Flood hazard (P1, P2, P3)

Flood hazard maps (pericolosità idraulica) show the areas that can be flooded by rivers, streams or the sea, in three scenarios set out by the EU Floods Directive:

• P3, high hazard: frequent floods, on average one every 20 to 50 years;
• P2, medium hazard: less frequent floods, on average one every 100 to 200 years;
• P1, low hazard: rare floods or extreme events.

The areas sit one inside the other: a P3 area is normally also P2 and P1. The official maps are in the flood risk management plans (Piani di gestione del rischio di alluvioni, PGRA) and in the PAI plans of the river basin district authorities; ISPRA combines them into a national map.

What "once every 100 years" means

Frequency is expressed as a return period: a flood with a 100-year return period has a 1% chance of happening in any given year. It is not a schedule: it can come twice in the same decade. Over a 30-year mortgage, the odds of seeing at least one become:

Other labels you will find on the maps

• H, M, L: the same three classes with the initials of the EU directive (high, medium, low), used in some flood risk management plans.
• Bands A, B, C: in the Po basin the PAI divides the land along rivers into river bands (fasce fluviali). Band A is where the flood flows, band B the area flooded by the reference flood, normally with a 200-year return period, band C the area reached by the most severe floods. In bands A and B the rules strictly limit new building.
• R1, R2, R3, R4: these are risk classes, not hazard classes. They combine the probability of a flood with what stands there (homes, roads, businesses, people) and run from R1, moderate, to R4, very high.

How to check a house

Search the address on ISPRA's IdroGEO platform, which combines the maps for all of Italy, or on the river basin district authority's map portal, which is the official reference. The report runs the same check on the building's cadastral plot and the 100 metres around it, with a map of the nearby flood areas: the steps are in is a house in Italy in a flood zone?.

A house in a P2 or P3 area is still a house you can buy, with a few more questions: whether there are basement rooms, how high the ground floor is, whether the building has flooded before and how it is insured. Read more in Flood zones P1, P2 and P3.