a chimney interrupts the slope of the roof, water naturally collects and flows around it, making this one of the most vulnerable areas for leaks. Properly installed flashings direct water safely onto the roof surface and away from the building structure, helping to prevent damp, timber decay, staining and damage to ceilings or roof timbers.

Why the Original Leadwork Needed Replacing

The roof failed because the original clay tiles had reached the end of their service life. Many tiles showed severe delamination and spalling, with the outer surface breaking away and exposing softer clay beneath due to long-term wetting and freeze-thaw damage. Heavy moss and lichen growth worsened the decay by trapping moisture and forcing apart joints. Several tiles were cracked, loose and uneven, suggesting that the underlying fixings or battens had corroded or deteriorated. As the tiles slipped and no longer overlapped properly, wind-driven rain was able to penetrate beneath the roof covering.

The chimney flashings on inspection were relatively new and is probably a repair to address a historical leakage at the chimney, but it would not solve the broader failure of the surrounding roof covering. The widespread tile decay, biological growth, and slipped courses indicate that water is likely entering through many areas of the roof rather than one isolated detail. Flashings therefore were replaced as part of whole roof covering replacement.

Leadwork Standards and Technical Approach

The flashings in this case were renewed as part of a wider restoration works involving the replacement of the ageing clay tile roof. It consists of lead sheet flashing installed around a brick chimney where it passes through the tile covering. The detail includes apron flashing at the front and downslope side of the chimney, step flashing along the sides that is integrated with the roof tiles, and a back pan or saddle flashing on the upslope side to direct water around the chimney. The vertical lead pieces inserted into the mortar joints of the brickwork are the counter flashing, which overlaps the roof flashing below to create a watertight seal.

The chimney flashings were replaced using new rolled lead sheet manufactured to BS EN 12588, ensuring correct purity, thickness and durability. Code 4 lead was used for the stepped cover flashings and Code 5 for the back gutter and apron, selected in accordance with BS 6915. The flashings were divided into separate lengths to accommodate thermal movement and prevent splitting. Each piece was chased into the mortar joints to the correct depth, wedged and pointed, rather than rigidly fixed. Laps, drips and welts were formed in accordance with Lead Sheet Association guidance, and the lead was fully supported over the tiled roof and timber substrate.

Specification of Works

Preparation and Removal

• Strip back the newly laid tiles around the chimney to expose the abutment.
• Mark out the positions of the apron, step flashing and back gutter.
• Cut and rake out the mortar joints in the chimney brickwork.
• Clean the chimney and roof surfaces to provide a sound fixing background.
• Check the tile alignment and support ready for installation of the new lead flashings.

Substrate and Support

• Provide a continuous timber backing beneath all lead aprons and back gutters.
• Ensure the roof tiles are firmly fixed and evenly aligned around the chimney abutment.
• Install suitable underlay or separation membrane beneath the lead to prevent abrasion and corrosion.

Leadwork Installation

• Install a Code 5 lead back gutter behind the chimney over a fully supported timber substrate.
• Fit a Code 5 lead apron flashing to the front of the chimney beneath the roof tiles.
• Install individual Code 4 lead step flashings to each course along the chimney sides.
• Chase the step flashings into the brick mortar joints to a minimum depth of 25 mm.
• Secure the lead in the chase with lead wedges at suitable centres.
• Form cover flashings in Code 4 lead to overlap and protect the step flashings.
• Provide minimum 100 mm laps between separate lead pieces in accordance with BS 6915.
• Point the chases with suitable lead-compatible mortar or sealant to complete the installation.

Finishing and Protection

• Re-lay and secure the surrounding roof tiles neatly against the completed lead flashings.
• Check all laps, joints and pointing to ensure the flashing detail is fully watertight.
• Apply patination oil to all exposed lead surfaces to protect against staining and oxidation.

The Materials

This project used a straightforward but important group of materials, each chosen for a clear purpose within the chimney flashing execution.

• Code 4 rolled lead sheet to BS EN 12588 rolled lead sheet
• Code 5 rolled lead sheet to BS EN 12588 rolled lead sheet
• Lead wedges for securing the flashings into the mortar joints
• Patination oil for protecting the finished leadwork
• Lead-compatible pointing mortar or sealant for sealing the chases
• Timber backing board or plywood support beneath the apron and back gutter
• Roofing underlay or separation membrane beneath the lead sheet
• Copper or stainless steel fixings for the timber support and tiles

If you are trying to understand which code of lead is right for a similar detail, our shoppable lead chart is a useful starting point. For broader product learning, see The Lead Handbook and our lead roofing guides.