Flashing for buildings and roofs

Map of flashings

Three rules that apply everywhere

1. Lap in the direction of flow. The higher sheet always laps over the lower one. That is why a chimney flashing is fitted front first, then the sides and finally the back. On valleys and eaves flashings the joints are made from the bottom up.

2. The metal must be able to move. Metals expand with heat – titanium-zinc, according to the manufacturer RHEINZINK, by 2.2 mm per metre for a 100 K change in temperature. Long components therefore need expansion joints and fixing with clips that allow movement. A long sheet screwed down rigidly will buckle or tear.

3. The metals must be compatible. Water running off copper dissolves zinc. Copper above galvanised steel, titanium-zinc or aluminium is a mistake no coating will fix. An overview of the pairs is in the material comparison.

Order of installation on a pitched roof

The roofing manufacturers’ installation guides (Ruukki, MASLEN) agree that components under the covering go on before it and components on the covering go on after it:

1. underlay drip flashing at the rafter ends,
2. roofing underlay, counter-battens, battens,
3. gutter brackets and gutters (before the first course of covering),
4. eaves flashing, valley, under-covering verge flashings, under-covering side flashings,
5. the roof covering,
6. ridge caps, over-covering verge flashings, over-covering side flashings.

Chimney flashing is fitted progressively with the covering around the chimney: first the front (lower) part, then the sides and the back, with the covering laid up to them. The exact order is set by the instructions for the particular system.

Choosing the material

Flashing should last at least as long as the covering it sits under or next to. Replacing a chimney flashing means taking up the surrounding covering. With metal roofing the flashings are made from the same sheet and colour; with clay tiles, pre-painted galvanised steel, aluminium, titanium-zinc or copper is used. See the comparison on the Materials page.

When you need the authorities

In practice, replacing flashing is treated as roof maintenance. Section 18(5)(a) of the new Building Act (No. 25/2025 Coll., stavebný zákon, as in force from 1 November 2025) lists repairing and replacing a roof covering or the surface of a flat roof among works that require neither a decision on the building plan (rozhodnutie o stavebnom zámere) nor a notification (ohlásenie). A notification to the building authority (stavebný úrad) is required, however, for maintenance that could affect the stability of the building, its fire safety, its appearance from public space or the environment, and for maintenance of a listed cultural monument (kultúrna pamiatka) (Section 18(4)(c) and (d)). Under Section 15, the building authority is the municipality.

In a heritage reserve or heritage zone (pamiatková rezervácia / zóna), even the owner of a property that is not itself a monument must ask the regional monuments board (krajský pamiatkový úrad) for a decision before altering the building (Section 32(5) of Act No. 49/2002 Coll. on the Protection of Monuments and Historic Sites). Changing the material or colour of flashing in a historic town centre is exactly such a case. Procedures for three cities: Bratislava, Košice, Prešov.

This is information, not legal advice. If you are unsure, ask your municipality’s building authority before you order any work.

Questions and answers

Do I need building permission to replace roof flashing?

Under Section 18(5)(a) of Building Act No. 25/2025 Coll., repairing and replacing a roof covering or the surface of a flat roof requires neither a decision on the building plan nor a notification. A notification is required, however, for maintenance that could affect stability, fire safety, the appearance of the building from public space or the environment, and for maintenance of a listed cultural monument (Section 18(4)). In a heritage area you need a decision from the regional monuments board in advance.

Can I combine copper and galvanised steel?

Not in a way that lets water run from copper onto galvanised steel, zinc or aluminium. The titanium-zinc manufacturers (RHEINZINK, VMZINC) and Lindab, in its warranty conditions, all rule out such run-off.