Chimney flashing

Chimney flashing means the roofer's or leadworker's labour in forming and fixing a weathering detail around a chimney stack on a pitched roof. It typically covers the following tasks:

• measuring the stack and the roof pitch, cutting and forming the lead or other flashing material to size,
• fitting the front apron at the lower face of the stack, which carries water out onto the roof covering below the chimney,
• fitting the side flashings, usually soakers under the tiles or slates with stepped cover flashings above, or a stepped flashing dressed over profiled tiles,
• forming the back gutter behind the stack, and on wider stacks a raised saddle (cricket) that splits the water to both sides,
• raking out the mortar joints, wedging the top edges of the flashings into the chase and pointing them,
• sealing where needed with a sealant made for roofing and checking all laps against the roof covering.

What the work does not normally include: lead sheet, flexible flashing rolls and other materials, repair or rebuilding of the stack above the roof, repointing or re-rendering the stack, new flaunching, chimney pots or cowls, replacing damaged tiles or slates around the stack, and repairs to battens and rafters. These items are agreed and priced separately.

Chimney flashing is usually carried out in three typical situations:

• On a new roof or a re-roof. The flashing is fitted once the tiles or slates have been laid around the stack, as part of the roofing and leadwork. Old flashings should not be reused when the roof is replaced.
• When the roof leaks around the chimney. This is the most common reason homeowners call a roofer. In practice the old lead has often split or lifted, the pointing has fallen out of the chase, or the flashing was never properly formed in the first place.
• When the stack is repaired or rebuilt. If the chimney above the roof is rebuilt, repointed or rendered, the old flashings have to come off and new ones are fitted after the repair.

The work is done in dry weather. Sealants and self-adhesive flashing rolls must not be applied to wet, frosty or dusty surfaces. Roofers tend to have more availability in spring and summer than in autumn, when many owners try to fix leaks before winter.

Before the roofer arrives, it helps to know whether the stack is in sound condition. If the render is falling off, bricks are spalling, the joints are badly eroded or the stack is leaning, the chimney needs repairing first. Flashing will not last on crumbling masonry. If the chimney serves a stove, open fire or boiler, it is worth asking a qualified chimney sweep to assess the flue whenever the stack itself is being altered.

How to tell that the flashing is leaking

A leak around a chimney often shows up with a delay, because water first soaks into the timber and insulation. Typical signs are:

• dark or damp patches on the ceiling or sloping ceiling next to the chimney breast, especially after wind-driven rain,
• wet or mouldy rafters and battens around the stack, visible from the loft,
• stained or flaking plaster on the chimney breast in the loft or top floor,
• lifted, split or missing lead visible from the ground,
• mortar or sealant missing from the chase at the top edge of the flashing.

Bear in mind that damp on a chimney breast is not always caused by failed flashing. It can also come from condensation in an unlined or unsuitable flue, from rain entering through the top of the stack, or from water soaking through porous brickwork or render above the roof. If the flashing looks sound, it is worth having the chimney checked by a chimney sweep or a chimney specialist.

Preparing the area around the stack

The roofer removes the old flashings and loose mortar, cleans the brickwork and checks the battens and the underlay around the stack. The underlay is turned up against the stack or dressed into the back gutter so that any water that gets under the tiles runs out and not into the roof timbers. With proprietary chimney systems, the manufacturer's own roof-penetration components are used. The underlay should not be stuck directly onto a hot flue surface, as manufacturers warn against this because of the fire risk. Timber in the roof must keep the clearance from the flue required by the chimney manufacturer and by building regulations on combustible materials.

Battens, support and roof covering

Extra battens or boards are added around the stack where the flashing needs support, and the back gutter is usually formed on a timber tilting fillet and back board. The tiles or slates are laid and cut up to the stack, leaving the gap specified by the tile manufacturer or the flashing system. For flexible lead-free flashings such as Wakaflex, the manufacturer states that unsupported gaps between the tiles and the stack should be under 50 mm, otherwise additional support is needed.

Front apron

The front apron goes on first. It lies over the tiles below the stack and discharges water onto the roof. On profiled tiles the lead is bossed or dressed into the tile profile, or a flexible flashing roll is moulded to it. On metal roofing the apron is connected according to the roofing system.

Side flashings

The side flashings run down both sides of the stack. With slates and plain tiles, soakers are slipped under each course and a stepped cover flashing is fixed over them. With profiled tiles, the stepped flashing is usually dressed over the tiles, or a secret gutter is formed beneath them to carry away water that gets under the covering. The flashing is turned up the side of the stack to the height given in leadwork guidance and by the tile manufacturer, and the steps are turned into the mortar joints. On metal roofing the side flashings connect to the seam or profile of the sheet.

Back gutter and saddle

Water running down the slope above the chimney stops and collects against the back of the stack. On a narrow stack a back gutter that discharges onto the side flashings is enough. On a wider stack a saddle (cricket) is formed behind it, a small pitched section in lead or on a boarded base that splits the water to both sides. Without it, water, leaves and in winter snow sit behind a wide chimney and, as the snow melts, water finds its way under the flashing.

Cover flashing and chase

The top edge of the flashing on the stack should never simply be smeared with sealant. The correct detail is in two parts. The apron, soakers and gutter upstands sit against the brickwork, and a cover flashing is turned into a raked-out mortar joint or cut chase, held with lead wedges and pointed with mortar or a lead sealant. The cover flashing laps over the upstands below, which can then move freely with temperature changes. Roofing guidance makes clear that self-adhesive tape or a bead of silicone on its own is not a permanent repair.

Sealing and final check

Any joints that need it are sealed with a sealant made for roofing, resistant to UV and, near flues, to higher temperatures. New lead is often treated with patination oil to limit white staining. Finally the roofer checks laps, fixings and the connection to the roof covering. Where possible, it is worth testing the detail with water from a hose.

Lead sheet

Lead is the traditional and still the most common material for chimney flashing in the UK. Rolled lead is graded by code according to its thickness. Code 4 is the usual choice for aprons, step and cover flashings on domestic chimneys, while back gutters, wider aprons and exposed positions often call for Code 5 or heavier. Code 3 is mainly used for soakers. Properly detailed lead has a very long service life, but it must be fitted in limited lengths with correct laps so it can expand and contract.

Zinc and copper

Zinc and copper are durable metals with a long life. Zinc gradually forms a protective patina, copper is the most resistant and the most expensive. They are mainly used on high-quality roofs, metal roofs in the same material and some heritage projects.

Coated steel and aluminium

Pre-formed flashings in colour-coated steel or aluminium are used mainly with metal roofing, where they are matched to the roof sheets. Aluminium is light and easy to form, but in contact with some other metals, such as copper, it is at risk of galvanic corrosion.

Lead-free flexible flashings and chimney systems

For tiles and profiled coverings, flexible lead-free flashing rolls are widely used, typically a polymer layer reinforced with aluminium mesh, such as Wakaflex and similar products. They are moulded to the tile profile by hand, are much lighter than lead and are often chosen where lead theft is a concern. Manufacturers of system chimneys also offer ready-made roof penetration kits with a collar and terminal, where the joint between flashing and stack is protected from rain. These are quicker to install but are designed for a specific chimney.

When choosing the material, make sure the flashing is not made from a metal that reacts with the roof covering or the gutters. Water running off copper onto zinc or galvanised steel below causes corrosion of the lower metal.

Differences by roof covering

Clay and concrete tiles are the most demanding, because the lead or flexible flashing has to follow the tile shape and a secret gutter or soakers are often needed under the side flashings. Natural and fibre-cement slates are usually detailed with individual soakers in each course and a stepped cover flashing above. With standing seam metal roofs, the flashing connects directly to the seams and the roofer forms it together with the roof covering. With profiled metal sheets, the flashing follows the profile and filler blocks or seals are fitted under the laps to stop capillary water. With shingles, step flashings are interleaved between the courses.

• Only an apron and a bead of silicone. In practice it happens that only one piece is fitted and the joint with the stack is filled with silicone or flashing tape. After a few years the sealant cracks and water gets in.
• Flashing fixed rigidly into the brickwork without a separate cover flashing. The metal cannot move, the mortar around it cracks and water runs behind the flashing.
• No saddle behind a wide stack. Water, leaves and snow sit behind the chimney and in heavy rain overflow the top of the back gutter.
• Upstands too low. In driving rain or with lying snow, water rises over the top edge of the flashing.
• Underlay not finished at the stack. Water that gets under the tiles runs down the underlay straight to the chimney and into the roof timbers.
• Lead lengths too long or laps too short. Thermal movement makes the lead split or creep, and water gets in at the joints.
• Incompatible metals. For example copper flashing above a galvanised or zinc gutter, which then corrodes quickly.
• Flashing on a defective stack. The flashing is wedged into crumbling joints or render, which comes away after a few winters, taking the flashing with it.
• Sealant on a wet or dusty surface. The sealant does not bond and the joint leaks from the first rain.

Chimney flashing is carried out by a roofer or a specialist leadworker. Membership of a trade body such as the National Federation of Roofing Contractors (NFRC) and training in leadwork in line with the Lead Sheet Association guidance are good signs of competence. This is work at height, so a safe system of work must be in place, typically scaffolding with edge protection, a chimney scaffold or other fall protection. Under UK work at height regulations and HSE guidance, the contractor is responsible for planning and carrying out the work safely.

If the building is listed or in a conservation area, check with the local planning authority whether consent is needed before changing the materials or appearance of the flashing.

If the repair involves a chimney that serves a heating appliance, for example rebuilding the top of the stack or replacing the pots, the condition of the chimney and flue should be assessed by a qualified chimney sweep, for instance a member of NACS, the Guild of Master Chimney Sweeps or APICS, or a HETAS approved sweep. The flashing itself does not require any inspection of this kind.

The finished work can be inspected from the scaffold, from the roof or at least with binoculars from the ground, and from the inside in the loft. It is worth going through these points:

• check that the front apron lies on top of the roof covering and discharges water out, not under the tiles,
• see whether the flashing is turned up the stack high enough and evenly on all sides,
• confirm that the top edges are covered by a cover flashing wedged and pointed into the joints, not just a strip of sealant,
• on a wider stack, check the back gutter and saddle and their fall to the sides,
• inspect the laps between pieces, whether they are closed and lapped in the direction of water flow,
• look in the loft after the first heavy rain to see whether the rafters and battens around the stack are dry,
• check that the flashing on the roof covering is not bent or damaged by foot traffic,
• confirm that the colour and material of the flashing suit the roof covering and the gutters.

On handover, ask for details of the materials used, for example the lead code or the flashing product, and the sealant. It is worth checking the flashing once a year, ideally in autumn when the gutters are cleared. Pointing and sealant wear over time, and renewing them early is far cheaper than repairing wet roof timbers.

The labour price in the NUMIQ price list is around €22 per linear metre of chimney flashing. The price covers the roofer's labour and small consumables, without lead sheet, flexible flashing or system components, which are priced as materials. Prices vary by region and city. The linear metre is measured around the perimeter of the stack, so a 60 × 60 cm chimney has a perimeter of 2.4 m.

The final cost is mainly affected by:

• the size of the stack and the number of chimneys on the roof,
• the pitch and height of the roof and the resulting need for scaffolding or other access,
• the type of roof covering, as profiled tiles and slates are more demanding to detail than flat metal sheet,
• the need for a back gutter with a saddle and for cutting chases in the brickwork,
• removal of the old flashing and repairs to battens, underlay and pointing,
• the material, since heavier lead codes, copper and zinc cost considerably more than a flexible lead-free flashing.

If the whole roof is being replaced, see the article on roof replacement. For building a new chimney, the article on the Schiedel chimney system will help. If the roof is in good condition and only needs surface protection, read the article on roof cleaning and coating. Further items can be found under sheet metal works.