Chimney flashing
A chimney is where everything that can fail on a roof comes together – masonry that moves differently from the roof structure, the covering, the underlay, snow piling up behind the stack and sealant that ages. Most chimney leaks are caused not by faulty material but by the order of work and the laps.
- Sheet
- Chimney flashing
- Revision
- Sources
- 10 items
- Type
- information guide
What a chimney flashing consists of
- chimney stack (masonry or a proprietary chimney system)
- roof covering
- roofing underlay, turned up and bonded to the chimney by at least 150 mm
- front apron flashing (zásterka) laid on the covering and turned up the chimney face
- back gutter flashing running under the covering, shedding water to the sides
- counter-flashing (movement) strip fixed into the chimney
- permanently elastic, UV-stable sealant in the top joint
- battens and counter-battens
- Drawing no.
- KV-01
- Title
- Chimney flashing, section along the fall
- Scale
- diagram, dimensions in mm
- Revision
- 14 October 2026
| Part | Function | Typical mistake |
|---|---|---|
| front apron flashing (zásterka) | carries water from the front face of the chimney onto the covering below | too short, or tucked under the covering instead of lying on it |
| side flashings | carry water down the sides of the chimney; on profiled roofing they have an upstand drainage channel | no channel, so water spills over the edge |
| back gutter (cricket) | catches water and snow above the chimney and sheds it to the sides | gutter too shallow, no tapered back gutter on a wider chimney |
| counter-flashing (movement strip) | covers the top edge of the flashing and is fixed into the chimney | missing, or sealed with sealant that is not UV-resistant |
| sealant in the top joint | closes the gap between the counter-flashing and the chimney | used instead of a proper lap |
| roofing underlay and drainage channel | a second line of defence under the metal | underlay not turned up or bonded to the chimney |
| chimney cladding and cowl | optional cover for the whole stack | unsuitable material for high flue gas temperatures |
Installation step by step
Summarised from the method statements by ASB.sk (2021, updated 2025) and Všetkoprestrechu.sk (2026). Individual systems differ in the details; the instructions of the particular manufacturer apply.
- Prepare the chimney. The chimney must be dry and free of cracks, and cleaned of moss, old sealant and the old flashing.
- Connect the underlay. The roofing underlay is turned up the chimney and bonded to it. ASB.sk gives an upstand of at least 150 mm on every side, Všetkoprestrechu.sk 10–15 cm, or a height of about 150 mm above the roof plane. The corners are taped with flexible butyl-rubber tape.
- Drainage channel above the chimney. A channel formed from underlay or sheet carries water from above the chimney into the next bay, which has no penetration.
- Battens and covering around the chimney. The covering is cut so that the front piece lies on the covering, never off it.
- Front flashing, then the sides, and finally the back – always from the bottom up, so that each higher piece laps over the one below.
- Fixing. The flashing is secured with clips; unnecessary holes through the sheet are tomorrow’s leaks.
- The counter-flashing (movement strip) is fixed into the chimney stack. For strips used with flexible flashing, ASB.sk gives screws at 200 mm centres.
- Sealant. The top joint is closed with a permanently elastic, UV-stable sealant.
Flashing material
- with metal roofing, the flashing is made from the same type and colour of sheet as the roof,
- with clay or concrete tiles, pre-painted galvanised steel, pre-painted aluminium, titanium-zinc or copper is used, or a flexible aluminium strip with a butyl layer,
- never copper above galvanised steel, titanium-zinc or aluminium – water from copper dissolves these metals (RHEINZINK, VMZINC); Lindab’s warranty conditions rule out contact between its sheet and copper or water from copper components.
To help you choose: material comparison and galvanic compatibility.
Why chimneys leak
| Symptom | Most common cause | What to check |
|---|---|---|
| damp patch below the chimney, worse in slanting rain | cracked sealant or loose counter-flashing | the top joint and the fixing of the strip |
| leaks when snow melts | the back: snow above the chimney, shallow back gutter | the back gutter and the drainage channel under it |
| damp chimney masonry inside even without rain | condensation in the flue, not the flashing | the chimney system, ventilation, cowl |
| water under the front apron in heavy rain | apron tucked under the covering or too short | whether the front apron lies on the covering |
| rust marks and pinholes in the sheet | galvanised steel at the end of its life, run-off from copper, scratches | the material and condition of the sheet |
| water running down a rafter from the chimney | no underlay turned up the chimney | where the underlay meets the chimney, seen from the loft |
A detailed method for tracking down the cause: Why a chimney leaks.
Cost
Prices are indicative, October 2026:
| Item | Range | Source |
|---|---|---|
| traditional flashing by a roofer, including materials | €200–250 | Všetkoprestrechu.sk, 8 January 2026, VAT not stated |
| flashing kit with installation | from €100 + 1–2 hours’ installation | Všetkoprestrechu.sk, 8 January 2026 |
| labour – chimney flashing | €18–45 per m excl. VAT | Numiq, 14 May 2026, materials not included |
| replacing / repairing a chimney flashing | €50–180 each, average €100 | flavio.sk, May 2025, VAT not stated |
| chimney flashing trim (materials) | €12 per m incl. VAT | Plechy Majer, 14 April 2026 |
| chimney cowl (materials) | €80–180 each incl. VAT | Plechy Majer, 14 April 2026 |
The price depends most on the size of the chimney and its height above the roof, the pitch and type of covering, access (scaffolding, a platform), the condition of the chimney crown, and whether the whole stack is clad as well. With clay tiles, add the cost of lifting and relaying the tiles around the chimney.
The authorities
Replacing a chimney flashing is routine roof maintenance. A notification to the building authority is needed for a listed cultural monument and for maintenance that could affect, for example, the fire safety of the building (Section 18(4) of Building Act No. 25/2025 Coll.) – so if work extends to the chimney itself or its upper part, check with the building authority first.
Questions and answers
- If the chimney leaks, is it enough to reseal the joint?
- Only if the cause really is cracked sealant in the top joint of the counter-flashing. If water is getting in at the back, under the front apron or through corroded metal, sealant only postpones the problem. First find out where the water is getting through.
- Is a traditional flashing by a roofer better than a chimney flashing kit?
- Both can be right. A traditional flashing is folded on site to suit the chimney and the covering; a kit is made to measure from the dimensions you supply and is quicker to fit. What decides it is the quality of the underlay connection, the back of the chimney and the counter-flashing.
- What material should a chimney cowl be made of?
- Všetkoprestrechu.sk recommends stainless steel or copper for chimney cowls because of the flue gas temperature. For the flashing itself, be guided by the roof covering and by galvanic compatibility with the other metals on the roof.
Technical sources
The sources this sheet draws on. We checked prices and values from standards on the date shown; for prices, whatever the source currently states always applies. Sources marked “in Slovak” or “in Czech” are only available in that language.
- Prepared by
- KlampiarskeVýrobky.sk
- Revision
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