Common roof flashing mistakes
This list was drawn up from manufacturers’ installation guides and warranty conditions. Almost every item is a reason a manufacturer will reject a warranty claim – and a reason the roof leaks.
- Sheet
- Flashing mistakes
- Revision
- Sources
- 10 items
- Type
- advice article
| # | Mistake | Consequence | How to avoid it |
|---|---|---|---|
| 1 | lap against the flow of water (lower sheet over the upper one) | water runs into the joint | install from the bottom up |
| 2 | a screw or nail in a surface where water flows (valley, gutter) | a direct leak | fix with clips along the edges |
| 3 | long components with no expansion joints | rippling, torn joints | expansion joints as specified by the manufacturer (KJG gutters every 15 m) |
| 4 | copper above galvanised steel, titanium-zinc or aluminium | the lower metal corrodes | the same metal, or a compatible pair |
| 5 | cutting with an angle grinder | burnt paint and zinc, rust; loss of warranty (Ruukki, Lindab) | snips, a hacksaw |
| 6 | swarf left on the roof | rust spots | sweep it off straight after drilling |
| 7 | no underlay drip flashing at the eaves | condensation drips onto the rafters | an underlay drip flashing on every roof |
| 8 | underlay not connected to the chimney | every defect in the metal becomes a leak | underlay turned up ~150 mm and bonded |
| 9 | no counter-flashing (movement strip) at the chimney | the most common chimney complaint according to Všetkoprestrechu.sk | strip fixed into the chimney + sealant |
| 10 | parapet coping with no fall towards the roof | streaks and algae on the facade | fall of min. 3° (PREFA) |
| 11 | short nose on a window sill | water down the render | projection of 30–40 mm (ZOFI) |
| 12 | valley on widely spaced battens | sagging, standing water | boarding, gaps under 10 cm (Ruukki) |
How to spot them
- at sign-off – a water test on the gutters, a look at the order of the laps, checking for screws in water-carrying surfaces,
- a year later – streaks on the facade, rust spots, cracked sealant,
- after winter – valleys, the back of the chimney, loose trims.
The sign-off checklist is on the Sheet-metal roofing work sheet.
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
- Spotted an error?
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